Abstract
This study uses a combination of conventional and high resolution field and laboratory methods to investigate processes causing attenuation of a hexavalent chromium (Cr(VI)) plume in sedimentary bedrock at a former industrial facility. Groundwater plume Cr(VI) concentrations decline by more than three orders of magnitude over a 900 m distance down gradient from the site. Internal plume concentrations generally exhibit stable to declining trends due to diffusive and reactive transport in the low permeability matrix as fluxes from the contamination source dissipate due to natural depletion processes and active remediation efforts. The strong attenuation is attributed to diffusion from mobile groundwater in fractures to immobile porewater in the rock matrix, and reactions causing transformation of aqueous Cr(VI) to low-solubility Cr(III) precipitates, confirmed by high spatial resolution rock matrix contaminant concentrations and comparisons with groundwater concentrations from multi-level sampling within the plume. Field characterization data for the fracture network and matrix properties were used to inform 2-D discrete-fracture matrix (DFM) numerical model simulations that quantify attenuation due to diffusion and reaction processes, which show consistency with field datasets, and provide insights regarding future plume conditions. The combination of field, laboratory and modeling evidence demonstrates effects of matrix diffusion and reaction processes causing strong attenuation of a Cr(VI) plume in a sedimentary bedrock aquifer. This approach has important implications for characterization of sites with Cr(VI) contamination for improved site conceptual models and remediation decision-making.
Highlights
Diffusion impedes remediation efforts due to slow rates of back diffusion from the matrix [2,3,4]. These effects have mostly been studied for sites contaminated by chlorinated solvent volatile organic chemicals (VOCs) such as tetrachloroethylene (PCE) and trichloroethylene (TCE), in which case matrix storage is enhanced by organic-carbon-dominated sorption
The primary goals of the study were to investigate the bedrock plume extent and degree of attenuation along the flowpath, maximum plume depth and Cr(VI) mass distribution including evidence for diffusion into the rock matrix by high resolution rock core sampling, and to assess whether evidence exists for redox reactions that reduce Cr(VI) to Cr(III)
HQ-wireline diamond bit rotary core drilling with a triple-tube core system that better preserves the in situ fracture distribution; (2) core logging for lithology, fractures and other features informing high resolution rock core sampling at fracture surfaces and varying distances into the rock matrix away from fractures; and borehole testing involving (3) use of inflatable packers to isolate discrete intervals for groundwater sampling and hydraulic testing during drilling [40,44]; (4) geophysical logging of the open borehole including fluid temperature/conductivity, caliper, natural gamma, resistivity and acoustic televiewer (ATV) imaging; (5) FLUTe transmissivity profiling during liner installation [35,45]
Summary
Diffusion impedes remediation efforts due to slow rates of back diffusion from the matrix [2,3,4]. These effects have mostly been studied for sites contaminated by chlorinated solvent volatile organic chemicals (VOCs) such as tetrachloroethylene (PCE) and trichloroethylene (TCE), in which case matrix storage is enhanced by organic-carbon-dominated sorption. Without substantial transformation reactions, these chlorinated solvent plumes are considered some of the most challenging sites to remediate and represent long-term liabilities [5,6]. It has been shown that biotic and/or abiotic degradation occurs, albeit slowly, in the rock matrix [7,8,9,10,11,12]
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