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

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.

Evaluation of contaminants of emerging concern attenuation through a vegetation filter managed using different operating conditions

In wastewater treatment using Vegetation Filters (VFs), natural processes reduce contaminants present in water although some of them can reach the environment. In this study, 39 contaminants of emerging concern (CECs) are evaluated in a pilot VF under different operating conditions during almost four years. The use of woodchip amendments and the change from surface irrigation through furrows to drip irrigation (and from weekly to daily water application) provide CEC concentration reductions in the water infiltrating through the vadose zone. Biodegradation is the main process taking place and has been favoured mainly by woodchip soil amendments and the increased residence. Median attenuation percentages of the CECs most frequently detected with highest concentrations in applied wastewater vary between 52% and 100% at the end of the study (at 45 cm depth). Among targeted CECs, caffeine, and its transformation product paraxanthine are the most attenuated. Flecainide and venlafaxine show a persistent behaviour. However, their leaching concentrations are very low (< 31 ng/L). Concerning the underlying aquifer, the groundwater quality in terms of CEC concentrations is conditioned by the surrounding area rather than the operation of the VF. Levels in groundwater are always below those in wastewater and infiltrating water.

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.

Dynamics and control mechanisms of inorganic nitrogen removal during wetting-drying cycles: A simulated managed aquifer recharge experiment

Managed aquifer recharge (MAR) systems can be operated intermittently through wetting-drying cycles to simultaneously improve the water supply and quality. Although MAR can naturally attenuate considerable amounts of nitrogen, the dynamic processes and control mechanisms of nitrogen removal by intermittent MAR remain unclear. This study was conducted in laboratory sandy columns and lasted for 23 d, including four wetting periods and three drying periods. The hydraulic conductivity, oxidation reduction potential (ORP), and leaching concentrations of ammonia nitrogen and nitrate nitrogen of MAR systems were intensively measured to test the hypothesis that hydrological and biogeochemical controls play an essential role in regulating nitrogen dynamics at different stages of wetting-drying cycles. Intermittent MAR functioned as a sink for nitrogen while providing a carbon source to support nitrogen transformations; however, it occasionally became a source of nitrogen under intense flushes of preferential flow. Nitrogen dynamics were primarily controlled by hydrological processes in the initial wetting phase and were further regulated by biogeochemical processes during the subsequent wetting period, supporting our hypothesis. We also observed that a saturated zone could mediate nitrogen dynamics by creating anaerobic conditions for denitrification and buffering the flush effect of preferential flow. The drying duration can also affect the occurrence of preferential flow and nitrogen transformations, which should be balanced when determining the optimal drying duration for intermittent MAR systems.

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.

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.

Denitrification kinetics during aquifer storage and recovery of drainage water from agricultural land

An aquifer storage transfer and recovery (ASTR) system was studied in which tile drainage water (TDW) was injected with relatively high NO₃ (about 14 mg/L) concentrations originating from fertilizers. Here we present the evolution of denitrification kinetics at 6 different depths in the aquifer before, and during ASTR operation. First-order denitrification rate constants increased over time before and during the first days of ASTR operation, likely due to microbial adaptation of the native bacterial community and/or bioaugmentation of the aquifer by denitrifying bacteria present in injected TDW. Push-pull tests were performed in the native aquifer before ASTR operation. Obtained first-order denitrification rate constants were negligible (0.00-0.03 d^-1) at the start, but increased to 0.17-0.83 d^-1 after a lag-phase of about 6 days. During the first days of ASTR operation in autumn 2019, the arrival of injected TDW was studied at 2.5 m distance from the injection well. First-order denitrification rate constants increased again over time (maximum >1 d^-1). Three storage periods without injection were monitored in winter 2019, fall 2020, and spring 2021 during ASTR operation. First-order rate constants ranged between 0.12 and 0.61 d^-1. Denitrification coupled to pyrite oxidation occurred at all depths, but other oxidation processes were indicated as well. NO₃ concentration trends resembled Monod kinetics but were fitted also to a first-order decay rate model to facilitate comparison. Rate constants during the storage periods were substantially lower than during injection, probably due to a reduction in the exchange rate between aquifer solid phases and injected water during the stagnant conditions. Denitrification rate constants deviated maximally a factor 5 over time and depth for all in-situ measurement approaches after the lag-phase. The combination of these in-situ approaches enabled to obtain more detailed insights in the evolution of denitrification kinetics during AS(T)R.

