Major issues regarding the efficiency of monitoring programs for nitrate contaminated groundwater

Impacts of large-scale irrigation and climate change on groundwater quality and the hydrological cycle: A case study of the Alqueva irrigation scheme and the Gabros de Beja aquifer system

This research aims to analyze the impacts of the large-scale Alqueva Irrigation System (AIS) on the water cycle in selected sub-basins and the underlying Gabros de Beja aquifer system (GBAS) in Southern Portugal. The Alqueva reservoir and irrigation project is one of the largest strategic water reservoirs in Western Europe and the AIS's primary source. The closure of the dam in 2002 resulted in significant changes to the region's land use and agricultural practices, shifting from predominantly rainfed dry cereals to intensively irrigated olive and almond orchards. Therefore, this study used SWAT+ to simulate water flows from 1934 to 2021 and examined the evolution of groundwater quality and its correlation with irrigation, using data from about 50 wells from 2002 to 2021. Kriging spatial interpolation, Mann-Kendal and Sen's trend tests and the correlation technique were used. The findings revealed several noteworthy trends. First, there was a significant historical decrease in precipitation, which can be attributed to climate change. The simulation indicated a decrease in runoff and recharge, along with an increase in actual evapotranspiration due to irrigation. Furthermore, the hydrogeostatiscal analysis showed that the aquifer experienced significant salinization after the AIS implementation. In contrast, a preponderant decreasing trend in nitrate concentration was observed, which may be attributed to (a) a decrease in fertilizer use, (b) dilution effects and (c) alteration in nitrates pathways due to changes in crop types. Finally, the correlation analysis suggested that nitrates and chlorides were highly correlated to actual evapotranspiration and precipitation evolution, which may be associated to irrigation. In conclusion, the large-scale irrigation implementation and climate change significantly altered the water cycle of the study region. Overall, these findings addressed existing knowledge gaps and provided valuable insights that can be extrapolated to draw conclusions and generalize climate change and irrigation's effects on fluvial ecosystems.

A parsimonious approach to predict regions affected by sewer-borne contaminants in urban aquifers

Leaky urban drainage networks (UDNs) exfiltrating wastewater can contaminate aquifers. Detailed knowledge on spatiotemporal distributions of water-dissolved, sewer-borne contaminants in groundwater is essential to protect urban aquifers and to optimize monitoring systems. We evaluated the effect of UDN layouts on the spreading of sewer-borne contaminants in groundwater using a parsimonious approach. Due to the UDN's long-term leakage behavior and the existence of non-degradable sewer-borne contaminants (equivalent to a conservative and constant contaminant source), we employed a concept of horizontal line sources to mimic the UDN layout. This does not require the consideration of bio-degradation processes or temporal delay and effectively bypasses the vadose zone, thus reducing computational requirements associated with a full simulation of leakages. We used a set of synthetic leakage scenarios which were generated using fractals and are based on a real-world UDN layout. We investigated the effects of typical leakage rates, varying groundwater flow directions, and UDN's layouts on the shape of the contaminant plume, disregarding the resulted concentration. Leakage rates showed minimal effects on the total covered plume area, whereas 89% of the variance of the plume's geometry is explained by both the UDN's layout (e.g., length and level of complexity) and groundwater flow direction. We demonstrated the potential of applying this approach to identify possible locations of groundwater observation wells using a real UDN layout. This straightforward and parsimonious method can serve as an initial step to strategically identify optimal monitoring systems locations within urban aquifers, and to improve sewer asset management at city scale.

Coupling the dual isotopes of water (δ2H and δ18O) and nitrate (δ15N and δ18O): A new framework for classifying current and legacy groundwater pollution

Nitrate contamination of groundwater is a concern globally, particularly in agricultural regions where decades of fertilizer nitrogen (N) use has led to a legacy of N accumulation in soils and groundwater. Linkages between current management practices and groundwater nitrate dynamics are often confounded by the legacy effect, and other processes unrelated to management. A coupled analysis of dual stable isotopes of water (δH2O = δ2H and δ18O) and nitrate (δNO3 - = δ15N and δ18O) can be a powerful approach to identify sources and processes responsible for groundwater pollution. To assess how management practices impact groundwater nitrate, we interpreted behavior of δH2O and δNO3 -, together with nitrate concentrations, in water samples collected from long-term monitoring wells in the Southern Willamette Valley (SWV), Oregon. The source(s) of nitrate and water varied among wells, suggesting that the nitrate concentration patterns were not uniform across the shallow aquifer of the valley. Analyzing the stability versus variability of a well's corresponding δH2O and δNO3 - values over time revealed the mechanisms controlling nitrate concentrations. Wells with stable δH2O and δNO3 - values and nitrate concentrations were influenced by one water source with a long residence time and one nitrate source. Variable nitrate concentrations of other wells were attributed to dilution with an alternate water source, mixing of two nitrate sources, or variances in the release of legacy N from overlying soils. Denitrification was not an important process influencing well nitrate dynamics. Understanding the drivers of nitrate dynamics and interaction with legacy N is crucial for managing water quality improvement. This case study illustrates when and where such coupled stable isotope approaches might provide key insights to management on groundwater nitrate contamination issues.

