Pumping tests in heterogeneous aquifers: An analytical study of what can be obtained from their interpretation using Jacob's method

Interpretation of pumping tests to estimate hydraulic parameter values is t ypically based on the assumption of aquifer homogeneity. The applicability of the traditional methods of interpretation in real aquifers can be questi oned, since the evaluation of the drawdown curves observed at different loc ations in a single test may not result in one consistent set of hydraulic p arameters. Thus most hydrogeologists tend to look at estimated transmissivi ties (T) as some average property of the medium, while estimated storativit ies (S) are disregarded in some cases, particularly when they are obtained from data measured at the pumping well. An analytical study of drawdown und er radially convergent flow toward a single point in heterogeneous aquifers shows that large time drawdown values form a straight line on a drawdown v ersus log time plot. Jacob's method consists of obtaining estimates for T a nd S from the slope and intercept of this line. We find that even in a hete rogeneous field, these estimates provide valuable information about the aqu ifer. Estimated T values for different observation points tend to converge to a single value, which corresponds to the effective T derived under paral lel flow conditions. Estimated storativities, however, display higher varia bility, but the geometric mean of the S,,, values can be used as an unbiase d estimator of the actual S. Thus it appears that although Jacob's method w as originally derived for homogeneous media, it can provide valuable inform ation in real aquifers.