Bias in ground-water data caused by well-bore flow in long-screen wells

Abstract

The results of a field experiment comparing water-quality constituents, specific conductance, geophysical measurements, and well-bore hydraulics in two long-screen wells and adjacent vertical clusters of short-screen wells show bias in ground-water data caused by well-bore flow in long-screen wells. The well screen acts as a conduit for vertical flow because it connects zones of different head and transmissivity, even in a relatively homogeneous, unconfined, sand and gravel aquifer where such zones are almost indistinguishable. Flow in the well bore redistributes water and solutes in the aquifer adjacent to the well, increasing the risk of bias in water-quality samples, failure of plume detection, and cross-contamination of the aquifer. At one site, downward flow from a contaminated zone redistributes solutes over the entire length of the long-screen well. At another site, upward flow from an uncontaminated zone masks the presence of road salt plume. Borehole induction logs, conducted in a fully penetrating short-screen well, can provide a profile of solutes in the aquifer that is not attainable in long-screen wells. In this study, the induction-log profiles show close correlation with data from analyses of water-quality samples from the short-screen wells; however, both of these data sets differ markedly from the biased water-quality samples from the long-screen wells. Therefore, use of induction logs in fully cased wells for plume detection and accurate placement of short-screen wells is a viable alternative to use of long screen wells for water-quality sampling.