Field and Laboratory Studies Linking Hydrologic, Geochemical, and Microbiological Processes and Enhanced Denitrification during Infiltration for Managed Recharge

We present linked field and laboratory
studies investigating controls
on enhanced nitrate processing during infiltration for managed aquifer
recharge. We examine how carbon-rich permeable reactive barriers (PRBs)
made of woodchips or biochar, placed in the path of infiltrating water,
stimulate microbial denitrification. In field studies with infiltration
of 0.2–0.3 m/day and initial nitrate concentrations of [NO3-N] = 20–28 mg/L, we observed that woodchips promoted
37 ± 6.6% nitrate removal (primarily via denitrification), and
biochar promoted 33 ± 12% nitrate removal (likely via denitrification
and physical absorption effects). In contrast, unamended soil at the
same site generated <5% denitrification. We find that the presence
of a carbon-rich PRB has a modest effect on the underlying soil microbial
community structure in these experiments, indicating that existing
consortia have 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, with and without a carbon-rich PRB. We find that
the influence of both PRB materials is diminished at higher infiltration
rates (>0.7 m/day) but can still result in denitrification. These
results demonstrate a quantitative relationship between infiltration
rate and denitrification that depends on the presence and nature of
a PRB. Combined results from these field and laboratory experiments,
with complementary studies of denitrification during infiltration
through other soils, suggest a framework for understanding linked
hydrologic and chemical controls on microbial denitrification (and
potentially other redox-sensitive processes) that could improve water
quality during managed recharge.