Identification of chlorinated solvents degradation zones in clay till by high resolution chemical, microbial and compound specific isotope analysis

The degradation of chlorinated ethenes and ethanes in clay till was investigated at a contaminated site (Vadsby, Denmark) by high resolution sampling of intact cores combined with groundwater sampling. Over decades of contamination, bioactive zones with degradation of trichloroethene (TCE) and 1,1,1-trichloroethane (1,1,1-TCA) to 1,2-<i>cis</i>-dichloroethene (<i>cis</i>-DCE) and 1,1-dichloroethane, respectively, had developed in most of the clay till matrix. <i>Dehalobacter</i> dominated over <i>Dehalococcoides</i> (<i>Dhc</i>) in the clay till matrix corresponding with stagnation of sequential dechlorination at <i>cis</i>-DCE. Sporadically distributed bioactive zones with partial degradation to ethene were identified in the clay till matrix (thickness from 0.10 to 0.22 m). In one sub-section profile the presence of <i>Dhc</i> with the <i>vcrA</i> gene supported the occurrence of degradation of <i>cis</i>-DCE and VC, and in another enriched δ¹³C for TCE, <i>cis</i>-DCE and VC documented degradation. Highly enriched δ¹³C for 1,1,1-TCA (25‰) and <i>cis</i>-DCE (−4‰) suggested the occurrence of abiotic degradation in a third sub-section profile.Due to fine scale heterogeneity the identification of active degradation zones in the clay till matrix depended on high resolution subsampling of the clay till cores. The study demonstrates that an integrated approach combining chemical analysis, molecular microbial tools and compound specific isotope analysis (CSIA) was required in order to document biotic and abiotic degradations in the clay till system.