Characterization of a managed aquifer recharge system using multiple tracers

Microplastics attenuation from surface water to drinking water: Impact of treatment and managed aquifer recharge - and identification uncertainties

River water can be used to recharge aquifers exploited for drinking water production. Several recent studies reported microplastics (MPs) in river water, and therefore, the potential contamination of groundwater by MPs is a growing concern among stakeholders and citizens. In this research, we investigate the fate of MPs (> 20 μm) along six different stages of a major Managed Aquifer Recharge (MAR)-water supply system in Switzerland. About 20 l of water were filtered using steel meshes at each location in triplicates. In the laboratory, MPs deposited on the anodisc filters were identified using Focal Plane Array (FPA) micro-Fourier-Transform-InfraRed (μFTIR) spectroscopy. The obtained hyperspectral data were processed using the imaging software Microplastics Finder for MPs identification and classification. Our results revealed a 20-fold decrease in MPs concentration from the Rhine River bed water (112 ± 27.4 MPs/l) to after the coagulation, flocculation and sedimentation (5.5 ± 2.2 MPs/l), a further 3-fold decrease to after the sand-filtration system (1.8 ± 0.9 MPs/l), corresponding to an overall removal efficiency of 98.4 %. The MPs concentrations remained low following MAR (2.7 ± 0.7 MPs/l) through a Quaternary gravel aquifer. Activated carbon filters did not substantially further reduce MPs concentrations. The percentage of fragments (≈95 %) prevailed over fibers (≈5 %) at all locations, with fibers being longer and more abundant in the river water. Overall, this study demonstrates the effectiveness of the treatment systems to remove MPs larger than 20 μm. Finally, we calculated an uncertainty in MPs concentrations of one order of magnitude depending on the user-defined parameters inside the MPs identification and classification model. The Quality Assurance/Quality Control approach followed during laboratory analysis highlighted an accumulation of surrogate particles at the edges of the disc, which would have an impact for MPs number upscaling.

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.

Climate change adaptation and mitigation measures for alluvial aquifers - Solution approaches based on the thermal exploitation of managed aquifer (MAR) and surface water recharge (MSWR)

As climate change adaptation strategies, both Managed Aquifer (MAR) and Surface Water Recharge (MSWR) are not only highly suitable tools to mitigate negative effects on water resources but also bear large potential for concomitant exploitation of thermal energy. They should thus form an integral part of any sustainable water resources management strategy. However, while at global scale general water resource adaptation and mitigation measures are discussed widely, measures that build on thermal exploitation of MAR and MSWR, and which are readily adaptable to various different local and regional scale conditions, have yet to be developed. Here, based on systematic numerical analyses of the sensitivity of groundwater and surface water recharge as well as water temperatures to climate change, we present adaptable implementation strategies of MAR and MSWR with concomitant exploitation of their thermal energy potential. Strategies and feasibility benchmarks for the exploitation of hydrologic and energetic potentials of MAR and MSWR were developed based on three hydrologically and hydrogeologically contrasting urban study sites near the city of Basel, Switzerland. Our studies show projected trends in the number of days when surface water temperatures exceed 25 °C examined for various streamflow and climate scenarios. We illustrate that local hydrogeologic settings and hydrological boundary conditions as well as legal aspects affect to which degree MAR and MSWR are suitable solutions as climate change adaptation measures. Optimal situations for exploiting the potential of seasonal heat storage in MAR and MSWR exist where subsurface travel times between the injection and the withdrawal or exfiltration point are between 4 and 8 months and legal limits allow a sufficiently large temperature spread. In such settings, the exploitable water flux and temperature spread of MAR and MSWR reaches a heat potential of 14 to 20 MW (i.e., corresponding to 3 to 7 wind power plants), and energetic exploitation becomes a suitable tool either for local low-temperature heat applications such as heating and hot water or for ecological use as a heat and water buffer in rivers affected by seasonal droughts. As a positive side effect, climate-induced warming of groundwater resources and temperature increases in drinking water withdrawals would be mitigated simultaneously.

