Pumping test analysis for a tidally forced aquifer

Abstract

The analysis of pumping test data for a shallow riverbank aquifer afforded the opportunity to compare standard methods for determining hydraulic conductivity (K) and storativity (S) with techniques based on the propagation of periodic fluctuations of head in the aquifer. Tidal fluctuations in water level at the riverbank were found to propagate approximately nondispersively for 200 m in the aquifer, allowing tidal phase lags and amplitude attenuations in the aquifer to be estimated from river levels by least-squares techniques. The least-squares attenuations and phases were used to correct measured pumping test drawdowns for tidal influences. Standard transient analyses of the corrected drawdowns were performed, yielding values of approximately 8 to 10 m/d for K and 0.002 for S. The low S value was taken to indicate semi-confined conditions. The tidal attenuation coefficients were used to determine the hydraulic diffusivity T/S, providing values that were integrated over the distance from the riverbank to the relevant monitoring wells. S values from the drawdown analyses then yielded K estimates ranging from 3 to 7 m/d. A three-dimensional finite-element simulation of the pumping tests showed that the estimated K and S values yielded simulated drawdowns in reasonable agreement with field observations.