Background/Objectives: Site monitoring using wells at chlorinated solvent and other contaminated sites suffers from a number of technical issues. Specifically, the resulting data density is often not sufficient to provide an adequate understanding of remediation effectiveness or potential contaminant excursions at point of compliance boundaries. Furthermore, the wells must be maintained to avoid biofouling and provide accurate data, and they must have been properly installed and eventually abandoned to avoid potential aquifer cross-contamination.
An improved monitoring approach is to limit the use of wells and leverage robust electrical monitoring data collected from the ground surface in continuous 2D planes and at depths to 1,000 feet or more. Temporal electrical resistivity imaging (TERI) involves burying electrode cables in a 2-foot-deep trench to protect them over time from vehicle traffic, wildlife, and potential vandalism. To further prepare for monitoring contaminated sites, research has been performed by EPA and academics at a non-contaminated site in Oklahoma to confirm installation methods and data quality. Additional technology deployment protocols were developed and field tested at a site in Colorado by a commercial provider.
Approach/Activities: Specifically, this project evaluated installation, performance, and durability/longevity of trenched-in monitoring cables at test sites with lengths from 50 meters to 1 kilometer. This project focused on cable construction and installation, monitoring on regular intervals and capturing storm (i.e., precipitation) events to evaluate data quality. The electrode cables were constructed using graphite electrodes to avoid degradation due to formation of patina or corrosion over time and during use.
The electrode cables were installed in the trench using electrically conductive grounding grout to ensure good contact with the buried electrodes and the earth to promote good data quality. The cables were trenched into soil and bedrock locations and completed using two different surface completion methods. Data acquisition was conducted over a period of up to three years to evaluate data quality.
Results/Lessons Learned: Cable installation originally required more field labor than expected which has resulted in modified design and updated installation protocols to increase speed of installation and decrease overall cost. These improved designs have been tested on a commercial basis to make the technology commercially viable.
The surface completion involved a PVC stub up from the cable trench to allow access to the cable end connectors to collect data using an earth resistivity meter. While high humidity does not affect well PVC in ground mount installation, it was more problematic for the electrical cable end connectors, so spray foam insulation was used to isolate moisture in the ground from the PVC stub up and connectors. Data quality was good through the experiment except during extreme drought where the ground desiccated to the depth of the trench. Subsequent rain events demonstrated that the cable reconnected with the subsurface to again provide high quality data.
The data results demonstrated the active locations of the flow system allowing better well placement and improved vadose zone monitoring of the sites. The collective work on this research and development project resulted in proved out methods sufficient for deployment that will facilitate more robust monitoring of remediation effectiveness at groundwater sites across a range of scales.
Background/Objectives: Conventional drilling and sampling typically involve widely spaced sampling locations that missed critical information about the precise distribution of contaminants. The advent of high-resolution sampling characterization (HRSC) helped increase data density but still has limitations, specifically when it comes to sub-surface lateral and vertical definition. HRSC collects data from multiple ~2-inch diameter boreholes across a site to gather high-resolution vertical data. These data are interpolated laterally to create useful hydrogeologic cross sections but for which critical information can still be missed due to lack of sufficient data density and lack of targeting with no pre-imaging event.
To address this challenge at a former strip mall dry cleaner release site in El Sobrante, California, continuous electrical resistivity imaging (ERI) data were collected to target follow-up HRSC direct push sampling as well as conventional drilling/sampling. All of these data were visualized in 2-D and 3-D to more fully define the lateral and vertical distribution of contaminants as well as site hydrogeologic conditions. The resulting robust conceptual site model (CSM) is informing remedial design for recalcitrant chlorinated ethenes. The Site is planned for brownfield redevelopment and investigation activities were funded by the California Environmental Protection Agency and Department of Toxic Substance Control’s (DTSC’s) Equitable Community Revitalization Grant (ECRG).