A data-driven modeling approach for the sustainable remediation of persistent arsenic (As) groundwater contamination in a fractured rock aquifer through a groundwater recirculation well (IEG-GCW®)

Persistent arsenic (As) pollution sources from anthropogenic activities pose a serious threat to groundwater quality. This work aims to illustrate the application of an innovative remediation technology to remove As from a heavily contaminated fractured aquifer at a historically polluted industrial site. Groundwater circulation well (GCW) technology was tested to significantly increase and accelerate the mobilization and removal of As in the source area. The GCW extracts and re-injects groundwater at different depths of a vertical circulation well. By pumping out and reinjecting in different screen sections of the well, the resulting vertical hydraulic gradients create recirculation cells and affect and mobilize trapped contaminants that cannot be influenced by traditional pumping systems. The first 45-m deep IEG-GCW® system was installed in 2020, equipped with 4 screen sections at different depths and with an above-ground As removal system by oxidation and filtration on Macrolite (Enki). A geomodeling approach supports both remediation and multi-source data interpretation. The first months of operation demonstrate the hydraulic effectiveness of the IEG-GCW® system in the fractured rock aquifer and the ability to significantly enhance As removal compared to conventional pumping wells currently feeding a centralized treatment system. The recirculation flow rate amounts to about 2 m³/h. Water pumped and treated by the GCW system is reintroduced with As concentrations reduced by an average of 20%-60%. During the pilot test, the recirculating system removed 23 kg As whilst the entire central pump-and-treat (P&T) system removed 129 kg, although it treated 100 times more water volume. The P&T plant removed 259 mg As per m³ of pumped and treated groundwater while the GCW removed 4814 mg As per m³ of the treated groundwater. The results offer the opportunity for a more environmentally sustainable remediation approach by actively attacking the contamination source rather than containing the plume.

3D dynamic model empowering the knowledge of the decontamination mechanisms and controlling the complex remediation strategy of a contaminated industrial site

Knowledge of the geology and hydrogeology of the polluted site emblematize a key requirement for environmental remediation, through assembling and synthesizing findings from various sources of physical evidence. In an increasingly virtual era, digital and geo-referenced metadata may serve as tools for collecting, merging, matching, and understanding multi-source information. The main goal of this paper is to emphasize the significance of a 3D hydrogeochemical model to the portrayal and the understanding of contamination dynamics and decontamination mechanisms at a highly contaminated industrial site. Some remediation measures are active on-site, due to the evidence-based presence of chlorinated solvents in groundwater. These are attributable to a slow-release source of pollutants in the saturated zone associated with very low permeability sediments. Therefore, in this research, a new technique for the remediation of secondary sources of dense non-aqueous phase liquid (DNAPL) contamination was investigated for the first time on a full-scale application. The combination of groundwater circulation wells (IEG-GCW®) and a continuous electron donor production device was set up to boost in situ bioremediation (ISB). A multi-phase approach was followed handling and releasing data during various remediation stages, from site characterization via pilot testing to full-scale remediation, thus allowing users to monitor, analyze, and manipulate information in 3D space-time. Multi-source and multi-temporal scenarios reveal the impact of ongoing hydraulic dynamics and depict the decontamination mechanisms in response to the interventions implemented over time, by quantifying the overall performance of the adopted strategies in terms of removal of secondary sources of pollution still active at the site.

An assessment of different methods to determine specific yield for estimating groundwater recharge using lysimeters

Estimation of groundwater recharge is considered crucial for the management of groundwater resources. The groundwater level fluctuation (GLF) method is a widely used approach to estimate groundwater recharge due to its simplicity and ease of implementation. However, the main source of uncertainty is the specific yield for the GLF method. Although there have been a lot of methods for determining specific yield, the performance of specific yield on the estimation of recharge remains unclear. We set up three lysimeters with different water table depths in the Guanzhong Basin, China. Soil moisture content along with the soil profile, water table depths, and rainfall data were measured continuously. These data provide us with accurately observed recharge and allow us to analyze the performance of specific yields from different methods in estimating recharge. The main results are: (1) The constant specific yield from the pumping tests, which is equal to the ultimate specific yield, significantly overestimated the observed recharge rates independent of water table depths. (2) The constant specific yield obtained from saturated soil moisture content minus field capacity tended to overestimate recharge under the shallow water table depths (less than 2 m), and vice versa; (3) The depth-dependent specific yield using the measured soil moisture content along with soil profile can obtain reliable recharge across all water table depths. (4) The accuracy of the depth-dependent specific yield obtained by the soil water retention curve relies on reliable parameters. The parameters α and n have to be taken into account carefully for determing the specific yield. Our results are important for the application of the GLF method to estimate recharge. More importantly, it is valuable for the sustainable management of groundwater recourses.

