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CARBON-BASED REACTIVE BARRIER FOR NITRATE REMEDIATION AT A FORMER SWINE CAFO
Citation:
WILKIN, R. T., S. R. HUTCHINS, T. R. LEE, AND B. SCROGGINS. CARBON-BASED REACTIVE BARRIER FOR NITRATE REMEDIATION AT A FORMER SWINE CAFO. Presented at 2nd International Conference on Environmental Science and Technology, Houston, TX, August 19 - 22, 2006.
Impact/Purpose:
To inform the public.
Description:
Nitrate (NO3-) is a common ground water contaminant related to agricultural activity, waste water disposal, leachate from landfills, septic systems, and industrial processes. This study reports on the performance of a carbon-based permeable reactive barrier (PRB) that was constructed for in-situ bioremediation of a ground water nitrate plume caused by leakage from a swine CAFO (concentrated animal feeding operation) lagoon. The swine CAFO, located in Logan County, Oklahoma, was in operation from 1992-1999. The overall site remediation strategy includes an ammonia recovery trench to intercept ammonia-contaminated ground water and a hay straw PRB which is used to intercept a nitrate plume caused by nitrification of sorbed ammonia. The PRB extends approximately 260 m to intercept the nitrate plume. The depth of the trench averages 6 m and corresponds to the thickness of the surficial saturated zone; the width of the trench is 1.2 m. Detailed quarterly monitoring of the PRB began in March, 2004, about 1 year after construction activities ended. Nitrate concentrations hydraulically upgradient of the PRB have ranged from 23 to 77 mg/L N, from 0 to 3.2 mg/L N in the PRB, and from 0 to 65 mg/L N hydraulically downgradient of the PRB. Nitrate concentrations have generally decreased in downgradient locations with successive monitoring events. Mass balance considerations indicate that nitrate attenuation is dominantly from denitrification but with some component of organic nitrogen production. Geochemical conditions within the wall support microbially mediated nitrate-, iron-, and sulfate-reduction. Concentrations of dissolved organic carbon have progressively decreased with time within the PRB, from >850 mg/L to <100 mg/L. This trend may be an indication of decreased substrate capability to support denitrification, iron-reduction, and sulfate-reduction. Ground water monitoring studies are continuing to evaluate whether decreased dissolved organic carbon concentrations are linked to decreasing performance of the PRB. Future studies will focus on the development of a site-specific hydrologic model using conservative tracers in order to estimate overall rates of denitrification for comparison with previous studies.