Managed Aquifer Recharge Systems Research Articles

Comment on “Numerical simulation of a managed aquifer recharge system designed to supply drinking water to the city of Amsterdam, The Netherlands”: paper published in Hydrogeology Journal (2023) 31: 1291–1309, by Pranisha Pokhrel, Yangxiao Zhou, Frank Smits, Pierre Kamps and Theo Olsthoorn

Comment on “Numerical simulation of a managed aquifer recharge system designed to supply drinking water to the city of Amsterdam, The Netherlands”: paper published in Hydrogeology Journal (2023) 31: 1291–1309, by Pranisha Pokhrel, Yangxiao Zhou, Frank Smits, Pierre Kamps and Theo Olsthoorn

Reply to Comment on “Numerical simulation of a managed aquifer recharge system designed to supply drinking water to the city of Amsterdam, The Netherlands”: paper published in Hydrogeology Journal (2023) 31:1291–1309, by Pranisha Pokhrel, Yangxiao Zhou, Frank Smits, Pierre Kamps and Theo Olsthoorn

Reply to Comment on “Numerical simulation of a managed aquifer recharge system designed to supply drinking water to the city of Amsterdam, The Netherlands”: paper published in Hydrogeology Journal (2023) 31:1291–1309, by Pranisha Pokhrel, Yangxiao Zhou, Frank Smits, Pierre Kamps and Theo Olsthoorn

Detection and mitigation of soil salinization risk from saline/brackish water aquaculture in coastal areas: an application of remote sensing and managed aquifer recharge

This study focuses on detecting and mitigating soil salinization in four coastal areas of the Mekong Delta (Vietnam). Salinity patterns in the soils of the Mekong River Delta are not random but linked to land use practices and distance to the sea. We examine two quick yet reliable remote sensing-based techniques to detect the coastal aquaculture area and separate it from the inland freshwater farmland. These techniques can eventually be used to identify locations with an elevated risk of salinization in other coastal regions. Finally, we investigate a salinization mitigation solution based on creating a managed aquifer recharge system along the buffer zone that separates the coastal aquaculture area from the inland freshwater agriculture area. The implementation of an infiltration pond system is technically feasible in the Mekong Delta provided that hydrogeological characteristics, the fresh-saline interface position, and freshwater demands are considered. The transitional zone between freshwater agriculture and brackish water aquaculture in Bac Lieu, Soc Trang, Tra Vinh and Ben Tre provinces is optimal for implementing an aquifer recharge/freshwater barrier scheme.

Site-specific condition evaluation for managed aquifer recharge (MAR) site selection in granitic aquifers, Ghana

ABSTRACT Water has prominence over all the usable natural resources to humankind due to its very significance to human life. Approximately, 70% of Ghana's populace relies on groundwater exploitation for freshwater consumption. Managed aquifer recharge is capable of sustaining water resources for all livelihood activities. Underperformance of MAR systems is often due to site-specific hydrogeological conditions turning out to be less favorable than anticipated. This study evaluates the site-specific conditions for MAR sites identification in granitic aquifers, integrating in situ hydrogeological factors and GIS-MCDA. One hundred and twenty-one datasets from different boreholes comprising hydro geophysics, borehole drilling and pump test reports were used. The constraint mapping results indicate 92% suitable for MAR application within the study area, implying available enormous bare lands for flooding recharge technique. The suitability analysis discloses that 48 and 51% of the studied area fall within very high and high suitable zones, respectively, showing convincing and great potential to support infiltration ponds for MAR technology application. The valuable information provided through this study can serve as a guide for MAR implementation and for sustainable groundwater resources management within the Upper East region of Ghana.

