Formatted Title
Using Multiple HRSC Technologies to Develop a Detailed CSM for a Complex Fractured Bedrock Site
Background/Objectives
Utilizing modern and innovative high resolution site characterization (HRSC) technologies to develop an accurate conceptual site model (CSM) is increasingly pertinent for remediation of fractured bedrock sites. Inherent complexities and variability of flow and contaminant transport in fractured bedrock can result in erroneous representations of site conditions and can stymie site cleanup efforts. As such, in situ remediation of fractured bedrock aquifers used to be considered near impossible. As recently as 2013, the US Department of Defense’s (DoD’s) environmental research arm (SERDP) wrote “One of DoD’s most challenging environmental restoration issues is determining how to deal with contaminants that have seeped into the fractures in bedrock and are a continuing source of groundwater contamination” (SERDP, 2013). The U.S. Geological Survey (USGS) noted that “remedial action is delayed or stymied by the complexity of contaminated fractured-rock aquifers”. This presentation will summarize multiple HRSC technologies and methodologies utilized at a complex bedrock site with chlorinated volatile organic compound (cVOC) contamination. The HRSC suite of technologies applied at the site included injectable tracer testing, downhole geophysical surveys, and remedial design characterization (RDC) via discrete soil, bedrock, and groundwater sampling. HRSC tools aided in developing an accurate CSM and was used to design a pilot-scale in situ remedial injection into the fractured bedrock.
Approach/Activities
Multiple technologies were employed to assist in developing an accurate and representative CSM at a complex bedrock aquifer site impacted with chlorinated solvents. An initial tracer injection test was completed using rhodamine dye and potassium bromide in 500 gallons of solution to understand real-world groundwater flow and transport conditions. A total of 40 discrete soil samples and six bedrock core samples were collected for inspection and baseline analyzed of CVOCs. Subsequent downhole borehole geophysical surveys were completed in the bedrock over a depth interval from 20 to 44 ft below grade develop a greater understanding of bedrock fracture conditions and flow patterns at the site. A review and interpretation of all upfront HRSC data collected was utilized to aid in the design of a 2,200 gallon pilot-scale remedial amendment injection of an activated carbon-based injectable (CBI) remedial amendment into four injection boreholes using pressure packer techniques to assess injection flow rates and pressures, injection radius of influence and treatment effectiveness.
Results/Lessons Learned
Reducing data gaps in a CSM for a complex bedrock aquifer site is essential for effective and timely remediation. Multiple lines of evidence were used to create a detailed CSM for the site and help develop a targeted remedial approach. The presentation will review the suite of HRSC technologies applied at the site, the data collected that assisted in constructing an accurate CSM, and the eventual pilot-scale in situ remedial approach that was designed in accordance with the detailed CSM. An extensive follow-up sampling and laboratory analytical program was completed at the site, results of which will be presented to highlight the importance of HRSC tools in understanding complex sites.