ALBERT

Notes: Two of the biggest drawbacks of using permeable reactive barriers (PRBs) to treat contaminated ground water are the high capital cost of installation, particularly when the contaminated ground water is deep below ground surface, and the uncertainty of whether or not PRBs remain effective for the long time scales (e.g., decades) needed for many contaminant plumes. The use of an injection-extraction treatment well pair (IETWP) for capture and treatment of contaminated ground water can circumvent these difficulties, while still providing many of the same advantages offered by PRBs. In this paper, the hydraulics of IETWPs and PRBs are compared, focusing primarily on the width of the captured plume. It is demonstrated that IETWPs act as hydraulic barriers in a manner similar to PRBs, and that IETWPs provide excellent plume capture. A mathematical expression is presented for the plume capture width of an IETWP oriented perpendicular to the ground water flow direction in a homogeneous aquifer. Also discussed are other practical considerations that might determine whether an IETWP is better suited than a PRB for a particular contaminated site; these considerations include operating and maintenance costs, and the conditions under which an IETWP system can be used for in situ remediation.

Notes: Two environmental tracer methods are applied to the Ti-Tree Basin in central Australia to shed light on the importance of recharge from floodouts of ephemeral rivers in this arid environment. Ground water carbon-14 concentrations from boreholes are used to estimate the average recharge rate over the interval between where the ground water sample first entered the saturated zone and the bore. Environmental chloride concentrations in ground water samples provide estimates of the recharge rate at the exact point in the landscape where the sample entered the saturated zone. The results of the two tracer approaches indicate that recharge rates around one of the rivers and an extensive flood-plain are generally higher than rates of diffuse recharge that occurs in areas of lower topographic relief. Ground water 2H/1H and 18O/16O compositions are all depleted in the heavier isotopes (δ2H = -67%0 to -50%0; 518O = -9.2%0 to -5.7%0) compared with the long-term, amount-weighted mean isotopic composition of rainfall in the area (δ2H = -33.8%0; δ18O = -6.3%0). This indicates that recharge throughout the basin occurs only after intense rainfall events of at least 150 to 200 mm/month. Finally, a recharge map is developed to highlight the spatial extent of the two recharge mechanisms. Floodout recharge to the freshest ground water (TDS 〈1000 mg/L) is ∼1.9 mm/year compared with a mean recharge rate of ∼0.2 mm/year to the remainder of the basin. These findings have important implications for management of the ground water resource.

Notes: The delineation of well capture zones is a basic component of ground water protection. The conventional methodology for capture zone delineation is backward advective particle tracking, often applied under the assumption of a two-dimensional aquifer. The suitability of the conventional approach for complex heterogeneous multi-aquifer systems was investigated, using the Waterloo Moraine aquifer system as an example. It was found that the conventional approach produces irregular particle tracks that require judgment to interpret in a meaningful way, and it can raise questions that may affect the credibility of the capture zone delineation. As an alternative, the potentially powerful but little-used backward-in-time advective-dispersive transport approach was investigated. A key advantage of this approach is its capability to represent local heterogeneities through the dispersion term. The dispersion process has a natural smoothing effect that results in unambiguous capture zones without the need for interpretation, thus enhancing credibility. The question of capture zone validation is also addressed. The meaning of a three-dimensional capture zone is considered, and it is shown that a fully three-dimensional representation of the system is crucial for valid results. The distinction between the maximum extent capture zone and the surface capture zone is also explained. In the case of complex heterogeneous systems, advective particle tracking can be used as an initial screening tool, whereas the more realistic backward-transport modeling approach can be used for final capture-zone delineation.

Notes: Tracer tests represent the most appropriate approach for assessing hydrodispersive parameters such as transversal and longitudinal dispersivities or kinematic porosity on an aquifer scale. They are generally carried out by injecting a tracer in a borehole and measuring its concentration over time in neighboring boreholes by extracted volume sampling or downhole measurements. Logging is one of the most suitable methods for evaluating fissured reservoirs. But short circuits between fractures with different hydraulic potential through boreholes induce mixing phenomena that cannot be avoided without packers. This mixing can shift the breakthrough curves deduced from the logs for each producing fracture and distort determination of their location.The method proposed in this paper aims at measuring the flow rate and the solute breakthrough for hydraulically active fractures, in open boreholes. It involves estimating a velocity profile along the borehole column by the analysis of two successive logs: a shift function according to depth is thus determined by comparison between log portions on each successive one. The velocity gradients reflect the inward or outward flow rates produced by each fracture. On the basis of these flow rates, it is possible to determine the mixing effects inside the borehole and then to plot unbiased breakthrough curves for each producing fracture.This method was applied at a granitic site in the eastern Pyrenees. In spite of some questionable limitations, the results showed that the method seems adapted to situations with many fractures. The precise hydraulic pattern which is obtained at the borehole scale is discussed in terms of a dual porosity model. Furthermore, interpretation of the breakthrough curves for fractures corrected for mixing effects revealed that Peclet numbers are strongly underestimated if this phenomenon is not considered.

Notes: Transport of a conservative compound and two sorbing compounds through fractured chalk was studied using flow-through columns consisting of chalk cores with a single subvertical fracture. Two types of chalk matrix were compared, an oxidized white chalk with low organic carbon content (0.2%), and a gray chalk with a higher organic carbon content (1.3%). Initial rapid breakthrough followed by a delayed approach to a relative concentration of unity for the conservative compound (2,6–difluorobenzoic acid [DFBA]) was clear evidence for diffusion into the porous chalk matrix. Matrix diffusion of DFBA was apparently much greater in the gray chalk columns than in the white chalk columns. Breakthrough curves (BTCs) of the sorbing compounds (2,4,6–tri-bromophenol [TBP] and ametryn [AME]) were retarded in all cases as compared to the conservative compound. Sorption retardation was far greater in the gray chalk as compared with the white chalk, in good agreement with results from batch sorption experiments. BTCs for the conservative compound were relatively nonhysteretic for both white and gray chalk columns. In contrast, BTCs for the sorbing compounds were hysteretic in all cases, demonstrating that sorption was not at equilibrium before desorp-tion began. These experiments suggest that on a field scale, transport of contaminants through fractures in chalk and other fractured porous media will be attenuated by diffusion and sorption into the matrix.

Notes: Models for ground water flow (MODFLOW) and particle tracking (MODPATH) were used to determine ground water flow patterns, principal ground water discharge and recharge zones, and estimates of ground water travel times in an unconfined ground water system of an outer coastal plain watershed on the Delmarva Peninsula, Virginia. By coupling recharge and discharge zones within the watershed, flowpath analysis can provide a method to locate and implement specific management strategies within a watershed to reduce ground water nitrogen loading to surface water. A monitoring well network was installed in Eyreville Creek watershed, a first-order creek, to determine hydraulic conductivities and spatial and temporal variations in hydraulic heads for use in model calibration. Ground water flow patterns indicated the convergence of flow along the four surface water features of the watershed; primary discharge areas were in the noontide portions of the watershed. Ground water recharge zones corresponded to the surface water features with minimal development of a regional ground water system. Predicted ground water velocities varied between 〈 0.01 to 0.24 m/day, with elevated values associated with discharge areas and areas of convergence along surface water features. Some ground water residence times exceeded 100 years, although average residence times ranged between 16 and 21 years; approximately 95% of the ground water resource would reflect land use activities within the last 50 years.

Notes: Ground water reservoirs in the Choshuichi alluvial fan, central western Taiwan, were investigated using direct-current (DC) resistivity soundings at 190 locations, combined with hydrogeological measurements from 37 wells. In addition, attempts were made to calculate aquifer transmissivity from both surface DC resistivity measurements and geostatistically derived predictions of aquifer properties.DC resistivity sounding data are highly correlated to the hydraulic parameters in the Choshuichi alluvial fan. By estimating the spatial distribution of hydraulic conductivity from the kriged well data and the cokriged thickness of the correlative aquifer from both resistivity sounding data and well information, the transmissivity of the aquifer at each location can be obtained from the product of kriged hydraulic conductivity and computed thickness of the geoelectric layer. Thus, the spatial variation of the transmissivities in the study area is obtained.Our work is more comparable to Ahmed et al. (1988) than to the work of Niwas and Singhal (1981). The first “constraint” from Niwas and Singhal's work is a result of their use of linear regression. The geostatistical approach taken here (and by Ahmed et al. [1988]) is a natural improvement on the linear regression approach.