Water quality aspects from Spanish sites to support managed aquifer recharge (MAR) guidelines not based on maximum allowable concentration standards

Most countries that have technical guidelines or regulations for artificial recharge or managed aquifer recharge (MAR), that include water quality aspects are based on the establishment of standards or Maximum Allowable Concentrations (MACs) to regulate the quality of the water percolated or injected into an aquifer. The number of parameters in these guidelines vary considerably (from 6 in Spain to 156 in the USA) and often apply to all aquifers within administrative boundaries (e.g., national territory), regardless of the nature of the receiving medium, the depth of the water table, and other key factors. Eleven MAR systems in Spain have been studied (eight operational and three experimental, with limited number of data from three sites), characterising both, the recharge water quality and the water resulting from the interaction processes recharge water-soil-unsaturated zone-saturated zone of the aquifer. In all cases, an improved effect on groundwater quality is observed, even though some parameters in the recharge water don’t comply with the standards employed in some European countries, where this article focuses. The article suggests that regulating water quality for MAR through MACs at national level gives room for another alternative approach specific for each site. It might be recommendable to establish local standards at the regional or aquifer-wide level to better reflect the diversity of groundwater occurrence. As per the article, sectoral water authorities could receive more decision-making power on granting permits for MAR based on the quality of the hydrogeological and risk studies for each request. This would help reduce the application of the precautionary principle when in granting permission.

Recarga artificial mediante pozos secos

Managed aquifer recharge (MAR) is a water management strategy that uses aquifers for the seasonal or inter-annual storage of exceeding surface water. Systems that implement artificial recharge use infiltration ponds or shallow or deep infiltration wells. While articifial recharge through infiltration ponds requires large areas and infiltration through deep wells demands high investment, infiltration through drywells, which inject water to the vadose zone, is often a preferred option because of its low cost and relatively easy implementation. We present results of detailed numerical simulations to assess the operation of a single drywell to implement an artificial recharge system. We demonstrate that depending upon the distribution and property of the geological materials that compose the subsurface, the operation of the well can produce quite different results in terms of effective recharge. We expect that these findings will help design real MAR systems and provide supporting information for decision-makers in charge of approving and financing such water management systems.

Developing managed aquifer recharge (MAR) to augment irrigation water resources in the sand and gravel (Crag) aquifer of coastal Suffolk, UK

Managed aquifer recharge (MAR) offers a potential innovative solution for addressing groundwater resource issues, enabling excess surface water to be stored underground for later abstraction. Given its favourable hydrogeological properties, the Pliocene sand and gravel (Crag) aquifer in Suffolk, UK, was selected for a demonstration MAR scheme, with the goal of supplying additional summer irrigation water. The recharge source was a 4.6 km drainage channel that discharges to the River Deben estuary. Trialling the scheme in June 2022, 12,262 m3 of source water were recharged to the aquifer over 12 days via a lagoon and an array of 565 m of buried slotted pipes. Groundwater levels were raised by 0.3 m at the centre of the recharge mound with an approximate radius of 250 m, with no detrimental impact on local water features observed. The source water quality remained stable during the trial with a mean chloride concentration (133 mg L−1) below the regulatory requirement (165 mg L−1). The fraction of recharge water mixing with the groundwater ranged from 69% close to the centre and 5% at the boundary of the recharge mound, leading to a reduction in nitrate-N concentration of 23.6 mg L−1 at the centre of the mound. During July–September 2022, 12,301 m3 of recharge water were abstracted from two, 18 m boreholes to supplement surface irrigation reservoirs during drought conditions. However, the hydraulic conductivity of the Crag aquifer (∼10 m day−1) restricted the yield and thereby reduced the economic viability of the scheme. Construction costs for the MAR system were comparatively low but the high costs of data collection and securing regulatory permits brought the overall capital costs to within 18% of an equivalent surface storage reservoir, demonstrating that market-based mechanisms and more streamlined regulatory processes are required to incentivise similar MAR schemes.

