EVALUATING AQUITARDS IN LEAKY AQUIFERS

Conventional methods of analyzing leaky aquifers usually rely on drawdown data from the pumped aquifer alone. When there is sufficient leakage from an overlying, and/or, underlying aquitard into the pumped aquifer, the curve of drawdown data will usually fall below the Theis curve. Hantush and Jacob (1955) developed their well known r/B solution that has been widely used to analyze this situation. However, they did not consider the effects of storativity, and in a later paper, Hantush (1960) presented a modified solution to include this factor. For some particular problems, such an approach is not sufficient to characterize a leaky system. Is the leakage coming only from the aquitard above the pumped aquifer, or is there some leakage from below? Furthermore, if the system is only slightly leaky and the pump test does not last long enough, the interpretation of the results may end up with r/B=0, meaning no leakage, when there may actually be significant leakage.

In his thesis research at Berkeley, Neuman (1966) developed a new solution for the transient behavior of an aquifer with a leaky aquitard that has been of considerable assistance on this problem. The method requires observation wells to be placed not only in the aquifer being pumped, but also in the confining layers (aquitards) above or below, depending on the nature of the problem. The ratio of the drawdown in the aquitard to that measured in the aquifer at the same time and the same radial distance from the pumping well can be used to evaluate the hydraulic properties of the aquitard. The new method is supported by theory and has been applied to a variety of field problems such as the evaluation of: a coastal groundwater basin in California, the tightness of an aquitard (caprock) overlying an aquifer that was being considered for underground storage of natural gas in Illinois, and the potential for inflows of water through an aquitard at the base of an open pit mine.