Multiple stable isotopes and geochemical approaches to elucidate groundwater salinity and contamination in the critical coastal zone: A case from the Bou-areg and Gareb aquifers (North-Eastern Morocco)

Integrated approach to understand the multiple natural and anthropogenic stresses on intensively irrigated coastal aquifer in the Mediterranean region

Understanding the major factors influencing groundwater chemistry and its evolution in irrigation areas is crucial for efficient irrigation management. Major ions and isotopes (δD-H2O together with δ18O-H2O) were used to identify the natural and anthropogenic factors contributing to groundwater salinization in the shallow aquifer of the Wadi Guenniche Plain (WGP) in the Mediterranean region of Tunisia. A comprehensive geochemical investigation of groundwater was conducted during both the low irrigation season (L-IR) and the high irrigation season (H-IR). The results show that the variation range and average concentrations of almost all the ions in both the L-IR and H-IR seasons are high. The groundwater in both seasons is characterized by high electrical conductivity and CaMgCl/SO4 and NaCl types. The dissolution of halite and gypsum, the precipitation of calcite and dolomite, and Na-Ca exchange are the main chemical reactions in the geochemical evolution of groundwater in the Wadi Guenniche Shallow Aquifer (WGSA). Stable isotopes of hydrogen and oxygen (δ18O-H2O and δD-H2O) indicate that groundwater in WGSA originated from local precipitation. In the H-IR season, the δ18O-H2O and δD-H2O values indicate that the groundwater experienced noticeable evaporation. The enriched isotopic signatures reveal that the WGSA's groundwater was influenced by irrigation return flow and seawater intrusion. The proportions of mixing with seawater were found to vary between 0.12% and 5.95%, and between 0.13% and 8.42% during the L-IR and H-IR seasons, respectively. Irrigation return flow and the associated evaporation increase the dissolved solids content in groundwater during the irrigation season. The long-term human activities (fertilization, irrigation, and septic waste infiltration) are the main drives of the high nitrate-N concentrations in groundwater. In coastal irrigation areas suffering from water scarcity, these results can help planners and policy makers understand the complexities of groundwater salinization to enable more sustainable management and development.

Multi-isotopes (δD, δ18Owater, 87Sr/86Sr, δ34S and δ18Osulfate) as indicators for groundwater salinization genesis and evolution of a large agricultural drainage lake basin in Inner Mongolia, Northwest China

Groundwater salinization, a major eco-environmental problem in arid and semi-arid areas, can accelerate soil salinization, reducing crop productivity and imbalances in ecosystem diversity. This study classified water samples collected from the Ulansuhai Lake basin into five clusters using self-organizing maps (SOM). On this basis, multiple isotopes (δ18Owater, δD, 87Sr/86Sr, δ18Osulfate and δ34S) and isotopic models (Rayleigh fractionation and Bayesian isotope mixing models) were used to identify and quantify the genesis and evolution of groundwater salinization. The results showed that the samples were brackish or saline water, and the hydrochemical types were dominated by Na + K-Cl (SO4). It has been proved that the processes associated with groundwater salinization in the Ulansuhai Lake basin were dominated by water-rock interaction and human inputs. Among them, evaporite dissolution contributed substantially to groundwater salinity. Furthermore, salt inputs from human activities cannot be negligible. Based on the model calculations, evaporite dissolution accounted for the most significant proportion of all sources, with a mean value of 53 %. In addition, human inputs from regular agricultural activities (28 % from sewage and manure and 8 % from fertilizers) constituted another vital source of groundwater salinization associated with extensive agricultural activities in the study area. This study's results can deepen our understanding of the genesis of groundwater salinization and the evolution of the agricultural drainage lake basin. This knowledge will assist the Environmental Protection Department in developing effective policies for groundwater management in the Yellow River Basin.

