This project's objective was to combine CO2 respiration, CO2 radiocarbon content and a zone of influence (ZOI) model to calculate chlorinated hydrocarbon degradation occurring at real Department of Defense (DoD) contaminated sites. Radiocarbon analysis measures the fraction of petroleum-source in the CO2 pool. To determine the degradation rate of contaminants of interest, the CO2 production rate is assessed by circulating groundwater well headspace through a CO2 trap over time or using a passive trap suspended in the wells’ headspace. A well ZOI can then be calculated using process based simulation models of gas transport in porous media. The ZOI model and CO2 production rate are used to determine the overall CO2 production rate per soil unit. Given the fraction of CO2 derived from a petroleum-source and CO2 produced per soil unit (cubic meter for example), the contaminant degradation rate can be determined over both time and space. This technology is straightforward and commercially-available, and provides two key answers for site management and remediation efficacy (both active and passive) that have not been available:
The technical approach involved measuring the CO2 production over unit time (respiration) and determining a radiocarbon age for that respired product. If the CO2 was radiocarbon-depleted relative to a background site where natural organic matter was the only respiratory substrate, the radiocarbon-depleted CO2 must therefore be derived from the petroleum-sourced contaminant. By coupling the respiration rate and the differential respiration product derived from the contaminant, a contaminant degradation rate can be determined. Developing a model based on site hydrogeologic parameters that determines the volume sampled during each respiration measurement allows scaling contaminant degradation rate spatially and interpolation between wells sampled as above allows site-wide contaminant degradation estimates.
At each site, time-point and well, respiration and 14CO2 data were compiled and analyzed to determine chlorinated volatile organic compound (cVOC) degradation (as trichloroethene carbon degraded m-3 d -1). cVOC degradation rates were calculated at three locations at the Naval Air Station North Island over the course of one year (each site). Respiration values along with site hydrogeologic parameters were used to create ZOI simulations for each well and time-point sampled. Ancillary measurements (cations, pH, organic acids) were used to ensure limestone (calcium carbonate) deposits did not interfere with radiocarbon analysis. Collected CO2 was combined according to well and respiration values for radiocarbon analyses due to the cost per sample.
Results from this SERDP project have been transitioned to ESTCP in which the project team will deploy various radiocarbon-based technologies at the same sites over the same timescales in order to cross validate radiocarbon techniques and assess their strengths and weaknesses given different site conditions, deployed remediation technologies and remedial management goals. The goal is to create a decision support tool encompassing these disparate but related characteristics with the ultimate goal to decrease overall uncertainty and increase cost effectiveness and cost avoidance.
Boyd, T.J., J. Gryzenia, D. Ramquist, and G.M. Wyatt. 2018. Short-term Respiration Measurements Coupled with CO2 Radiocarbon Analysis Provide Fuel Degradation Rates at an Underground Fuel Storage Tank Leak Site. US Naval Research Laboratory, Washington, DC, NRL--6110/052.
Boyd, T.J., M.J. Pound, D. Lohr, and R.B. Coffin. 2013. Radiocarbon-depleted CO2 Evidence for Fuel Biodegradation at the Naval Air Station North Island (USA) Fuel Farm Site. Environmental Science: Processes & Impacts, 15(5):912-918.
Boyd, T.J., M.J. Pound, R.H. Cuenca, Y. Hagimoto, and M.T. Montgomery. 2014. Radiocarbon Allows Direct Determination of Fuel and Industrial Chemical Degradation at ER Sites. ER News, US Naval Facilities, Port Hueneme, CA, 15:13-14.
Boyd, T.J., M.T. Montgomery, R.H. Cuenca, and Y. Hagimoto. 2014. CO2 Radiocarbon Analysis to Quantify Organic Contaminant Degradation, MNA, and Engineered Remediation Approaches. United States Naval Research Laboratory, NRL/MR/6110--14-9539.
Boyd, T.J., M.T. Montgomery, R.H. Cuenca, and Y. Hagimoto. 2015. Combined Radiocarbon and CO2 Flux Measurements used to Determine In Situ Chlorinated Solvent Mineralization Rate. Environmental Science: Processes & Impacts, 17:683-692.
Boyd, T.J., M.T. Montgomery, R.H. Cuenca, and Y. Hagimoto. 2016. Measuring Carbon-based Contaminant Mineralization Using Combined CO2 Flux and Radiocarbon Analyses. Journal of Visualized Experiments, 116:e53233.