Identifying Impervious Boundary and Aquifer Parameters from Pump-Test Data

The common field problem of identifying and locating impervious aquifer boundaries is generally solved by analyzing the pumptest data. The estimate of the distance between the image well and the observation well (r) is required for locating the boundary. Popular methods of estimating r either use the concept of fully developed two straight lines or rely on curve matching. Curvematching methods are subjective and involve errors due to personal judgment. A very long duration pump test is required in order that the second straight line is developed. In many cases, the first straight line is not developed at all, especially when the observation well is at a greater distance from the pumped well. In this paper, a derivative-based robust optimization method has been proposed for the identification of aquifer parameters (transmissivity, T and storage coefficient, S) and r from the drawdowns observed at an observation well. It returns unbiased estimates of the parameters (T, S, and r) even with a poor initial guess. The method can yield the estimates of the parameters from shorter duration pump-test data, thus saving time and money involved in conducting the pump test with the purpose of identifying r. It is observed that the hydraulic diffusivity (T/S) of the aquifer and r can be estimated even when the constant rate of pumping is not known.