Formatted Title
Use of Passive Groundwater/Mass Flux Meters to Support Natural Attenuation in Complex Geologic/Hydrogeologic Conditions
Background/Objectives
Groundwater impacted with chlorinated volatile organics, at a former manufacturing facility, has been historically monitored to identify the potential for detection near the site boundary. Based on the ongoing review of historical groundwater monitoring data, a program to evaluate natural attenuation versus other potential remedial alternatives, should they be required, has been implemented. A deep dive into the historical data collected through 2022 was completed that identified that natural attenuation mechanisms were active at the site. Further supplemental studies were completed to refine the site conceptual model and provide multiple lines of evidence (LOEs) to support a natural attenuation evaluation at the Site. The supplemental investigation included source identification soil borings, high-resolution geophysics, seismic borehole logging, microbiological sampling using multiple analytical approaches (using qPCR with standard and small (nanoliter) sample volumes to compare results), carbon stable isotope analysis (CSIA), along with a focused groundwater monitoring program to gather VOC and other natural attenuation groundwater analytical data. These results yielded a more thorough understanding of subsurface conditions given the complex geologic/hydrogeologic site setting. Prior to considering any additional remedial options at the site a detailed evaluation of groundwater and contaminant mass flux was implemented to collect additional LOEs to 1) bolster the current understanding of subsurface conditions with respect to natural attenuation; 2) further refine the current conceptual site model (CSM); and 3) focus the implementation of any warranted additional remedial action to mitigate the potential migrations of VOCs beyond the property.
Approach/Activities
To collect additional data related to groundwater and contaminant mass flux, the revised conceptual site model (CSM) and historical data re-evaluation results were used to design a supplemental data gathering effort to utilizing a series of strategically placed passive flux meters (PFMs) within two identified areas (Areas A & B) of groundwater impacts at the Site. A total of twenty-two (22) PFMs were deployed in nine (9) individual monitoring wells (five locations in Area A and 4 locations in Area B). The PFMs were deployed in three distinct groundwater flow zones (Overburden, weathered bedrock, and bedrock) and along the center line of the identified groundwater impacts to provide detailed areal and vertical data resolution. Following the initial PFM deployment, the PFMs were retrieved after approximately 5 weeks of exposure to site groundwater. For each of the twenty-two PFMs, up to three individual samples were collected from each of the individual, 5-ft long PFMs. These data were evaluated to develop a detailed vertical interpretation of groundwater and contaminant mass flux. These data along with the other multiple LOEs were combined with the revised CSM to prepare a comprehensive presentation of current site conditions and to guide future decision making related to natural attenuation and/or the feasibility for implementing additional remedial alternatives at the site.
Results/Lessons Learned
The results from the PFMs will be available by the end of 2023 and findings from the completion of the revised CSM, the evaluation of the supplemental groundwater data, and the other multiple LOEs will be presented and discussed with respect to the evaluation of natural attenuation versus additional remedial measures. Additionally, information will be presented relating how the data has been utilized to inform the next steps that are planned for the Site with respect to pilot testing in focused areas of the various water-bearing units based on the results of the overall subsurface evaluation. The data evaluation is currently being completed and will be available for presentation in 2024. Lastly, lessons learned related to the deployment and retrieval of the PFMs will be provided within the context of the site-specific conditions encountered during the study.