ABSTRACT EAR-9506403 Allen-King Clay-rich aquitards and constructed clay-liners provide low permeability barriers to contaminant migration and are used for hazardous waste disposal because of the margin of protection provided to underlying groundwater resources. Sorption in the aquitard matrix retards transport and provides a sink for uptake and release of organic contaminant mass over time. However, the mechanisms of sorption in clay-rich aquitards are not well understood. Recent observations, for example, have shown that perchloroethene (PCE) sorption in several natural clay-rich and relatively low carbon aquitards is significantly under-predicted by existing methods. It is hypothesized that in natural, low-fraction organic content (foc) and clay-rich aquitards, organic matter is the primary sorbing medium for nonionic organic compounds (NIOCs) such as PCE. The objectives of the proposed research are: (1) to determine the relative importance of organic carbon and/or direct sorption to clay minerals; (2) to determine the effects of differences in oxidation and distribution of organic matter on the magnitude and form of sorptionO and (3) to test for competitive sorption between similar solutes. Using the Planning Grant, the hypothesis will be tested using two contrasting natural aquitard samples with similar clay mineralogy and foc, but markedly different PCE sorption behavior. The results will provide insights into sorption mechanisms and bases for establishing the best methodologies for a more comprehensive study. The objective is to improve mechanistic understanding of NIOC sorption in natural low-carbon geologic samples, thereby allowing more accurate assessments of contaminant transport for groundwater resource protection and remediation.
