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Austin Road Landfill Appendix D Contaminant Plume Characterization Report Aquifer Test Methods Effects of Delayed Drainage In addition to regional fluctuations in the water table surface,there can be pumping-induced factors that also effect the drawdown results in unconfined aquifers. This can occur when fine grained sediments exist below the water table surface and within the areas of influence of the cone of depression from a pumping well. As pumping occurs and the water table surface drops,the sediments within the cone of depression become dewatered. Coarse grained sediments lose almost all of the water within the open pore spaces of their matrix very quickly. Fine grained sediments such as silt and clay,however,drain relatively slowly. These fine grained materials have a large water-holding capacity. Slow drainage of the fine grained materials into the still-saturated portions of the aquifer reduces the rate of aquifer drawdown due to pumping and can have the same effect as a recharge event. After a sufficient length of pumping,the dewatered sediments will have drained most of their water and the drawdown in the aquifer will increase to a rate more representative of the sediments within the aquifer. This phenomena is referred to as delayed gravity drainage. It appears as a flattening of the drawdown curves from observation wells over the time interval in which the fine grained sediments are draining. Depending on the amount of drawdown in the aquifer and how much of the dewatered sediments are fine grained materials,the delayed gravity drainage effects,when present, will dampen out from hours to days after pumping begins. Delayed gravity drainage effects do not occur during the recovery phase of an aquifer test,since the materials are being resaturated as the water table surface rises. The presence of and effects from delayed gravity drainage were evaluated in the Pumping-1 and Pumping-2 phases of the ARL aquifer test by examining plots of drawdown from each of the observation wells and reviewing borehole lithologic logs from the tested wells. Based on these analyses it appears that delayed gravity drainage effects are minor for the wells tested. Effects of Superposition A final correction to the water level drawdown data relates to the four cycles of recovery and pumping that were used in the ARL aquifer test. The effects of pumping(and recovery)on water levels in an aquifer slowly propagate outward with increasing during of the pumping or recovery phase. When a new pumping/recovery phase begins,the effects on water levels from the previous pumping/recovery phases are still active in the aquifer and it is the combined effects from the - previous and current pumping that manifest themselves as the drawdown in the aquifer. The combined effects of different pumping/recovery phases is described by the principle of superposition(Reilly et a1,1987). The effects from previous pumping/recovery activities must be removed from the data trends in the current test phase in order to evaluate the aquifer's response due to pumping or recovery during the current phase. The.methods used to account for the effects of superposition are described below. Aquifer Test Analysis Methods Aquifer Analysis Equations The Theis analytical solution is a widely-used method for determining aquifer T and S values for transient(time-varying)drawdown due to pumping. It was developed for situations of constant CDM Camp Dresser&McKee D-9 W:IREPORTS\STOCKTO"RLPWME.98WPPD.WPD CS 01095E Cary of Stockton 19o&55m%