Predicting algal blooms: Are we overlooking groundwater?

Significant advances in understanding and predicting freshwater algal bloom dynamics have emerged in response to both increased occurrence and financial burden of nuisance and harmful blooms. Several factors have been highlighted as key controls of bloom occurrence, including nutrient dynamics, local hydrology, climatic perturbations, watershed geomorphology, biogeochemistry, food-web control, and algal competition. However, a major research gap continues to be the degree to which groundwater inputs modulate microbial biomass production and food-web dynamics at the terrestrial-aquatic interface. We present a synthesis of groundwater related algal bloom literature, upon which we derive a foundational hypothesis: long residence times cause groundwater to be geochemically and biologically distinct from surface water, allowing groundwater inputs to modulate algal bloom dynamics (growth, decline, toxicity) through its control over in-stream water chemistry. Distinct groundwater chemistry can support or prevent algal blooms, depending on specific local conditions. We highlight three mechanisms that influence the impact of groundwater discharge on algal growth: 1) redox state of the subsurface, 2) extent of water-rock interactions, and 3) stability of groundwater discharge. We underscore that in testing hypotheses related to groundwater control over algal blooms, it is critical to understand how changes in land use, water management, and climate will influence groundwater dynamics and, thus, algal bloom probabilities. Given this challenge, we argue that advances in both modeling and data integration, including genomics data and integrated process-based models that capture groundwater dynamics, are needed to illuminate mechanistic controls and improve predictions of algal blooms.

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.

Combined method of 3H/3He apparent age and on-site helium analysis to identify groundwater flow processes and transport of perchloroethylene (PCE) in an urban area

Urban groundwater management requires a thorough and robust scientific understanding of flow and transport processes. ³H/³He apparent ages have been shown to efficiently help provide important groundwater-related information. However, this type of analysis is expensive as well as labor- and time-intensive, and hence limits the number of potential sampling locations. To overcome this limitation, we established an inter-relationship between ³H/³He apparent groundwater ages and ⁴He concentrations analyzed in the field with a newly developed portable gas equilibrium membrane inlet mass spectrometer (GE-MIMS) system, and demonstrated that the results of the simpler GE-MIMS system are an accurate and reliable alternative to sophisticated laboratory based analyses. The combined use of ³H/³He lab-based ages and predicted ages from the ³H/³He-⁴He age relationship opens new opportunities for site characterization, and reveals insights into the conceptual understanding of groundwater systems. For our study site, we combined groundwater ages with hydrochemical data, water isotopes (¹⁸O and ²H), and perchloroethylene (PCE) concentrations (1) to identify spatial inter-aquifer mixing between artificially infiltrated groundwater and water originating from regional flow paths and (2) to explain the spatial differences in PCE contamination within the observed groundwater system. Overall, low PCE concentrations and young ages occur when the fraction of artificially infiltrated water is high. The results obtained from the age distribution analysis are strongly supported by the information gained from the isotopic and hydrochemical data. Moreover, for some wells, fault-induced aquifer connectivity is identified as a preferential flow path for the transport of older groundwater, leading to elevated PCE concentrations.

The spatial extent and timescales of bank infiltration and return flows in an upland river system: Implications for water quality and volumes

The temporary storage of river water in riverbanks and its subsequent return to the river is an important part of the hydrological cycle. Detailed multi-year head and electrical conductivity (EC) variations document the spatial and temporal scales of bank infiltration and return flows in the Ovens catchment, southeast Australia. The study represents one of only a few that has integrated geochemical data and hydraulic heads over several hydrological cycles in a catchment unimpacted by groundwater extraction. The river water infiltrates only a few metres to tens-of-metres into the riverbanks and dilution of regional groundwater in the banks is commonly observed for several weeks to months following high river stages. Two-dimensional numerical modelling shows that the extent and timescales of bank infiltration and return flows are governed mainly by the regional head gradients, hydraulic conductivities, and the height of the stream pulse. Variable topography in river catchments will result in significant variations in the volumes of bank storage and the timescales that these bank waters are retained. The combination of field observations and numerical modelling illustrates that parts of the riverbanks may store water for several months to years and reconnection of rivers to the regional groundwater will occur in different parts of the catchment at different times. In much of the catchment the bank storage zones act as a buffer against contaminated regional groundwater and bank return flows will sustain streamflow and riverine ecosystems during dry periods.

