Characterization of Chlorinated Ethenes Contamination Using Quantitative and Qualitative Evaluation Methods in a Complex Groundwater System

Abstract

Characteristics of contamination before and after the remedial action in a complex groundwater system contaminated with chlorinated ethenes such as trichloroethylene (TCE) were identified using quantitative and qualitative evaluation methods. The study site is located at an industrial complex of Wonju city, Korea, adjacent to a stream. Contamination by trichloroethylene (TCE) was detected in 1990s and had been investigated for contaminant identification and remediation. The precise information for initial mass and concentration of TCE spilled at the source zone was unknown. For this reason, it was hard to characterize sources that caused pollution and to find appropriate remediation strategies. Analytical solutions which can assess and quantify the impacts of partial mass reduction on groundwater are used to estimate the unknown residual TCE source mass and dissolved concentration as time passed using long-term monitoring data. Initial spilled TCE mass (1,000 kg) and dissolved concentration (150,000 μg/L) were assessed with analytical solutions. The results of this study supports that an analytical solution can be applied to give the quantitative information for initial residual source mass and dissolved concentration at contaminated sites even when there are no document on the source history such as spilled amount and concentration. In scale of a locally heterogeneous stream watershed, if contaminated groundwater discharges to a stream, stream water pollution near the contamination source also can cause stream contamination problem. In this respect, the fate and transport of chlorinated ethenes around a stream in an industrial complex were evaluated using the concentration of each component, and hydrogeochemical, microbial, and compound-specific carbon isotope data. Groundwater geochemical data indicate that aerobic conditions prevail in the upgradient area of the studied aquifer, whereas conditions become anaerobic in the downgradient. An increasing trend in the molar fraction of cis-1, 2-dichloroethene (cis-DCE) and vinyl chloride (VC) was observed in the downgradient zone of the study area. The enriched δ13C values of TCE and depleted values of cis-DCE in the stream zone, compared to those of the source zone, also suggest biodegradation of volatile organic compounds (VOCs). Microbial community structures in monitoring wells adjacent to the stream zone were analyzed using 16S rRNA gene-based pyrosequencing to identify the microorganisms responsible for biodegradation. The multilateral approaches used in this study indicate that contaminants around the stream were naturally attenuated by active anaerobic biotransformation processes. At the main source zone of this study site, remediation technologies such as soil vapor extraction, soil flushing, biostimulation, and pump-and-treatment have been applied to eliminate the contaminant sources of TCE and to prevent the migration of TCE plume from remediation target zones. At each remediation target zone, the aqueous concentrations of TCE plume present at and around the main source areas decreased significantly as a result of remedial action. Variations in the contaminant flux across three transect lines and statistical trend in variation of TCE concentration using Sens slope estimator were analyzed to identify the effect of remediation during the long period after the remedial action. After the remediation, the effect of remediation took place clearly at each transect line located at the main source zone and industrial complex and also was represented on results of Sens slope estimator. By tracing a time-series of plume evolution, a greater variation in the TCE concentrations was detected at the plumes near the source zones compared to the relatively stable plumes in the downstream. The difference in the temporal trend of TCE concentrations between the plumes in the source zone and those in the downstream could have resulted from intensive remedial actions taken at the main source zones. The efficiency of the intensive remediation actions performed was demonstrated using quantitative evaluation methods such as analytical solutions with a long-term monitoring data set. From results of quantitative evaluation using analytical solutions, it is evaluated that the intensive remedial action had effectively been performed with the removal efficiency of 70 % for the residual source mass during the remediation period. The evaluation methods mentioned in this study will be helpful in assessing the long-term performance of remediation technologies at contaminated sites by chlorinated ethenes.