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.

Denitrifying bioreactor microbiome: Understanding pollution swapping and potential for improved performance

Denitrifying woodchip bioreactors are a best management practice to reduce nitrate-nitrogen (NO₃ -N) loading to surface waters from agricultural subsurface drainage. Their effectiveness has been proven in many studies, although variable results with respect to performance indicators have been observed. This paper serves the purpose of synthesizing the current state of the science in terms of the microbial community, its impact on the consistency of bioreactor performance, and its role in the production of potential harmful by-products including greenhouse gases, sulfate reduction, and methylmercury. Microbial processes other than denitrification have been observed in these bioreactor systems, including dissimilatory nitrate reduction to ammonia (DNRA) and anaerobic ammonium oxidation (anammox). Specific gene targets for denitrification, DNRA, anammox, and the production of harmful by-products are identified from bioreactor studies and other environmentally relevant systems for application in bioreactor studies. Lastly, cellulose depletion has been observed over time via increasing ligno-cellulose indices, therefore, the microbial metabolism of cellulose is an important function for bioreactor performance and management. Future work should draw from the knowledge of soil and wetland ecology to inform the study of bioreactor microbiomes.

Denitrification in Intrinsic and Specific Groundwater Vulnerability Assessment: A Review

Several groundwater vulnerability methodologies have been implemented throughout the years to face the increasing worldwide groundwater pollution, ranging from simple rating methodologies to complex numerical, statistical, and hybrid methods. Most of these methods have been used to evaluate groundwater vulnerability to nitrate, which is considered the major groundwater contaminant worldwide. Together with dilution, the degradation of nitrate via denitrification has been acknowledged as a process that can reduce reactive nitrogen mass loading rates in both deep and shallow aquifers. Thus, denitrification should be included in groundwater vulnerability studies and integrated into the various methodologies. This work reviewed the way in which denitrification has been considered within the vulnerability assessment methods and how it could increase the reliability of the overall results. Rating and statistical methods often disregard or indirectly incorporate denitrification, while numerical models make use of kinetic reactions that are able to quantify the spatial and temporal variations of denitrification rates. Nevertheless, the rating methods are still the most utilized, due to their linear structures, especially in watershed studies. More efforts should be paid in future studies to implement, calibrate, and validate user-friendly vulnerability assessment methods that are able to deal with denitrification capacity and rates at large spatial and temporal scales.

Modeling the effects of temperature on the migration and transformation of nitrate during riverbank filtration using HYDRUS-2D

Riverbank filtration is a natural aquifer-based process. The nitrogen dynamics in a riverbank filtration system are affected by many factors, including temperature, water quality, and travel time, which cannot be quantified easily. In this study, a field experiment was conducted to investigate nitrogen transport during riverbank filtration. The HYDRUS-2D software package was used to investigate and quantify the factors that affect the fate of nitrogen. The effects of temperature, water quality, and travel time on nitrate transport were considered. The model was calibrated and validated using field experimental data from the river water and groundwater during riverbank filtration at different periods. The results showed that HYDRUS-2D adequately simulated nitrate transport during riverbank filtration. The denitrification rate constant exhibited a positive exponential relationship with temperature. An empirical formula describing this relationship in riverbank filtration was developed and validated. In addition, the denitrification rate can be quantified within a specified temperature data range under field conditions. Compared with indoor experimental conditions, for the same temperature, there was a 10-fold increase in the denitrification rate constant under field conditions. The results showed that most of the nitrate removal occurred in the riparian zone at high temperatures during riverbank filtration. We concluded that the fate of nitrate in the riparian zone is strongly controlled by groundwater temperature. Travel time also plays an important role in nitrate removal during riverbank filtration.