Microplastic removal in managed aquifer recharge using wastewater effluent

Microplastic (MP) pollution has emerged as a pressing environmental issue, with its impacts on ecosystems and human health yet to be fully understood. This study aims to investigate the presence and distribution of MPs in the soil of a managed aquifer recharge (MAR) system, built with different reactive barriers of natural materials and irrigated with the secondary effluent of a wastewater treatment plant (WWTP). MPs were extracted from reactive barrier material following an approach based on the density separation of MPs with posterior oxidant digestion, combined with visual and chemical characterisation by Fourier-Transform Infrared Spectroscopy (FTIR). The results revealed the widespread occurrence of MPs in the MAR soil samples. MPs concentration in the different barrier materials ranged from 60 to 236 n kg−1. The most dominant morphologies were fragments (60%) and fibers (17%), and the most abundant colour was white (51%), followed by transparent MPs (20%). Polypropylene (PP) was detected in all the samples with an abundance of 47%, followed by polyethylene (PE, 34%). The interplay of barrier composition significantly influences the retention of MPs, with compost (T5) and woodchips (T4) exhibiting the most notable retention rates. Remarkably, the outer layers of the reactive barriers display superior retention compared to the deeper layers.The findings of this study demonstrate the good performance of the MAR system in retaining MPs and contribute to the growing body of knowledge on MPs pollution in freshwater systems while providing insights into the dynamics of MPs transport and accumulation in soil. Such information can inform the development of effective wastewater management strategies to mitigate the impacts of these pollutants on water resources and safeguard the environment.

Fates of potentially persistent and mobile organic substances in embedded outdoor columns for artificial groundwater recharge simulation

Persistent and mobile organic micropollutants (OMP) are ubiquitously found in the aquatic environment and have a high propensity to distribute in water resources and are difficult to remediate. Managed aquifer recharge systems such as artificial groundwater recharge, produce high-quality drinking water by removing numerous OMP from the source water. In this study, the fates of selected emerging and potentially persistent and mobile OMP were investigated in outdoor columns for artificial groundwater recharge simulation. Breakthrough curves of OMP were modeled to differentiate between sorption and bio-transformation. The study showed that selected OMP were persistent in the surface water and no photo-degradation was observed, except for diclofenac. The trends of dissolved organic carbon concentrations and UV light absorption at 254 nm wavelength suggest elevated biological activity in the first 0.3 m of the columns. The study revealed that the bio-transformation of cyanoguanidine, valsartan acid and diclofenac correlated with the biological activity in the sand columns. Benzyltrimethylammonium, n-(3-(dimethylamino)-propyl)methacrylamide, 1,3-di-o-tolylguanidine, 1,3-diphenylguanidine and melamine were completely eliminated within the first 0.3 m, likely due to sorption. Less mobile compounds such as carbamazepine and adamantan-1-amine also showed sorption. Sorption was also observed for diclofenac, likely due to decreased pH along the column depth. Retardation factors of several OMP were higher in the first 0.3 m of the columns, likely due to higher organic carbon contents compared to the remaining depth. Six organic substances (for example 2-acrylamido-2-methylpropane sulfonate and dimethylbenzene sulfonate) were persistent and mobile throughout the experiment. Overall, this study reveals the vital role of pH and sand organic carbon for sorption and residence time and biological activity for OMP elimination.

Numerical simulation of a managed aquifer recharge system designed to supply drinking water to the city of Amsterdam, The Netherlands

Managed aquifer recharge (MAR) is increasingly used to secure drinking water supply worldwide. The city of Amsterdam (The Netherlands) depends largely on the MAR in coastal dunes for water supply. A new MAR scheme is proposed for the production of 10 × 106 m3/year, as required in the next decade. The designed MAR system consists of 10 infiltration ponds in an artificially created sandbank, and 25 recovery wells placed beneath the ponds in a productive aquifer. Several criteria were met for the design, such as a minimum residence time of 60 days and maximum drawdown of 5 cm. Steady-state and transient flow models were calibrated. The flow model computed the infiltration capacity of the ponds and drawdowns caused by the MAR. A hypothetical tracer transport model was used to compute the travel times from the ponds to the wells and recovery efficiency of the wells. The results demonstrated that 98% of the infiltrated water was captured by the recovery wells which accounted for 65.3% of the total abstraction. Other sources include recharge from precipitation (6.7%), leakages from surface water (13.1%), and natural groundwater reserve (14.9%). Sensitivity analysis indicated that the pond conductance and hydraulic conductivity of the sand aquifer in between the ponds and wells are important for the infiltration capacity. The temperature simulation showed that the recovered water in the wells has a stable temperature of 9.8–12.5 °C which is beneficial for post-treatment processes. The numerical modelling approach is useful and helps to gain insights for implementation of the MAR.

