Comparing Rock Matrix Contaminant Profiles Downgradient of a DNAPL Source after 10 Years of Groundwater Dissolution
Tuesday, October 3, 2017: 10:10 a.m.
Prior to 1970, over 70,000 L of dense non-aqueous phase liquids (DNAPLs) were released into the subsurface at a site in south central Wisconsin. The mixed organic DNAPL migrated through unconsolidated glacial sediments and shallow sedimentary bedrock, eventually accumulating about 56 meters below ground in a fractured sandstone. The objective of this study was to characterize the temporal evolution of contaminant mass in the source zone by comparing two co-located rock core volatile organic compound (VOC) concentration profiles, one collected in 2003 and the other in 2014. The rock core VOC profiles provided depth discrete and detailed (at least 1 sample/foot of core) quantification of the contaminant mass in the rock matrix. The 2003 core shows relatively uniform rates of mass accumulation with depth for most contaminants; whereas, the 2014 core shows highly variable rates of mass accumulation with depth, particularly in the shallow rock units. These results indicate variable attenuation rates for specific depth horizons. Comparison of total mass estimates for each core indicate an apparent mass loss of ~ 80%, most of which occurs in the shallow bedrock units. Assessment of specific contaminants shows declines in concentration for parent ethanes and ethanes, dichlormethane, and MIBK and increases in concentrations for daughter products (e.g., chloroethane, vinyl chloride). Core and borehole geophysical logs and hydraulic testing provide site-specific parameters for evaluating the role of various attenuation processes (e.g., dispersion, diffusion, sorption, abiotic and biotic degradation) influencing source zone fluxes and longevity. The insights from this temporal comparison will inform a process-based site conceptual model and improve remedial technology assessments.