Notes: This paper presents a modeling analysis of the geochemical evolution of a contaminated sandy aquifer at a uranium mill tailings site in the western United States. The tailings pond contains fluids having a pH of 1.5 to 3.5 and high levels of As, Be, Cd, Cr, Pb, Mo, Ni, Se, 226Ra, 228Ra, 230Th, 238U, and 234U. Seepage of tailings fluids into the aquifer has formed a low-pH ground water plume. The reclamation plan is to install a low-permeability cover on the tailings pond to stop the seepage and allow the plume to be attenuated by reactions with the aquifer matrix and flushed by uncontaminated upgradient ground water. To evaluate this reclamation scenario, ground water and sediment core samples were analyzed along one flowpath. Speciation-solubility and mass-transfer modeling revealed two sets of chemical reactions for acid seepage and flushing, respectively. The current concentrations and distribution of ground water constituents can be interpreted as being controlled by stepwise pH-buffer reactions with calcite, amorphous aluminum hydroxide, and amorphous iron hydroxides. These buffer reactions divide the aquifer into zones of near-constant pH, separated by interface zones. For the flushing stage, it is predicted that reactions with surface-bound species will dominate the reaction paths, and more pore volumes are required to neutralize the plume than predicted by models that do not consider surface reactions. Direct mineralogical and surface analysis is needed to substantiate this assertion.

Notes: Redox conditions exercise important controls on water chemistry in the red-bed Sherwood Sandstone Aquifer of the English East Midlands. A distinct redox boundary exists some 3 to 5 km downgradient of the onset of confined conditions, defined by a 300 mV drop in Eh and complete reaction of dissolved oxygen. The aerobic aquifer contains polluted water with high nitrate concentrations and organic carbon significantly above background concentrations (〉 0.2 mg/L). Concentrations of Fe, Mn, and Mo are highest in reducing ground water. As, Sb, Se, and U show a residence-time-dependent increase in aerobic ground water, but are much lower under reducing conditions. Iron oxides are believed to play a key role in determining the spatial patterns in many of these trace elements as a result of Eh- and pH-controlled sorption/desorption reactions, as well as some reductive dissolution in the confined aquifer. Fresh ground water persists in the confined aquifer to approximately 30 km downgradient of the redox boundary. However, SO4 concentrations increase progressively along the flowline as a result of the dissolution of gypsum or anhydrite. Concentrations of available organic carbon are low in ground water (1 mg/L or less) and are also likely to be limited in the sediments; conditions are insufficiently reducing for significant sulphate reduction to have taken place. Only in the extreme down-gradient (eastern) part of the aquifer do conditions become sufficiently reducing with some evidence of sulphate reduction. In this part of the aquifer, ground water is more saline (TDS values up to 10 g/L) and is believed to be composed substantially of older formation water. This has distinctive concentrations of several redox-influenced trace elements, with relatively high Fe, Mn, As, and Sb, occasional high Cr, and low Mo relative to the confined fresh ground water upgradient.

Notes: Investigation techniques for karst flow systems are based mainly on the study of different signals leaving the system caused by natural or induced external influences. Each signal represents one of the systems outputs (e.g., hydraulic, chemical, physical, or isotopic responses) that reflect the characteristics of the entire system.In this paper, we present a method to infer information about the structure of karst systems. It is based on a simultaneous analysis of chemical and hydraulic responses. Beside the classical piston flow at the beginning of a flood pulse, we define a chemically based recession flow phase. During this phase, field data show that the concentration of total dissolved solids can be considered as an exponential function of the logarithm of flow. This relationship allows two parameters to be defined, one of which is dependent on the structure and degree of development of the karst conduit network, the other is dependent mainly on bioclimatic factors. Data collected from seven karst springs are used to support ideas introduced in the paper.

Notes: Vertical profiles of tetrachloroethene (or perchloroethylene, PCE) and trichloroethene (TCE) were used to validate a diffusion process in a natural aquitard at Dover Air Force Base, Delaware. PCE and TCE distributions in the aquitard underlying an unconfined aquifer were sampled from core tubes obtained at four times over the course of a 35–month field investigation within “test cells” that were isolated from the surrounding ground water by means of grout-sealed steel sheetpile barriers (Mackay et al. 2000). For the final 23 months of this period, boundary conditions at the aquiferJaquitard interface were such that a “back diffusion” of contaminants from the aquitard was induced. Modeling predictions of concentration changes were made on the basis of the earliest coring results and an assumption of sorption-retarded diffusion and using laboratory information about sorption and diffusion characteristics of the media. The predictive modeling was complicated by the fact that “initial” and “final” PCE and TCE distributions in the aquitard were measured at different (albeit proximate) coring locations, such that results reflect spatial variations in aquitard characteristics. This problem was solved by means of an inverse interpretation that involved spatial “translation” of observed profiles on the basis of the laboratory characterizations and assuming a common aquifer-side contaminant history. Predictions indicated substantial change in PCE and TCE concentrations within the upper aquitard (near the aquiferJaquitard interface) and the development of a back-diffusion profile up into the aquifer. Modeling also predicted comparatively minor profile changes in the deeper aquitard, and especially in the deep layer where sorption was strongest. All of these predicted effects were observed in the coring results. Although not exact, the agreement between predictions and observations was sufficiently good to justify the basic tenets of the diffusion model and to support a conclusion that major processes of advection and Jor transformation were unimportant within the 35-month time scale of this work.

Notes: Lineaments derived from three image types (1:80,000 black and white, 1:58,000 color infrared, and 1:250,000 side-looking airborne radar) were compared to water-bearing features within a 9.6 km section of tunnel being constructed through foliated crystalline metamorphic bedrock in a glaciated region of eastern Massachusetts. Lineaments drawn by three observers during two independent trials (N = 9137) were reduced to three sets (one per image type) of coincident lineaments (N = 794). Thirty-five coincident lineaments crossed the tunnel. Nineteen discrete flow zones, each producing ≥ 19 L/min, were identified in the tunnel and used to quantify the reliability of lineament analysis as a method of predicting water-bearing features in glaciated metamorphic rocks.Thirteen (68%) of the flow zones correlate with coincident lineaments, six zones correlate with more than one image type, and one zone correlates with all three image types. Overall, without additional corroborating evidence, it is difficult to interpret in advance which lineaments will result in a successful correlation with water-producing zones in the subsurface and which ones will not. Most of the observed flow (80%) correlates with northwest-trending coincident lineaments; however, the majority of the flow (67%) associated with these lineaments is produced from structures that strike to the north or northeast. In addition, only 15 of the 35 coincident lineaments correlate with the flow zones, indicating that 20 lineaments are not associated with any appreciable flow. Six flow zones are undetected by the lineament analysis.

Notes: A simple analytical model is presented for predicting subsurface gas flow to a vadose-zone well in response to atmospheric pressure fluctuations (barometric pumping). The effective radial permeability (kr) in the vicinity of the well is determined during model calibration using less than two weeks worth of data. By combining the flow solution with a solution for the vertical gas pressure, only atmospheric pressure data are required to predict the induced flow through a well. The ability to quantitatively predict naturally induced flow in vadose-zone wells by simple and inexpensive measurements is invaluable for systems using barometric pumping for remediation.

Notes: In the UK, the disposal of wastes from the production of potable water has been an increasing problem for water companies since the introduction of the Waste Management Licensing Regulations in 1994. This paper considers alternative management strategies. The importance of investigating each works individually is stressed because of variations in sludge properties and disposal costs. Of the disposal options, the growing of turf is environmentally sustainable and economically attractive, whereas direct spreading on agricultural land and use in soil production can be successful. Other uses, e.g. in ceramics, are at various stages of development; co-disposal with sewage is an option, but hidden costs need to be assessed.

