Surface Geophysical Surveys Used to Image Electrically Conductive Groundwater Contaminants and Lithology in the Atlantic Coastal Plain of Delaware

Surface geophysical surveys, direct current (DC) resistivity of drilling core, lithologic logs, and vertical profiling of groundwater specific conductance (SC) measured during drilling were collectively analyzed to identify zones of electrically conductive contaminated groundwater, and lithology, at an industrial spill site on Atlantic Coastal Plain sediments in Delaware. Site contaminants include various dissolved and dense non-aqueous phase (DNAPL) chlorobenzenes, benzene, and hydrochloric acid. The elevated SC of contaminated groundwater is primarily due to the presence of hydrochloric acid. Discontinuous confining layers and complex heterogeneous hydrogeologic properties within the unconfined Columbia aquifer and upper sand layer of the semi-confined Potomac aquifer form downward and lateral migration pathways for contaminated groundwater and DNAPL. Shallow contaminated groundwater is widespread in the Columbia aquifer at the site and was observed discharging in seeps to surface water in fringing marshes. Using multiple lines of evidence, contaminated surface water was interpreted to be recharging to groundwater and discrete zones in the Potomac Aquifer.

Geophysical detection of contaminants at this site is due to the contrast in electrical conductivity between contaminated and unaffected groundwater.  Results were analyzed in the context of a detailed geologic framework due to the presence of electrically-conductive, low permeability clay that must be distinguished from higher permeability silts or sands containing electrically conductive groundwater.

An image of lithologic and groundwater SC characteristics at the site was produced with DC resistivity surveys. Many of the electrically resistive features include unsaturated or cemented and uncontaminated saturated portions of the Columbia Formation and Potomac Formation paleochannel sand and gravel deposits with low SC groundwater. Multi-channel analysis of surface waves (MASW) surveys and lithologic logs were used to differentiate between electrically-conductive low permeability clays and the permeable sands and gravels with electrically conductive ground water.