Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

Use of High-Resolution Site Characterization to Aid Closure Decision-Making

Background/Objectives:

Trihydro was asked to evaluate closure options for a former oil production waste management facility located in rural Louisiana. The site groundwater quality meets LDEQ standards except for residual LNAPL sheen in four wells. Prior risk-based standards were calculated which were compared to soil data quality from vadose zone, which indicated the presence of total petroleum hydrocarbon impacts were below applicable standards. However, under the Louisiana Risk Evaluation and Corrective Action Program (RECAP), closure of the environmental matters at the site by the agency is not possible due to the residual LNAPL related to total petroleum hydrocarbon (TPH) impacts. Past remedial activities included excavation of approximately 11,700 cubic yards of TPH-impacted soils within the former waste pond area in 1991. Historical total fluids groundwater/LNAPL extraction was conducted by others from 1996 to 2011 from eight extraction wells to attempt to remove LNAPL. However, due to the low permeability of clayey soils and low LNAPL transmissivity, the effectiveness of the extraction system was limited and very low quantities of free product were recovered. As of 2021, four monitoring wells had LNAPL sheen present and six monitoring wells had TPH-DRO concentrations greater than 10% of the solubility limit, indicating that LNAPL may be present. In 2021, Trihydro began a focused site investigation to further assess the vertical and horizontal extent of LNAPL impacts and develop a path to closure.

Approach/Activities:

Well redevelopment was conducted in 2021 to extract potentially entrained LNAPL in the sand pack; however, LNAPL sheen returned to the wells. In order to better understand the nature and extent of LNAPL impacts, Trihydro recommended the use of high-resolution fluorescence tools to collect essentially continuous results from the vadose and saturated zones to a depth of 26 feet below ground surface. The selected driller tested an LNAPL sample from the site using both UVOST and OiHPT laser induced fluorescence tools to select the detector with the best response; OiHPT equipped with a 275-nanometer ultraviolet light source was selected as the technology of choice. A total of 42 borings were advanced over two phases of LNAPL delineation. The lateral and vertical extent of the LNAPL impacts were evaluated along with electrical conductivity for grain size analysis and hydraulic profiling tool pressure. Nearly continuous fluorescence results were evaluated real-time to identify step-out boring locations. Concurrent soil sampling was conducted in select areas to compare to the OiHPT results and prior soil sampling data to evaluate natural attenuation progress. The raw data from the OiHPT was then imported into Leapfrog, a 3-D data visualization software, to better assess and visualize the LNAPL distribution and plan for treatment area quantities.

Results/Lessons Learned:

The site was evaluated in three dimensions using the OiHPT results with special attention to the four monitoring wells that historically had LNAPL sheen and six monitoring wells had TPH-DRO concentrations greater than 10% of the solubility limit. A threshold of 60% fluorescence was used to categorize soils in Leapfrog as impacted or not impacted and the respective volumes were interpolated between boreholes. The OiHPT results and 60% fluorescence screening criteria identified two vertically distinct LNAPL impacted areas, one in the vadose zone and one sitting on top the water table in the saturated zone. The Leapfrog model was used to quickly compile over 5,500 OiHPT data points and visualize the impacted areas relative to previous treatment areas, and concluded that the LNAPL impact extent was far more discrete than previously understood. These data resulted in an expanded suite of remedial options which could be considered as part of the remedy selection process to achieve closure within the allocated budget. The impact volumes generated in Leapfrog were further assessed using other GIS tools to calculate treatment volumes and various individual scenarios targeted at final site closure. This information was then used to evaluate remedial options and estimate costs relative to LDEQ corrective action screening criteria.

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Adam Wiest