ARCHIVED REPORTS_1999 - Laserfiche WebLink

Home Browse Search

Austin Road Landfill Appendix D Contaminant Plume Characterization Report Aquifer Test Methods pumping fiom a well screened across the entire saturated thickness of the aquifer. Other assumptions,listed above,are that water is released instantaneously from storage with the onset of pumping,that the aquifer is homogenous,isotropic and of infinite areal extent,that the prepumping water table is flat and that there are no other sources of recharge or discharge other than the pumping well being tested(Kruseman and de Ridder,1990). When these conditions are met then the test drawdown data should follow an exponential trend when both time and drawdown are plotted on logarithmic scales,and the following equations can be used(Theis,1935): T = 4Q W(u) and S _ 47 to r2 _.. where Q is the well pumping rate,W(u)is the Theis well function,s is the aquifer drawdown,t is the time after which pumping began,r is the distance between the pumping and observation wells and u is defined as r 2S - U = — 4Tt It was observed that at later stages of a pumping test and at locations more distant from the pumping well the cone of depression expands at steady logarithmic rate with time. This occurs when the aquifer's u value is small,typically less than about 0.05. Under these conditions the drawdown data take on a straight line trend when drawdown are plotted on a linear scale and time on a logarithmic scale(Cooper and Jacob,1946). The Theis equation is simplified under these -- conditions to the following: T = 2.3Q 4ndr and S - 2.25Tto = r2 - where delta s is the drawdown measured over one log cycle and to is the time where a straight line from the drawdown data intersects the y-axis corresponding to zero drawdown. Evaluating aquifer test data using this method allows for evaluation of recharge and discharge events by observing when the drawdown curve deviates from the predicted straight line. This attribute is particularly important in the ARL aquifer test due to the multiple cycles of recovery and pumping that occurred as part of the test,and because of heavy rainfall events that occurred during the test. CDM Camp Dresser&McKee D-10 W:\REPORTS'STOCKTOMARLPLUME.gMP PD.W PD CS ®1®8cJ7 City of Stockton 1909SS.000f,