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.

Understanding and predicting physical clogging at managed aquifer recharge systems: A field-based modeling approach

Managed aquifer recharge (MAR) techniques are in demand to cope with water scarcity challenges posed by climate change and groundwater overexploitation. One of the long-lasting technical issues associated with MAR systems is physical clogging. The intrusion and deposition of external fines during water recharge reduce the infiltration capacity of the site over time. Operation and maintenance (O&M) costs are experienced directly at the site to restore the original efficiency of infiltration rates. Thus, investors need reliable estimations of the risk of clogging during the planning of the site. As a rule, in MAR design, the main parameter of concern for physical clogging is the total suspended solids (TSS), and most clogging models rely on experiment calibrations in 1D sand columns. However, secondary processes can control the development and spatial distribution of physical clogging in field conditions. The proposed work aims to detect key clogging factors directly in the field and to model these processes for reproducibility at other sites. The fieldwork is conducted at the two-stage infiltration basin in Suvereto (Tuscany, Italy). Spatial factors are included in the analysis (i.e. basin topography) to explain clogging patterns in the field altered by erosion processes. The observed clogging profiles at two sampled locations exhibiting clogging are replicated by a mathematical model. Based on the computation of annual erosion rates in the pond and fines’ redistribution, the exceeding fines’ contents over depth are validated with an RMSE of 2.53% and 12.53%. The infiltration capacity of the site is estimated to reach a stable value of 90% of the initial infiltration capacity over 20 years, given the Suvereto basin features. The model's parameterisation from field measurements represents a great advantage over existing clogging models due to its transferability to other MAR sites. The assessment of the risk of clogging supported by field characterization and numerical modelling is cost-effective and assists the deduction of O&M schemes for MAR sites.

Risk assessment for the prevention of managed aquifer recharge (MAR) facility failure during the operation and the expansion phases

This comprehensive risk assessment, done for a MAR facility in Tarnów, southern Poland, covers two MAR techniques: infiltration ditches and induced bank filtration during the operation and planned potential expansion phases. Two methods were applied: the semi-quantitative risk assessment matrix and the probabilistic risk assessment – fault tree analysis (PRA-FTA), and their advantages and limitations analysed. Both methods examine a range of technical and non-technical risks that may pose a threat to the efficient functioning of the MAR facility in Tarnów, which supplies about 25% of the city’s population with drinking water. It was found that 34 of the 65 risks analysed could pose a low to high risk for the Świerczków well field. The predominant risks potentially reducing MAR efficiency or leading to MAR failure are technical ones. According to the PRA-FTA method, if no preventive measures are taken, there is a 99.5 and 99.3% probability of the MAR system failing during the operational and expansion phases, respectively. The results indicate that the most serious risk is associated with the insufficient initial removal from the recharge river water of suspended solids, which clog infiltration ditches. The risks scored as medium relate to the threat of groundwater inflow from an extensive industrial zone, lying to the east of the well field, and to climate change negatively affecting the quantity of groundwater resources. This is the first example in Poland of a risk assessment for an existing MAR facility and its expansion, based on a broad spectrum of technical and non-technical risks present at the Świerczków well field, using two methods: the risk assessment matrix and PRA-FTA. This case study is a practical example of how to perform a risk analysis for a MAR site. It can make an important contribution to existing knowledge in this field and enrich the subject literature, a particularly relevant aspect given the widespread use of MAR techniques in Central Europe for the supply of drinking water to the public.