Notes: Model Validation — Perspectives in Hydrological Science. Edited by Malcolm G. Anderson and Paul D. Bates. Groundwater Lowering in Construction -- A Practical Guide, by P. M. Cashman and M. Preene. Decentralised Sanitation and Re-Use -- Concepts, Systems and Implementation. Edited by P. Lens, G. Zeeman and G. Lettinga. The EU Water Framework Directive: An Introduction, by Peter Chave. Applied Hydrogeology (Fourth edition), by C. W. Fetter. Springs and Bottled Waters of the World. Edited by P. E. LaMoreaux and J. T. Tanner. Rapid Detection Assays for Food and Water. Edited by Stuart A. Clark, K. Clive Thompson, C. William Keevil and Mark S. Smith. Urban Rivers: Our Inheritance and Future. Edited by Geoff Petts, John Heathcote and Dave Martin. Compensating for Wetland Losses under the Clean Water Act. Microbial Pathogens and Disinfection By-products in Drinking Water: Health Effects and Management of Risks. Edited by G. F. Gaun, F. S. Hauchman and D. E. Robinson. Strategic Environmental Assessment in Transport and Land Use Planning, by Thomas B. Fisher. Economics of Water Resources -- The Contributions of Dan Yaron, by Ariel Dinar and David Zilberman. The Precautionary Principle in the Twentieth Century. Edited by Paul Harremoes et al. Transport Modelling in Hydrogeological Systems. By J. David Logan.

Notes: The paper identifies the mass and volume components of waterworks’sludge. The mass components arise from (i) the quality of the raw water treated, (ii) the chemicals used for treatment, and (iii) the efficiency with which those chemicals are utilised. Operation of the water-treatment processes and choice of treatment chemicals has an impact on the volume of sludge produced.The paper also identifies the roles in sludge mass and volume minimisation of (a) the choice and management of raw-water source, (b) the choice and management of coagulant and other treatment chemicals, sludge recycle and coagulant recovery, and (c) the choice and operation of water and sludge-treatment processes.

Notes: Experience of plumbosolvency control in the soft waters of north-west England is presented, and expectations for complying with the new lead standards are indicated. The influence of natural organic matter on the solubility of lead is highlighted - in particular its importance in the setting of target pH values in phosphated waters. An analysis of a large quantity of regulatory lead-compliance data confirms the benefit of the introduction of phosphate. At a distribution zone level, the impact and benefit of a change in plumbosolvency conditions on regulatory data are demonstrated using a non-parametric statistical technique. To assist in the operational management of plumbosolvency conditions, a number of performance metrics have been developed; these quantify (in a simple way), the quantity, variability and delivery of phosphate, and pH. The expected benefit from plumbosolvency control has been assessed in several ways, indicating that reasonable compliance against 25 μg/l might be achievable, but against 10 μg/l this is more questionable.

Notes: This study, which is based on responses from over 300 individuals in England and Wales, explores how a willingness to use a water-recycling system of specific configuration varies against a number of factors which are linked to system design, system scale, system context, and water-conservation behaviour. Findings show that there is broad willingness to accept in-house water recycling provided that public health is not compromised.

Notes: The environmental quality of many rivers on the Somerset Levels and Moors has been reduced because of past river-engineering practices and changes in land use. A new holistic and restorative approach to river management is now helping to improve the quality of aquatic ecosystems. This paper presents the results of a study which assessed the impact of past river and floodplain management on the Land Yeo, Somerset, with the objective of recommending measures for future management. The main findings are that (a) the ecological interest of the river has been reduced due to channel re-sectioning and flow diversion, and (b) the flow diversion is also causing water-quality problems such as dissolved-oxygen reduction. Most of this degradation could be reversed by the adoption of a number of channel and riparian-enhancement measures and by revising the annual maintenance programme.

Notes: This paper details the results of a survey of UK potable water supply organisations, which was undertaken in 1998 on behalf of UK Water Industry Research. An extrapolation of the data indicated that, in 1997, about 131 000 tDS of water-treatment works sludge were produced in the UK. The relevant methods and associated costs are reported. A number of companies are using, or developing, novel waste-management methods for water-treatment works sludges, such as incorporation into bricks and spreading on agricultural land, and these are discussed.

Notes: This paper discusses the problem of permeation of organic chemicals through plastic water-supply pipes, resulting in drinking-water contamination. Case studies are reported where contamination has resulted in taste and odour problems for customers, and might pose a risk to health. It is therefore recommended that (a) guidance is provided for water companies and building contractors, specifying when it is inappropriate to use unprotected plastic pipes for the provision of drinking water, and (b) measures are taken to ensure adherence to this guidance.

Notes: Increasingly strict legislation, escalating costs and external pressures are causing the water industry to look for alternative strategies to agricultural recycling for the disposal of biosolids. Although the development of the ‘safe sludge matrix’has encouraged agricultural recycling, negative press has hampered this outlet and public perception is seen as a major threat. Accordingly, many water companies are looking towards thermal-reduction technologies and have invested in thermal dryers and sludge incinerators; however, such technology has high capital costs and complex maintenance and operator requirements. An alternative method, i.e. co-combustion, is becoming more popular in mainland Europe. Unlike incineration/drying, the capital expenditure is low and operator/maintenance risks are minimised.This paper (i) discusses a case study in which 40 000 dry tonnes of sludge per annum have been successfully combusted since 1996, and (ii) explores how this experience could be applied in a UK situation.

Notes: A revision of the Regulations regarding the use of sewage sludge in agriculture has forced a change in emphasis in sludge treatment from stabilisation to achieving pathogen kill. Previous work carried out by United Utilities Service Delivery highlighted temperature as being a key factor in pathogen kill in mesophilic anaerobic digestion and a possible important factor in pathogen destruction in secondary digestion, which is normally carried out in large open tanks where temperature control is not possible. It was found that temperature is a key factor in the rate of pathogen de-activation under isothermal conditions. However, in the full-scale secondary digester trial under winter conditions it was found that there was a significant difference between the surface temperature and the bulk temperature within the sludge; at the surface the temperature decreased more quickly. There was no significant difference between the performance of the ‘old’and ‘new’design of concrete tank with respect to wall thickness, cooling rates and E. coli reduction rate. The glass-coated steel-tank design showed a marked increase in the cooling rate and, despite the low ambient temperatures and fluctuating sludge temperature, a minimum 1-log reduction in E. coli was achieved.

Notes: This paper presents a case study which describes water-availability problems and corresponding public-health implications for a rural area in Nigeria. A water-availability assessment was carried out in eight villages which are controlled by the Jalingo Local Government of Taraba State. Questionnaires were used to obtain data from sixty households (approximately 650 participants) concerning issues of water-supply sources, water quantity, water quality and community participation in water-resources projects. It was found that (i) community well water is the major source of water, (ii) 35% of the inhabitants depend on well water during the wet season, and (iii) 69% depend on it during the dry season. About 80% of the households have access to less than 30 litres of water per person per day and, because of low water availability, there is a prevalence of water-related diseases.

Notes: Particle counters are widely used in the water industry and generate a significant amount of data. To be useful to operators, such data need rapid interpretation. This paper describes how poor individual filter performance can be detected from particle-size analysis of the combined filtrate from a block of six rapid-gravity filters. Relationships between particle counts in the 2–5 μm size range and total count are explored, and the concept of an onsite real-time alarm system, monitoring differential particle-count data from combined filtrate, is developed.Consideration is given to the frequency of data collection. Whilst all major events were detected by collecting regimes with collection frequencies varying from 5 to 45 mins, it was found that the governing factor in determining collection frequency is the response time required by the operator to an incident.

Notes: Springs are unique freshwater habitats and represent ideal sites in which to examine the relationship between environmental variables and macro-invertebrate communities. The results of three case studies infer that species assemblages are determined by the physical habitat conditions within a spring and the ecological requirements of the component species. The composition of the macro-invertebrate community changes significantly along the longitudinal gradient of a spring-brook, as predicted by ecological theory. The results of the case studies are examined in relation to the conservation and protection of these little-studied habitats, and suggestions are made as to how to achieve this in the Wye Valley.

Notes: The Water Framework Directive was implemented in December 2000 and has to be transposed into Scottish legislation by 2003. The Directive provides a major opportunity to enhance the water environment of Scotland and to improve its management through the implementation of river-basin management planning with stakeholder and public engagement and the delivery of a programme of environmental-improvement measures to deliver good ecological status. Whilst the Directive provides unparalleled opportunities, there are significant challenges to its successful implementation in Scotland, where a step-change in managing the water environment will be required. This paper explores some of the challenges which include technical aspects, scale and timing issues, and capacity limitations.

