A horizontal permeable reactive barrier stimulates nitrate removal and shifts microbial ecology during rapid infiltration for managed recharge

Hydrogeochemical changes during artificial groundwater well recharge

Artificial groundwater recharge is a relatively economic and efficient method for solving shortages and uneven spatial-temporal distribution of water resources. Changes in groundwater quality during the recharge process are a key issue that must be addressed. Identifying the hydrogeochemical reactions that occur during recharge can be vital in predicting trends in groundwater quality. However, there are few studies on the evolution of groundwater quality during artificial recharge that comprehensively consider environmental, chemical, organic matter, and microbiological indicators. Based on an artificial groundwater recharge experiment in Xiong'an New Area, this study investigated the hydrogeochemical changes during groundwater recharge through a well. The results indicate that (1) as large amounts of recharge water (RW) were injected, the groundwater level initially rose rapidly, then fluctuated slowly, and finally rose again. (2) Water quality indicators, dissolved organic matter (DOM), and microbial communities were influenced by the mixture of RW and the background groundwater before recharge (BGBR), as well as by water-rock interactions, such as mineral dissolution-precipitation and redox reactions. (3) During well recharge, aerobic respiration, nitrification, denitrification, high-valence manganese (Mn) and iron (Fe) minerals reduction dissolution, and Mn2+ and Fe2+ oxidation-precipitation occurred sequentially. (4) DOM analysis showed that protein-like substances in the BGBR were the main carbon sources for aerobic respiration and denitrification, while humic-like substances carried by the RW significantly enhanced Mn and Fe minerals reduction dissolution. Therefore, RW quality significantly affects groundwater quality after artificial groundwater well recharge. Controlling indicators, such as dissolved oxygen (DO) and DOM, in the RW can effectively reduce harm to groundwater quality after recharge. This study is of theoretical and practical significance for in-depth analysis of the evolution of groundwater quality during artificial well recharge, prediction of trends in groundwater quality during and after recharge and ensuring groundwater quality safety.

Molecular and Dual-Isotopic Profiling of the Microbial Controls on Nitrogen Leaching in Agricultural Soils under Managed Aquifer Recharge

Nitrate (NO₃^-) leaching is a serious health and ecological concern in global agroecosystems, particularly those under the application of agricultural-managed aquifer recharge (Ag-MAR); however, there is an absence of information on microbial controls affecting NO₃^- leaching outcomes. We combine natural dual isotopes of NO₃^- (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) with metagenomics, quantitative polymerase chain reaction (PCR), and a threshold indicator taxa analysis (TITAN) to investigate the activities, taxon profiles, and environmental controls of soil microbiome associated with NO₃^- leaching at different depths from Californian vineyards under Ag-MAR application. The isotopic signatures demonstrated a significant priming effect (P < 0.01) of Ag-MAR on denitrification activities in the topsoil (0-10 cm), with a 12-25-fold increase of ¹⁵N-NO₃^- and ¹⁸O-NO₃^- after the first 24 h of flooding, followed by a sharp decrease in the enrichment of both isotopes with ∼80% decline in denitrification activities thereafter. In contrast, deeper soils (60-100 cm) showed minimal or no denitrification activities over the course of Ag-MAR application, thus resulting in 10-20-fold of residual NO₃^- being leached. Metagenomic profiling and laboratory microcosm demonstrated that both nitrifying and denitrifying groups, responsible for controlling NO₃^- leaching, decreased in abundance and potential activity rates with soil depth. TITAN suggested that Nitrosocosmicus and Bradyrhizobium, as the major nitrifier and denitrifier, had the highest and lowest tipping points with regard to the NO₃^- changes (P < 0.05), respectively. Overall, our study provides new insight into specific depth limitations of microbial controls on soil NO₃^- leaching in agroecosystems.

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.

A comprehensive review on permeable reactive barrier for the remediation of groundwater contamination

Groundwater quality is deteriorating due to contamination from both natural and anthropogenic sources. Traditional "Pump and Treat" techniques of treating the groundwater suffer from the disadvantages of a small-scale and energy-intensive approach. Permeable reactive barriers (PRBs), owing to their passive operation, offer a more sustainable strategy for remediation. This promising technique focuses on eliminating heavy metal pollutants and hazardous aromatic compounds by physisorption, chemisorption, precipitation, denitrification, and/or biodegradation. Researchers have utilized ZVI, activated carbon, natural and manufactured zeolites, and other by-products as reactive media barriers. Environmental parameters, i.e., pH, initial pollutant concentration, organic substance, dissolved oxygen, and reactive media by-products, all influence a PRB's performance. Although their long-term impact and performance are uncertain, PRBs are still evolving as viable alternatives to pump-and-treat techniques. The use of PRBs to remove anionic contaminants (e.g., Fluoride, Nitrate, etc.) has received less attention since precipitates can clog the reactive barrier and hinder groundwater flow. In this paper, we present an insight into this approach and the tremendous implications for future scientific study that integrates this strategy using sustainability and explores the viability of PRBs for anionic pollutants.

