Double-flow behavior observed in well tests of an extremely heterogeneous mine-spoil aquifer

A radial-flow investigation was performed to examine hydraulic characteristics of a heterogeneous unconfined minespoil aquifer formed by reclamation of surface-mine overburden. Slug test estimates of hydraulic conductivity (K) range over four orders of magnitude (from 10(-6) to 10(-2) m/s) and display two prominent statistical modes, ascribed to fine matrix and coarse rubble zones, respectively. A large proportion of wells displayed either of two anomalous slug-test response patterns, attributable to zones of high K around or near the well. Slug-test estimates of matrix K appear unbiased, but those of high-conductivity zones are thought to contain error induced by local heterogeneity and drainage of storage from near the water table. Numerical simulation of slug tests suggests that high-K zones, vertically continuous to the water table and either in the annulus of (e.g. well skin) or in close proximity to the well, may produce response similar to that observed. It is not possible to distinguish well-skin from heterogeneity effects based on the nature of slug test response alone. Pumping yields, however, are generally higher for wells with anomalously rapid slug-test response and suggest that, in some cases, the heterogeneity extends to large distance from the well. Despite their local region of influence, slug tests in such extremely heterogeneous aquifers may encounter highly conductive zones that may reduce the reliability of their results if they permit water-table drainage to the well. (C) 1997 Elsevier Science B.V.