ALBERT

Notes: The performance of air sparging systems, as measured by the predicted region of airflow, was investigated by conducting numerical simulations with a multiphase modeling program (TETRAD). The simulations employed standard two-phase flow theory, and included pressure-dependent calculations of the compressibility of water and air. Simulations tested the response of air sparging systems to variations in geological properties (intrinsic permeability and anisotropy of permeability) as well as different injection scenarios (depth, pressure, and rate of injection).Three stages of flow behavior are predicted following initiation of air injection. These are: (1) an initial transient period of growth in the lateral and vertical limits of airflow (expansion stage); (2) a second transient period of reduction in the lateral limits of airflow (collapse stage); and (3) a steady-state stage, during which the system remains static as long as injection parameters do not change. In homogeneous media the geometry of the region of airflow changes from a teardrop-or bell-shaped configuration to a shape that is roughly conical during the expansion stage. During the collapse stage, air is preferentially diverted to established regions of high effective air permeability, and ground water resaturates the remainder of the original region of influence. For pilot testing it is important to realize that measurements of the lateral extent of airflow expansion and collapse stages can be misleading because they will differ from the limits established at steady-state. The time required for a particular system to progress through the transient stages and establish steady-state behavior can vary from hours to years, and is dependent on the permeability structure of the aquifer, injection depth, and injection rate. Under homogeneous conditions the maximum width of the region of airflow attained during the transient expansion stage was substantially greater than the width attained at steady-state.The width of the steady-state region of airflow is significantly affected by all the variables investigated, except injection depth, which appears to primarily influence the transient behavior. The simulations further show that the vertical permeability and anisotropy of the aquifer are very important variables, and warrant routine assessment during design of sparge systems. Simulation of heterogeneous media demonstrate that complex airflow patterns will occur as a result of air ponding beneath, and flow around, zones of low air permeability.

Notes: Continuous remediation monitoring using sensors is potentially a more effective and inexpensive alternative to current methods of sample collection and analysis. Gaseous components of a system are the most mobile and easiest to monitor. Continuous monitoring of soil gases such as oxygen, carbon dioxide, and contaminant vapors can provide important quantitative information regarding the progress of bioremediation efforts and the area of influence of air sparging or soil venting. Laboratory and field tests of a commercially available oxygen sensor show that the subsurface oxygen sensor provides rapid and accurate data on vapor phase oxygen concentrations. The sensor is well suited for monitoring gas flow and oxygen consumption in the vadose zone during air sparging and bioventing. The sensor performs well in permeable, unsaturated soil environments and recovers completely after being submerged during temporary saturated conditions. Calibrations of the in situ oxygen sensors were found to be stable after one year of continuous subsurface operation. However, application of the sensor in saturated soil conditions is limited. The three major advantages of this sensor for in situ monitoring arc as follows: (1) it allows data acquisition at any specified time interval; (2) it provides potentially more accurate data by minimizing disturbance of subsurface conditions; and (3) it minimizes the cost of field and laboratory procedures involved in sample retrieval and analysis.

Notes: The analysis of dissolved contaminant transport away from source areas containing nonaqueous phase liquids is an important component in the evaluation of the risk posed to downgradient receptors, of monitoring plan development, and of remedial alternative selection. Typically, individual source-plume systems are evaluated on a site-specific basis; however, much can be learned by looking at the collective data from multiple sites. This has been done with leaking underground storage tank sites in California, Texas, and Florida, where the analyses led to important conclusions concerning dissolved MTBE (methyl tertiary butyl ether) and benzene plume behaviors that could not be drawn from individual site data sets. In these previous works, each site data set was reduced to a plume length and then a statistical analysis of plume lengths was performed. In this paper, the use of multiple-site collective data analyses is also pursued; however, rather than use a single metric plume length analysis, a composite plume data analysis approach is proposed and examined. This alternate approach provides a means for making use of data from ground water plumes that are incompletely defined. In addition, few of the assumptions required by single plume studies for interpolation of data are necessary. The composite plume approach is especially well suited to the analysis of large sites or regions with multiple source areas within similar hydrogeochemical settings. A case study of petroleum hydrocarbon transport at a large former oil field with more than 90 source areas illustrates the application and benefits of this approach.