Managed aquifer recharge implementation criteria to achieve water sustainability

Depletion of groundwater is accelerated due to an increase in water demand for applications in urbanized areas, agriculture sectors, and energy extraction, and dwindling surface water during changing climate. Managed aquifer recharge (MAR) is one of the several methods that can help achieve long-term water sustainability by increasing the natural recharge of groundwater reservoirs with water from non-traditional supplies such as excess surface water, stormwater, and treated wastewater. Despite the multiple benefits of MAR, the wide-scale implementation of MAR is lacking, partly because of challenges to select the location for MAR implementation and identify the MAR type based on site conditions and needs. In this review, we provide an overview of MAR types with a basic framework to select and implement specific MAR at a site based on water availability and quality, land use, source type, soil, and aquifer properties. Our analysis of 1127 MAR projects shows that MAR has been predominantly implemented in sites with sandy clay loam soil (soil group C) and with access to river water for recharge. Spatial analysis reveals that many regions with depleting water storage have opportunities to implement MAR projects. Analyzing data from 34 studies where stormwater was used for recharge, we show that MAR can remove dissolved organic carbon, most metals, E. coli but not efficient at removing most trace organics, and enterococci. Removal efficiency depends on the type of MAR. In the end, we highlight potential challenges for implementing MAR at a site and additional benefits such as minimizing land subsidence, flood risk, augmenting low dry-season flow, and minimizing salt-water intrusion. These results could help identify locations in the water-stressed regions to implement specific MAR for water sustainability.

Genomic studies on natural and engineered aquatic denitrifying eco-systems: A research update

Excess nitrogenous compounds in municipal or industrial wastewaters can stimulate growth of denitrifying bacteria, in return, to convert potentially hazardous nitrate to inorganic nitrogen gas. To explore the community structure, distributions and succession of functional strains, and their interactions with other microbial communities, contemporary studies were performed based on detailed genomic analysis. This mini-review updated contemporary genomic studies on denitrifying genes in natural and engineered aquatic systems, with the constructed wetlands being the demonstrative system for the latter. Prospects for the employment of genomic studies on denitrifying systems for process design, optimization and development of novel denitrifying processes were discussed.

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.

Denitrification in wetlands: A review towards a quantification at global scale

Research to understand the nitrogen cycle has been thriving. The production of reactive nitrogen by humans exceeds the removal capacity through denitrification of any natural ecosystem. The surplus of reactive nitrogen is also a significant pollutant that can shift biological diversity and distribution, promotes eutrophication in aquatic ecosystems, and affects human health. Denitrification is the microbial respiration in anoxic conditions and is the main process that removes definitively nitrates from the ecosystem by returning of reactive nitrogen (Nr) to the atmosphere as N₂ and N₂O emissions. This process occurs in the oceans, aquatic ecosystems and temporary flooded terrestrial ecosystems. Wetlands ecosystems are rich in organic matter and they have regular anoxic soil conditions ideal for denitrification to occur. In the current paper, we provide a meta-analysis that aims at exploring how research around global nitrogen, denitrification and wetlands had evolved in the last fifty years. Back in the time, wetland ecosystems were seen as non-exploitable elements of the landscape, and now they are being integrated as providers of ecosystem services. A significant improvement of molecular biology techniques and genetic extraction have made the denitrification process fully understood allowing constructed wetlands to be more efficient and popular. Yet, large uncertainties remain concerning the dynamic quantification of the global denitrification capacity of natural wetland ecosystems. The contribution of the current investigation is to provide a way forward for reducing these uncertainties by the integration of satellite-based Earth Observation (EO) technology with parsimonious physical based models.