Deconvolution and compensation of mass spectrometric overlap interferences with the miniRUEDI portable mass spectrometer

The miniRUEDI is a portable mass spectrometer system designed for on-site analysis of gases in the environment during field work and at remote locations. For many gas species (e.g., He, Ar, Kr, N₂, O₂, CO₂) the ion-current peak-heights measured with the mass spectrometer can usually be calibrated in terms of the partial pressures by simple peak-height comparison relative to a gas standard with well known partial pressures. However, depending on the composition of the analysed gases, the ion currents measured at certain m/z ratios may result from overlapping signals of multiple species (for example CH₄, O₂ and N₂ at m/z=15 and 16; or Ne, Ar and H₂O at m/z=20).

Here, we present a method extension to the existing miniRUEDI peak-height comparison in order to resolve such overlap interferences:

• We developed and tested a data processing procedure for accurate deconvolution and compensation of such mass-spectrometric overlap interferences.

• The method was incorporated into the miniRUEDI open-source software (ruediPy).

• The method substantially improves the analytical accuracy in situations where mass-spectrometric interferences cannot be avoided.

A simple model for evaluating isotopic (18O, 2H and 87Sr/86Sr) mixing calculations of mine - Impacted surface waters

This study was aimed at identifying and quantifying mixing proportions in surface waters downstream of historical Cu-W-F skarn mine tailings at Yxsjöberg, Sweden, using ¹⁸O, ²H, and ⁸⁷Sr/⁸⁶Sr isotopes. In addition, a simple mathematical model was developed to evaluate the consistency of the mixing calculations. Hydrochemical and isotopic data from 2 groundwater wells, 6 surface water and 2 rainwater sampling sites, spanning 6 sampling campaigns between May and October were used. Three mixed surface waters downstream of the tailings were identified, namely: C7, C11 and C14. C7 was directly influenced by groundwater from the tailings whereas C11 was also subsequently influenced by C7. C14 on the other hand, had contributions from C11. Sequential mixing calculations indicated that the contribution of the groundwater to C7 ranges from 1 to 17%. The subsequent contribution of C7 to C11 varied from 49 to 91% whereas C14 had contributions of C11 ranging between 16 and 56%. A strong agreement between the model data (MD) and measured raw data (RD) for C11 and C14 indicated the accuracy of the mixing calculations. Variations between the MD and RD at C7, however, was mainly due to sorption and reductive processes underneath the tailings, which tend to attenuate the amount of dissolved ions reaching the surface waters, resulting in a low ionic contribution of the tailings groundwater to the surface water. The low ionic contribution of the groundwater to C7 suggested that although the tailings impoundment is of environmental concern, its impact on the downstream surface waters is small. The results of this study suggest that mixing calculations in surface waters involving a closed system such as groundwater (as an end-member) must be treated with caution. It is recommended that the interpretation of such mixing results must be coupled with detailed knowledge of the potential hydrogeochemical processes along its flow paths.

A global-scale dataset of direct natural groundwater recharge rates: A review of variables, processes and relationships

Groundwater recharge indicates the existence of renewable groundwater resources and is therefore an important component in sustainability studies. However, recharge is also one of the least understood, largely because it varies in space and time and is difficult to measure directly. For most studies, only a relatively small number of measurements is available, which hampers a comprehensive understanding of processes driving recharge and the validation of hydrogeological model formulations for small- and large-scale applications. We present a new global recharge dataset encompassing >5000 locations. In order to gain insights into recharge processes, we provide a systematic analysis between the dataset and other global-scale datasets, such as climatic or soil-related parameters. Precipitation rates and seasonality in temperature and precipitation were identified as the most important variables in predicting recharge. The high dependency of recharge on climate indicates its sensitivity to climate change. We also show that vegetation and soil structure have an explanatory power for recharge. Since these conditions can be highly variable, recharge estimates based only on climatic parameters may be misleading. The freely available dataset offers diverse possibilities to study recharge processes from a variety of perspectives. By noting the existing gaps in understanding, we hope to encourage the community to initiate new research into recharge processes and subsequently make recharge data available to improve recharge predictions.