Nitrogen fate during agricultural managed aquifer recharge: Linking plant response, hydrologic, and geochemical processes

Agricultural managed aquifer recharge (Ag-MAR, on-farm recharge), where farmland is flooded with excess surface water to intentionally recharge groundwater, has received increasing attention by policy makers and researchers in recent years. However, there remain concerns about the potential for Ag-MAR to exacerbate nitrate (NO₃^-) contamination of groundwater, and additional risks, such as greenhouse gas emissions and crop tolerance to prolonged flooding. Here, we conducted a large-scale, replicated winter groundwater recharge experiment to quantify the effect of Ag-MAR on soil N biogeochemical transformations, potential NO₃^- leaching to groundwater, soil physico-chemical conditions, and crop yield. The field experiment was conducted in two grapevine vineyards in the Central Valley of California, which were each flooded for 2 weeks and 4 weeks, respectively, with 1.31 and 1.32 m³ m^-2 of water. Hydrologic, geochemical, and microbial results indicate that NO₃^- leaching from the first 1 m of the vadose zone was the dominant N loss pathway during flooding. Based on pore water sample and N₂O emission data, denitrification played a lesser role in decreasing NO₃^- in the root zone but prolonged anoxic conditions resulted in a significant 29 % yield decrease in the 4-week flooded vineyard. The results from this research, combined with data from previous studies, are summarized in a new conceptual model for integrated water-N dynamics under Ag-MAR. The proposed model can be used to determine best Ag-MAR management practices.

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.

Sensitivity analysis of factors influencing pollutant removal from shallow groundwater by the PRB method based on numerical simulation

Permeable reactive barrier (PRB) is one of the most promising in situ treatment methods for shallow groundwater pollution. However, optimal design of PRB is very difficult due to a lack of comprehensive understanding of various complex influencing factors of PRB remediation. In this study, eight of the main factors of PRB, including hydraulic gradient I, permeability coefficient KPRB of PRB material, PRB length L, PRB width W, PRB distance from pollution source Dist., the ratio of the maximum adsorption capacity to Langmuir constant of PRB material Qmax/KL, the discharge rate of pollution source DR, and recharge concentration RC were investigated, to carry out the sensitivity analysis of PRB removal efficiency. The simulation experiments for Morris analysis were designed, and pollutant removal efficiency was numerically simulated by coupling MODFLOW and MT3DMS under two scenarios of high and low permeability and dispersivity. For a typical low permeability with low dispersity medium, the sensitivity ranking of factors from high to low is DR, RC, I, W, L, Dist., Qmax/KL, and KPRB, and for a typical high permeability with a high dispersity medium, the sensitivity ranking of factors from high to low is I, W, DR, Qmax/KL, L, RC, Dist., and KPRB. When considering multiple factors in PRB design, the greater the KPRB, L, W, Qmax/KL is, the higher the removal efficiency is; the greater the RC, I is, the lower the removal efficiency is. The rest factors remain ambiguous enhancement to removal efficiency.

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.

Nitrogen behavior during artificial groundwater recharge through ponds: A case study in Xiong'an New Area

The Xiong'an New Area (XA) was established as a development hub in China. Excessive exploitation of groundwater has caused a series of environmental and geological problems, restricting further development of XA. The widely distributed ponds in this area have been targeted as convenient and efficient sites of artificial groundwater recharge. However, nitrogen accumulation in the shallow vadose zone associated with agricultural activities may pose environmental risks to groundwater during the recharge and infiltration process. Therefore, this study investigated the effects, transfer, and transformation of nitrogen during artificial groundwater recharge. The aeration zone is thick and the medium comprises fine particles, with total nitrogen and nitrate accumulation mainly in the shallow aeration zone. In indoor experiments, the nitrate removal rate reached 83.5% when organic carbon in the source water was increased by 10 mg/L. For Baigou diversion river water(BW) with slightly higher (14.46 mg/L) and lower (5.04 mg/L) nitrate contents, the nitrate content decreased by 26.0% (10.70 mg/L) and 26.8% (3.69 mg/L), respectively, after 150 days. When the water head was increased by 20 cm to increase the recharge rate, the time required for nitrate and ammonium to reach the maximum and equilibrium concentration was reduced by 50%. These findings indicate that nitrogen concentration in the source water, aeration zone media, and groundwater should be considered in pond replenishment. It is also necessary to control the concentration of organic carbon and the rate of recharge, which would provide guidance for other similar projects.

