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Austin Road Landfill Appendix D Contaminant Plume Characterization Report Aquifer Test Methods _ • a pumping well whose well screen penetrates the full saturated thickness of the aquifer,and • a water surface which remains static over the duration of the test except as affected by the pumping well Other assumptions unique to a certain aquifer test evaluation method are discussed in the next section. To see if any of the above general assumptions were violated during the test and if correction factors were needed,the data underwent several types of preprocessing and evaluation as described below. Aquifer thickness evaluation The assumption of a constant aquifer saturated thickness is based on the difference of the static (nonpumping)water table surface and the base of the aquifer. Historic groundwater data show a depth to water of about 80 feet and a decrease in the water surface elevation of about 4 feet over the area of the aquifer test observation wells. Lithologic results from deeper boreholes in the area show a clay-rich layer starting at about 125 feet below ground surface,encountered at the MW-6 and MW-7 locations and in temporary borehole ARL-IA drilled as part of concurrent site investigations (Figure 1),. These three locations form a triangle covering most of the area comprising the observation well network. The consistent depth of the base of the aquifer and the relatively small change in ground water surface elevation makes the assumption of a constant aquifer thickness a reasonable one over the scale of the site. The difference in e depth to water and depth to the clay layer is the aquifer saturated thickness of the uppermost aquifer layer. Based on information presented above,the average saturated thickness is about 45 feet. Borehole storage evaluation The issue of borehole storage is relevant in estimating the time over which aquifer drawdown data from the Pumping-1 and Pumping-2 test phases may be influenced by water being evacuated from the well boreholes. This effect was evaluated using the method presented by Walton(1987),which estimates the time after pumping starts for which the effects of well bore storage on measured drawdown become minimal(less than 1 percent). Using reported borehole diameters of 13 inches for wells EW-1 and EW-2(EMCON, 1991),pump eductor pipe diameters of 4 inches and an assumed aquifer K value of 100 feet per day,this method indicates that borehole storage effects are insignificant after about 17 minutes. Due to the length of each pumping test phase,the assumption of no wellbore storage can also assumed to be met. Constant pumping rate evaluation The assumption of a constant pumping rate at the test well(EW-1)was evaluated by plotting pumping rates as a function of time. The aquifer test analysis methods described below require that the pumping rate fluctuates less than about 10 percent over the region that the data are analyzed (Stallman,1971). Totalizing flow meters installed downstream of each well pump were used to determine pumping rates over time periods ranging from one minute to several hours. The calculated pumping rates remained reasonably consistent for a given test phase,although there appear to be differences in the pumping rate of each well when they are operating alone or together. CDM Camp Dresser&McKee D-6 - W.\REPORTS\STOCKTOWRLPLUME.9MPPD.WPD CS 010853 City of Stockton 190855.0006