A review of organic and inorganic amendments to treat saline-sodic soils: Emphasis on waste valorization for a circular economy approach

Soil salinization poses a significant challenge to the sustainable advancement of agriculture on a global scale. This environmental issue not only hampers plant growth and soil fertility but also hinders the advancement of the national economy due to restrictions on plant development. The utilization of organic and/or inorganic amendments has demonstrated the ability to mitigate the detrimental impacts of salt stress on plant life. At the outset, this review, in addition to summarizing current knowledge about soil amendments for saline-sodic soils, also aims to identify knowledge gaps requiring further research. The organic or inorganic amendments modify soil conditions and impact plant development. For instance, organic amendments have the potential to improve the structure of the soil, augment its capacity to retain water, and stimulate microbial activity. As this occurs, salts gradually leach through the porous structure of the soil. Conversely, inorganic amendments, such as gypsum and phosphogypsum, displace sodium from soil-negative sorption sites reducing the salinity, they also increase base saturation, altogether positively impacting plant growth conditions. This review emphasizes that, under adequate rates, the combination of organic and inorganic amendment has a high potential to enhance the poor physicochemical properties of saline-sodic soils, thereby reducing their salinity. Consequently, an in-depth examination of the mineral composition, texture, and chemical composition of the soil is required to choose the most effective amendment to implement. Future research necessitates a thorough investigation of techno-economic and life cycle assessment, with active involvement from stakeholders, to enhance the decision-making process of the amendments in specific localities.

Multienvironmental tracers in coastal aquifer (Morocco): A window into groundwater mixing and risk to contamination

In many coastal areas in Morocco, groundwater (GW) constitutes an important water supply for human activities. Intensive pumping makes GW highly susceptible to contamination, affecting its quality and then human health. This work aims to assess and improve the application of environmental isotopes in exploring the connections between GW recharge and discharge, as well as to identify the direction, age, and speed of GW flow, in the coastal aquifer system of the Akermoud plain. A total of 23 boreholes and wells were sampled during two sampling campaigns in 2017 and 2018, including 11 samples from the shallow aquifer and 12 samples from the deep aquifer. A set of chemical and isotopic tracers (δ18 O, δ2 H, 3 H, δ13 C, 14 C, and 3 He) is used to track water and solute from input to output of the investigated system. Stable isotopes distinguish recharge at different altitudes for the shallow and deep aquifers. Both aquifers reveal consistently low values of 3 H (between 0.3 and 0.9 tritium units) and from 28% to 64% of modern carbon for six boreholes. According to 14 C correction models, GW has ages ranging from 3300 to 11,000 years before present. GW flows from SSE to NNW and discharge along the Atlantic coast of Akermoud plain with a velocity ranging from 0.41 to 1.8 m/year. PRACTITIONER POINTS: The use of environmental tracers helps determine the origin of salinity and identify the recharge area. Investigating the MRT of groundwater resources is essential, especially in arid regions. Evaluating the efficiency of isotopic tracing is crucial in assessing the risk of groundwater contamination. The findings provide insights for stakeholders to promote more sustainable groundwater management in coastal areas.

Nitrate sources and transformation processes in groundwater of a coastal area experiencing various environmental stressors

In coastal salinized groundwater systems, contamination from various nitrate (NO3) inputs combined with complex hydrogeochemical processes make it difficult to distinguish NO3 sources and identify potential NO3 transformtation processes. Effective field-based NO3 studies in coastal areas are needed to improve the understanding of NO3 contamination dynamics in groundwater of such complex coastal systems. This study focuses on a typical Mediterranean coastal agricultural area, located in Tunisia, experiencing substantial NO3 contamination from multiple anthropogenic sources. Here, multiple isotopic tracers (δ18OH2O, δ2HH2O, δ15NNO3, δ18ONO3, and δ11B) combined with a Bayesian isotope MixSIAR model are used (i) to identify the major NO3 sources and their contributions, and (ii) to describe the potential NO3 transformation processes. The measured NO3 concentrations in groundwater are above the natural baseline threshold, suggesting anthropogenic influence. The measured isotopic composition of NO3 indicates that manure, soil organic matter, and sewage are the potential sources of NO3, while δ11B values constrain the NO3 contamination to manure; a finding that is supported by the results of MixSIAR model revealing that manure-derived NO3 dominates over other likely sources. Nitrate derived from manure in the study area is attributed to organic fertilizers used to promote crop growth, and livestock that deposit manure directly on the ground surface. Evidence for ongoing denitrification in groundwaters of the study area is supported by an enrichment in both 15N and 18O in the remaining NO3, although isotopic mass balances between the measured and the theoretical δ18ONO3 values also suggest the occurrence of nitrification. The simultaneous occurrence of these biogeochemical processes with heterogeneous distribution across the study area reflect the complexity of interactions within the investigated coastal aquifer. The multiple isotopic tracer approach used here can identify the effect of multiple NO3 anthropogenic activities in coastal environments, which is fundamental for sustainable groundwater resources management.

