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CO2 Radiocarbon Analysis to Quantify Organic Contaminant Degradation, MNA, and Engineered Remediation Approaches
Memorandum rept. 1 Oct 2012-30 Sep 2013
NAVAL RESEARCH LAB WASHINGTON DC CHEMICAL DYNAMICS AND DIAGNOSTICS BRANCH
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A series of coupled measurements was made at the Naval Air Station North Island NASNI Installation Restoration Site 5, Unit 2 during July and August 2013. Preliminary samples were taken in March 2013. Coupled measurements included CO2 respiration rate, proportion of the CO2 attributable to chlorinated hydrocarbon CH mineralization and a zone of influence ZOI model. This coupling led to calculating CH degradation per unit time per unit area. These coupled measurements represent the first analysis where carbon is followed from contaminant to final degradation product CO2 directly without need for inference or lines of evidence. In order to determine a mass removal for this study, the average contaminant mineralization rate 0.673 g C m-3 d-1 was multiplied by the total area within the sampling grid 1,225 m2 then multiplied by the ZOI depth to give the total carbon mass removal from the CH pool 14.5 g C d-1. The wells on site are screened at the groundwatervadoze zone interface and the vertical ZOI characteristics are likely very different above and below the water table vadoze versus saturated zone. However, if the assumption is made that the entire surface soil lens sandy silt to a depth of approximately 3 meters has similar CH degradation rates, an optimistic estimate of 246 g C d-1 over the entire sampled site area can be calculated. In terms of dissolved CH, an estimate of 0.100 g CH carbon L-1 was assumed based on reported values from 2009. Over the site, within the ZOI, residence time for CH averaged just over 4 years, assuming no additional input desorbing from soils. At the lowest measured turnover rates, residence time was calculated at 2,400 years and the highest measured rate, 11 months.
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