Hydrology, geochemistry, and microbiology data from meter-scale infiltration experiments exploring the impact of a woodchip soil amendment on nitrate removal during infiltration

We present results from field experiments linking hydrology, geochemistry,
and microbiology during infiltration at a field site that is used for
managed aquifer recharge (MAR). These experiments measured how a
horizontal permeable reactive barrier (PRB) made of woodchips impacted
subsurface nitrate removal and microbial ecology. Concentrations of
dissolved organic carbon consistently increased in infiltrating water
below the PRB, but not in un-amended native soil. The average nitrate
removal rate in soils below the PRB was 1.5 g/m2/day NO3-N, despite rapid
infiltration (up to 1.9 m/d) and a short fluid residence time within the
woodchips (≤6 h). In contrast, 0.09 g/m2/day NO3-N was removed on average
in native soil. Residual nitrate in infiltrating water below the PRB was
enriched in δ15N and δ18O, with low and variable isotopic enrichment
factors that are consistent with denitrification during rapid
infiltration. Many putative denitrifying bacteria were significantly
enhanced in the soil below a PRB; Methylotenera mobilis and genera
Microbacterium, Polaromonas, and Novosphingobium had log2 fold-changes of
+4.9, +5.6, +7.2, and +11.8, respectively. These bacteria were present
before infiltration and were not enhanced in native soil. It appears that
the woodchip PRB contributed to favorable conditions in the underlying
soil for enhanced nitrate removal, quantitatively shifting soil microbial
ecology. These results suggest that using a horizontal PRB could improve
water quality during rapid infiltration for MAR.