ALBERT

Notes: Observation wells and access holes for neutron probe use were drilled by a jet-percussion drill rig in coarse alluvial material near Tucson, Arizona. The method combines the jetting action which effectively removes loose materials with the percussion necessary to break up tighter formations and large particles. A unique feature of the equipment is the provision for simultaneous drilling and driving the casing to keep the hole open in loose formations. Washed samples of the material being drilled can be obtained from the recirculating water system. Drilling rates were from seven to ten feet per hour, and costs including casing were less than $1.50 per foot. Maximum depth is about 100 feet.Hydrologic study of a ground-water reservoir requires access to the aquifer. Observations are made where the water table intersects the land surface, such as at springs or seeps, or through existing wells. However, wells are not always found in desired locations or are pumped so heavily that their usefulness as observation wells is impaired. Measurements are also made in the unsaturated zone with neutron moisture measuring equipment, which requires a small diameter cased access hole for the neutron probe.An observation well needs a diameter only large enough for access by measuring devices and must be immediately responsive to changes in aquifer water level. For neutron probe use, the required diameter is about two inches, and the well must have no significant effect on moisture movement in the unsaturated zone. To meet these requirements the drilling process should have no lasting effect on the formation surrounding the well. Finally, since the wells have no economic utility, construction cost must be low.A network of observation wells and neutron probe access holes was drilled as part of a project to evaluate the natural recharge from Rillito Creek near Tucson, Arizona. The formation along the creek bed is a coarse alluvial outwash containing almost no clay, considerable running sand, and some large gravel and boulders. Commercial churn (percussion) and hydraulic rotary drills, commonly used for drilling in this area, were first tried but encountered much difficulty in the loose materials.Previous investigators used jetting equipment drill small diameter holes in a variety of alluvi formations (Cederstrom and Tibbitts, 1961; Bowma: 1911; and Pillsbury and Christiansen, 1947). Howeve in most cases, either open hole drilling was used because the fine material content and the limited numb. of boulders or large gravel particles made this methc possible, or drilling mud was used to maintain th hole. The jetting method uses water pumped unde pressure through hollow rods and a drill bit to loose the material and carry it to the surface. The jettin water receives little aid from the drill in loosenin the material to be removed. Usually the drill cutting are settled out in a tank or basin, and the water i recirculated.

Notes: A two-dimensional, passive element electric analog model was used to determine the spatial distribution of natural ground-water recharge in the Santa Cruz River basin of southern Arizona. Existing records were used to draw a ground-water level contour map of the area in its undeveloped condition. The model was made to duplicate these contours by varying inputs from known locations of recharge. Recharge distribution as determined by the model was consistent with previous estimates, but no direct correlation was found with the contributing drainage area.

Notes: The paper describes an integrated lake and catchment model which was developed and tested against data from the Rutland Water catchment (River Nene and Welland) in Eastern England. The model uses export coefficients to predict annual diffuse losses of phosphorus from catchment land use, which are temporally disaggregated using empirical relationships between phosphorus load and discharge in headwater streams. Dynamic inputs of phosphorus are incorporated into a hydrologically based, multi-reach distributed catchment model which also simulates sewage discharges, abstractions and reservoirs/ lakes. The model allows a comparison of simulated output with observed discharge and water-quality data from various points within the river system.

Notes: An integrated lake and catchment model was used to simulate reservoir volume and phosphorus concentrations in Rutland Water over a period of seventeen years, taking into account changes in reservoir output and phosphorus sedimentation resulting from the addition of ferric sulphate. Application of the model for critical-period analysis and the setting of environmental targets and trigger values are discussed, and an example is given showing the impact of changing reservoir output on the frequency distribution of phosphorus using an eighty-year set of river-discharge data. The impact of phosphorus removal at point sources in the catchment on phosphorus concentrations in the river and reservoir is simulated and compared to results from SIMCAT - a steady-state model which was developed by the Environment Agency. Results are also compared with observed data following the implementation of the EC urban waste water treatment Directive, at the end of 1998.

Notes: Groundwater residence-time survey results on 21 public water supplies in the chalk aquifer in southern England are compared with a previous Cryptosporidium risk assessment which was carried out on the same supplies for regulatory-compliance purposes in 1999. The results indicate that residence-time indicators could provide useful corroborative evidence for rapid recharge hazard - not only in those settings already identified by microbiological surveillance, but also in the more difficult-to-identify situation where potential rapid pathways have been identified but the bacteriological indicators are negative or ambiguous. However, groundwater-mixing processes under pumping conditions are complex, especially in the chalk, and will always require interpretation informed by an understanding of the local hydrogeological and operational setting.

