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Austin Road Landfill Section 3 Contaminant Plume Characterization Report Field Investigation Results 3.4 Aquifer Pumping Test Data perform each phase of the testing and the data analysis are resented in The methods used tope p g y P Appendix D,Aquifer Test Methods. The aquifer characteristics determined from the tests are described below. These aquifer parameters are used to understand the hydrogeologic setting and to develop the capture zone analysis provided in Section 4.0. Aquifer transmissivity(T),hydraulic conductivity(K) and storage coefficient(S)values were calculated based on water level data collected during the aquifer test at the ARL. These results were used to evaluate the extent of plume capture achieved by extraction wells EW-1 and EW-2. The aquifer system in the immediate vicinity of the ARL consists of two sandy aquifer layers separated by a clay-rich layer(Figure 3-1). For the shallow aquifer the average aquifer T value is approximately 10,200 ft2/day,with an aquifer K value estimated as 255 ft/day based on an average _ saturated thickness of the shallow aquifer of 40 feet. The average aquifer storage coefficient is approximately 4 x 105,indicating that the shallow aquifer is under confined to semi-confined conditions in the area of the test observation wells. The aquifer K value estimated for the deeper aquifer system is approximately 105 ft/day. This value is assumed to be lower than the actual aquifer K,due to complicating effects which increase the uncertainty of the calculated results. These effects include partial penetration of the pumping wells and effects of the intervening clay layer between the pumping wells and the deeper observation wells. Based on the observation well data collected during the aquifer pump test,the aquifer is influenced by pumping at distances at up to approximately 1700 feet away from extraction well E -1,in both the cross-gradient(east-west)and downgradient(north)directions. Extraction well E -2 has an influence on the aquifer of approximately 1000 feet in the downgradient direction and possibly up to 1600 feet in the cross gradient direction. The deeper aquifer is also influenced by pumping from the shallow aquifer,as evidenced by drawdowns in deeper monitoring wells located close to extraction wells. The areas of influence of extraction wells EW-1 and EW-2 are much larger, however,than the areas of capture of each extraction well. The area of capture is defined as the aquifer extent,or area,in which groundwater flows to,and is extracted by,a specific pumping well. - 3.5 Contaminant Fate and Transport Compounds dissolved in water,such as the VOCs detected north of the ARL,tend to migrate along with the prevailing direction of groundwater flow. The rate of compound migration is affected by any attenuating mechanisms that may be present in the part of the aquifer where the contaminants are present. These attenuating mechanisms may be physical,chemical or biological and generally act to reduce the compound concentrations,the compound migration rate,or both. Physical mechanisms include dilution effects from water which recharges the aquifer from overlying sources or underlying aquifer zones,and from dispersion within the aquifer as a compound spreads out during transport. For the compounds present in groundwater north of the ARL,the chemical mechanism likely to be most significant is compound sorption onto the aquifer soils. This leads to a slowing down of the contaminant plume relative to the rate of groundwater flow and can reduce the overall contaminant mass in groundwater as some of the compounds adsorb onto the aquifer soils. Biological reactions occurring at the site would lead to a breakdown of the VOCs into less chlorinated products,such as PCE breaking down into TCE. CDM Camp Dresser&McKee CS 010777 3-7 W:\REPORTS\STOCKTOMARLPLUME.98\SEC3.W PD — Gry of Stockton 190855-0006