Dating groundwater with dissolved silica and CFC concentrations in crystalline aquifers

Deep denitrification: Stream and groundwater biogeochemistry reveal contrasted but connected worlds above and below

Excess nutrients from agricultural and urban development have created a cascade of ecological crises around the globe. Nutrient pollution has triggered eutrophication in most freshwater and coastal ecosystems, contributing to a loss in biodiversity, harm to human health, and trillions in economic damage every year. Much of the research conducted on nutrient transport and retention has focused on surface environments, which are both easy to access and biologically active. However, surface characteristics of watersheds, such as land use and network configuration, often do not explain the variation in nutrient retention observed in rivers, lakes, and estuaries. Recent research suggests subsurface processes and characteristics may be more important than previously thought in determining watershed-level nutrient fluxes and removal. In a small watershed in western France, we used a multi-tracer approach to compare surface and subsurface nitrate dynamics at commensurate spatiotemporal scales. We combined 3-D hydrological modeling with a rich biogeochemical dataset from 20 wells and 15 stream locations. Water chemistry in the surface and subsurface showed high temporal variability, but groundwater was substantially more spatially variable, attributable to long transport times (10-60 years) and patchy distribution of the iron and sulfur electron donors fueling autotrophic denitrification. Isotopes of nitrate and sulfate revealed fundamentally different processes dominating the surface (heterotrophic denitrification and sulfate reduction) and subsurface (autotrophic denitrification and sulfate production). Agricultural land use was associated with elevated nitrate in surface water, but subsurface nitrate concentration was decoupled from land use. Dissolved silica and sulfate are affordable tracers of residence time and nitrogen removal that are relatively stable in surface and subsurface environments. Together, these findings reveal distinct but adjacent and connected biogeochemical worlds in the surface and subsurface. Characterizing how these worlds are linked and decoupled is critical to meeting water quality targets and addressing water issues in the Anthropocene.

Determination of groundwater origins and vulnerability based on multi-tracer investigations: New contributions from passive sampling and suspect screening approach

Knowledge about groundwater origins and their interactions with surface water is fundamental to assess their vulnerability. In this context, hydrochemical and isotopic tracers are useful tools to investigate water origins and mixing. More recent studies examined the relevance of contaminants of emerging concern (CECs) as co-tracers to distinguish sources contributing to groundwater bodies. However, these studies focused on known and targeted CECs a priori selected regarding their origin and/or concentrations. This study aimed to improve these multi-tracer approaches using passive sampling and qualitative suspect screening by exploring a larger variety of historical and emerging concern contaminants in combination with hydrochemistry and water molecule isotopes. With this objective, an in-situ study was conducted in a drinking water catchment area located in an alluvial aquifer recharged by several water sources (both surface and groundwater sources). CECs determined by passive sampling and suspect screening allowed to provide in-depth chemical fingerprints of groundwater bodies by enabling the investigation of >2500 compounds with an increased analytical sensitivity. Obtained cocktails of CECs were discriminating enough to be used as chemical tracer in combination with hydrochemical and isotopic tracers. In addition, the occurrence and type of CECs contributed to a better understanding of groundwater-surface water interactions and highlighted short-time hydrological processes. Furthermore, the use of passive sampling with suspect screening analysis of CECs lead to a more realistic assessment and mapping of groundwater vulnerability.

A hillslope-scale aquifer-model to determine past agricultural legacy and future nitrate concentrations in rivers

The long-term fate of agricultural nitrate depends on rapid subsurface transfer, denitrification and storage in aquifers. Quantifying these processes remains an issue due to time varying subsurface contribution, unknown aquifer storage and heterogeneous denitrification potential. Here, we develop a parsimonious modelling approach that uses long-term discharge and river nitrate concentration time-series combined with groundwater age data determined from chlorofluorocarbons in springs and boreholes. To leverage their informational content, we use a Boussinesq-type equivalent hillslope model to capture the dynamics of aquifer flows and evolving surface and subsurface contribution to rivers. Nitrate transport was modelled with a depth-resolved high-order finite-difference method and denitrification by a first-order law. We applied the method to three heavily nitrate loaded catchments of a crystalline temperate region of France (Brittany). We found that mean water transit time ranged 10-32 years and Damköhler ratio (transit time/denitrification time) ranged 0.12-0.55, leading to limited denitrification in the aquifer (10-20%). The long-term trajectory of nitrate concentration in rivers appears determined by flows stratification in the aquifer. The results suggest that autotrophic denitrification is controlled by the accessibility of reduced minerals which occurs at the base of the aquifer where flows decrease. One interpretation is that denitrification might be an interfacial process in zones that are weathered enough to transmit flows and not too weathered to have remaining accessible reduced minerals. Consequently, denitrification would not be controlled by the total aquifer volume and related mean transit time but by the proximity of the active weathered interface with the water table. This should be confirmed by complementary studies to which the developed methodology might be further deployed.

