SPATIAL VARIABILITY WITHIN A MULTICOMPONENT ORGANIC CONTAMINANT PLUME DISCHARGING TO A POND

From 1950 to 1970, onsite operations at a chemical recycling company resulted in multiple releases of NAPL-phase organic chemicals to the subsurface. The resulting mixed organic plume migrates through heterogeneous Quaternary deposits and sedimentary bedrock towards a manmade pond. The spatial nature of the groundwater contamination approaching the pond was investigated along two cross sections orthogonal to groundwater flow: one located 20 m upgradient from the pond and the other within the pond, 3-4 m from the upgradient edge. Detailed groundwater sampling and hydraulic head monitoring was done along the upland cross section using a new 15 port depth-discrete multilevel system installed in 4 boreholes, 18 m to 26 m deep. Groundwater sampling along the cross-section within the pond was conducted using a direct push sampling tool (Waterloo Profiler) with 50 - 60 cm vertical spacing in each hole. On the upland cross section, the contaminant distributions show narrow zones of clean and contaminated groundwater as well as zones of separation between contaminant groups, where peak concentrations of chlorinated ethenes are exclusive of peak chlorinated ethane, ketone, and BTEX concentrations. Degradation along the migration path from the source to the cross-section may explain the observed pattern, but flow tubes of varying length and velocity emanating from source zone NAPL-residual of varying age, size, and location likely also play a role. Groundwater flow continues from the upland cross-section directly toward the pond cross-section, however the pond cross-section displays a different contaminant distribution. While degradation may be an important factor in the differences seen between the two transects, the most plausible cause involves complex groundwater flow convergence and divergence caused by variability in the geologic media. Blending of contaminant mass from different flow tubes during sample extraction may also impact the concentrations in each cross-section. Hydraulic head monitoring indicates the pond is a flow-through system. Contamination concentrations in the recharge-zone of the pond are up to 5 orders of magnitude lower than on the plume cross-sections, and therefore, the sediment on the pond bottom is likely where strong contaminant attenuation by degradation occurs.