Ground Water Monitoring and Remediation最新文献

This study presents a methodology for evaluating natural and enhanced degradation behavior at complex dense non-aqueous phase liquid (DNAPL) sites via automated processing of compound-specific isotope analysis (CSIA) groundwater datasets developed over several years of sampling. The method utilizes CSIA datasets from two Australian sites, the Orica Botany Bay Facility in Matraville, New South Wales, and a former chemical manufacturing facility in Victoria, to gain qualitative and semi-quantitative insights into natural and enhanced attenuation processes. An automated workflow was developed to evaluate CSIA data trends to assess degradation mechanisms and rates, providing insight into contaminant attenuation progress across large, complex sites. In this study, isotopic enrichment factors were estimated based on temporal groundwater concentration and CSIA data at individual well locations, and those enrichment factors were evaluated alongside corresponding geological, microbial, and geochemical data to identify areas where attenuation plays a significant role in contaminant mass reduction. Interpretation of the CSIA data was ground-truthed by comparing to enrichment factors in the academic literature, assessing other evidence supporting degradation activity, and considering aspects of the site conceptual model that could affect isotopic behavior. By applying an automated workflow to CSIA datasets, the findings of this study demonstrate a valuable standardized approach to gain useful knowledge on the contribution of monitored natural attenuation (MNA) and enhanced biodegradation to contaminant mass reduction at complex sites. The study also illustrates some complexities associated with DNAPL sites (e.g., multiple sources of mass, multiple degradation mechanisms) that need to be considered when interpreting CSIA data.

Among the most challenging sites to remediate are those where the groundwater is contaminated with large, dilute plumes of tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cDCE), and vinyl chloride (VC). Monitored natural attenuation (MNA) may be a viable strategy, but relevant mechanisms such as abiotic degradation can be difficult to document. The objective of this study was to evaluate the use of a carbon-14 (¹⁴C) assay to measure the rate constants for degradation of chlorinated ethenes in contaminated aquifers using samples of soil and groundwater from three sites. Use of ¹⁴C-labeled compounds makes it possible to quantify degradation by measuring the accumulation of degradation products that are otherwise difficult to discern from background levels (e.g., ¹⁴CO₂ and ¹⁴C-labeled soluble compounds). The soil and groundwater samples were added to serum bottles; one set of the microcosms was incubated in the absence of oxygen, another set in the presence of oxygen. After injecting purified ¹⁴C-PCE, ¹⁴C-TCE, or ¹⁴C-cDCE, unlabeled compounds were added to bring the initial concentrations to ~200–1700 μg/L. The microcosms were placed on a tumbling device to ensure gentle agitation during incubation. At weekly intervals over 42 days, 5 mL liquid samples were withdrawn, filtered, and sparged to remove the unreacted ¹⁴C-labeled parent compound. The amounts of ¹⁴C products that accumulated were used to calculate pseudo-first-order rate constants that ranged from 0.0092 to 0.24 per year. In a companion paper by Wilson et al. (2025), the rate constants are evaluated as a line of evidence for assessing the applicability of MNA.

This study estimates Natural Source Zone Depletion (NSZD) rates of Light Nonaqueous Phase Liquid (LNAPL) at a former refinery site in Central California. The site’s complex hydrogeology, which includes a thick vadose zone, significant groundwater level fluctuations, and previous long-term remediation using a soil vapor extraction system and an air sparging system, created unique bioremediation conditions. Notably, this study is one of the first, and among only a few, to apply the Single Stick Method for NSZD rate calculation in a full-scale active remediation site. The method, which transforms real-time continuous temperature data from six locations over 9 months into NSZD rates, captures subsurface heating associated with NSZD and surface heating and cooling at each test well, eliminating the need for background corrections. Three distinct phases were observed in the vadose zone during the study: Phase 1—Post-Active Remediation (immediately after remediation system shut-down); Phase 2—Thermal Shift (3 months after remediation system shut-down); and Phase 3—Steady-State Thermal Conditions (6 months after remediation system shut-down). The study showed a transition from widespread elevated heat signals to thermal shifts, followed by stable thermal conditions. The sitewide average NSZD rate was estimated at 7300 gal per acre per year (gal/acre/year) during Phase 1, 3500 gal/acre/year during Phase 2, and 2700 gal/acre/year during Phase 3. The results underscore the importance of excluding early-stage data influenced by active remediation to avoid artificially high rates. This study highlights the value of long-term monitoring for sites with prior active remediation systems and demonstrates the utility of the Single Stick Method for NSZD rate estimation, offering valuable insights into natural attenuation processes.

Chlorinated ethenes are common contaminants in aerobic water-supply aquifers, and they can form large dilute plumes. Often, concentrations attenuate naturally along the flow path, and the bulk first-order rate constant for attenuation with travel time in the aquifer (k_na) can be used to determine whether the concentrations at a point of compliance will be below the cleanup goal. If concentrations are below the goal, k_na provides the U.S. EPA the first line of evidence for Monitored Natural Attenuation (MNA) as a remedy. However, monitoring wells may fail to sample important areas of a plume; thus, the first line of evidence alone is often not convincing for MNA acceptance. Consequently, there is a need for methods that provide additional lines of evidence to support the use of MNA in aerobic water-supply aquifers. A recently developed ¹⁴C-assay addresses this gap by directly quantifying natural abiotic degradation of chlorinated ethenes through the accumulation of radiolabeled polar products. Specifically, the assay provides rate constants for abiotic degradation within 6 weeks, offering both a quantitative measure of degradation (the second U.S. EPA line of evidence for MNA) and proof that contaminants are being transformed (the third line of evidence). Here, the ¹⁴C-assay was used to evaluate abiotic degradation at three sites. At one site k_na was 0.89 ± 0.094 per year. The rate constant for abiotic TCE degradation in a ¹⁴C-assay using sediment from the site was 0.27 ± 0.11 per year, and the rate constant for cDCE degradation was 1.57 ± 0.39 per year. These results demonstrate that the ¹⁴C-assay can provide robust additional lines of evidence supporting MNA in aquifers where abiotic degradation would otherwise be overlooked.