Pumping Test Evaluation of Stream Depletion Parameters

Abstract

Descriptions are given of a pumping test and a corresponding analysis that permit calculation of all five hydrogeological parameters appearing in the Hunt (2003) solution for stream depletion caused by ground water abstraction from a well beside a stream. This solution assumes that flow in the pumped aquifer is horizontal, flow in the overlying aquitard or system of aquitards is vertical, and the free surface in the top aquitard is allowed to draw down. The definition of an aquitard in this paper is any layer with a vertical hydraulic conductivity much lower than the horizontal hydraulic conductivity of the pumped aquifer. These "aquitards" may be reasonably permeable layers but are distinguished from the pumped aquifer by their hydraulic conductivity contrast. The pumping test requires a complete set of drawdown measurements from at least one observation well. This well must be deep enough to penetrate the pumped aquifer, and pumping must continue for a sufficient time to ensure that depleted streamflow becomes a significant portion of the well abstraction rate. Furthermore, two of the five parameters characterize an aquitard that overlies the pumped aquifer, and values for these parameters are seen to be dependent upon the initial water table elevation in the aquitard. The field test analyzed herein used a total of eight observation wells screened in the pumped aquifer, and measurements from these wells gave eight sets of parameters that are used in a sensitivity analysis to determine the relative importance of each parameter in the stream depletion calculations.