Diné citizen science: Phytoremediation of uranium and arsenic in the Navajo Nation

Mid-20th century mining in Naabeehó Bináhásdzo (Navajo Nation) polluted soil and groundwater with uranium and arsenic. The Diné and other indigenous residents of this region use groundwater for drinking, livestock, and irrigation, creating a serious environmental health risk. Currently, many individuals and communities on the Navajo Nation must purchase and transport treated water from hours away. Sunflowers (Helianthus annuus) preferentially take up uranium and arsenic, potentially representing a tool to remove these contaminants through on-site, low-cost phytoremediation. This study reports the results of a collaboration among researchers, high school students, teachers, and tribal leaders to analyze water chemistry and perform a phytoremediation experiment. In 2018 and 2019, we compiled existing data from the Navajo Nation Environmental Protection Agency (NNEPA) and collected samples from surface and groundwater. We then used sunflower seedlings grown in local soil to assess whether phytoremediation could be effective at removing arsenic and uranium. For the NNEPA-sampled wells, 9.5% exceeded the maximum contaminant level for uranium (30 μg per liter) and 16% for arsenic (10 μg per liter). For the new samples, uranium was highest in surface pools, suggesting leaching from local soil. Unlike studies from humid regions, sunflowers did not decrease uranium and arsenic in soil water. Instead, there was no change in arsenic concentration and an increase in uranium concentration in both planted and control treatments, attributable to weathering of uranium-bearing minerals in the desert soil. Because much of global uranium mining occurs in arid and semiarid regions, the ineffectiveness of phytoremediation on the Navajo Nation emphasizes the importance of prevention and conventional remediation. More generally, the participatory science approach created meaningful relationships and an important collaboration between a tribal chapter and a university, providing both cultural and scientific experiential learning opportunities for Diné high school students, undergraduate researchers, and senior personnel.

Multi Frequency Isotopes Survey to Improve Transit Time Estimation in a Situation of River-Aquifer Interaction

The Barthelasse alluvial aquifer is used to supply water to 180,000 inhabitants. The pumping field is located less than 200 m from the Rhône and is 100% fed by water from the Rhône, which makes it particularly vulnerable to any pollution from the Rhône. Between the Rhône and the pumping field is a Girardon unit, an arrangement that can be found regularly along the banks of the lower and middle reaches of the Rhône, and whose role is to stabilise the banks (alluvial deposits) and to facilitate river navigation. In order to know the transfer times between the Rhône and the pumping field, fortnightly monitoring was carried out over a hydrological year, as well as hourly monitoring during a flood in the winter of 2019. The Rhône shows a cyclicality in its isotopic signature with enrichment in heavy isotopes during the winter period, particularly during floods, and a depletion during the summer period. This variation is found well within the associated alluvial aquifer. The application of LPMs models showed that the average transfer time between the Rhône and the Girardon unit was 20 days and 50 days between the Rhône and the Barthelasse pumping. This study highlighted the importance of using several sampling frequencies to consider the diversity of hydrological situations. For the Rhône, event-based monitoring (flooding) proved to be relevant to account for isotopic variability throughout the year. This work also highlighted the impact of the disruption of hydraulic exchanges between the river and the water table caused by the presence of the Girardon unit in terms of the propagation of contaminants.

What do we need to predict groundwater nitrate recovery trajectories?

Nitrate contamination affects many of the Earth's aquifers and surface waters. Large-scale predictions of groundwater nitrate trends normally require the characterization of multiple anthropic and natural factors. To assess different approaches for upscaling estimates of nitrate recovery, we tested the influence of hydrological, historical, and biological factors on predictions of future nitrate concentration in aquifers. We tested the factors with a rich hydrogeological dataset from a fractured bedrock catchment in western France (Brittany). A sensitivity analysis performed on a calibrated model of groundwater flow, denitrification, and nitrogen inputs revealed that trends in nitrate concentration can effectively be approximated with a limited number of key parameters. The total mass of nitrate that entered the aquifer since the beginning of the industrial period needs to be characterized, but the shape of the historical nitrogen input time series can be largely simplified without substantially altering the predictions. Aquifer flow and transport processes can be represented by the mean and standard deviation of the residence time distribution, offering a tractable tool to make reasonable predictions at watershed to regional scales. Apparent sensitivity to denitrification rate was primarily attributable to time lags in oxygen depletion, meaning that denitrification can be simplified to an on/off process, defined only by the time needed for nitrate to reach the hypoxic reactive layer. Obtaining these key parameters at large scales is still challenging with currently available information, but the results are promising regarding our future ability to predict nitrate concentration with integrated monitoring and modeling approaches.

