Between a Rock and a Hard Place: Improving the Modeling of Chlorinated Solvent Natural Attenuation in Bedrock Aquifers

Natural attenuation has the potential to effectively remediate TCE in bedrock aquifers, albeit slowly, with in situ half-lives that can range from 80 to 800 days if dehalogenating bacteria are present. Nutrient distribution, mineral speciation, habitat availability and seasonal geochemical cycling may limit microbial colonization. To demonstrate natural attenuation in the zone between plume and receptor, these factors must be addressed along the groundwater flow pathway. In this paper, a method is developed to assess natural attenuation of chlorinated organic compounds with greater statistical confidence. Recent developments in microcosm methods for the evaluation of bedrock aquifers are employed for more statistically significant laboratory evaluations, and then comparisons made of field measures for model verification. Analysis of data from a bedrock aquifer consisting of meta-siltstone and meta-sandstone in Southeast New Hampshire is used for illustration, complete with cost evaluation and comparison of error probability versus expended cost.