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.

Soil characteristics and redox properties of infiltrating water are determinants of microbial communities at managed aquifer recharge sites

In this study, we conducted a meta-analysis of soil microbial communities at three, pilot-scale field sites simulating shallow infiltration for managed aquifer recharge (MAR). We evaluated shifts in microbial communities after infiltration across site location, through different soils, with and without carbon-rich amendments added to test plots. Our meta-analysis aims to enable more effective MAR basin design by identifying potentially important interactions between soil physical-geochemical parameters and microbial communities across several geographically separate MAR basins. We hypothesized infiltration and carbon amendments would lead to common changes in subsurface microbial communities at multiple field sites but instead found distinct differences. Sites with coarser (mainly sandy) soil had large changes in diversity and taxa abundance, while sites with finer soils had fewer significant changes in genera, despite having the greatest increase in nitrogen cycling. Below test plots amended with a carbon-rich permeable reactive barrier, we observed more nitrate removal and a decrease in genera capable of nitrification. Multivariate statistics determined that the soil texture (a proxy for numerous soil characteristics) was the main determinant of whether the microbial community composition changed because of infiltration. These results suggest that microbial communities in sandy soil with carbon-rich amendments are most impacted by infiltration. Soil composition is a critical parameter that links between microbial communities and nutrient cycling during infiltration and could influence the citing and operation of MAR to benefit water quality and supply.

Enhanced cycling of nitrogen and metals during rapid infiltration: Implications for managed recharge

We present results from a series of plot-scale field experiments to quantify physical infiltration dynamics and the influence of adding a carbon-rich, permeable reactive barrier (PRB) for 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 faster infiltration (>1 m/day), through coarse and carbon-poor soils, and how adding 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 dissolved organic carbon, nitrite, manganese and iron, decreases in loads of nitrate and ammonium, and variable changes in arsenic. These trends differed considerably from those seen during infiltration through native soil without a carbon-rich PRB. Use 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 all of the plots included increases in the relative abundances of microbes in 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 other soil types, flow rates, and system scales, show how water quality can be improved during infiltration for managed recharge, even during rapid infiltration, with a carbon-rich soil amendment.

Denitrification during infiltration for managed aquifer recharge: Infiltration rate controls and microbial response

Managed aquifer recharge (MAR) systems can be designed and operated to improve water supply and quality simultaneously by creating favorable conditions for contaminant removal during infiltration through shallow soils. We present results from laboratory flow-through column experiments, using intact soil cores from two MAR sites, elucidating conditions that are favorable to nitrate (NO₃) removal via microbial denitrification during infiltration. Experiments focused on quantitative relations between infiltration rate and the presence or absence of a carbon-rich permeable reactive barrier (PRB) on both amounts and rates of nitrate removal during infiltration and associated shifts in microbial ecology. Experiments were conducted using a range of infiltration rates relevant to MAR (0.3-1.4 m/day), with PRBs made of native soil (NS), woodchips (WC) and a 50:50 mixture of woodchips and native soil (MIX). The latter two (carbon-rich) PRB treatments led to statistically significant increases in the amount of nitrate removed by increasing zero-order denitrification rates, both within the PRB materials and in the underlying soil. The highest fraction of nitrate removal occurred at the lowest infiltration rates for all treatments. However, the highest nitrogen mass removal (∆N_L) was observed at 0.4-0.7 m/day for both the WC and MIX treatments. In contrast, the maximum ∆N_L for the NS treatment was observed at the lowest infiltration rates measured (~0.3 m/day). Further, both carbon-rich PRBs had a substantial impact on the soil microbial ecology in the underlying soil, with lower overall diversity and a greater relative abundance of groups known to degrade carbon and metabolize nitrogen. These results demonstrate that infiltration rates and carbon availability can combine to create favorable conditions for denitrification during infiltration for MAR and show how these factors shape and sustain the microbial community structures responsible for nutrient cycling in associated soils.