Shift of nitrate sources in groundwater due to intensive livestock farming on Jeju Island, South Korea: With emphasis on legacy effects on water management

Time lags between anthropogenic nitrogen inputs and their impacts to nitrate levels cause a misunderstanding for sources and subsequently misguide the groundwater management.We investigated the hydrochemical data of groundwater samples (n = 172 from 49 wells) with chlorofluorocarbons (CFCs)-based groundwater age dating and stable N (δ¹⁵N) and O isotopes (δ¹⁸O) of nitrate to assess the legacy effect of livestock farming to groundwater in an agricultural area where intensive livestock farming started in the 1970s and illegal dumping of manure wastewater in a lava cave was revealed in 2015. Approximately 90% of the groundwater samples had nitrate concentrations exceeding the natural threshold (5.5 mg/L NO₃^-) for nitrate contamination and 34% exceeded the World Health Organization's guideline for drinking water quality (44.3 mg/L), indicating severe nitrate contamination. The δ¹⁵N_NO3 values (5.5 to 24.3‰) in groundwater exceeding the threshold of nitrate showed that livestock manure was a major nitrate source, while ammonium fertilizer also seemed influential given the δ¹⁵N_NO3 values in the overlapping fields of N sources. Factor analysis of hydrochemical data also supported nitrate contamination by manure as well as by plant farming in the study area. Based on the spatial distribution of nitrate levels and δ¹⁵N_NO3, livestock farming affected nitrate contamination by illegal manure dumping in the leakage cave. According to a Bayesian mixing model, the contribution of manure wastewater was 33.5 to 81.8% as of 2015-2018, with the rest from fertilizers. Meanwhile, the groundwater ages showed negative correlations with both nitrate levels (r = -0.90) and δ¹⁵N_NO3 values (r = -0.74) on a log scale, consistent with the increasing N release from livestock farming since the 1960s. In particular, the median value of δ¹⁵N_NO3 rapidly increased to 9.2‰ in groundwater recharged between the late 1970s and early 1990s when N production exponentially increased, implying a significant effect of livestock farming after the 1980s. Groundwater quality is expected to deteriorate over the next several decades based on the groundwater ages (> 23.5 years), the increased N production from livestock farming, and the legacy effect of N. Long-term groundwater management plans (> 25 years) are required to decrease N loads in the study area, because it takes time for management practices to take effect. The study results are a good reference for groundwater management in regions with a source shift to livestock farming under intensive livestock production systems. Moreover, the chronological study using historical N production, groundwater age data, and dual nitrate isotopes can be applied to other regions with multiple N sources and their shifting for identifying sources and estimating time lags.

The Shallow and Deep Hypothesis: Subsurface Vertical Chemical Contrasts Shape Nitrate Export Patterns from Different Land Uses

Eutrophication has threatened water resources worldwide, yet mechanistic understanding on controls of nutrient export remains elusive. This work tests the shallow and deep hypothesis: subsurface vertical chemical contrasts regulate nitrate export patterns under different land use conditions. We synthesized data from 228 watersheds and used reactive transport modeling (500 simulations) under broad land use, climate, and geology conditions. Data synthesis indicated that human perturbation has amplified chemical contrasts in shallow water (e.g., soil water) versus deep waters (e.g., groundwater), inducing primarily flushing patterns (concentrations increase with streamflow) in agriculture lands and dilution patterns (concentrations decrease with streamflow) in urban watersheds. Results revealed a quantitative relationship between export patterns and shallow-versus-deep concentration contrasts, underscoring the often-overlooked role of nutrient distribution over depth. Results challenge the commonly held perception that legacy stores in agricultural lands induce chemostasis where concentrations vary negligibly with streamflow. They suggest that nitrate concentrations from agricultural lands will escalate during large hydrological events, which can exacerbate nutrient export problems as flooding events intensify in the future climate.

