Evaluation of Passive Vapor Diffusion Samplers to Quantify Acetylene, Ethene, and Ethane in Groundwater

Acetylene, ethene, and ethane are products from the degradation of trichloroethene (TCE) in contaminated aquifers. However, the volatility of these gases makes it challenging to avoid excessive losses during sampling. The objective of this study was to compare the quantification of acetylene, ethene, and ethane using passive vapor diffusion (PVD) samplers vs. conventional low-flow groundwater collection. Samples were obtained from 8 to 13 monitoring wells at three sites that show potential for biotic and abiotic degradation of TCE in fractured rock aquifers. Method reporting limits (MRLs) for the PVD samplers were 0.25 μg/L for acetylene (0.0094 μM) and 0.28 μg/L for ethene and ethane (0.0099 and 0.0092 μM, respectively); the MRLs for conventional low-flow groundwater samples were ~40% higher. For two of the sites, the maximum concentrations of acetylene, ethene, and ethane obtained with the PVD samplers were comparable to the conventional low-flow samples. The frequency of detection for these gases with the PVD samplers was also comparable to conventional low-flow groundwater sampling. For one of the sites with higher levels of acetylene (maximum of 13 μg/L), the concentrations from the PVD samplers were approximately twofold higher than those with conventional low-flow groundwater sampling. Based on robust detection of acetylene, ethene, and/or ethane, it appears likely that degradation of TCE is occurring at the three sites that were sampled. The use of PVD samplers can reduce the possibility of false negative results to provide another line of evidence in support of natural attenuation.