Abiotic Reaction and Diffusion of Chlorinated Solvents in Fe2+ Bearing Intact Limestones and Mudstones

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Diffusion of groundwater contaminants into and out of water-saturated low permeability rock matrices is a very slow process and responsible for the limited success of many subsurface remediation activities. Chlorinated solvents are still among the most relevant groundwater contaminants forming large scale plumes especially in urban space despite decades of clean-up efforts. Slow diffusion results in very long residence times in the rock matrix and thus slow abiotic reactions involving Fe2+ bearing minerals (e.g. iron sulfides, magnetite, phyllosilicates) may become effective in transformation of chlorinated compounds and should be considered in assessment of natural attenuation potentials. Goals of this proposal is to measure abiotic transformation of tri- and perchloroethene during slow diffusion in rock samples from fractured aquifers and from aquitards. Since reaction rates of the parent compounds are likely too slow to be measurable experimentally, focus is on identification and analysis of transformation intermediates and products (e.g. partly chlorinated acetylenes, ethenes and ethane). Reactive diffusion experiments have to be performed with intact rock samples since crushing of minerals (e.g. quartz, pyrite) may introduce reactive surfaces resulting potentially in dehalogenation of the parent compounds. In contrast to earlier studies reactions will be related to types of minerals and their original size in the rock matrix. Results are not only relevant for long-term behavior of chlorinated solvent plumes, but in general important for radioactive waste disposal or chemical weathering (oxidation) of reduced rocks.