Nitrogen Removal Capacity of Microbial Communities Developing in Compost- and Woodchip-Based Multipurpose Reactive Barriers for Aquifer Recharge With Wastewater

Global water supplies are threatened by climate changes and the expansion of urban areas, which have led to an increasing interest in nature-based solutions for water reuse and reclamation. Reclaimed water is a possible resource for recharging aquifers, and the addition of an organic reactive barrier has been proposed to improve the removal of pollutants. There has been a large focus on organic pollutants, but less is known about multifunctional barriers, that is, how barriers also remove nutrients that threaten groundwater ecosystems. Herein, we investigated how compost- and woodchip-based barriers affect nitrogen (N) removal in a pilot soil aquifer treatment facility designed for removing nutrients and recalcitrant compounds by investigating the composition of microbial communities and their capacity for N transformations. Secondary-treated, ammonium-rich wastewater was infiltrated through the barriers, and the changes in the concentration of ammonium, nitrate, and dissolved organic carbon (DOC) were measured after passage through the barrier during 1 year of operation. The development and composition of the microbial community in the barriers were examined, and potential N-transforming processes in the barriers were quantified by determining the abundance of key functional genes using quantitative PCR. Only one barrier, based on compost, significantly decreased the ammonium concentration in the infiltrated water. However, the reduction of reactive N in the barriers was moderate (between 21 and 37%), and there were no differences between the barrier types. All the barriers were after 1 year dominated by members of Alphaproteobacteria, Gammaproteobacteria, and Actinobacteria, although the community composition differed between the barriers. Bacterial classes belonging to the phylum Chloroflexi showed an increased relative abundance in the compost-based barriers. In contrast to the increased genetic potential for nitrification in the compost-based barriers, the woodchip-based barrier demonstrated higher genetic potentials for denitrification, nitrous oxide reduction, and dissimilatory reduction of nitrate to ammonium. The barriers have previously been shown to display a high capacity to degrade recalcitrant pollutants, but in this study, we show that most barriers performed poorly in terms of N removal and those based on compost also leaked DOC, highlighting the difficulties in designing barriers that satisfactorily meet several purposes.

Field-applied biochar-based MgO and sepiolite composites possess CO2 capture potential and alter organic C mineralization and C-cycling bacterial structure in fertilized soils

Agricultural soils contribute a significant amount of anthropogenic CO₂ emission, a greenhouse gas of global environmental concern. Hence, discovering sustainable materials that can capture CO₂ in cultivated soils is paramount. Since the effect of biochar on C mineralization/retention in fertilized soils is unclear, we produced biochar-based MgO and sepiolite-nanocomposites with CO₂ capture potential. The field-scale impacts of the modified-biochars were evaluated on net C exchange rate (NCER) periodically for 3 months in fertilized plots. The effects of the modified-biochar on organic-C mineralization, the activities, and dynamics of C-cycling-related 16S rRNA which are unknown, were investigated. Results revealed an initial rapid and higher cumulative CO₂ emission from the sole fertilizer treatment (F). Unlike the biochar treatment (BF), the successful incorporation of MgO/Mg(OH)₂ nanoparticles into the matrix and surface of biochar, and the potential formation of MgCO₃ with soil CO₂, mitigated CO₂ emission, especially in the MgO-modified biochar (MgOBF), compared to the sepiolite-biochar treatment (SBF). Compared to F and BF, the higher C retention as MgCO₃ in the modified biochar treatments led to an increase in cellulase activity, stimulation of key C-cycling-related bacteria, and the expression of genes associated with starch, sucrose, amino sugar, nucleotide sugar, ascorbate, aldarate, cellulose, and chitin degradation, thus, increasing organic C mineralization. Among the modified-biochar treatments, higher C mineralization was recorded in SBF, resulting in increased cumulative CO₂ emission, despite its initial capture for up to 42 days. However, MgOBF was effective in capturing soil-derived CO₂, despite the increased C mineralization compared to biochar. The changes in soil moisture and temperature significantly regulated NCER. Also, the modified biochars positively influenced the distribution of C-cycling-related bacteria by improving soil pH and available nutrients. Among the modified biochars, the observed higher mitigation effect of MgOBF on NCER indicated that it could be preferably applied in agricultural soils.

