Threshold amounts of organic carbon needed to initiate reductive dechlorination in groundwater systems

Abstract

Aquifer sediment and groundwater chemistry data from 15 Department of Defense facilities located throughout the United States were collected and analyzed with the goal of estimating the amount of natural organic carbon needed to initiate reductive dechlorination in groundwater systems.Aquifer sediments were analyzed for hydroxylamine and NaOH-extractable organic carbon, yielding a probable underestimate of potentially bioavailable organic carbon (PBOC). Aquifer sediments were also analyzed for total organic carbon (TOC) using an elemental combustion analyzer, yielding a probable overestimate of bioavailable carbon. Concentrations of PBOC correlated linearly with TOC with a slope near one. However, concentrations of PBOC were consistently five to ten times lower than TOC. When mean concentrations of dissolved oxygen observed at each site were plotted versus PBOC, it showed that anoxic conditions were initiated at approximately 200 mg/kg of PBOC. Similarly, the accumulation of reductive dechlorination daughter products relative to parent compounds increased at a PBOC concentration of approximately 200 mg/kg. Concentrations of total hydrolysable amino acids (THAA) in sediments also increased at approximately 200 mg/kg, and bioassays showed that sediment CO 2 production correlated positively with THAA. The results of this study provide an estimate for threshold amounts of bioavailable carbon present in aquifer sediments (approximately 200 mg/kg of PBOC; approximately 1,000 to 2,000 mg/kg of TOC) needed to support reductive dechlorination in groundwater systems. O

Modeling the Effects of Naturally Occurring Organic Carbon on Chlorinated Ethene Transport to a Public Supply Well

“…By varying the assumed fraction of bioavailable POC, it was found that the closest match to observed DO concentrations (Figure ) was obtained by assigning a fraction of bioavailable POC to be 0.15. That value is consistent with previous studies on POC bioavailability (Chapelle et al ; Thomas et al ).…”

“…Previous studies have clearly demonstrated the importance of naturally occurring organic carbon driving the reductive dechlorination and natural attenuation of CEs in groundwater systems (U.S. EPA ; Bradley ; Chapelle et al ). However, it has not previously been feasible to couple the oxidation of naturally occurring DOC and POC coupled to CE reduction in order to model the vulnerability of public supply wells to contamination (Focazio et al ; Frind et al ).…”

“…This is particularly important for CEs that are subject to microbial reductive dechlorination processes that can be driven by naturally occurring sources of dissolved organic carbon (DOC) and particulate organic carbon (POC) (U.S. EPA ; Bradley ; Rectanus et al ). It has long been observed that aquifers containing little natural organic carbon tend to have oxic conditions that inhibit reductive dechlorination, whereas aquifers containing more abundant natural organic carbon tend to be anoxic and support efficient reductive dechlorination (Wiedemeier et al ; Chapelle et al ). Building quantitative models that are based on a mass balance between electron donors such as organic carbon and electron acceptors (EAs) such as dissolved oxygen (DO), and CEs, however, is challenging (Feinstein and Thomas ).…”

“…The second step is to couple the mobility and persistence of the concerned contaminant, which in this case is perchloroethene (PCE), with ambient reduction/oxidation (redox) conditions in the aquifer. PCE is subject to reductive dechlorination when concentrations of DO decrease below approximately 0.5 mg/L (Chapelle et al ). Thus, in order to realistically simulate PCE transformation, it is necessary to consider the delivery, transport, and biological uptake of DO in the aquifer.…”

Long‐Term Evaluation of Mulch Biowall Performance to Treat Chlorinated Solvents

“…During the 2015 field investigation, 21 soil borings were advanced to approximately 35 feet (10.7 m) bgs with a sonic drill at locations adjacent to each of the upgradient, biowall, and downgradient groundwater monitoring wells at seven of the eight transects sampled for groundwater; soil borings were not advanced at transect BE‐09 due to conflicts with existing utilities. Potentially bioavailable organic carbon (PBOC) from each of the 21 soil borings was analyzed at the laboratory of Dr. Mark Widdowson at the Virginia Polytechnic Institute and State University (Virginia Tech) by the PBOC methodology documented in Rectanus et al (), Chapelle et al (), Thomas et al (), and Lebrón et al (2013).…”