Groundwater recharge sites and pollution sources in the wine-producing Guadalupe Valley (Mexico): Restrictions and mixing prior to transfer of reclaimed water from the US-México border

Rapid depletion of aquifers in semiarid and arid regions threatens water security. This holds true especially in emerging countries where insufficient knowledge about aquifer systems precludes the implementation of advanced management measures, such as managed aquifer recharge. This study deals with the generation of baseline knowledge for the assessment of aquifers in arid and semiarid regions where artificial recharge with reclaimed water gains increasing impetus. The Guadalupe aquifer in Baja California provides water to 57% of the Mexican wine industry. Recent plans foresee a partial replenishment of its depleted groundwater reserves by transferring treated waste water from the Mexico-USA border for irrigation. The aquifer demonstrated to have a rapid response by rising the water table of about +20 m in relation to natural recharge under an intense rainfall period of 236 mm. Two predominant recharge sources were identified based on a geochemical multi-tracer approach: (a) water of modern age (<5 yr, >1.8 TU) and mixed water of recent-submodern age (³H 0.8-1.8 TU), and (b) sub-modern waters that were recharged before 1952 (³H < 0.5 TU). Water of the first type originate in the main Guadalupe stream, which has a more depleted average δ¹⁸O isotope value (-7.8‰) than average local rainwater (-2.0‰). The stream water initially has a Na-HCO₃ composition and recharges the entire Calafia zone and most groundwater along the riverbed across the valley. Water of the second type is mostly derived from hill-slope groundwater that has a stable isotope composition of mixed local rainwater and a NaCl composition. High total dissolved solids >2 g l^-1 together with enriched NO₃^- and Se concentrations characterize groundwater in the downstream the Porvenir zone. The geochemical age of this older, hill-slope groundwater suggests that its replenishment takes at least several decades when it becomes exhausted.

Evaluating Bank-Filtration Occurrence in the Province of Quebec (Canada) with a GIS Approach

Due to the abundance of surface water in the province of Quebec, Canada, it is suspected that many groundwater wells are pumping a mixture of groundwater and surface water via induced bank filtration (IBF). The regulatory framework in Quebec provides comprehensive guidelines for the development and monitoring of surface water and groundwater drinking water production systems. However, the regulations do not specifically address hybrid groundwater-surface water production systems such as IBF sites. More knowledge on the use of IBF in the province is needed to adjust the regulations with respect to the particularities of these systems. In order to provide a first evaluation of municipal wells potentially using IBF and the corresponding population served by these wells, a Geographic Information Science framework (GISc) was used to implement an IBF spatial database and calculate the distance from each well to the nearest surface water body. GISc is based on open source GIS programs and openly available data, to facilitate the reproducibility of the work. From this provincial scale approach, we show that nearly one million people are supplied by groundwater from municipal wells located <500 m from a surface water body, and half a million have a significant probability to be supplied by IBF wells. A more focused look at the watershed scale distribution of wells allows us to improve our interpretations by considering the aquifer type and other regional factors. This approach reveals strong spatial variability in the distribution of wells in proximity to surface water. Of the three selected regions, one has a high potential for IBF (Laurentides), one requires additional information do draw precise conclusions (Nicolet), and the third region (Vaudreuil-Soulanges) is unlikely to have widespread use of IBF. With this study, we demonstrate that extensive use of IBF is likely and that there is a need for improved understanding and management of these sites in order to properly protect the drinking water supply.

