Quantifying In Situ Transformation Rates of Chlorinated Ethenes by Combining Compound-Specific Stable Isotope Analysis, Groundwater Dating, And Carbon Isotope Mass Balances

We determined in situ reductive transformation rates of tetrachloroethene (PCE) in a contaminated aquifer by combining compound-specific carbon stable isotope analysis (CSIA) of the contaminants with tracer-based (H-3-He-3) groundwater dating. With increasing distance from the source, PCE was gradually transformed to trichloroethene (TCE), cis-dichloroethene (cDCE), and vinyl chloride (VC). Using the in situ determined carbon isotopic enrichment factor of -3.3 +/- 1.2 parts per thousand allowed for quantification of the PCE-to-TCE transformation based on isotopic (delta C-13) shifts. By combining these estimates of the extent of PCE transformation with measured groundwater residence times (between 16 and 36 years) we calculated half-lives of 2.8 +/- 0.8 years (k = 0.27 +/- 0.09 yr(-1)) for the PCE-to-TCE transformation. Carbon isotope mass balances including the chloroethenes PCE, TCE, cDCE, and VC(delta C-13(Sigma(CEs)))enabled an assessment of complete PCE dechlorination to nonchlorinated products. Shifts of delta C-13(Sigma(CEs)) at the fringe of the plume of more than 25 parts per thousand pointed to dechlorination beyond VC of up to 55 +/- 17% of the chloroethene mass. Calculated rates for this multistep dechlorination were highly variable throughout the aquifer (k = 0.4 +/- 0.4 yr(-1)), suggesting that PCE reduction to nonchlorinated products occurred only in locally restricted zones of the investigated site.