Linking nitrate removal, carbon cycling, and mobilization of geogenic trace metals during infiltration for managed recharge.

We present results from a series of laboratory column studies investigating the impacts of infiltration dynamics and the addition of a soil-carbon amendment (wood mulch or almond shells) on water quality during infiltration for flood-managed aquifer recharge (flood-MAR). Recent studies suggest that nitrate removal could be enhanced during infiltration for MAR through the application of a wood chip permeable reactive barrier (PRB). However, less is understood about how other readily available carbon sources, such as almond shells, could be used as a PRB material, and how carbon amendments could impact other solutes, such as trace metals. Here we show that the presence of a carbon amendment increases nitrate removal relative to native soil, and that there is greater nitrate removal in association with longer fluid retention times (slower infiltration rates). Almond shells promoted more efficient nitrate removal than wood mulch or native soil, but also promoted the mobilization of geogenic trace metals (Mn, Fe, and As) during experiments. Almond shells in a PRB likely enhanced nitrate removal and trace metal cycling by releasing labile carbon, promoting reducing conditions, and providing habitat for microbial communities, the composition of which shifted in response. These results suggest that limiting the amount of bioavailable carbon released by a carbon-rich PRB may be preferred where geogenic trace metals are common in soils. Given the dual threats to groundwater supplies and quality worldwide, incorporating a suitable carbon source into the soil for managed infiltration projects could help to generate co-benefits and avoid undesirable results.