Prototype Field Method for Diffusion and Sorption Coefficients and CVOC Reaction Rates in Low-Permeability Strata

The U.S. Geological Survey and the University at Buffalo, in cooperation with the U.S. Navy and the Strategic Environmental Research and Development Program, developed an in-situ field method for measurement of diffusion and sorption coefficients and reaction rates of chlorinated volatile organic compounds (CVOCs) in low-hydraulic-conductivity (low-K) sedimentary strata. A diffusion tracer test is initiated by replacing native groundwater between two packers, which isolate low-K strata from fractured strata, with native groundwater from which most of the CVOCs have been removed by sparging. Standard hydraulic tests prior to the test evaluate whether the isolated strata are sufficiently low-K such that advection will be insignificant. Groundwater samples are collected with a zero-net-volume, closed-loop protocol that further minimizes advection into and out of the isolated interval. Sample frequency ranges from sub-daily initially to at least weekly for three to four months. Biodegradation reaction rates and diffusion and sorption coefficients are estimated by inverse modeling of borehole CVOC and nonreactive tracer concentrations. The method is applicable to open intervals of wells that expose low-K strata, and have pre-test water quality data, including CVOC concentrations. A limitation is that groundwater levels must be shallow enough to allow peristaltic (suction) pumping from the interval.