The Significance of Groundwater Table Inclination for Nature-Based Replenishment of Groundwater-Dependent Ecosystems by Managed Aquifer Recharge

Managed aquifer recharge (MAR) is an increasingly popular technique; however, the significance of groundwater flow dynamics is rarely examined in detail regarding MAR systems. In general, a high hydraulic gradient is not favoured for MAR implementation, as it causes higher water loss and mixing of recharge water with native groundwater. However, during groundwater-dependent ecosystem (GDE) rehabilitation, these hydraulic gradient-driven flow processes can be taken advantage of. The aim of this research is to test this hypothesis by evaluating the effect of groundwater table inclination, topography, and other local characteristics on MAR efficiency from the perspective of GDE restoration. MAR efficiency was examined from recharge to discharge area in a simple half-basin based on theoretical flow simulations, using GeoStudio SEEP/W software. Different scenarios were compared to analyse the groundwater level increase and the infiltrated water volumes and to assess the efficiency of MAR based on these parameters in each scenario. The theoretical results were applied to a close-to-real situation of Lake Kondor, a GDE of the Danube-Tisza Interfluve (Hungary), which dried up in the past decades due to groundwater decline in the area. Based on the results, initial hydraulic head difference, model length, and hydraulic conductivity are the most critical parameters regarding water level increase at the discharge area. The water amount needed for increasing the water table is mainly influenced by the thickness of the unsaturated zone and the material properties of the aquifer. The findings can help better understand MAR efficiency in light of local groundwater flow processes and contribute to optimising MAR systems. The results of the study suggest that, if water is infiltrated at the local recharge area, the water table will also increase at the corresponding discharge area, which positively effects the connected GDEs. This approach can serve as a nature-based solution (NBS) to sustain sensitive ecosystems in changing climatic conditions.

The fate of nitrification and urease inhibitors in simulated bank filtration

The application of nitrification and urease inhibitors (NUI) in conjunction with nitrogen (N) fertilizers improves the efficiency of N fertilizers. However, NUI are frequently found in surface waters through leaching or surface runoff. Bank filtration (BF) is considered as a low-cost water treatment system providing high quality water by efficiently removing large amounts of organic micropollutants from surface water. The fate of NUI in managed aquifer recharge systems such as BF is poorly known. The aim of this work was to investigate sorption and degradation of NUI in simulated BF under near-natural conditions. Besides, the effect of NUI on the microbial biomass of slowly growing microorganisms and the role of microbial biomass on NUI removal was investigated. Duplicate sand columns (length 1.7 m) fed with surface water were spiked with a pulse consisting of four nitrification (1,2,4-triazole, dicyanodiamide, 3,4-dimethylpyrazole and 3-methylpyrazole) and two urease inhibitors (n-butyl-thiophosphoric acid triamide and n-(2-nitrophenyl) phosphoric triamide). The average spiking concentration of each NUI was 5 μg/L. Experimental and modeled breakthrough curves of NUI indicated no retardation for any of the inhibitors. Therefore, biodegradation was identified as the main elimination pathway for all substances and was highest in zones of high microbial biomass. Removal of 1,2,4-triazole was 50% and n-butyl-thiophosphoric acid triamide proved to be highly degradable and was completely removed after a hydraulic retention time (HRT) of 24 h. 50% of the mass recovery for nitrification inhibitors except for 3,4-dimethylpyrazole was observed at the effluent (4 days HRT). In addition, a mild effect of NUI on microbial biomass was noted. This study highlights that the degradation of NUI in BF depends on HRT and microbial biomass.

Evaluation of Potential Seawater Intrusion in the Coastal Aquifers System of Benin and Effect of Countermeasures Considering Future Sea Level Rise