Pathline Density Distributions in a Null-Space Monte Carlo Approach to Assess Groundwater Pathways

A null-space Monte-Carlo (NSMC) approach was applied to account for uncertainty in the calibration of the hydraulic conductivity (K) field for a three-dimensional groundwater flow model of a major water supply system in Switzerland. The approach generates different parameter realizations of the K field using the pilot point methodology. Subsequently, particle tracking (PT) was applied to each calibrated model, and the resulting particles are interpreted as the spatial pathline density distribution of multiple sources. The adopted approach offers advantages over classical PT which does not provide a means for treating uncertainty originating from the incomplete description of subsurface heterogeneity. Uncertainty in the K field is shown to strongly influence the spatial pathline distribution. Pathline spreading is particularly evident in locations where the information content of the head observations does not sufficiently constrain the estimated parameters. Despite the predictive uncertainty, the pumped drinking water at the study site is most likely dominated by artificially-infiltrated groundwater originating from the local infiltration canals. The model suggests that within the well field, the central pumping wells could be extracting regional groundwater, although the probability is relatively low. Nevertheless, a rigorous uncertainty assessment is still required since only a few realizations resulted in flow paths that support the field observations. Model results should therefore not be based on only one model realization; rather, an uncertainty analysis should be carried out to provide a sufficiently large suite of equally probable simulations that include all potential sources and pathways.

Solute Reactive Tracers for Hydrogeological Applications: A Short Review and Future Prospects

Tracer testing is a mature technology used for characterizing aquatic flow systems. To gain more insights from tracer tests a combination of conservative (non-reactive) tracers together with at least one reactive tracer is commonly applied. The reactive tracers can provide unique information about physical, chemical, and/or biological properties of aquatic systems. Although, previous review papers provide a wide coverage on conservative tracer compounds there is no systematic review on reactive tracers yet, despite their extensive development during the past decades. This review paper summarizes the recent development in compounds and compound classes that are exploitable and/or have been used as reactive tracers, including their systematization based on the underlying process types to be investigated. Reactive tracers can generally be categorized into three groups: (1) partitioning tracers, (2) kinetic tracers, and (3) reactive tracers for partitioning. The work also highlights the potential for future research directions. The recent advances from the development of new tailor-made tracers might overcome existing limitations.

A review of threats to groundwater quality in the anthropocene

Awareness concerning sustainable groundwater consumption under the context of land use and climate change is gaining traction, raising the bar for adequate understanding of the complexities of natural and anthropogenic processes and how they affect groundwater quality. The heterogeneous characteristics of aquifers have hampered comprehensive source, transport and contaminant identification. As questions remain about the behavior and prediction of well-known groundwater contaminants, new concerns around emerging contaminants are on the increase. This review highlights some of the key contaminants that originate from anthropogenic activities, organized based on land use categories namely agricultural, urban and industrial. It further highlights the extensive overlap, in terms of both provenance as well as contaminant type, between the different land use sectors. A selection of case studies from literature that describe the continued concern of established contaminants, as well as new and emerging compounds, are presented to illustrate the many qualitative threats to global groundwater resources. In some cases, the risk of groundwater contamination lacks adequate gravity, while in others the underlying physical and societal processes are not fully understood and activities may commence without adequately considering potential impacts. In the agricultural context, the historic and current application of fertilizers and plant protectants, use of veterinary pharmaceuticals and hormones, strives to safeguard the growing food demands. In the context of a sprawling urban environment, waste, human pharmaceuticals, and urban pesticide outputs are increasing, with adequate runoff and sanitation infrastructure often lagging. Finally, industrial activities are associated with accidental leaks and spills, while the large-scale storage of industrial byproducts has led to legacy contaminants such as those stemming from raw mineral extraction. With this review paper, we aim to underscore the need for transdisciplinary research, along with transboundary communication, using sound science and adaptive policy and management practice in order to procure sustainable groundwater quality.

Characteristics of hydrochemistry and nitrogen behavior under long-term managed aquifer recharge with reclaimed water: A case study in north China