Notes: Horizontal air sparging (HASP) wells offer several potential advantages compared to linear arrays of vertical air sparging wells. For some of these advantages to be realized, however, HASP wells must be able to deliver air uniformly along the length of the well. HASP wells can fail to deliver air uniformly for either engineering or geological reasons.A 58 m (190-foot) long HASP well, with a 15 m (50-foot) long screen interval, was designed, installed, and tested in eolian dune sand. The relative uniformity of the geologic medium allowed specific evaluation of the impact of the well design on air delivery. A variety of monitoring approaches were used during a six-day pilot test. Pressure drop within the sparge well was found to be negligible through the screen interval of the well. Soil gas pressure and ground water mounding responses were very similar at both ends of the well screen, suggesting relatively uniform air delivery throughout. Electrical resistance tomography results confirmed that airflow in the formation was similar at both ends of the screen interval and that the principal region of airflow was within 1.5 m (5 feet) of the axis of the well. Increased dissolved oxygen was primarily limited to a region within 2.3 m (7.5 feet) of the well and occurred throughout the length of the screen interval. These monitoring results show that HASP wells, properly constructed and installed, can supply air in a generally uniform manner along their length.

Notes: An estimation of the volume of light nonaqueous phase liquids (LNAPL) is often required during site assessment, remedial design, or litigation. LNAPL volume can be estimated by a strictly empirical approach whereby core samples, distributed throughout the vertical and lateral extent of LNAPL, are analyzed for LNAPL content, and these data are then integrated to compute a volume. Alternatively, if the LNAPL has obtained vertical equilibrium, the thickness of LNAPL in monitoring wells can be used to calculate of LNAPL in monitoring wells can be used to calculate LNAPL volume at the well locations if appropriate soil and LNAPL properties can be estimated.A method is described for estimating key soil and LNAPL properties by nonlinear regression of vertical profiles of LNAPL saturation. The methods is relatively fast, cost effective, and amenable to quantitative analysis of uncertainty. Optionally, the method allows statistical determination of best-fit values for the Van Genuchten capillary parameters (n, αoil-water and αoil-air), residual water saturation and ANAPL density. The sensitivity of the method was investigated by fitting field LNAPL saturation profiles and then determining the variation in misfit (mean square residual) as a function of parameter value for each parameter. Using field data from a sandy aquifer, the fitting statistics were found to be highly sensitive to LNAPL density, αoil-water and αoil-air moderately sensitive to the Van Genuchten n value, and weakly sensitive to residual water saturation. The regression analysis also provides information that can be used to estimate uncertainty in the estimated parameters, which can then be used to estimate uncertainty in calculated values of specific volume.

Notes: Air sparging is a relatively new technique for the remediation of ground water contaminated with petroleum hydrocarbons. In this technique, air is injected below the water table, beneath the contaminated soil. Remediation occurs by a combination of contaminant partitioning into the vapor phase and enhanced biodegradation. The air is usually removed by vacuum extraction in the vadose zone.The efficiency of remediation from air sparging is a function of the air flow pattern, although the distribution of the injected air is still poorly understood. Cross-borehole resistivity surveys were performed at a former service station in Florence, Oregon, to address this unknown. The resistivity measurements were made using six wells, one of which was the sparge well. Data were collected over a two-week period during and after several air injections, or sparge events. Resistivity images were calculated between wells using an algorithm that assumes axially symmetric structures. The movement of the injected air through time was defined by regions of large increases in resistivity, greater than 100 percent from the background. During early sparge times, air moved outward and upward from the injection point as it ascended to the unsaturated zone. At later sparge times, the air flow reached a somewhat stable cone-shaped pattern radiating out and up from the injection point. Two days after sparging was discontinued, a residue of entrained air remained in the saturated zone, as indicated by a zone of 60 to 80 percent water saturation.