In the present study, a three-dimensional SEAWAT model was developed to generally simulate the impact of climate change and anthropogenic activities on seawater intrusion (SWI) in the coastal region of Benin by the end of 2050. The model was calibrated and validated from 2015 to 2020, considering groundwater head and salt concentration measured in 30 wells. After calibration, a sensitivity analysis was performed with the model parameters (hydraulic conductivity, recharge, storage coefficient and boundary conditions). For the calibration, model computed and observed values displayed good correlation, approximatively 0.82 with a root mean square error (RMSE) of 0.97 m and 13.38 mg/L for groundwater head and salt concentration, respectively. The simulation results indicate that freshwater head had declined by 1.65 m from 2015 to 2020 (taking reference from the average groundwater head in 2015: 27.08 m), while the seawater intrusion area increased in the same period by an average of 1.92 km2 (taking reference from the seawater intrusion area in 2015: 20.03 km2). The model is therefore used to predict groundwater level decline and seawater intrusion area increase by the end of 2050, considering the predicted sea level rise (SLR) and estimated groundwater pumping rate. Furthermore, the interface fresh groundwater–saltwater change was studied using the SHARP interface developed by USGS in 1990. The interface variation was found to be influenced by the distance from shoreline, sea level, groundwater level and geological formation hydraulic conductivity. Finally, the 3D model was used to simulate the effect of a managed aquifer recharge system on reducing SWI rate in the study region.

Managed Aquifer Recharge as a Low-Regret Measure for Climate Change Adaptation: Insights from Los Arenales, Spain

In view of heightened climate change (CC), adaptation strategies are imperative to diminish the impacts on social and environmental assets. Two approaches are commonly used to formulate adaptation measures, namely bottom-up and top-down, each with inherited limitations. A sound bridge between both approaches is low-regret adaptive measures, which result in win-win scenarios, as they provide solutions to current pressures and contribute to building CC adaptive capacity. Managed aquifer recharge (MAR) is a term that includes a series of techniques that enhance groundwater storage for later use or environmental purposes. MAR is often mentioned in the literature as a CC adaptation measure. Nonetheless, few examples explicitly prove this point. We show through the Los Arenales MAR systems (Central Spain) that MAR is a low-regret CC adaptive measure. We evaluate a series of social and environmental challenges that MAR systems contribute to solving, as well as their attributes that diminish the expected impacts of CC in the study area. MAR in the Los Arenales groundwater body has resulted in an overall increase in groundwater levels; a reduction in groundwater pumping energy and costs and CO2 emissions; restoration of a surface water body; improvement in rural population indexes; and enhanced groundwater demand control and CC adaptive capacity through irrigation communities. To cope with CC, the Los Arenales MAR systems can be operated even if decreasing streamflow precludes the use of river water surpluses; they provide surface storage volume to mitigate flooding; and they decrease the impacts of droughts and water scarcity. This research proves that MAR is a water management tool capable of providing solutions to several pressures simultaneously in the present and future, an attribute particularly useful when dealing with adaptation gaps in developing countries, rural areas, or regions lacking long-term climatic data.

Predictive modeling of pharmaceutical product removal by a managed aquifer recharge system: Comparison and optimization of models using ensemble learners

Pharmaceutical products (PPs) are emerging water pollutants with adverse environmental and health-related impacts, owing to their toxic, persistent, and undetectable microscopic nature. Globally, increasing scientific knowledge and advanced technologies have allowed researchers to study PP-associated problems and their removal for water reuse. Experimental modeling methods require laborious, lengthy, expensive, and environmentally hazardous lab-work to optimize the process. On the other hand, predictive machine learning (ML) models can trace the complex input-output relationship of a process using available datasets. In this study, ensemble ML techniques, including decision tree (DT), random forest (RF), and Xtreme gradient boost (XGB), were used to explore PP (diclofenac, iopromide, propranolol, and trimethoprim) removal by a managed aquifer recharge (MAR) system. The model input parameters included characteristics of reclaimed water and soil used in the columns, pH, dissolved organic carbon, operating time, nitrogen dioxide, sulfate, nitrate, electrical conductivity, manganese, and iron. The selected PP removal was the model output. Datasets were collected through a one-year experimental study of continuous MAR system operation to predict the removal of PPs. DT, RF, and XGB models were then developed for one of the selected compounds and tested for the others to check the reliability of the ML model results. The developed models were assessed using statistical performance matrices. The experimental results showed >80% removal of propranolol and trimethoprim; however, removal of diclofenac and iopromide was only ≈50% by the MAR system. The proposed DT and RF models presented higher coefficients of determination (R2 ≥ 0.92) for diclofenac, propranolol, and trimethoprim than for iopromide (R2 ≤ 0.63). In contrast, the XGB model showed better results for diclofenac, iopromide, propranolol, and trimethoprim, with R2 values of 0.92, 0.72, 0.96, and 0.97, respectively. Therefore, XGB could be the best predictive model to provide insight into the adaptation of ML models to predict PP removal by the MAR system, thereby minimizing experimental work.