Notes: Bulk age determinations, based upon chlorofluorocarbons and sulphur hexafluoride measurements of samples from twenty-one chalk groundwater supplies in southern England, Indicate that waters of relatively recent age predominate In both unconfined and partially confined situations. Water from pumping stations located on chalk below Palaeogene cover can be distinguished hydrochemically, and a likely interpretation is that these supplies are receiving a small proportion of recharge via induced via induced leakage. Whilst water which is abstracted from the chalk always involves mixing processes, for a sub-set of confined supplies, ‘piston’ flow could be inferred as a dominant mechanism - resulting in bulk groundwater ages of a few decades. Other supplies are the product of complex mixing. Although low-level chlorofluorocarbon enrichment was encountered for half the catchments sampled, they and sulphur hexafluoride appear to provide independent corroboration to microbiological indicators of the presence of rapid recharge.

Notes: This research develops a methodology for parameter structure identification in ground water modeling. For a given set of observations, parameter structure identification seeks to identify the parameter dimension, its corresponding parameter pattern and values. Voronoi tessellation is used to parameterize the unknown distributed parameter into a number of zones. Accordingly, the parameter structure identification problem is equivalent to finding the number and locations as well as the values of the basis points associated with the Voronoi tessellation. A genetic algorithm (GA) is allied with a grid search method and a quasi-Newton algorithm to solve the inverse problem. GA is first used to search for the near-optimal parameter pattern and values. Next, a grid search method and a quasi-Newton algorithm iteratively improve the GA's estimates. Sensitivities of state variables to parameters are calculated by the sensitivity-equation method. MODFLOW and MT3DMS are employed to solve the coupled flow and transport model as well as the derived sensitivity equations. The optimal parameter dimension is determined using criteria based on parameter uncertainty and parameter structure discrimination. Numerical experiments are conducted to demonstrate the proposed methodology, in which the true transmissivity field is characterized by either a continuous distribution or a distribution that can be characterized by zones. We conclude that the optimized transmissivity zones capture the trend and distribution of the true transmissivity field.

Notes: The preliminary evaluation of the first direct-use (low enthalpy) geothermal well in the United Kingdom included core analysis, geophysical logging, and drill-stem testing. The effective aquifer is restricted to a few relatively thin loosely cemented bands in the Triassic sandstones at a depth of approximately 1660 m. With an aquifer temperature of at least 67°C the initial assessment of transmissivity, 18 m2/day, suggested that a thermal yield of approximately 2.5 MW would be obtainable. The results are in reasonable agreement with those of a subsequent long-term production test.

Notes: A method for collection and analysis of well cuttings samples to aid in ground-water resources evaluation of certain types of alluvial aquifers is described. The method standardizes collection procedures which minimize disruption of the drilling process and allows for easier laboratory interpretation. Analysis techniques for soils are adapted to differentiate between the formations encountered. Wet and dry colors, particle size analysis, acid reaction, plus examination of mineral constituents permits correlation of formation characteristics from wells constructed at different times and by different drillers. An example of the procedure for an area northwest of Tucson, Arizona is given.

Notes: The function of a septic tank-soil absorption system is to purify the wastewater discharged from the home before reaching the ground water. However, failure often occurs in the soil absorption field. Failure can originate from two causes: (1) inadequate infiltration of effluent into the soil, due to soil clogging or an increase in loading which results in surfacing septic tank effluent, and (2) inadequate purification in the soil during percolation because of short travel times which can be due to presence of very permeable, shallow soils or to local overloading. Inadequate purification may result in pathogenic pollution of private well-water supplies.Failures of the second type can be prevented through proper design and operation of the soil absorption field. Studies have shown that 3 feet (90 cm) of unsaturated soil are adequate in purifying septic tank wastes with the exception of nitrogen removal. However, 4-inch (10 cm) diameter perforated drain pipe commonly used for distribution leads to local overloading near the point of inlet whereas other areas in the seepage system do not receive effluent at all. This leads to locally high flow rates of the waste through the soil reducing the soil's efficiency for pathogenic organism removal. Uniform distribution of the septic tank effluent over the entire soil absorption field is necessary to relieve this situation.One method by which uniform distribution can be achieved over a large area is through a pressurized system. By properly sizing the diameters of the pipes and the number and diameter of the orifices in the distribution laterals, the head losses across the orifices will be great enough to cause the entire network to fill before the liquid is applied to the soil. This system combines the advantages of dosing with uniform distribution.Three systems have been tested under laboratory and field conditions. The network is easy to manufacture and can be quickly sized. The design guidelines and results of laboratory and field testing are discussed.