A Field Study of Non-Reactive Transport Behavior in a Heterogeneous Aquifer Unit

Abstract

Two dual-well, forced gradient, tracer experiments have been conducted at a Superfund site as part of an effort to better define the efficiency of ongoing "pump and treat" remediation. An injection/extraction well couplet, 24.5 ft. in length, was used to generate steady flow through a 20 ft. thick semi-confined aquifer unit beginning at 140 ft. BGS. The unit grades downward from a fine-medium sand to a clayey gravel with cobbles up to 4 inches in nominal diameter. Bromide breakthrough was monitored at the extraction well and at four vertically discrete zones in a centerline monitor well located 13.8 feet from the injection well using an innovative multilevel sampler. Calculated hydraulic conductivities based on the tracer data range from 1 to 43 ft./day, and compare well to an aquifer pump-test value of 30.1 ft./day. One-dimensional transport modeling was used to estimate values of dispersivity ranging from 1 to 2 ft. for the medium sand.