Monitoring and inverse modelling of hydrogeochemical processes during managed aquifer recharge in Southwestern Bangladesh

Managed Aquifer Recharge (MAR) has been applied as Aquifer Storage, Transfer, and Recovery (ASTR) to provide fresh drinking water for local communities at 99 locations in southwest Bangladesh since 2009. Aerobic freshwater from ponds is filtered and subsequently infiltrated into anaerobic shallow brackish aquifers. At approximately 45% of these sites, relatively higher levels of Fe and As were observed in recovered water, which requires a better understanding of the hydrogeochemical processes that govern the Fe, Mn, and As levels in these MAR systems. Therefore, two representative sites with As above (74 ± 11 μg/L at site GMF11) and below (19 ± 6 μg/L at site JJS91), the Bangladesh drinking water standard of 50 μg/L were weekly monitored on hydrochemical changes from Dec 2017 to Dec 2018. Hydrogeochemical processes occurring during storage were quantified with inverse and forward geochemical mass balance models developed with PHREEQC. The following processes explained the changes in water quality: 1) mixing of infiltration water with native groundwater (∼90%:∼10%); 2) consumption of O2 by a) dissolved Fe2+ that subsequently precipitated as Fe(OH)3 at GMF11 and by b) dissolved and sedimentary organic matter (OM) at site JJS91; 3) reduction of SO4 coupled to the oxidation of OM at both sites; and 4) mixing corrosion and freshening induced cation-exchange (Ca sorption; Na desorption) triggering calcite and siderite dissolution at GMF11. Dissolution of these carbonate minerals occurred to a lesser extent at JJS91, while cation exchange (Na sorption; Ca desorption) suggested that the freshwater was displaced by brackish groundwater because of inadequate infiltration at JJS91. Distinct pH values in recovered water reflected the dominance of Fe2+ versus OM oxidation. Siderite dissolution led to 4.3 ± 3.1 and 1.0 ± 0.5 mg/L Fe in recovered water at GMF11 and JJS91, respectively. Elevated As and Mn levels in recovered water were caused for max. 20% by mixing with native groundwater and for min. 80% by mobilization processes, mainly by desorption of As from Fe-oxides and by the dissolution of Mn-bearing siderite. Recommendations are provided to improve recovered water quality.

Alleviating drought and water scarcity in the Mediterranean region through managed aquifer recharge

Drought and water scarcity can significantly impair the sustainable development of groundwater resources, a scenario commonly found in aquifers in the Mediterranean region. Water management measures to address these drivers of groundwater depletion are highly relevant, especially considering the increasing severity of droughts under climate change. This study evaluates the potential of managed aquifer recharge (MAR) to offset the adverse effects of drought and water scarcity on groundwater storage. Los Arenales aquifer (central Spain), which was unsustainably exploited for irrigation in the second half of the twentieth century, is employed as a case study. Two neighbouring zones within this aquifer are contrasted, namely, Los Arenales (LA) and Medina del Campo (MC). The primary difference between them in terms of water resources management is the wide-scale implementation of MAR systems in LA since the early 2000s. Several groundwater statistical methods are used. Groundwater-level trend analysis and average piezometric levels show in LA a faster recovery of aquifer storage and less susceptibility to drought compared to MC. On the other hand, standardised precipitation indexes and standardised groundwater level indexes of detrended groundwater-level time series, which do not include the effects of MAR, show that LA can be more negatively affected by drought and groundwater abstraction. The sharper recovery of piezometric levels in LA when considering MAR, and bigger drought impacts observed when the effects of this measure are removed, demonstrate that MAR can effectively alleviate the impacts of water scarcity and drought, providing an adaptation solution to climate change worldwide.