Due to the quality difference between reclaimed water and natural groundwater, managed aquifer recharge (MAR) with reclaimed water may pose environmental risks. A river infiltration of reclaimed water for groundwater recharge in north China has been in operation for over 10 years. To investigate the actual impact on native groundwater under long-term MAR, 10-year monitoring data of recharge water and groundwater were analyzed. Due to the effect of recharge, the hydrochemical type of groundwater rapidly changed from Ca-Mg-HCO₃ into Na-HCO₃ which was the type of recharge water. Cl^- was used as a conservative tracer in a physical mixing model, and the mixing was concluded to be dominant in the groundwater hydrochemical change under long-term MAR. The hydraulic travel time to the 30 m depth was determined to be about 6.5 months by obtaining the best-fit linear cross correlation between the concentrations of Cl^- in recharge water and those in groundwater. In application of this method, the monitoring wells should be located downstream and as close as possible to the recharge site (e.g., <50 m). Based on the travel time, behaviors of total nitrogen (TN), NO₃-N, NO₂-N, and NH₄-N were determined by attenuation factor (A_f). As the main nitrogen compound, NO₃-N was well attenuated under high hydraulic load, resulting in the A_f > 1, with an attenuation rate of 99.6%. The A_f < 1 of NH₄-N indicated the additional input of NH₄-N in groundwater. Fluctuations of NH₄-N in recharge water exceeded 4 mg/L changes sorption equilibrium, resulting in the sorption/desorption of NH₄-N in soil-groundwater system. The concentration of NH₄-N in groundwater increased in the later period of monitoring. The overall attenuation rate of NH₄-N was 26.3%. These findings contributed to improving the environmental benefits of this MAR site and provided guidance for other similar projects.

In-situ mass spectrometry improves the estimation of stream reaeration from gas-tracer tests

The estimation of gas-exchange rates between streams and the atmosphere is of great importance for the fate of volatile compounds in rivers. For dissolved oxygen, this exchange process is called reaeration, and its accurate and precise estimation is essential for the quantification of metabolic rates. A common method for the determination of gas-exchange rates is through artificial gas-tracer tests with a proxy gas. We present the implementation of a portable gas-equilibrium membrane inlet mass spectrometer (GE-MIMS) to record concentrations of krypton and propane injected as tracer compound in the context of a gas-tracer test. The field-compatible GE-MIMS uses signals of atmospheric measurements for concentration standardization, and allows recording the dissolved-gas concentrations at a high temporal resolution, leading to overall low measurement uncertainty. Furthermore, the in-situ approach avoids loss of gas during the steps of sampling, transport, storage, and analysis required for ex-situ gas measurements. We compare obtained gas-exchange rate coefficients, reaeration and derived metabolic rates from the in-situ measurements to results obtained from head-space sampling of propane followed by laboratory analysis, and find much lower uncertainties with the in-situ method.

Institutional Feasibility of Managed Aquifer Recharge in Northeast Ghana

As part of global efforts to address the challenges that are confronting groundwater for various purposes (including irrigation), engineering methods such as Managed Aquifer Recharge (MAR) have been adopted. This wave of MAR has engulfed some parts of Northern Ghana, characterized by insufficient groundwater for dry-season irrigation. Inspired by the strides of these schemes, the paper assesses the institutional feasibility of MAR methods in the Atankwidi catchment where dry-season farmers may lose their source of livelihood due to limited access to groundwater. We used both primary and secondary data, together with policy documents, to address the following questions: (i) What provisions and impacts formal government institutions had for MAR, and; (ii) what catchment-level institutions exist which may influence MAR. The results show that formal government institutions do not prohibit the adoption of MAR in the country. Among these institutions, it is realized that laws/legislative instruments provide sufficient information and support for MAR than policies and administrative agencies. Moreover, catchment-level institutions which are informal in the form of taboos, rules, norms, traditions, and practices, together with local knowledge play a significant role as far as groundwater issues in the catchment are concerned, and are important for the adoption of MAR methods.

Identifying Karst Aquifer Recharge Areas using Environmental Isotopes: A Case Study in Central Italy

Water resources management is one of the most important challenges worldwide because water represents a vital resource for sustaining life and the environment. With the aim of sustainable groundwater management, the identification of aquifer recharge areas is a useful tool for water resources protection. In a well-developed karst aquifer, environmental isotopes provide support for identifying aquifer recharge areas, residence time and interconnections between aquifer systems. This study deals with the use of environmental isotopes to identify the main recharge area of a karst aquifer in the Upper Valley of Aniene River (Central Italy). The analysis of 18O/16O and 2H/H values and their spatial distribution make it possible to trace back groundwater recharge areas based on average isotope elevations. The Inverse Hydrogeological Balance Method was used to validate spring recharge elevations obtained by the use of stable isotopes. Areas impacted by direct and rapid rainfall recharge into the study area were delineated, showing groundwater flowpaths from the boundaries to the core of the aquifer. The results of this study demonstrate the contribution that spatial and temporal isotope changes can provide to the identification of groundwater flowpaths in a karst basin, taking into account the hydrogeological setting.