Enhanced Cycling of Nitrogen and Metals During Rapid Infiltration : Implications for Managed Recharge

We present results from a series of plot-scale field tests to quantify physical infiltration dynamics and the influence of adding a carbon-rich permeable reactive barrier (PRB) to the cycling of nitrogen and associated trace metals during rapid infiltration for managed aquifer recharge (MAR). Recent studies suggest that adding a bio-available carbon source to soils can enhance denitrification rates and associated N load reduction during moderate-to-rapid infiltration (≤ 1 m/day). We examined the potential for N removal during rapid infiltration (> 1 m/day), through coarse and carbon-poor soils, and how the presence of a carbon-rich PRB (wood chips) affects subsurface redox conditions and trace metal mobilization. During rapid infiltration, plots amended with a carbon-rich PRB generally demonstrated modest increases in subsurface loads of DOC, nitrite, manganese and iron, and decreases in loads of nitrate, arsenic, and ammonium. These trends varied considerably from those seen during infiltration through native soil. Overall, the application of a carbon-rich soil amendment increased the fraction of dissolved N species that was removed at equivalent inflowing N loads. There is evidence that N removal took place primarily via denitrification. Shifts in microbial ecology following infiltration in the all plots included increases in the relative abundances of the families Comamonadaceae , Pseudomonadaceae , Methylophilaceae, Rhodocyclaceae and Sphingomonadaceae , all of which contain genera capable of carrying out denitrification. These results, in combination with studies that have tested additional soil types and flow rates, indicate that both water quality and quantity can be improved during infiltration for managed recharge, even during rapid infiltration