Notes: The Flood Estimation Handbook, which was published in 1999, has substantially replaced the Flood Studies Report for design flood estimation in the UK. Although there are substantial differences between the handbook and the report, the philosophy of the rainfall-runoff method has been retained in the handbook. However, there are differences in the equations that have been used to estimate parameters of the rainfall-runoff method.This paper provides a comprehensive evaluation of the performance of the Flood Estimation Handbook rainfall-runoff method for flood-frequency estimation, using data from eighty-eight catchments in the UK. The performance is evaluated by reference to the flood quantiles obtained from single-site frequency analysis, and comparison is made with the performance of the Flood Studies Report methodology. In general terms, the handbook over-estimates flood quantiles, with respect to (a) those estimated from observed records and (b) the report's quantiles. The paper explores the causes of these over-estimations and seeks linkages with particular catchment characteristics.

Notes: Anguilla is a UK overseas (non-independent) territory in the Caribbean. It is a small middle-income island with a population of 13 000, and recent statistics indicate that it is more dependent on tourism than any other country or territory in the world. Rapid population growth and development have placed more emphasis on the need for human health and environmental protection. For the first time, a systematic water-quality monitoring programme has been developed, based on the realities of the local situation, e.g. low resource base, limited capabilities, dependence on household rainwater catchment, storage cisterns, and a poor water-supply network. Development of the programme incorporated the key themes of pragmatic, realistic, incremental, responsive, and consultative approaches.

Notes: Southern Water has implemented a large capital-investment programme in order to meet the requirements of the EU Urban Wastewater Treatment Directive. Within Southern Water, the Directive will result in a doubling in the quantity of wastewater sludge to be treated. In order to minimise the amount of sludge to be transported and produce a stabilised sludge which is suitable for recycling to agricultural land, the company's sludge strategy is based on treatment by mesophilic anaerobic digestion.This paper (a) highlights some of the consequences of operating a high-rate digestion plant, and (b) focuses on areas such as depositions in thickened sludge digester feed lines, the start-up procedures for high-rate digesters, foaming, hair and fibre build-up problems and treatment failure caused by toxicity inhibition. All these problems have been corrected or controlled and have resulted in Southern Water revising and updating its design manuals and standards, to prevent such future difficulties.

Notes: The paper describes a project which was carried out to establish whether particle counts can be used to predict the risk of Cryptosporidium and Giardia breakthrough at a large water-treatment works. The project was based at a direct river-abstraction works which treats up to 120 MI/d. On-line particle counters were installed following three of the rapid-gravity filters, and a fourth counter was used to measure particles in spot samples of raw and clarified water.There was a clear relationship between particle counts and the occasional low-level (oo)cyst (crypto oocysts and giardia cysts) occurrence in the filter samples, and no (oo)cysts were found in the final water. There appear to be complex inter-relationships between particle counts and other parameters such as raw-water conditions, environmental factors and treatment-plant variables.

Notes: This paper demonstrates the economic value of recycling treated biosolids to land in terms of (a) the effect on agricultural yield and (b) the avoidance of environmental and social costs of using conventional mineral fertilisers. Replicated field trials show that yield increases can be achieved when biosolids are used in conjunction with mineral fertilisers, compared with using conventional fertilisers. Relevant environmental and social costs of the production and use of both nitrogen and phosphate fertilisers and recycled biosolids are estimated. By recycling biosolids to agricultural land, the external costs associated with conventional mineral fertilisers can be fully recovered. The total replacement value of biosolids is the sum of the values of the increased yields and the replaced external costs. For digested cake this is equal to £2.64/tonne and for granules £10.85/tonne; in both cases the replacement value exceeds the external costs of biosolids. These costs and benefits should be taken into account in the development of environmental policy related to wastewater treatment.

Notes: This paper describes an appraisal methodology which has been developed and applied specifically to identify options for alleviating surface-water acidification in Wales in the short, medium and longer terms. It involves screening and scoping of options, ranging from those that might result in environmental improvement several years on from a policy change, to more immediate measures with results that can be seen on the ground - albeit only at a local level. The method relies on gaining consensus from a wide range of experts and involves (a) identification of options, (b) deciding on decision factors, (c) elicitation of scores for the impacts of each option on particular issues/decision factors, and (d) trade-off analysis using a non-parametric statistical technique. It is hoped that the lessons derived from this case study will be of use in further development and application of the methodology.

Notes: The increasingly common occurrence of eutrophic conditions in shallow lakes and reservoirs, including the presence of potentially toxic blue-green algae, has given rise to water-quality concerns throughout the world. The need to undertake active catchment management to improve raw-water quality is a key component of the new EC Water Framework Directive.This paper describes the investigations which were undertaken in Tallinn (the capital of Estonia, the northernmost of the Baltic States) to develop an integrated approach to catchment and lake management, in order to improve the quality of the surface-water resource which provides 90% of the total drinking-water needs of the city. Several tools, including ‘geographical information systems’and spreadsheet-based financial and water-resources models, were developed to investigate the source of the water-quality problems. Practical solutions comprising an innovative and cost-effective catchment-management approach to overcome the seasonal raw-water quality issues, whilst also reducing treatment plant operating costs, are described. Some of the recommendations have been tested by Tallinn Water following the investigations, and some improvements in raw and treated water quality have been attained.

Notes: Water Pollution and Water Quality Law, by W. Howarth and D. McGilivray. The Law of International Watercourses, by Stephen C. McCaffrey.

Notes: The use of hybrid reed-bed treatment systems to treat domestic sewage has become widespread in mainland Europe and the UK. However, in Ireland, the application of combined vertical and horizontal beds is still at an experimental stage. The existing septic-tank system at Colecott (County Dublin) was upgraded by the construction of vertical and horizontal reed-beds in series. This paper discusses the rationale behind its selection, design, construction and operation.

Notes: In 1996, Thames Water launched a range of peat-free compost products based on biosolids. In order to keep up with demand and to maintain the high standards of product quality, the company has investigated higher-rate composting processes and optimisation of the maturation stage. Agitated-bay composting systems are the preferred in-vessel technology for large sludge treatment centres using woodchip as a bulking agent. These automated systems (i) accelerate the composting process, (ii) maximise throughput on a smaller footprint, and (iii) achieve better control of odour emissions.A maturation trial with composted material from the US Filter agitated-bay system confirmed that a horticultural-grade product could be attained. Small-scale maturation trials showed that seeding with mature green waste compost can accelerate nitrification to such a rate that near zero levels of ammonia can be achieved after one month of maturation.

Notes: Since 1982, Danish legislation on streams and drainage has included clauses which make it possible to implement river restoration. In this paper, the Danish shift in policy from drainage to the return of wetlands is described. To illustrate the new beginning on catchment management, two major river-restoration schemes, i.e. the River Brede demonstration project and the River Skjern Nature project, are presented. Both projects have been implemented and consist of many parts, including (a) returning the straight, regulated rivers back to their former meanders, (b) introducing better hydraulic interaction between the river and its meadows, and (c) establishing former lakes, bogs, ponds and marshes. Also, an increase in security against flooding has resulted, as the regulated ‘water-motorways’have become more natural, with buffer-zones and retention of water in the restored features.

Notes: Rainwater Catchment Systems for Domestic Supply: Design, Construction and Implementation, by John Gould and Erik Nissen-Petersen.Hydraulic Gates and Valves, by Jack Lewin.Water Quality - Processes and Policy. Edited by S. T. Trudgill, D. E. Walling and B. C. Webb.A Celebration of Engineering: Key Papers from the Last Century.Management of Shared Water Resources: The Israeli-Palestinian Case with an International Perspective. Edited by E. Feitelson and M. Haddad.El Nino. The Weather Phenomenon That Changed The World, by Ross Couper-Johnston.Dictionary of Environmental Science and Technology, by Andrew Porteous.Life Style: A Parable of Sharing, by A. H. Dammers.Mendel's Demon: Gene Justice and the Complexity of Life, by Mark Ridley.Population Ecotoxicology, by Michael C. Newman.Upgrading Water Treatment Plants, by E. G. Wagner and R. G. Pinheiro.Water Resources Engineering (First Edition), by Larry W. Mays.Arsenic in Drinking Water: 2001 Update. National Research Council. National Research Council, National Academic Press, Washington DC.Air Quality Assessment and Management: A Practical Guide, by D. Owen Harrop.