Geochemical stability of zero-valent iron modified raw wheat straw innovatively applicated to in situ permeable reactive barrier: N2 selectivity and long-term denitrification

The zero-valent iron (ZVI) modified wheat straw materials are widely used for treating groundwater by permeable reactive barrier (PRB). We report the performance of a field-scale PRB filled with ZVI modified wheat straw materials for nitrate (NO3-)-contaminated groundwater. In lab-scale PRB filled with ZVI modified wheat straw material, NO3- concentration entering the PRB was varied (27.80-59.86 mg L-1) according to the in situ NO3- contamination. A stable NO3- removal rate of 90% was achieved at a controlled hydraulic retention time of 22 days, together with a proportion of denitrifying bacteria up to 34.37%. The field-scale PRB filled with ZVI modified wheat straw material was successful at removing NO3- from groundwater (removal percentages ≥60%) at a groundwater flow rate of 0.01 m3 d-1. Monitoring of groundwater within this PRB provided evidences that the nitrogen gas (N2) selectivity increased with lower ammonia (NH4+) generated from ZVI reduction of NO3-, and few emission of NO2- present due to denitrification capacity in this PRB. The results are finally compared with the few others reported existing PRBs for nitrate-contaminated groundwater worldwide, and demonstrated that the ZVI modified wheat straw material would be an effective fillings for field PRB to remediate groundwater.

Fungal pellets immobilized bacterial bioreactor for efficient nitrate removal at low C/N wastewater

In this study, fungal pellets immobilized denitrifying Pseudomonas stutzeri sp. GF3 was cultivated to establish a bioreactor. The denitrification effect of fixed bacteria with fungal pellets was tested by response surface methodology (RSM). Analysis of the bioreactor showed that the denitrification efficiency reached 100% under the optimal conditions and the denitrification efficiency of the actual wastewater treatment in the stable phase reached 95.91%. Moreover, the organic matter and functional groups in the bioreactor under different C/N conditions were analyzed by fluorescence excitation-emission matrix (EEM) spectra and Fourier transform infrared spectroscopy (FTIR), which revealed that metabolic activities of denitrifying bacteria were enhanced with the increase of C/N. The morphology and structure of bacteria immobilized by fungal pellets explored by scanning electron microscope (SEM) showed the filamentous porous fungal pellets loaded with bacteria. Community structure analysis by high-throughput sequencing demonstrated that strain GF3 might was the dominant strain in bioreactor.

Denitrification using permeable reactive barriers with organic substrate or zero-valent iron fillers: controlling mechanisms, challenges, and future perspectives

Nitrate as a diffusive agricultural contaminant has been causing substantial groundwater quality deterioration worldwide. In situ groundwater remediation techniques using permeable reactive barriers (PRBs) have attracted increasing interest. Particularly, PRBs based on biological denitrification, using the organic substrate as a biostimulator, and chemical nitrate reduction, using zero-valent iron (ZVI) as a reductant, are two major PRB approaches for groundwater denitrification. This review paper analyzed the published studies over the past 10 years (2010-2020) using laboratory, modeling, and field-scale approaches to explore the performance and mechanisms of these two types of PRBs. Important factors affecting the denitrification efficiencies as well as the influential mechanisms were discussed. Several research gaps have been identified and further research needs are discussed in the end.

A pilot study characterizing gravesoil bacterial communities a decade after swine decomposition