History traces of HCHs and DDTs by groundwater dating and their behaviours and ecological risk in northeast China

Organochlorine pesticides legacies, such as hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT), remained in sediments or soils due to their difficulty in decomposition, especially in the agricultural areas where pesticides were widely used historically. Different from the little disturbed depositional environment of lake, it was difficult for rivers to explore the timing of DDT and HCH inputs through dating sediment cores as records. Based on groundwater dating, this study ascertained the historic pollution of DDT and HCH in Taizi River basin. HCH and DDT residues in groundwater were consistent with the historical production and usage, which increased from the 1950s to the 1980s and declined from the 1980s to the 1990s. Moreover, the partitioning behaviours of HCHs and DDTs in surface water and suspended particulate matter were discussed. It was revealed that β-HCH and o,p'-DDT were more likely to attach to suspended particulate matter than other isomers. Furthermore, species sensitivity distribution curves were generated using 54 toxicity data records to assess the risk of HCHs and DDTs in water and suspended particulate matter. These results indicated that p,p'-DDT in surface water posed a high risk to 95% of the aquatic life in the long run.

Stratification of reactivity determines nitrate removal in groundwater

Although groundwater is a critical source of drinking water and irrigation, it has been polluted worldwide by agriculture, industry, and domestic activity. Because assessing groundwater quality and recovery rates is challenging, we developed a method for determining where and how quickly nitrate is removed in aquifers using just a few point measurements of groundwater chemistry. This methodology opens new avenues for characterizing catchment-scale nutrient dynamics, including nitrogen, carbon, and silica, with existing datasets for ecosystems around the globe. Understanding the subsurface structure of reactivity would also improve estimates of recovery time frames for polluted ecosystems and inform sustainable limits for anthropogenic activity.

Biogeochemical reactions occur unevenly in space and time, but this heterogeneity is often simplified as a linear average due to sparse data, especially in subsurface environments where access is limited. For example, little is known about the spatial variability of groundwater denitrification, an important process in removing nitrate originating from agriculture and land use conversion. Information about the rate, arrangement, and extent of denitrification is needed to determine sustainable limits of human activity and to predict recovery time frames. Here, we developed and validated a method for inferring the spatial organization of sequential biogeochemical reactions in an aquifer in France. We applied it to five other aquifers in different geological settings located in the United States and compared results among 44 locations across the six aquifers to assess the generality of reactivity trends. Of the sampling locations, 79% showed pronounced increases of reactivity with depth. This suggests that previous estimates of denitrification have underestimated the capacity of deep aquifers to remove nitrate, while overestimating nitrate removal in shallow flow paths. Oxygen and nitrate reduction likely increases with depth because there is relatively little organic carbon in agricultural soils and because excess nitrate input has depleted solid phase electron donors near the surface. Our findings explain the long-standing conundrum of why apparent reaction rates of oxygen in aquifers are typically smaller than those of nitrate, which is energetically less favorable. This stratified reactivity framework is promising for mapping vertical reactivity trends in aquifers, generating new understanding of subsurface ecosystems and their capacity to remove contaminants.

Hedgerows reduce nitrate flux at hillslope and catchment scales via root uptake and secondary effects

Agricultural contamination of groundwater with nitrate (NO₃^-) is one of the most widespread and pressing environmental issues. The preservation and planting of hedgerows around agricultural fields can reduce NO₃^- flux, but the efficacy of hedgerows depends on the amount of NO₃^- in soil and groundwater, hydrological flowpath and timing, and biogeochemical conditions surrounding and below roots. Quantifying these parameters is a major challenge, usually requiring involved and destructive fieldwork. Here, we present a new analytical method to characterize NO₃^- stratification using water chemistry sampled during piezometer slug tests. We tested this method with a network of wells in a hillslope intersected by an oak hedgerow during high- and low-water conditions, respectively spring and autumn. We found that hedgerows had a strong seasonal effect on near-surface NO₃^- dynamics in the proximity of the root system, reducing annual hillslope-level fluxes by 26 to 63%, comparable to NO₃^- removal from cover crop techniques. Hedgerow root uptake accounted for two-thirds of this reduction, with the remaining third attributable to secondary effects, potentially hedgerow-induced microbial retention or denitrification due to increased organic carbon and heterogeneous redox conditions in the rooting zone. However, a simple scaling exercise suggested that at the catchment level, hedgerow NO₃^- removal has a smaller effect (ca 1-10% reduction of annual flux), due to the large legacy of NO₃^- in the aquifer from past fertilizer application. These results suggest that while hedgerows cannot immediately solve problems of past groundwater contamination, protection and reestablishment of hedgerow networks could substantially accelerate recovery of groundwater quality on decadal timescales.