Approach/Activities: This remedial design characterization (RDC) work involved a five (5)-step approach: Step 1 included integrating a very limited historical conventional sampling data set into 3-D visualization software. Step 2 included ERI field data acquisition via GeoTrax Survey™ specialty ERI technology to scan the subsurface to a depth of ~90 feet. Step 3 involved integration of electrical imaging data and interpreting these combined multiple lines of evidence relative to updated CSM hypotheses and remaining data gaps. Step 4 was a targeted confirmation drilling (CD) program to confirm the geological, chemical, and biological composition of anomalous zones detected by the ERI data and to facilitate data calibration and interpretation. Multiple field events were performed via HRSC membrane interface hydraulic profiling tool (MiHPT), direct push soil borings/temporary monitoring, permanent monitoring well installation, and active and passive soil gas sampling. Step 5 involved integration of all of the abovementioned data sets in 2-D and 3-D to interpret an updated CSM to inform the remedial design process and focus the subsequent remediation program.
Results/Lessons Learned: This combined RDC approach achieved project goals with only nine (9) direct push soil borings and seven (7) conventional monitoring wells over a ~2-acre Site. The horizontal and vertical extents of dissolved phase tetrachloroethylene (PCE) impacts to groundwater were better defined by targeted confirmation drilling. Three (3) zones of impacts and two (2) preferential flowpaths were identified and mapped based on the integrated data sets.
Prior to conducting this RDC work, the primary contaminant flowpath was presumed to coincide with a known buried paleo stream channel. Unexpectedly, the CVOC impacts in shallow groundwater were found to be laterally constrained along the clay depocenter and not the buried stream channel, and impacts in deeper groundwater were found to be constrained to an identified syncline fold axis. Without the integrated RDC data, this critical information would have likely been missed.
Obtaining regulatory and grant funding approval was initially challenging since ER approaches are not a commonly used technique but through meetings and presentations the stakeholders agreed the combined remedy was the best approach. An Analysis of Brownfield Cleanup Alternatives is currently being completed, and this presentation is anticipated to discuss the selected remedy for the Site.
Background/Objectives: Since the inception of environmental remediation practices, subsurface characterization has relied primarily on drilling wells and borings, collecting data from these points, and interpolating results to infer geologic structure and contaminant distribution. However, the majority of environmental sites are geologically complex, ranging from heterogeneous fluvial deposits to fractured bedrock and karst systems, each characterized by preferential flow pathways that are difficult, if not impossible, to delineate using well data alone.
As a result, multiple rounds of drilling, sampling, and analysis often fail to produce sufficiently resolved conceptual site models (CSMs). Industry data suggest that well spacing on the order of five to ten meters or more is typically inadequate for resolving heterogeneity in such complex settings, frequently leading to incomplete or ineffective remediation efforts. Obtaining well data of sufficient data density is often not economically viable so leveraging higher resolution geophysical data to compliment and guide the drilling work is a more appropriate option to consider at many project sites. This presentation discusses case studies for two different sites where advanced electrical resistivity imaging (ERI) data was used to target new monitoring wells and successfully identify source zones as well as complex geology and flowpaths which were previously not well understood using only conventional drilling and sampling data.
Approach/Activities: The first site involves a large hydrocarbon plume within the Ogallala Formation, migrating from a nearby industrial facility. In this region, the Ogallala reaches approximately 500 feet in thickness and contains numerous paleochannels with highly variable permeability. Specialized electrical resistivity imaging (ERI) was conducted across a ~3-mile by ~1-mile area to depths of ~540 feet below ground surface. By integrating targeted boring log data with resistivity imagery, the project team identified and mapped the primary paleochannels serving as LNAPL migration pathways. This integrated dataset was instrumental in developing a robust groundwater flow model that will guide targeted and cost-effective remedial design.
The second site is a refinery located in the northeastern United States which exhibited hydrocarbon seeps discharging from the adjacent riverbank. Despite years of drilling and modeling, the origin and migration mechanisms of the seeps remained unclear. Land based and marine based ERI surveys were implemented to delineate the source and migration pathways of the hydrocarbons. The results indicated that contamination was migrating beneath the existing sentinel monitoring well network and discharging at the riverbank. Confirmation drilling encountered approximately ten feet of free product substantially below the water table, and beneath the base of the existing wells. When integrated with a regional geologic model, the data revealed a Pleistocene-age paleochannel serving as the primary migration pathway, with evidence suggesting that the plume originated offsite.
