DUAL-MODE ISOTHERM MODELING OF CHLORINATED SOLVENT SORPTION TO SURFICIAL AQUIFER SEDIMENT

Sorption to organic matter in aquifer sediments affects the transport and fate of hydrophobic organic contaminants in groundwater. Kerogen is a particularly important form of organic matter because it exhibits nonlinear sorption and is ubiquitous in sedimentary rocks.

Previous studies in our lab have determined that the primary sorbing lithocomponents in the surficial aquifer at Borden CFB, Ontario are dark carbonate grains. For this work, Ordovician carbonate rocks from southwestern Ontario were used in place of these lithocomponents because they exhibit very similar sorption behavior to the in situ aquifer grains and allow us to exclude the non-sorbing bulk aquifer material from our experiments.

Single-solute sorption isotherms for trichloroethene (TCE) were collected for source rock materials that represent the sorbents at Borden. The isotherm data for these source rock materials were best fit by dual-mode isotherm models. Nonlinear sorption behavior is described by a dual-mode model in which solid-phase partitioning and concentration-dependent adsorption mechanisms occur simultaneously in the organic matter phase. Our results suggest that these models will offer improved sorption modeling across the broad aqueous concentration ranges encountered in the proximity of a contaminant plume.

A typical sorption coefficient (K_OC­) _­for TCE was calculated to be 93 L·kg^-1·oc^-1 based on a linear free energy relationship (LFER) in the form of log K_OC = a log K_OW + b, where a and b are regression coefficients and K_OW is the octanol-water partition coefficient for TCE. The LFER value is more than an order of magnitude lower than the coefficients determined from our sorption data at environmentally relevant, low aqueous concentrations. Sorption affinity increases significantly beyond that predicted by the LFER as aqueous sorbate concentration decreases. When considering site remediation, our isotherm models suggest that sorption at low concentration may cause the extended mass storage and tailing that is observed as an aquifer is flushed to low aqueous concentrations and allowed to relax.