Vertebrate decomposition leads to an efflux of fluids rich with biochemicals and microbes from the carcass into the surrounding soil affecting the endogenous soil bacterial community. These perturbations are detectable in soils associated with carcasses (gravesoil) and influence soil bacterial ecology for years after the decomposition event, but it is unknown for how long. Measuring these impacts over extended timescales is critical to expanding vertebrate decomposition's role in the ecosystem and may provide useful information to forensic science. Using 16S rRNA gene amplicon data, this study surveyed bacterial composition in terrestrial soils associated with surface-exposed swine decomposition for 10 years after carcass placement. This pilot study utilizes the increased statistical power associated with repeated measure/within-subjects sampling to analyze bacterial diversity trends over time. Our results demonstrate that the soil bacterial diversity was significantly impacted by decomposition, with this impact being localized to the area underneath the carcass. Bacterial community dissimilarity was greatest 12 months postmortem before beginning recovery. Additionally, random forest regressions were utilized to determine 10 important genera for distinguishing decomposition timepoints, an important component of forensic investigations. Of these 10 genera, four were further analyzed for their significant relative abundance shifts underneath the carcass. This pilot study helps expand the current knowledge of long-term effects of carcass decomposition on soil bacterial communities, and is the first to our knowledge to characterize these communities temporally from placement through a decade of decomposition.

Mobilization of Arsenic and Other Naturally Occurring Contaminants during Managed Aquifer Recharge: A Critical Review

Population growth and climate variability highlight the need to enhance freshwater security and diversify water supplies. Subsurface storage of water in depleted aquifers is increasingly used globally to alleviate disparities in water supply and demand often caused by climate extremes including floods and droughts. Managed aquifer recharge (MAR) stores excess water supplies during wet periods via infiltration into shallow underlying aquifers or direct injection into deep aquifers for recovery during dry seasons. Additionally, MAR can be designed to improve recharge water quality, particularly in the case of soil aquifer treatment and riverbank filtration. While there are many potential benefits to MAR, introduction of recharge water can alter the native geochemical and hydrological conditions in the receiving aquifer, potentially mobilizing toxic, naturally occurring (geogenic) contaminants from sediments into groundwater where they pose a much larger threat to human and ecosystem health. On the basis of the present literature, arsenic poses the most widespread challenge at MAR sites due to its ubiquity in subsurface sediments and toxicity at trace concentrations. Other geogenic contaminants of concern include fluoride, molybdenum, manganese, and iron. Water quality degradation threatens the viability of some MAR projects with several sites abandoning operations due to arsenic or other contaminant mobilization. Here, we provide a critical review of studies that have uncovered the geochemical and hydrological mechanisms controlling mobilization of arsenic and other geogenic contaminants at MAR sites worldwide, including both infiltration and injection sites. These mechanisms were evaluated based on site-specific characteristics, including hydrological setting, native aquifer geochemistry, and operational site parameters (e.g., source of recharge water and recharge/recovery cycling). Observed mechanisms of geogenic contaminant mobilization during MAR via injection include shifting redox conditions and, to a lesser extent, pH-promoted desorption, mineral solubility, and competitive ligand exchange. The relative importance of these mechanisms depends on various site-specific, operational parameters, including pretreatment of injection water and duration of injection, storage, and recovery phases. This critical review synthesizes findings across case studies in various geochemical, hydrological, and operational settings to better understand controls on arsenic and other geogenic contaminant mobilization and inform the planning and design of future MAR projects to protect groundwater quality. This critical review concludes with an evaluation of proposed management strategies for geogenic contaminants and identification of knowledge gaps regarding fate and transport of geogenic contaminants during MAR.

Combined removal of organic micropollutants and ammonium in reactive barriers developed for managed aquifer recharge

Groundwater is an important drinking water resource. To ensure clean drinking water, managed aquifer recharge (MAR) could be an attractive solution when recharging with treated wastewater. The installation of reactive barriers, e.g. with compost or other organic materials at MAR facilities, may improve pollutant removal. To link pollutant transformation processes and microbiology in reactive barriers, we simulated infiltration through different sand-compost mixtures using laboratory columns with depth-specific sampling of water and barrier material. We also evaluated the effect of inoculation with activated sludge. Our focus was on the simultaneous removal of organic micropollutants and nitrogen species, with parallel monitoring of the development of microbial communities. During 17 weeks of operation, the columns were fed with synthetic wastewater containing five organic micropollutants (1-2 µg/L each) and ammonium (2 mg N/L). Unique communities developed in the columns in relation to barrier material, with high effects of compost addition and minor effect of inoculation. Removal of the micropollutant paracetamol (acetaminophen) occurred in all columns, while sulfamethoxazole was only removed in columns with 50% compost. By contrast, limited removal was observed for sulfadiazine, carbamazepine and diuron, with the latter two displaying transient removal, attributed sorption. Oxygen was depleted within the top few cm of the columns when compost was present, but this was sufficient to remove all ammonium through nitrification. The fate of accumulated nitrate at deeper layers depended on the fraction of compost, with more compost leading to removal of nitrate by denitrification, but also by dissimilatory nitrate reduction to ammonium, hampering the overall nitrogen removal efficiency. Introducing compost as reactive barrier in MAR facilities has a large effect on the microbial communities and processes, but whether it will provide overall cleaner water to the underlying aquifer is uncertain and will depend very much on the type of pollutant.

