Geochemical and Hydrogeological Impacts of a Wood Particle Barrier Treating Nitrate and Perchlorate in Ground Water

Abstract

Detailed distribution of nitrate (NO 3 ) adjacent to a permeable reactive layer comprising coarse wood particles with high hydraulic conductivity ( K ∼ 1 cm/s) showed that ground water converged into the layer, allowing treatment of aquifer zones located up to 1.1 m below the bottom of the layer. NO 3 ‐N of 6 to 99 mg/L was highly attenuated (generally <0.1 mg/L) within the first third of the 4‐m‐wide reactive layer. Hydraulic retention time estimates based on borehole dilution velocity values indicated a nitrate reaction rate of approximately 10 mg N/L/d at the ground water temperature of 10°C. In addition to denitrification, sulfate reduction, reductive dissolution of ferric and manganic oxyhydroxide minerals, and calcite dissolution likely occurred within the layer. The plume water continued to evolve in the downgradient area, where geochemical modeling suggested that CO 2 degassing likely occurred, resulting in pH increase and possibly the precipitation of calcite, siderite, and rhodochrosite. At a distance of 4 to 6 m downgradient, the plume water had returned to a composition similar to the upgradient ground water, but with lower NO 3 , SO 4 , and Ca concentrations. The reactive layer was also successful in treating trace quantities (∼1 μg/L) of perchlorate (ClO 4 ) present in the aquifer. The wood particle layer used in this study was of low cost and was simple to install and maintain; thus larger scale aquifer remediation could be feasible, particularly at sites where nitrate contamination occurs at shallow depths.