Evaluating degradation of hexachlorcyclohexane (HCH) isomers within a contaminated aquifer using compound-specific stable carbon isotope analysis (CSIA)

• First CSIA field study for monitoring in situ biodegradation of HCH isomers.
• Confirmation of HCH sources in a contaminated aquifer using isotope data.
• Estimation of time- & distance-dependent biodegradation rate constants for HCH isomers.
• Exploration of temporal variations in HCH biodegradation & natural attenuation.
• Demonstration of CSIA as a monitoring tool for field site management & remediation concepts.

The applicability of compound-specific stable carbon isotope analysis (CSIA) for assessing biodegradation of hexachlorocyclohexane (HCH) isomers was investigated in a contaminated aquifer at a former pesticide processing facility. A CSIA method was developed and tested for efficacy in determining carbon isotope ratios of HCH isomers in groundwater samples using gas chromatography – isotope ratio mass spectrometry (GC-IRMS). The carbon isotope ratios of HCHs measured for samples taken from the field site confirmed contaminant source zones at former processing facilities, a storage depot and a waste dump site. The 13C-enrichment in HCHs provided evidence for biodegradation of HCHs especially downstream of the contaminant source zones. CSIA from monitoring campaigns in 2008, 2009 and 2010 revealed temporal trends in HCH biodegradation. Thus, the impact and progress of natural attenuation processes could be evaluated within the investigated aquifer. Calculations based on the Rayleigh-equation approach yielded levels of HCH biodegradation ranging from 30 to 86 %. Moreover, time- and distance-dependent in situ first-order biodegradation rate constants were estimated with maximal values of 3 × 10−3 d−1 and 10 × 10−3 m−1 for α-HCH, 11 × 10−3 d−1 and 37 × 10−3 m−1 for β-HCH, and 6 × 10−3 d−1 and 19 × 10−3 m−1 for δ-HCH, respectively. This study highlights the applicability of CSIA for the assessment of HCH biodegradation within contaminated aquifers.

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