Dense non-aqueous phase liquid chlorinated contaminant detected far from the source release area in an aquifer

Abstract

Here we confirm migration of chlorinated solvents tetrachloroethene and trichloroethene, and co-disposed elemental mercury as dense non-aqueous phase liquid in an aquifer at a scale larger than previously documented in the literature, over 650 m from original surface discharge. This finding enhances the conceptual model explaining extensive contaminated plume persistence by adding structurally controlled dense non-aqueous phase liquid to known mechanisms such as source dissolution and matrix back-diffusion. Following injections of oxidants at a depth greater than 50 m, which effectively destroyed dissolved chlorinated contaminants during a groundwater remediation pilot test in South Carolina, we measured excess chloride higher than attributable to pre-test dissolved concentrations of chlorinated contaminants, and oxidative releases of mercury, which is an opportunistic tracer of non-aqueous phase solvent transport when in its elemental form. The results suggest potentially targeted destruction strategies in disconnected non-aqueous phase liquid accumulation areas may reduce remediation timeframe and support cleanup of sites previously considered technically impracticable.