Processes governing treatment of contaminants in low permeability zones

Abstract

Herein we embrace the premise that aquifers are commonly composed of transmissive and low k (permeability) zones. Contaminants stored and subsequently released from low k zones sustain aqueous phase plumes for problematic periods. Processes governing the occurrence and treatment of contaminants in low k zones are advanced via conceptual models and a laboratory tank study. A two-dimensional sand tank with interbedded low k clay layers is flushed for 92 days with water spiked with 100 mg/L fluorescein, a proxy for chlorinated solvent contamination, and 67 mg/L bromide, a conservative tracer. Given active sources, fluorescein and bromide diffuses into the clay layers. Subsequently, the tank is flushed with water for 38 days. Water only flushing illustrates how the release of contaminants stored in low k zones sustains downgradient plumes. Next, an alkaline persulfate solution (40,000 mg/L persulfate at pH 11) is delivered to the tank for eight days. A fiber optic cable, placed on the glass wall of the sand tank, and a spectrometer with an ultraviolet light source are used to track depletion of fluorescein in transmissive sand and low k clay zones through time. Lastly, the tank is flushed with water only for 69 days to evaluate the efficacy of treatment with respect to mitigating releases from low k zones. Results indicate that flushing the tank with an alkaline persulfate solution, at a laboratory-scale, was effective in depleting fluorescein in both the transmissive and low k zones. Novelly, results capture concurrent transport of reactants and contaminants in domains governed by advection in transmissive zones and diffusion in low k zones.