Notes: The ‘water-balance calculation’is a valuable method of assessing the integrity of many components of a company's distribution system and is used in (a) evaluating the overall performance of the system and (b) assessing, for example, leakage within distribution zones or district metered areas. However, the quality of data which are derived from balance calculations reflects the quality of the input data, and each input source (whether metered or estimated) is subject to uncertainty.This paper explores the impact of input uncertainty on the water-balance calculation, and uses Anglian Water's annual balance as an example. The principal contributors to overall uncertainty are identified, and these show that the overall uncertainty depends upon only a few of the many inputs. Identifying these critical inputs has enabled Anglian Water staff to target the areas of their auditing chains that will yield greatest benefit to the reporting process.

Notes: Deep observation boreholes in the vicinity of active production wells in Honolulu, Hawaii, exhibit the anomalous condition that fluid-column electrical conductivity logs and apparent profiles of pore-water electrical conductivity derived from induction conductivity logs are nearly identical if a formation factor of 12.5 is assumed. This condition is documented in three boreholes where fluid-column logs clearly indicate the presence of strong borehole flow induced by withdrawal from partially penetrating water-supply wells. This result appears to contradict the basic principles of conductivity-log interpretation. Flow conditions in one of these boreholes was investigated in detail by obtaining flow profiles under two water production conditions using the electromagnetic flowmeter. The flow-log interpretation demonstrates that the fluid-column log resembles the induction log because the amount of inflow to the borehole increases systematically upward through the transition zone between deeper salt water and shallower fresh water. This condition allows the properties of the fluid column to approximate the properties of water entering the borehole as soon as the upflow stream encounters that producing zone. Because this condition occurs in all three boreholes investigated, the similarity of induction and fluid-column logs is probably not a coincidence, and may relate to aquifer response under the influence of pumping from production wells.

Notes: The use of small volume elastic collection bags (condoms) has become popular in seepage meter studies in recent years, despite minimal field or laboratory validation of their use and, specifically, the impact of their elasticity on seepage measurements. A laboratory study was initiated after field results using small elastic collection bags produced seepage data that did not correlate with hydrometric data. The laboratory data demonstrate that condoms undergo significant mechanical relaxation during seepage measurement times typically observed in field settings. Unlike conventional nonelastic collection bags, which mechanically relax over several minutes, the condoms suffered from a slow mechanical relaxation or equilibration. Over nine hours, condoms gained 43 mL of water, -50% of maximum workable volume (between mechanical relaxation effect and elastic limit), under stagnant flow conditions. This long-term equilibration invalidates simple subtraction of equilibration volumes from collection volumes as a correction technique. Previously published studies using flexible small-volume elastic measurement bags (condoms) have not reported a mechanical relaxation effect. Overall, because the condom's small workable volume and inherent variability, we would not recommend any small-volume elastic measurement bags for quantitative seepage measurements.

Notes: The capture zone or contributing area of a ground water extraction well can be defined as that portion of the aquifer from which the well draws its water. Accurate delineation of capture zones is important in many ground water remediation applications and in the definition of wellhead protection areas. Their mathematical delineation is often simplified by using quasi-steady-state models based on time-weighted average pumping rates and background hydraulic gradients. We present a new semianalytic approach for the definition of capture zones under transient-flow conditions. We then use this approach to evaluate the effects of time variations in the direction of the background hydraulic gradient on capture. Results are presented in the form of capture efficiency maps (CEMs). Although the area contributing to a given well is found to generally expand relative to the steady-state average capture zone when the gradient direction varies, the zone of 100% capture may expand or contract depending on site-specific conditions. We illustrate our CEM approach by applying it to the design of a plume containment system.

Notes: In time-varying ground water remediation, the lack of an optimal control algorithm to simultaneously consider fixed costs and time-varying operating costs makes it nearly impossible to obtain an optimal solution. This study presents a novel algorithm that integrates a genetic algorithm (GA) and constrained differential dynamic programming (CDDP) to solve this time-varying ground water remediation problem. A GA can easily incorporate the fixed costs associated with the installation of wells. However, using a GA to solve for time-varying policies would dramatically increase the computational resources required. Therefore, the CDDP is used to handle the subproblems associated with time-varying operating costs. A hypothetical case study that incorporates fixed and time-varying operating costs is presented to demonstrate the effectiveness of the proposed algorithm. Simulation results indicate that the fixed costs can significantly influence the number and locations of wells, and a notable total cost savings can be realized by applying the novel algorithm herein.

Notes: The effect of increasing the amount of hydraulic conductivity (K) data on the level of uncertainty about contaminant concentration predictions was examined. Contaminant concentrations were predicted using Monte Carlo numerical simulations with progressively more K information, and were compared to reference data obtained from a physical experiment. Increasing the amount of K data used in generating the aquifer's K field improved the measure of confidence in the predicted contaminant concentration. The normalized concentration interval with 95% confidence decreased from 1.29 to 0.75 when the amount of K data increased from 20 to 71. However, the trade-off is the increase in the costs of aquifer investigation. Conducting Monte Carlo numerical simulations may help design an optimal aquifer investigation scheme, so that it can provide enough input data to predict concentrations with the desired level of confidence and at the same time avoid the excessive costs of aquifer investigation.

Notes: Ground water chemistry data collected over a six-year period show that the distribution of contaminants and redox processes in a shallow petroleum hydrocarbon-contaminated aquifer has changed rapidly over time. Shortly after a gasoline release occurred in 1990, high concentrations of benzene were present near the contaminant source area. In this contaminated zone, dissolved oxygen in ground water was depleted, and by 1994 Fe(lll) reduction and sulfate reduction were the predominant terminal electron accepting processes. Significantly, dissolved methane was below measurable levels in 1994, indicating the absence of significant methanogenesis. By 1996, however, depletion of solid-phase Fe(lll)-oxyhydroxides in aquifer sediments and depletion of dissolved sulfate in ground water resulted in the onset of methanogenesis. Between 1996 and 2000, water-chemistry data indicated that methanogenic metabolism became increasingly prevalent. Molecular analysis of 16S-rDNA extracted from sediments shows the presence of a more diverse methanogenic community inside as opposed to outside the plume core, and is consistent with water-chemistry data indicating a shift toward methanogenesis over time. This rapid evolution of redox processes reflects several factors including the large amounts of contaminants, relatively rapid ground water flow (∼0.3 m/day [∼1 foot/day]), and low concentrations of microbially reducible Fe(lll) oxyhydroxides (∼ 1 umol/g) initially present in aquifer sediments. These results illustrate that, under certain hydrologic conditions, redox conditions in petroleum hydrocarbon-contaminated aquifers can change rapidly in time and space, and that the availability of solid-phase Fe(lll)-oxyhydroxides affects this rate of change.

Notes: An analytical model of stream/aquifer interaction is proposed that predicts drawdown in an aquifer with leakage from a finite-width stream induced by pumping from a well. The model is formulated based on the assumptions of stream partial penetration, a semipervious streambed, and distributed recharge across a finite-width stream. Advantages of the analytical solution include its simple structure, consisting of the Theis well function with integral modifications. The solution is derived for the semi-infinite domain between the stream and pumping well, which is of primary interest to hydrogeologists. Previous stream/aquifer analytical models are compared to the analytical solution based on dimensionless drawdown profiles. Drawdown in the aquifer near a wide stream was found to be less than that predicted by a solution that ignored stream width. Deviations between the proposed analytical solutions and previous solutions increase as stream width increases. For a hypothetical stream/aquifer system, the proposed analytical solution was equivalent to prior solutions when the ratio of the distance between the stream and aquifer to the stream width was greater than 25. This analytical solution may provide improved estimates of aquifer and streambed leakage parameters by curve fitting experimental field drawdown data.

Notes: The interaction between a gaining stream and a water-table aquifer is studied at an outwash plain. The aquifer is hydraulically well connected to the stream. Pumping tests were carried out in 1997 and 1998 in two wells 60 m from the stream, screening different depths of the aquifer. Drawdown was measured on both sides of the stream. Hydraulic head, drawdown, and stream depletion data were analyzed using numerical flow models. Similar models were fitted to each of two different data sets: Model A was fitted to steady-state hydraulic head and streamflow gain data not influenced by pumping; and model B was fitted to drawdown data measured during the 1998 pumping test. Each calibrated model closely fits its calibration data; however, predictions were biased if model A was used to predict the calibration data of model B, and vice versa. To further test the models, they were used to predict streamflow depletion during the two pumping tests as well as the drawdown during the 1997 test. Neither of these data were used for calibration. Model A predicted the measured depletions fairly accurately during both tests, whereas the predicted drawdowns in 1997 were significantly larger than actually measured. Contrary to this, the 1997 drawdowns predicted by model B were nearly unbiased; the predicted depletions deviate significantly from the measured depletions in 1997, but they compare well with the observations in 1998. Thus, although field work and analyses were extensive and done carefully to develop a ground water flow model that could predict both drawdown and streamflow depletion, the model predictions are biased. Analyses indicate that the deviations between model and data may be because of error in the models' representations of either the release of water from storage or of the hydrology in the riparian zone.