An overview of nitrate sources and operating processes in arid and semiarid aquifer systems

Nitrate concentration in most aquifers in arid and semi-arid areas has increased in the past several decades as a result of human activities. Under the predominantly oxic conditions of these aquifers, denitrification is inhibited, allowing nitrate, a soluble and stable form of nitrogen (N), to accumulate. Because of its close association with municipal and agricultural wastes, nitrate is commonly used as an indicator of anthropogenic contamination. Aquifers affected by agricultural waste may contain salts from irrigation returns and herbicides in addition to nitrates. Preventing leakage from soil to deeper parts of the aquifer is thus a priority in the sustainable management of aquifers in arid and semiarid areas. Studies report a wide range of nitrate concentrations distributed non-uniformly within the aquifer, with roughly 40% and 20% of sampled wells exceeding 50mg/L nitrate in shallow and deep parts of the aquifer respectively. In aquifers at risk of becoming contaminated, nitrate isotopes (δ¹⁵N, δ¹⁸O, Δ¹⁷O) can be used to identify the source of nitrogen as mineral or organic fertilizer, sewage, or atmospheric deposition. A variety of mathematical models (crop, hydrological, geochemical, or a combination of them) have been successful in identifying best practices that minimize N leakage without negatively affecting crop yield. In addition, field research in crop management, e.g., conservation agriculture, has yielded promising results in determining the adequate dosage and time of application of fertilizers to reduce N losses. Examples of key dryland aquifers impacted by nitrate are discussed, and some of the most pressing challenges to achieve sustainability are presented.

Three-dimensional modeling of nitrate-N transport in vadose zone: Roles of soil heterogeneity and groundwater flux

Contamination of groundwater from nitrogen fertilizers in agricultural lands is an important environmental and water quality management issue. It is well recognized that in agriculturally intensive areas, fertilizers and pesticides may leach through the vadose zone and eventually reach groundwater. While numerical models are commonly used to simulate fate and transport of agricultural contaminants, few models have considered a controlled field work to investigate the influence of soil heterogeneity and groundwater flow on nitrate-N distribution in both root zone and deep vadose zone. In this work, a numerical model was developed to simulate nitrate-N transport and transformation beneath a center pivot-irrigated corn field on Nebraska Management System Evaluation area over a three-year period. The model was based on a realistic three-dimensional sediment lithology, as well as carefully controlled irrigation and fertilizer application plans. In parallel, a homogeneous soil domain, containing the major sediment type of the site (i.e. sandy loam), was developed to conduct the same water flow and nitrate-N leaching simulations. Simulated nitrate-N concentrations were compared with the monitored nitrate-N concentrations in 10 multi-level sampling wells over a three-year period. Although soil heterogeneity was mainly observed from top soil to 3 m below the surface, heterogeneity controlled the spatial distribution of nitrate-N concentration. Soil heterogeneity, however, has minimal impact on the total mass of nitrate-N in the domain. In the deeper saturated zone, short-term variations of nitrate-N concentration correlated with the groundwater level fluctuations.

Preparation of nitrate-selective porous magnetic resin and assessment of its performance in removing nitrate from groundwater

Nitrate-selective, porous magnetic anion-exchange resin (NS-PMAER) with enhanced affinity and higher selectivity for nitrate was synthesized, characterized and its performance in nitrate removal was investigated. The results show that NS-PMAER consists of spherical particles with an average size of 200 μm. It has mean pore diameter, total pore volume, and BET specific surface area of 21.38 nm, 0.3605 cm³/g, and 67.455 m²/g, respectively. The specific saturation magnetization of NS-PMAER was about 10.79 emu/g. Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS) results indicate that NS-PMAER has selectivity for nitrate higher than that of MIEX® resin; its coefficients of selectivity toward nitrate for nitrate and sulfate are 20.978 and 6.769, respectively, higher than those of MIEX® resin (1.256 and 4.342, respectively). Its working exchange capacity was 72.41 mg/mL. Column-exchange experiments' results suggest that it could be easily regenerated using 1.5 mol/L sodium chloride solution for a contact time of 30 min. Its recovery rate stayed at > 95% even after five rounds of recycling. Results of the pilot test indicate that NS-PMAER could effectively remove nitrate in groundwater, and ensure that nitrate concentrations of effluent to meet the guideline limit for drinking water by the World Health Organization.