Biochar-fertilizer interaction modifies N-sorption, enzyme activities and microbial functional abundance regulating nitrogen retention in rhizosphere soil

The impact of the excessive use of N fertilizer remains an environmental problem of global concern. The effect of biochar on soil N retention is still unclear, and knowledge on how a mixture of biochar and fertilizer (B-F) influence N-sorption, N-cycling enzymes activities, diversity and functional abundance of organisms regulating N-retention in rhizosphere soil is poorly understood. Therefore, biochars derived from bamboo, rice straw, cow and pig manure were characterized, and their interactions with NPK fertilizer were evaluated. Results showed that while the effect of biochar on N retention varied among biochar types, such variations increased after B-F. Unlike NH₄⁺ retention, NO₃^- retention by biochar in fertilized soil was poor (<8 weeks), but were however increased after longer periods (15 weeks) in B-F due to plant uptake, sorption and stimulation of N-cycling enzymes activities. This stimulation proved that N-fertilizer provided substrates for N-cycling organisms which was confirmed by the dominance of Proteobacteria, Chloroflexi, Actinobacteria, and Gemmatimonadetes which are important in soil N-cycling, despite the reductions in total diversity, class, phyla and genera abundance of bacterial 16SrRNA genes by B-F. This suggested that B-F induced specific organisms involved in N-cycling, which out-competed other organisms not involved in N-cycling. The provision of substrates by N-fertilizer in B-F for bacterial groups involved in N-cycling modified the rhizosphere microbial structure. The abundance of N-cycling organisms was regulated by the persistence among dominant groups, soil pH, total N, and microbial colonization induced by different biochars interacting with fertilizer which led to enhanced N-retention.

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.

Modeling the fate of UV filters in subsurface: Co-metabolic degradation and the role of biomass in sorption processes

Ultraviolet filters (UVFs) are emerging organic compounds found in most water systems. They are constituents of personal care products, as well as industrial ones. The concentration of UVFs in the water bodies in space and time is mostly determined by degradation and sorption, both processes being determinant of their bioavailability and toxicity to ecosystems and humans. UVFs are a wide group of compounds, with different sorption behavior expected depending on the individual chemical properties (pK_a,K_oc,K_ow). The goal of this work is framed in the context of improving our understanding of the sorption processes of UVFs occurring in the aquifer; that is, to evaluate the role of biomass growth, solid organic matter (SOM) and redox conditions in the characterization of sorption of a set of UVFs. We constructed a conceptual and a numerical model to evaluate the fate of selected UV filters, focused on both sorption and degradation. The models were validated with published data by Liu et al. (2013), consisting in a suite of batch experiments evaluating the fate of a cocktail of UVs under different redox conditions. The compounds evaluated included ionic UV filters (Benzophenone-3; 2-(3-t-butyl-2-hydroxy-5-methylphenyl)5-chloro-benzotriazole; 2-(2'-hydroxy-5'-octylphenyl)-benzotriazole) and neutral ones (octyl 4-methoxycinnamatte; and octocrylene).

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.

A novel manganese oxidizing bacterium-Aeromonas hydrophila strain DS02: Mn(II) oxidization and biogenic Mn oxides generation

The extensive applications of biogenic manganese oxides (BioMnOx) generated by manganese oxidizing bacteria (MOB) have attracted considerable attentions. In this study, we report on a novel MOB that has been isolated from sediments and identified as Aeromonas hydrophila strain DS02. The Mn(II) oxidation activity of strain DS02 under Mn(II) stress and the application of the associated BioMnOx products were investigated. Nearly 90.0% (495 mg L^-1) of the soluble Mn(II) were removed and 45.6% (240 mg L^-1) was converted to Mn(III/IV). Fitting the XPS data showed that Mn(IV)-oxide is the major component (82.0%) of the flake-shaped BioMnOx, corresponding to an average Mn oxidation number of 3.71. When the BioMnOx were coupled with the PMS activation, a 99.5% catalytic degradation of 2,4-dimethylaniline was observed after 80 min, revealing a high degradation efficiency.