Notes: The determination of anisotropy in a phreatic aquifer by means of a pumping test can be carried out, under given conditions, with common analysis methods. This paper develops and applies an amended method based on a technique proposed by Boulton (1970). This new method enables a quick, simple determination of vertical hydraulic conductivity, given drawdown data from a pumping well and at least one observation well, that is applicable to confined phreatic aquifers.

Notes: One of the mechanisms of sudden particle release from grain surfaces in natural porous media is a decrease in salt concentration of the permeating fluid to below the critical salt concentration. Particle release can cause a change in hydraulic conductivity of the matrix, either by washing out the fines and thus increasing the pore sizes or by the plugging of pore constrictions. The phenomenon of permeability changes as a result of particle detachment was investigated in a series of column experiments. Coarse and fine sediments from the Hanford Formation in southeast Washington were tested. Columns were subject to a pulse of highly saline solution (NaNO3) followed by a fresh water shock causing particle release. Outflow rates and changes in hydraulic head as well as electric conductivity and pH were monitored over time. No permeability decrease occurred within the coarse matrix alone. However, when a thin layer of fine sediment was embedded within the coarse material (mimicking field conditions at the Hanford site), permeability irreversibly decreased to 10% to 20% of the initial value. Evidence suggests that most of this permeability decrease was a result of particles detached within the fine layer and its subsequent clogging. An additional observation was a sudden increase in pH in the outflow solution, generated in situ during the fresh water shock. Because layered systems are common in natural settings, our results suggest that alteration between sodium solution and fresh water can lead to particle release and subsequently reduce the overall permeability of the matrix.

Notes: Complex aquifer systems are often modeled with quasi-three-dimensional models, which consider two-dimensional horizontal flow in the aquifers and one-dimensional vertical flow through aquitards. When the aquifer system consists of a phreatic aquifer and one or more semiconfined aquifers connected by aquitards, the discrete model consists of a nonlinear system of algebraic equations, because the transmissivity of the phreatic aquifer depends on the phreatic head. If the water extraction is very high, the phreatic aquifer can be depleted and the equations of the model must be modified accordingly. There are not simple and general criteria to state if the phreatic aquifer is depleted before solving the system of equations. Therefore, the iterative procedures (e.g., relaxation methods), used to find the solution to the forward problem, must handle these particular conditions and can suffer several problems of convergence. These problems can be caused by the choice of the initial head values or of the relaxation coefficient of the iterative algorithms; however, they can also be caused by the nonexistence or nonuniqueness of the solution to the system of nonlinear equations. The study of existence and uniqueness of the general problem is very difficult and, therefore, we consider a simplified problem, for which the discrete model can be handled analytically. The results of the numerical experiments show that the solution to the forward problem can be nonunique. Only for some cases it is possible to invoke physical arguments to eliminate tentative solutions.

Notes: Recession curves contain information on storage properties and different types of media such as porous, fractured, cracked lithologies and karst. Recession curve analysis provides a function that quantitatively describes the temporal discharge decay and expresses the drained volume between specific time limits (Hall 1968). This analysis also allows estimating the hydrological significance of the discharge function parameters and the hydrological properties of the aquifer. In this study, we analyze data from perennial springs in the Judean Mountains and from others in the Galilee Mountains, northern Israel. All the springs drain perched carbonate aquifers. Eight of the studied springs discharge from a karst dolomite sequence, whereas one flows out from a fractured, slumped block of chalk. We show that all the recession curves can be well fitted by a function that consists of two exponential terms with exponential coefficients α1 and α2. These coefficients are approximately constant for each spring, reflecting the hydraulic conductivity of different media through which the ground water flows to the spring. The highest coefficient represents the fast flow, probably through cracks, or quickflow, whereas the lower one reflects the slow flow through the porous medium, or baseflow. The comparison of recession curves from different springs and different years leads to the conclusion that the main factors that affect the recession curve exponential coefficients are the aquifer lithology and the geometry of the water conduits therein. In normal years of rainy winter and dry summer, α1 is constant in time. However, when the dry period is longer than usual because of a dry winter, α1 slightly decreases with time.

Notes: A method to predict aquifer vulnerability to pesticide contamination at the subregional scale was developed. The assessment method was designed to incorporate relevant hydrologic and pesticide-transport information and to use generally available data. The method assumes steady-state advection of pesticides in the vadose zone, including sorption and biological decay. The solution is presented as a vulnerability index (VI) that increases as the aquifer vulnerability increases. The hydrologic input data for the VI model are obtained from the soil survey geographic database. Pesticides were grouped into three leachability classes using a leachabiiity ratio (half-life divided by organic carbon partition coefficient). Pesticide transformation is assumed to occur in the surface layer. The influence of vertical transport in the remainder of the vadose zone has been incorporated by applying a multiplying factor to the VI that varies with depth to ground water. Hydraulic conductivity is used as a surrogate for soil-water velocity for practical purposes. The performance of the VI model is evaluated using ground water data from Weld County, Colorado. The model is demonstrated to be successful at predicting relative vulnerability, defined as the magnitude of pesticide concentration and percent of wells in a unit that exhibit a pesticide detection. Areas of low, medium, and high vulnerability are assigned. Results indicate that the vulnerability classifications are most dependent on the leachability ratio, hydraulic conductivity, and organic carbon content.

Notes: Ground water discharge to the Great Lakes around the Lower Peninsula of Michigan is primarily from recharge in riparian basins and proximal upland areas that are especially important to the northern half of the Lake Michigan shoreline. A steady-state finite-difference model was developed to simulate ground water flow in four regional aquifers in Michigan's Lower Peninsula: the Glaciofluvial, Saginaw, Parma-Bayport, and Marshall aquifers interlayered with the Till/“red beds,” Saginaw, and Michigan confining units, respectively. The model domain was laterally bound by a continuous specified-head boundary, formed from lakes Michigan, Huron, St. Clair, and Erie, with the St. Clair and Detroit River connecting channels.The model was developed to quantify regional ground water flow in the aquifer systems using independently determined recharge estimates. According to the flow model, local stream stages and discharges account for 95% of the overall model water budget; only 5% enters the lakes directly from the ground water system. Direct ground water discharge to the Great Lakes' shorelines was calculated at 36 m3/sec, accounting for 5% of the overall model water budget. Lowland areas contribute far less ground water discharge to the Great Lakes than upland areas. The model indicates that Saginaw Bay receives only ∼1.13 m3/sec ground water; the southern half of the Lake Michigan shoreline receives only ∼2.83 m3/sec. In contrast, the northern half of the Lake Michigan shoreline receives more than 17 m3/sec from upland areas.

Notes: Study of ground water in the Southern High Plains is central to an understanding of the geomorphology, deposition of economic minerals, and climate change record in the area. Ground water has controlled the course of the Canadian and Pecos rivers that isolated the Southern High Plains from the Great Plains and has contributed significantly to the continuing retreat of the westward escarpment. Evaporative and dissolution processes are responsible for current plateau topography and the development of the signature 20,000 small playa basins and 40 to 50 large saline lake basins in the area. In conjunction with eolian processes, ground water transport controls the mineralogy of commercially valuable mineral deposits and sets up the distribution of fine efflorescent salts that adversely affect water quality. As the water table rises and retreats, lunette and tufa formation provides valuable paleoclimate data for the Southern High Plains. In all these cases, an understanding of ground water processes contributes valuable information to a broad range of geological topics, well beyond traditional interest in water supply and environmental issues.

