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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 (NO 3 ) 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

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 [NO 3 -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.

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. [ABSTRACT FROM AUTHOR]

Managed aquifer recharge (MAR) can increase groundwater supply and, under some conditions, improve groundwater quality simultaneously. However, the conditions under which water quality improvements can be achieved during infiltration for MAR have not been systematically examined in a spatially explicit manner. This work aims to address that gap by synthesizing observations from laboratory tests, field experiments and operational facilities at four MAR sites within the Pajaro Valley, California, USA to develop a predictive model of nitrate removal during infiltration. We compare two machine learning approaches; random forest and boosted regression trees, to multiple linear regression. The preferred model uses boosted regression trees, with four predictor variables (initial nitrate concentration of infiltrating water, residence time in the soil, soil organic carbon content, and soil percent clay). We apply this model to simulate the spatial distribution of potential nitrate removal (NRp) across a heterogeneous and mixed‐use landscape, finding that >87% of the region has the potential to remove nitrate during infiltration. To link potential nitrate removal capacity to available water for MAR, we combine a map of NRp with independently simulated hillslope runoff, and find that potential load reduction is highest in urban areas (median = 18.8 kg‐N/yr) where large runoff volumes are collocated with soils having high nutrient cycling capacity compared to forested and agricultural areas (median = 1.6 and 3.5 kg‐N/yr, respectively). We analyse potential load reduction across dry, normal, and wet precipitation scenarios, which shows that urban areas have the potential to yield large load reductions (median = 11.3 kg‐N/yr) even under relatively dry conditions. However, much of the generation of elevated nitrate in runoff is associated with agricultural activity. These results suggest that the urban–agricultural interface represents a crucial link between hydrologic and biogeochemical cycles where water supply and quality goals may be met simultaneously. The technical approach used in this study is highly flexible and can help guide decisions in resource management and identify promising MAR sites. More broadly, this approach also elucidates heterogeneity in biogeochemical cycling across complex landscapes. [ABSTRACT FROM AUTHOR]

Abstract 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 NO 3 -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 NO 3 -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 log 2 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. Graphical abstract Image 1 Highlights • A horizontal permeable reactive barrier (PRB) was tested for impacts on N-cycling. • The PRB was associated with enhanced nitrate removal during rapid infiltration. • Putative denitrifying microbial clades were enhanced in soils below a PRB. • Geochemical, isotopic, and microbial data are consistent with denitrification. • Using a horizontal woodchip PRB could improve water quality during managed recharge. [ABSTRACT FROM AUTHOR]