Illumina MiSeq from subsurface soil samples collected from managed aquifer recharge plots and soil cores

We present linked field and laboratory studies investigating controls on enhanced nitrate removal during infiltration for managed aquifer recharge. We examine the influence of carbon-rich permeable reactive barriers (PRBs) made of woodchips or biochar, placed in the path of infiltrating water to stimulate microbial denitrification. In field studies with infiltration at 0.2-0.4 m/day and initial concentration of [NO3-N] = 20-28 mg/L, we observed that woodchips promoted 37±5.6% nitrate removal (primarily via denitrification), and biochar promoted 33±15% nitrate removal (via denitrification and physical absorption effects), whereas unamended soil at the same site generated <4% denitrification. We find that the presence of a PRB only has a modest effect on the underlying soil microbial community structure at this site, indicating that the existing microbial community has the capability to carry out denitrification given favorable conditions. In laboratory studies using intact cores from the same site, we extend the results to quantify how infiltration rate influences denitrification. We find that the effect of both PRB materials is diminished at higher infiltration rates (>0.7 m/day), but still measurable for woodchips. Combined results from these field and laboratory experiments, along with results of other studies of denitrification during infiltration suggest a framework for understanding the interconnected hydrologic and chemical controls on microbial denitrification that could improve water quality during managed recharge.