Notes: While significant progress has been made in the theoretical development of the simulation/optimization (S/O) approach for ground water remediation design, its application to large, field-scale problems has remained limited. To demonstrate the applicability and usefulness of the S/O approach under real field conditions, an optimization demonstration project was conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts, involving the design of a pump-and-treat system for the containment and cleanup of a large trichloroethylene (TCE) plume. The optimization techniques used in this study are based on evolutionary algorithms coupled with a response function approach for greater computational efficiency. The S/O analysis was performed parallel to a conventional trial-and-error analysis based on simulation alone. The results of this study demonstrate that not only would it be possible to remove more TCE mass under the same amount of pumping assumed in the trial-and-error design, but also substantial cost savings could be achieved by reducing the number of wells needed and adapting dynamic pumping. In spite of the large model size of more than 500,000 nodes and a long planning horizon of 30 years, the optimization modeling was carried out successfully on desktop PCs. This field demonstration project clearly illustrates the potential benefits of applying optimization techniques in remediation system design.

Notes: In ground water flow and transport modeling, the heterogeneous nature of porous media has a considerable effect on the resulting flow and solute transport. Some method of generating the heterogeneous field from a limited dataset of uncertain measurements is required. Bayesian updating is one method that interpolates from an uncertain dataset using the statistics of the underlying probability distribution function. In this paper, Bayesian updating was used to determine the heterogeneous natural log transmissivity field for a carbonate and a sandstone aquifer in southern Manitoba. It was determined that the transmissivity in m2/sec followed a natural log normal distribution for both aquifers with a mean of -7.2 and - 8.0 for the carbonate and sandstone aquifers, respectively. The variograms were calculated using an estimator developed by Li and Lake (1994). Fractal nature was not evident in the variogram from either aquifer. The Bayesian updating heterogeneous field provided good results even in cases where little data was available. A large transmissivity zone in the sandstone aquifer was created by the Bayesian procedure, which is not a reflection of any deterministic consideration, but is a natural outcome of updating a prior probability distribution function with observations. The statistical model returns a result that is very reasonable; that is homogeneous in regions where little or no information is available to alter an initial state. No long range correlation trends or fractal behavior of the log-transmissivity field was observed in either aquifer over a distance of about 300 km.

Notes: In a typical ground water flow model, lakes are represented by specified head nodes requiring that lake levels be known a priori. To remove this limitation, previous researchers assigned high hydraulic conductivity (K) values to nodes that represent a lake, under the assumption that the simulated head at the nodes in the high-K zone accurately reflects lake level. The solution should also produce a constant water level across the lake. We developed a model of a simple hypothetical ground water/lake system to test whether solutions using high-K lake nodes are sensitive to the value of K selected to represent the lake. Results show that the larger the contrast between the K of the aquifer and the K of the lake nodes, the smaller the error tolerance required for the solution to converge. For our test problem, a contrast of three orders of magnitude produced a head difference across the lake of 0.005 m under a regional gradient of the order of 10−3 m/m, while a contrast of four orders of magnitude produced a head difference of 0.001 m. The high-K method was then used to simulate lake levels in Pretty Lake, Wisconsin. Results for both the hypothetical system and the application to Pretty Lake compared favorably with results using a lake package developed for MODFLOW (Merritt and Konikow 2000). While our results demonstrate that the high-K method accurately simulates lake levels, this method has more cumbersome postprocessing and longer run times than the same problem simulated using the lake package.

Notes: The desorption of contaminants from soils/sediments is one of the most important processes controlling contaminant transport and environmental risks. None of the currently adopted desorption models can accurately quantify desorption at relatively low concentrations; these models often overestimate the desorption and thus the risks of hydrophobic organic chemicals, such as benzene and chlorinated solvents. In reality, desorption is generally found to be biphasic, with two soil-phase compartments. A new dual-equilibrium desorption (DED) model has been developed to account for the biphasic desorption. This model has been tested using a wide range of laboratory and field data and has been used to explain key observations related to underground storage tank plumes. The DED model relates the amount of a chemical sorbed to the aqueous concentration, with simple parameters including octanol-water partition coefficient, solubility, and fractional organic carbon; thus, it is the only biphasic model, to date, that is based on readily available parameters. The DED model can be easily incorporated into standard risk and transport models. According to this model, many regulatory standards of soils and sediments could be increased without increasing the risks.

Notes: Complex stratigraphy can be difficult to simulate in MODFLOW models. MODFLOW uses a structured grid that requires that each grid layer be continuous throughout the model domain. This makes it difficult to explicitly represent common features such as pinchouts and embedded seams in a MODFLOW model. In this paper, we describe a method for automatically generating MODFLOW-compatible grids from boundary-representation solid models. Solid models are data structures developed originally for computer-aided design applications that define the geometry of three-dimensional objects. Solid models can be used to represent arbitrarily complex stratigraphy. The elevations defined by the solids are then extracted from the solids in a manner that preserves the continuous-layer requirement imposed by MODFLOW. Two basic approaches are described: The first method adjusts the MODFLOW grid dimensions (layer elevations) to fit the solid model boundaries, and the second method creates a regular MODFLOW grid and adjusts the material properties to match the changes in stratigraphy. One of the main benefits of using solid models to define stratigraphy for MODFLOW models is that it provides a grid-independent definition of the layer elevations that can be used to immediately re-create the MODFLOW grid geometry after any change to the grid resolution.

Notes: The Lisse effect is a rarely noted phenomenon occurring when infiltration caused by intense rain seals the surface soil layer to airflow, trapping air in the unsaturated zone. Compression of air by the advancing front results in a pressure increase that produces a water-level rise in an observation well screened below the water table that is several times as large as the distance penetrated by the wetting front. The effect is triggered by intense rains and results in a very rapid water-level rise, followed by a recession lasting a few days. The Lisse effect was first noted and explained by Thal Larsen in 1932 from water-level observations obtained in a shallow well in the village of Lisse, Holland. The original explanation does not account for the increased air pressure pushing up on the bottom of the wetting front. Analysis of the effect of this upward pressure indicates that a negative pressure head at the base of the wetting front, Ψf, analogous to that postulated by Green and Ampt (1911) to explain initially rapid infiltration rates into unsaturated soils, is involved in producing the Lisse effect. Analysis of recorded observations of the Lisse effect by Larsen and others indicates that the water-level rise, which typically ranges from 0.10 to 0.55 m, should be only slightly larger than |Ψf| and that the depth of penetration of the wetting front is no more than several millimeters.

Notes: In this study, a new method-the asymmetric dipole-flow test—is proposed and tested for characterization of conductive properties and structure of fractured aquifers. Analytical solutions were developed and then used for interpretation of a modification of the dipole-flow test with a single packer at the Bissen Quarry test site (Wisconsin, USA). The asymmetric dipole-flow tests were conducted by packing a well at various elevations, and fluids were pumped from the upper section (chamber) of the well to the lower section (chamber). The head was then monitored at 11 observation points and in both sections of the well, and the conductivities of the well segments were determined. The tests at seven packer elevations in the well were rapid (less than one hour to reach steady state). The asymmetric dipole-flow test demonstrates the potential to quantify heterogeneities of a fractured aquifer and delineate the applicability of the continuum and discrete approaches for conceptualization of ground water flow.

Notes: The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only 25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one-dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 μm) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (∼50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters.

Notes: Within the ground water community, the idea persists that if one can estimate the recharge to a ground water system, one then can determine the size of a sustainable development. Theis addressed this idea in 1940 and showed it to be wrong-yet the myth continues. The size of a sustainable ground water development usually depends on how much of the discharge from the system can be “captured” by the development. Capture is independent of the recharge; it depends on the dynamic response of the aquifer system to the development. Ground water models were created to study the response dynamics of ground water systems; it is one of the principal reasons hydrogeologists model.

Notes: The migration of five dense nonaqueous phase liquids (DNAPLs) through a single fracture in a clay aquitard was numerically simulated with the use of a compositional simulator. The effects of fracture aperture, fracture dip, matrix porosity, and matrix organic carbon content on the migration of chlorobenzene, 1,2-dichloroethylene, trichloroethyiene, tetrachloroethylene, and 1,2-dibromoethane were examined. Boundary conditions were chosen such that DNAPL entry into the system was allowed to vary according to the stresses applied. The aperture is the most important factor of those studied controlling the migration rate of DNAPL through a single fracture embedded in a clay matrix. Loss of mass to the matrix through diffusion does not significantly retard the migration rate of the DNAPL, particularly in larger aperture fractures (e.g., 50 urn). With time, the ratio of diffusive loss to the matrix to DNAPL flux into the fracture approaches an asymptotic value lower than unity. The implication is that matrix diffusion cannot arrest the migration of DNAPL in a single fracture. The complex relationships between density, viscosity, and solubility that, to some extent, govern the migration of DNAPL through these systems prevent accurate predictions without the use of numerical models. The contamination potential of the migrating DNAPL is significantly increased through the transfer of mass to the matrix. The occurrence of opposite concentration gradients within the matrix can cause dissolved phase contamination to exist in the system for more than 1000 years after the DNAPL has been completely removed from the fracture.

