Quantifying Matrix Diffusion and Redox Effects on Hexavalent Chromium Plume Conditions in a Fractured Mudstone
Tuesday, October 3, 2017: 8:30 a.m.
It is well known that diffusion driven mass transfer between fractures and the rock matrix controls plume transport and fate in fractured sedimentary bedrock aquifers. Effects can be positive since mass transfer from groundwater flowing in fractures to the rock matrix reduces rates of plume migration and mass flux to receptors, and negative since mass stored in the matrix acts as an impediment to groundwater restoration. While much attention has been focused on chlorinated solvents, there has been little focus on matrix diffusion effects at metals contaminated sites. This study involves a Superfund site in New Jersey, where a hexavalent chromium plume emanates from a former electroplating facility and migrates within a fractured mudstone aquifer discharging to a river. Hexavalent chromium is a contaminant of concern at many sites and can be highly mobile in aquifer systems; however redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to immobile Cr(III) precipitates. A discrete fracture network (DFN) framework was applied for characterization, including collection of detailed profile of Cr mass distribution in the low permeability matrix by subsampling continuous cores with development of new laboratory extraction techniques to quantify both mobile Cr(VI) in porewater and Cr(III) precipitates. Supporting field data included geophysical and hydrophysical methods for fracture network and flow system characterization and installation of a multilevel system for depth-discrete hydraulic head and groundwater sampling for Cr(VI), hydrochemistry and Cr-isotopes. Results show matrix diffusion and reaction processes are influencing the plume including Cr(VI) reduction and immobilization in the rock matrix as Cr(III), which can significantly enhance plume attenuation in combination with matrix diffusion and limit potential for back diffusion if the reduction is irreversible. This presentation will provide an update on study results and implications.