Results/Lessons Learned: In both instances, integrating advanced electrical imaging data with well data yielded high-resolution CSMs that effectively characterized heterogeneous geologic conditions where traditional drilling-based methods alone were insufficient. The fundamental distinction with this approach involves a number of factors including increasing data density with continuous 2D imaging technology, leveraging imagery to target confirmation drilling locations, performing significant data integration in 2D and 3D, and developing robust geology interpretations. Applying this approach yielded more detailed and reliable CSMs for both sites, enhancing decision-making efficiency and reducing overall costs and timelines for remedial actions.
Background/Objectives: Remedial substrate injection (light remediation) and thermal (heavy remediation) are commonly used for in situ treatment of contaminated sites. An important component of these remediation programs is having a reliable way to assess the effectiveness of the technology in terms of cleanup progress.
Temporal (e.g., time-lapse) electrical resistivity imaging (TERI) technology has been demonstrated to more robustly characterize NAPL, and dissolved plume distribution and state of bioactivity/weathering as compared to traditional monitoring wells only. When these electrical data sets are collected over time (i.e., 4D/temporal/time-lapse imaging) for site monitoring purposes, valuable insights about preferential flowpaths, plume migration/behavior, radius of influence, and overall success of remedial actions can be better understood by analyzing changes in electrical properties of the subsurface.
Approach/Activities: Successful electrical monitoring has been performed at multiple injection and thermal remediation sites using both single-event post-injection imaging or multiple-event temporal imaging (e.g. pre- and post-remediation). Use of TERI to better understand actual injectate distribution and effectiveness of thermal remediation over time has produced a number of helpful lessons. First, electrodes ideally remain in place throughout the monitoring period to maintain low temporal noise and ensure data quality. Second, channeling and vertical migration of injectate are common occurrences. Finally, electrical signatures can change over time in response to subsurface reactions. This presentation will highlight the advantages of the method through a discussion of several project examples which provided information about the benefits and challenges of subsurface electrical imaging and data integration.
Results/Lessons Learned: Results to date indicate that the use of high data density TERI technology for monitoring light and heavy remediation efforts provides a higher degree of certainty regarding technology effectiveness relative to established site cleanup goals.
OBJECTIVE: This learning lab is intended to educate and solicit feedback from environmental professionals regarding the use of electrical subsurface scanning technology for remedial design characterization (static imaging) of chlorinated solvent plumes to allow project stakeholders to close sites faster and at lower cost. This subsurface imaging technology can also be deployed over time (temporal imaging) to monitor point of compliance and other site issues at high data density.
Site managers (engineers, geologists, hydrogeologists, others), responsible parties, regulators, and chlorinated solvent professionals will benefit from this Learning Lab which will have hands on examples of equipment used as well as examples of 3D data visualization.
DESCRIPTION: Successful remediation of chlorinated solvent impacts starts with characterizing the spatial distribution of contaminants in the subsurface. Understanding the contaminant source area, preferential flowpaths, lithologic controls, etc. on sites impacted with chlorinated solvents is important as this determines available options and related costs for remedial design, implementation, and, ultimately, site closure.
Traditional site characterization methods using monitoring wells alone offer only a limited view of critical contaminant pathways with an extremely limited sample size relative to the size of the site as a whole. This inherent low data density site characterization methodology leaves numerous data gaps which often yield an incomplete or skewed understanding of actual subsurface conditions. Resulting conceptual site models (CSMs) which are incorrect or incomplete negatively impact the effectiveness and cost of remedial efforts. Industry research has demonstrated repeatedly that traditional methods fail to characterize these sites effectively.
Laboratory and field studies have demonstrated the effectiveness of electrical imaging in mapping subsurface chlorinated solvent plumes. These modern tools provide a detailed view of the electrical structure of plumes, bioactivity and previous remedial efforts, both laterally and vertically. This learning lab will explore how environmental professionals can leverage electrical imaging to characterize and monitor chlorinated solvent plumes over time which enables greater project success and limits trailing liabilities for project stakeholders.