Notes: Steady and transient seepage problems were investigated numerically using a dimensionless formulation for water flow in variably saturated, two-dimensional anisotropic and homogenous porous media. The dimensionless formulation combines the aspect ratio and the anisotropy ratio in one dimensionless parameter, M, and accounts for capillarity effects using the ratio of the domain height to the height of capillary fringe as another dimensionless parameter, a. Two domain geometries were considered: rectangular and trapezoidal. It was found that M and a play major roles in the development and the height of the seepage face. The height of the steady-state seepage face increased with the decreasing value of a.The effects of capillarity on the development of the transient seepage face were investigated by lowering the open water level on one side of the domain at various uniform velocities while the other side was kept at a constant value. It was found for most scenarios that decoupling between the water table and the open water level first occurred for domains with large a. In other words, the seepage face heights of these systems were larger than those with a small a. These results are the opposite of the steady-state results and are due to the fact that drainage of the pores was the major mechanism controlling the drop of the transient water table, especially at earlier times.A criterion for the decoupling of the water table from the falling open water level (i.e., the formation of the transient seepage face) was developed by Dracos (1965). It states that decoupling does not occur as long as the falling speed, VF, of the open water level is less than Vd= Ko sin2 B/φ where Ko is the saturated hydraulic conductivity of the soil, φ is the porosity, and β is the angle of the exit face with the horizontal (i.e., β=π/2 implies a vertical face). Our investigation revealed that decoupling occurs for smaller falling velocities than Vd, and is caused by, among others, the fact that the water table is not tangent to the exit face but rather intersects it at a nonzero angle.

Notes: Model ground water ages based on chlorofluorocarbons (CFCs) and tritium/helium-3 (3H/3He) data were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province of Pennsylvania. The fracture geometry of the gently east plunging fold is very regular and consists predominately of south dipping to subhorizontal to north dipping bedding-plane parting and east striking, steeply dipping axial-plane spaced cleavage. In the area of the piezometer arrays, which trend north-south on the north limb of the fold, north dipping bedding-plane parting is a more dominant fracture set than is steeply south dipping axial-plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding-plane parting has younger 3H/3He and CFC model ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. In predominantly unmixed samples there is a strong positive correlation between age of the young fraction of water and dissolved sodium concentration. The travel times inferred from the model ages are significantly longer than those previously calculated by a ground water flow model, which assumed isotropically fractured layers parallel to topography. A revised model factors in the directional anisotropy to produce longer travel times. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground water models in other areas underlain by similarly tilted or folded sedimentary rock, such as elsewhere in the Valley and Ridge or the early Mesozoic basins.

Notes: Numerical modeling techniques were used to simulate stream-aquifer interactions from seasonal ground water pumping. We used stream-aquifer models in which a shallow stream penetrates the top of an aquifer that discharges ground water to the stream as base flow. Because of the pumping, the volume of base flow discharged to the stream was reduced, and as the pumping continued, infiltration from the stream to the aquifer was induced. Both base-flow reduction and stream infiltration contributed to total stream depletion. We analyzed the depletion rates and volumes of the reduced base flow and induced stream infiltration during pumping and postpumping periods. Our results suggested that for a shallow penetrating stream with a low streambed conductance, base-flow reduction accounts for a significant percentage of the total stream depletion. Its residual effects in postpumping can last very long and may continue into the next pumping season for areas where recharge is nominal. In contrast, the contribution of the induced stream infiltration to the total stream depletion is much smaller, and its effects often become negligible shortly after pumping was stopped. For areas where surface recharge replenishes the aquifer, the residual effects of base-flow reduction and thus its depletion volume will be significantly reduced. A stream of large conductance has a high hydraulic connection to the aquifer, but the relationship between stream conductance and stream depletion is not linear.

Notes: Ground water exchange affects the ecology of surface water by sustaining stream base flow and moderating water-level fluctuations of ground water-fed lakes. It also provides stable-temperature habitats and supplies nutrients and inorganic ions. Ground water input of nutrients can even determine the trophic status of lakes and the distribution of macrophytes. In streams the mixing of ground water and surface water in shallow channel and bankside sediments creates a unique environment called the hyporheic zone, an important component of the lotic ecosystem. Localized areas of high ground water discharge in streams provide thermal refugia for fish. Ground water also provides moisture to riparian vegetation, which in turn supplies organic matter to streams and enhances bank resistance to erosion. As hydrologists and ecologists interact to understand the impact of ground water on aquatic ecology, a new research field called “ecohydrology” is emerging.

Notes: Recent widely accepted findings respecting the origin and nature of specific yield in unconfined aquifers rely heavily on water level changes observed during two pumping tests, one conducted at Borden, Ontario, Canada, and the other at Cape Cod, Massachusetts. The drawdown patterns observed during those tests have been taken as proof that unconfined specific yield estimates obtained from long-duration pumping tests should approach the laboratory-estimated effective porosity of representative aquifer formation samples. However, both of the original test reports included direct or referential descriptions of potential supplemental sources of pumped water that would have introduced intractable complications and errors into straightforward interpretations of the drawdown observations if actually present. Searches for evidence of previously neglected sources were performed by screening the original drawdown observations from both locations for signs of diagnostic skewing that should be present only if some of the extracted water was derived from sources other than main aquifer storage. The data screening was performed using error-guided computer assisted fitting techniques, capable of accurately sensing and simulating the effects of a wide range of non-traditional and external sources. The drawdown curves from both tests proved to be inconsistent with traditional single-source pumped aquifer models but consistent with site-specific alternatives that included significant contributions of water from external sources. The corrected pumping responses shared several important features. Unsaturated drainage appears to have ceased effectively at both locations within the first day of pumping, and estimates of specific yield stabilized at levels considerably smaller than the corresponding laboratory-measured or probable effective porosity. Separate sequential analyses of progressively later field observations gave stable and nearly constant specific yield estimates for each location, with no evidence from either test that more prolonged pumping would have induced substantially greater levels of unconfined specific yield.

Notes: Laboratory experiments involving both homogeneous and heterogeneous porous media are used to demonstrate that fluid flow and solute transport will occur regularly in the capillary fringe (CF), including both vertical (upward as well as downward) and horizontal flow velocities. Horizontal flow above the water table appears to be limited primarily to the region of high water saturation (i.e., the CF), an observation supported by numerical modeling and consistent with the literature. Beyond observations presented in prior literature, it was observed that exchange of water within the CF with water below the water table is active, with flux both from the CF downward across the water table and from the region below the water table, upward into the CF. This flux is enhanced by the presence of physical heterogeneity. These findings strongly contrast the common conceptualization of predominantly downward vertical fluid flow through the unsaturated zone, with transition to fully three-dimensional flow only below the water table. Based on these observations, it is suggested that the CF may affect, far more significantly than is usually assumed, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated ground water flow.

Notes: A major tool used in the design of wellhead protection areas is the delineation of a capture zone for a pumping well by use of a simple, steady-state analytic solution. This simple approach has been useful for many small municipalities because of the high costs associated with obtaining the hydrogeologic information needed for detailed numerical modeling. This analytic solution, however, is deterministic, and uncertainty in the mean value estimates of the hydraulic parameters used in this model can be a major source of error in predicting capture zones. To address this problem, a statistical theory was developed for including the uncertainty in the transmissivity and the magnitude and direction of the hydraulic head gradient in the analytic solution for both the ultimate and time-dependent capture zone for an arbitrary reliability level. To demonstrate the method and investigate the effect of varying magnitudes of uncertainty on time-dependent capture zones, the method is applied to three synthetic data sets based on data from the Borden Aquifer in Ontario, Canada. In general, the results show that uncertainty in the length of the time-dependent capture zone at a given reliability level is dependent on the uncertainty in the magnitude of the mean regional flow, which is equal to the transmissivity multiplied by the hydraulic head gradient; uncertainty in the maximum width of the capture zone is dependent primarily on the uncertainty in the mean direction of the regional flow.

Notes: A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O,2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.