Assessing Contaminant Mass Discharge Uncertainty With Application of Hydraulic Conductivities Derived From Geoelectrical Cross-Borehole Induced Polarization and Other Methods

A new methodology was developed to support contaminant mass discharge (CMD)-based risk assessment of groundwater contamination downgradient of point source zones. Geoelectrical cross-borehole induced polarization (IP) data were collected at a site undergoing in situ remediation of chlorinated solvents for determining 2D hydraulic conductivity (K) distributions with an inversion model resolution of 0.15 m (vertically) x 0.50 m (horizontally) in three control planes from 10 to 20 m depth. Additionally, 18 slug tests and 31 grain size distribution analyses (GSA) from the control planes, were used for K-estimation. The geometric means and variance of the IP, slug test, and GSA derived K-estimates were consistent with previously studied sandy aquifers. Furthermore, the vertical variation in K between two geological settings, a sandy till and a meltwater sand formation, was clearly identified by the IP K-estimates. The vertical variation was backed up by hydraulic profiling tool (HPT) measurements. Random realizations of CMD were simulated based on the cross-borehole IP derived K-values. For comparison, the CMD was also estimated with a geostatistical conditional simulation approach, using the data from slug tests and GSAs. The high IP resolution captured the small scale variations in K across the transects and led to CMD predictions with a narrow uncertainty interval, whereas slug test and GSA either under- or overestimated the magnitude of the areas with the highest CMD. Applying the geophysical cross-borehole method for estimating K-distributions in addition to traditional methods would improve CMD-based risk assessment and evaluation of remediation performance at contaminated sites.