Water resource management: a comparative evaluation of Brazil, Rio de Janeiro, the European Union, and Portugal

This paper presents an overview of water resource management in Brazil, in particular the state of Rio de Janeiro, and in the European Union, with an emphasis on member country Portugal. The study examines the primary laws, governing bodies and water resource plans. The paper describes the concerns and interests of the scientific community and other sectors of society with regard to water resource management. The paper also draws attention to challenges and opportunities concerning the main objective of water resource management, which is to ensure the availability of water of high quality and sustainable quantity. Additionally, it also mentions good and poor management practices. Among the concerns highlighted are integrated water resource management and water resource monitoring. The objective of this study was to contribute to water resource management processes. The primary reasons for this study are the growing scarcity of freshwater in the world, recurrent problems in managing this resource and a desire to contribute to the improvement of the current situation. The study of water management in different contexts allows for a greater understanding of the subject, thereby assisting the decision-making of managers and society in general with regard to environmental quality and ecological and human health. There is an increasing interest in efficient water resource management, which creates a demand for information on the subject. Both Brazil and the European Union are facing problems related to quantity and quality of water. Problems like scarcity of freshwater, contamination, salinization, and floods. This makes the realities of them quite close, despite the physical distance between them. In general, Brazil, Rio de Janeiro, the European Union and Portugal have similar water resource management requirements. If these regions are to supply a consistent quantity of high-quality water to present and future generations, then they need effective laws and plans, efficient managing agencies, political interest and economic resources. Investments in research and developing water resource management plans are inefficient measures if they are not implemented with special emphasis on monitoring and inspection.

Understanding the fate of sanitation-related nutrients in a shallow sandy aquifer below an urban slum area

We hypothesized that wastewater leaching from on-site sanitation systems to alluvial aquifers underlying informal settlements (or slums) may end up contributing to high nutrient loads to surface water upon groundwater exfiltration. Hence, we conducted a hydro-geochemical study in a shallow sandy aquifer in Bwaise III parish, an urban slum area in Kampala, Uganda, to assess the geochemical processes controlling the transport and fate of dissolved nutrients (NO3, NH4 and PO4) released from on-site sanitation systems to groundwater. Groundwater was collected from 26 observation wells. The samples were analyzed for major ions (Ca, Mg, Na, Mg, Fe, Mn, Cl and SO4) and nutrients (o-PO4, NO3 and NH4). Data was also collected on soil characteristics, aquifer conductivity and hydraulic heads. Geochemical modeling using PHREEQC was used to determine the level of o-PO4 control by mineral solubility and sorption. Groundwater below the slum area was anoxic and had near neutral pH values, high values of EC (average of 1619μS/cm) and high concentrations of Cl (3.2mmol/L), HCO3 (11mmol/L) and nutrients indicating the influence from wastewater leachates especially from pit latrines. Nutrients were predominantly present as NH4 (1-3mmol/L; average of 2.23mmol/L). The concentrations of NO3 and o-PO4 were, however, low: average of 0.2mmol/L and 6μmol/L respectively. We observed a contaminant plume along the direction of groundwater flow (NE-SW) characterized by decreasing values of EC and Cl, and distinct redox zones. The redox zones transited from NO3-reducing in upper flow areas to Fe-reducing in the lower flow areas. Consequently, the concentrations of NO3 decreased downgradient of the flow path due to denitrification. Ammonium leached directly into the alluvial aquifer was also partially removed because the measured concentrations were less than the potential input from pit latrines (3.2mmol/L). We attributed this removal (about 30%) to anaerobic ammonium oxidation (anammox) given that the cation exchange capacity of the aquifer was low (<6meq/100g) to effectively adsorb NH4. Phosphate transport was, on the other hand, greatly retarded and our results showed that this was due to the adsorption of P to calcite and the co-precipitation of P with calcite and rhodochrosite. Our findings suggest that shallow alluvial sandy aquifers underlying urban slum areas are an important sink of excessive nutrients leaching from on-site sanitation systems.