Formatted Title

Geochemical Evaluation of Arsenic in Groundwater and the Interaction of Organic-Rich Sediments and Underlying Beach Sands at a Former Petroleum Release Site

Background/Objectives

APTIM is conducting a basewide geochemical evaluation of metals concentrations in soil and groundwater to support the site characterization and eventual closure of a former petroleum release site at a Department of Defense installation located on a barrier island along the U.S. Gulf Coast.  Arsenic concentrations in groundwater remained elevated above the USEPA MCL of 0.10 milligrams per liter (mg/L) within and beyond the historical footprint of the former total petroleum hydrocarbon plume.  Previous sampling has documented that the arsenic concentrations are higher in shallow Holocene-aged organic peat deposits located in some areas of the installation and lower in the underlying Pleistocene-aged beach sands.  It is hypothesized that periodic fluctuations in the redox state within saturated peat layers are interacting with arsenic and iron- and sulfur-containing minerals contained in the peat to “flush” arsenic into the underlying “clean” saturated sand intervals, where it is able to be transported downgradient with the groundwater flow.  The objective of the geochemical evaluation is to identify whether the arsenic concentrations in the peat and underlying saturated intervals are naturally occurring and whether the concentrations exceeding the MCL are being driven by changes to the redox state caused by lingering influences from the petroleum plume or are related to the presence or absence of peat layers within or near the elevated detections.

Approach/Activities

Trace elements, defined as having concentrations less than 100 milligrams per kilogram, naturally associate with specific soil-forming minerals, and geochemical evaluations are predicated on these known associations.  For example, in most uncontaminated oxic soils, arsenic exhibits an almost exclusive association with iron oxide minerals.  APTIM’s geochemical evaluation will be performed in general accordance with methodology recommended in the Interstate Technology Regulatory Council (ITRC) Soil Background and Risk Assessment guidance published in 2022.  The methodology involves the comparison of various trace elements to major element ratios plotted for the data set to identify groups of samples falling on a common trend that are likely to represent the same source for the metals being evaluated.  Additional parameters such as redox state, pH, turbidity, soil organic content, etc. can also be incorporated to strengthen the lines of evidence linking one or more groups of samples.  New groundwater monitoring wells have been installed in areas on or surrounding the base that are believed to be relatively free from anthropogenic contamination and represent a cross section of areas where the Holocene-aged peat layers will be present in some of the locations and absent in others.  During installation, soil samples were collected from any peat or organic rich sediment layer encountered and from any underlying “clean” sands.  The soil samples were analyzed for 23 target analyte list (TAL) metals and total organic carbon.  Groundwater samples will be collected from the newly installed wells as well as selected existing wells in two sampling events approximately 6 months apart and analyzed for dissolved and total TAL metals, total organic carbon, total dissolved solids, total suspended solids, and field parameters (dissolved oxygen, oxidation/reduction potential, conductivity, pH, temperature, and turbidity).  The accumulated results from the two sampling events will be used to generate a background data set for use in the geochemical evaluation. 

Results/Lessons Learned

Upon completion of all sampling, the dataset will be geochemically evaluated in general accordance with the ITRC guidance described above.  It is anticipated that the results will identify key relationships between arsenic concentrations and areas of high and low organic content.  The presentation will summarize the data evaluation and provide key conclusions from the geochemical evaluation in the context of identifying whether the arsenic contamination at the subject site is likely to be naturally occurring or driven by an anthropogenic release. These conclusions will result in either closure of the site and discontinuation of sampling if the arsenic is found to be naturally occurring, or will trigger additional investigation to further delineate and characterize the arsenic contamination for selection of an appropriate remedy if the contamination is found to be anthropogenically sourced.  Lessons learned for future evaluation planning will also be presented as appropriate.