Theme: 10. Characterization, Fate and Transport

Topic Area: 10h. HRSC Suites of Tools to Improve CSMs

Formatted Title:

HRSC Techniques Used to Identify Preferential Flow Paths and Inform Monitoring Well Screened Intervals in a Heterogeneous Aquifer

Background/Objectives:

The focus of the field investigation presented in this abstract is to characterize the nature and extent of a volatile organic compound (VOC) groundwater plume in a heterogeneous shallow aquifer with localized high-conductivity flow pathways in order to select additional, successful remedies. The site is a contaminated groundwater plume, separated into two operable units (OUs). OU1 includes the area of a known, significant source of source of tetrachloroethene (PCE) contamination, and OU2 includes two other known source areas and areas outside of the known sources with impacted soil, sediment, and groundwater. Contamination in OU2 includes volatile organic compounds, namely PCE and degradation products, and petroleum hydrocarbons. The site geologic/hydrogeologic framework includes quaternary alluvium, from surface to approximately 15 feet below ground surface (bgs), quaternary glacial outwash deposits, below the alluvium to approximately 50 feet bgs, and Miocene nonmarine clay below the glacial outwash deposits. The nonmarine clay serves as an aquitard separating shallow and deep aquifers at the site. The glacial outwash deposits are considered the shallow aquifer and are further separated into an upper zone and a deep zone, separated by a till layer within the glacial outwash deposits. The contamination and current investigation are focused on the shallow aquifer. Following previous investigations at the site, an interim remedy at OU1 was approved and began in 2021 involving soil, sediment, and groundwater sampling, in situ thermal technology, enhanced aerobic biodegradation, and performance monitoring.

Approach/Activities:

Beginning in August 2023, more than 30 exploratory borings were drilled using direct push technology (DPT) up to 50 feet bgs. Borings were located along three transects downgradient of OU1 and across the OU2 plume. Soil and groundwater samples were collected during the advancement of the exploratory DPT borings and evaluated for VOC concentrations in real time using a portable micro gas chromatograph - photoionization detector (FROG-5000). Based on the results of the FROG analysis, select soil and groundwater samples were sent to an accredited lab for analysis. Generally, samples with concentrations above 50 parts per billion (ppb) were sent to the lab, and occasionally samples not detected above the reporting limit were sent to the lab for confirmation of the non-detect result. Four locations were also selected for shallow (5 feet bgs) soil vapor probes. Multiple groundwater, surface water, and sediment sampling have been conducted concurrently with the drilling to provide supplemental data sets for this investigation. Field work was paused in November 2023 due to weather and site conditions. At the time of this conference, field work will recommence, and will include installation of casings with roto-sonic technology to approximately 50 feet bgs at select previously-drilled exploratory DPT borings. Locations to be cased have been chosen based on lithology, proximity to the local surface water, and VOC concenctrations. The cased borings will be used to conduct downhole geophysical logging which will include natural gamma, electromagnetic, and nuclear magnetic resonance. Data collected during the geophysical logging will provide depth-discrete, high resolution hydraulic conductivity estimates, soil moisture content, and lithology. Additional exploratory borings may be drilled and cased in areas at the site where additional high resolution data would be beneficial. Future work will include monitoring well installation using roto-sonic technology, and additional groundwater, surface water, sediment, and soil vapor sampling events.

Results/Lessons Learned:

Significant changes in grain size and variable layers within the shallow aquifer allow for localized, highly conductive flow pathways in area of the contaminated groundwater plume. These highly conductive flow pathways could potentially allow for the contaminant plume to travel downgradient at higher rates than originally estimated based on previous aquifer testing. The localized flow pathways could also potentially focus contamination at certain areas within the site, such as local creeks or private wells. In order to select the most effective remedy, it is imperative to fully understand these flow pathways throughout the site. Based on preliminary VOC concentration data and lithology, the local creek may be a source of seasonal groundwater-surface water interaction and preferential flow pathways downgradient in the plume. The high-resolution data collected during the geophysical logging and NMR logging at select locations will be evaluated along with the lithologic and chemical data obtained during drilling of the exploratory borings and surface water and groundwater sampling events to determine the most effective locations and screened depths for monitoring wells, and provide further evidence or lack thereof that the local creek is a source of groundwater-surface water interaction and preferential flow pathways.. Monitoring well placement within these highly conductive flow pathways will provide the most useful groundwater data going forward. Data collected during the investigation will also provide crucial updates to the current conceptual site model (CSM) and allow for the most effective remedy to be selected.

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August Welch