Variability of chlorinated-solvent sorption associated with oxidative weathering of kerogen

Abstract

The perchloroethylene (PCE) sorption distribution coefficient ( K d, measured at 40–100 μg/l solution concentration in batch experiments) ranged from 4.0 to 134 l/kg over the sampled thickness (1.0–6.0 m depth) of a moderately-low organic carbon (0.002–0.005), fractured and weathered diamict (poorly sorted, glacially deposited) aquitard. The organic carbon-normalized K d ( K oc) for deeper, unoxidized (reduced) samples from this deposit (log K oc=4.4–4.59) were generally consistent with the kerogen in the underlying Devonian-age shale (log K oc=4.57), a likely source rock for these sediments. The similarities between these samples suggest that the sorption capacity of the kerogen was minimally altered during transport and deposition, and that humic substances were not co-deposited with the kerogen in the glacially-deposited sediments. The K oc within the diamict was correlated to the oxidation state of the organic matter, characterized by the H/O ratio through elemental analysis. The K oc for more oxidized samples from shallower depth samples (log K oc=3.35–3.65) were approximately an order of magnitude less than those determined for the reduced samples. The K oc for 1.5- to 2.5-cm thick oxidation haloes surrounding fractures and characterized by visible Fe-oxidation, were approximately 20–30% lower than the adjacent unaltered matrix samples. The linear trend observed between log K oc and log H/O is similar to, but steeper than, the relation reported previously, which represents increasing sorption magnitude with increasing diagenesis in coal and shale samples. The spatial distribution of the K oc within this aquitard, in concert with qualitative geologic observations, suggests that oxidative weathering of the native kerogen exerts an important control on hydrophobic organic compound sorption.