Analysis of hydrochemical characteristics and assessment of organic pollutants (PAH and PCB) in El Fahs plain aquifer, northeast of Tunisia

The availability of good quality groundwater constitutes a major concern in many developing countries. The El Fahs shallow aquifer, northeastern Tunisia, is an important source of water supply for various economic sectors in the agricultural region. The intensive exploitation of this groundwater has led to its quality degradation. In fact, assessment of water quality degradation is very useful in planning the conservation and management practices of water resources in this watershed. This research aims to evaluate the groundwater quality and its suitability for irrigation uses, identify the main processes to assess their chemical composition, and investigate the potential sources of persistent organic pollutants (POPs). The hydrogeochemical investigation is thus conducted by collecting groundwater samples and analyzing their physicochemical characteristics. Polycyclic aromatic hydrocarbons (16 PAHs) and polychlorinated biphenyls (7 PCBs) were determined in groundwaters from nine stations. The sampling took place in July 2020. The relative abundance of ions was Na > Mg > Ca > K for cations and Cl > SO4 > HCO3 for anions. The groundwater exhibits two predominant hydrochemical facies: Ca-Mg-Cl/SO4 and Na-Cl. The relevant recorded pollutant is nitrate, which was generally far above values of pollution thresholds indicating the influence by the intensive agricultural activity. The suitability for irrigation purposes was assessed using several parameters (EC, SAR, %Na, TH, PI, Mh, and Kr). As a matter of fact, the results mentioned that the majority of the samples are unsuitable for irrigation uses. An analysis of the organic pollutants indicates that the total PAH and PCB concentrations are above the permissible values. Therefore, a considerable predominance of naphthalene and PCB28 was observed in order to discriminate between pyrolitic and petrogenic PAH sources; low-molecular-weight (LPAH)/high-molecular-weight (HPAH) ratio was calculated. Results showed that PAHs were mainly of petrogenic origin. The results revealed also that the chemical composition of groundwater is influenced by evaporation process, ion exchange, and water-rock interaction during the flow. A high risk of organic contamination has been highlighted linked to anthropogenic activities which have exerted increasing pressure on groundwater quality. The presence of organic pollutants in groundwater is becoming a serious threat to the environment and human health.

Identifying the factors controlling surface water and groundwater chemical characteristics and irrigation suitability in the Yarkant River Basin, northwest China

This study investigated the geochemical processes and the suitability for irrigation purposes of surface water and groundwater in the Yarkant River Basin, northwest China. The results showed that the surface water was moderately alkaline and neutral to slightly alkaline in the groundwater. The mean values for most ions in the groundwater were higher than those in the surface water. Geochemical method and hydrogen and oxygen isotope revealed that hydrochemical were mainly affected by dissolved evaporite minerals, ion exchange processes, and anthropogenic activities. Still, the geochemical processes of in surface water and groundwater were different. Additionally, three and four factors were extracted by factor analyses of surface water and groundwater, respectively, which distinguished the hydrochemical from natural origins and anthropogenic activities in more detail. The surface water and some groundwater in the south of the study area were suitable for irrigation processes. However, many groundwaters in the north were unsuitable for irrigation, because of their high sodium levels and salinity. The study results provide a theoretical basis for the sustainable utilization of regional water resources.

Groundwater quality for irrigation in an arid region-application of fuzzy logic techniques

Groundwater is the main source to answer the irrigation supply in several arid and semi-arid areas. In the present work, groundwater quality for irrigation purposes in the arid region of Menzel Habib (Tunisia) for thirty-six groundwater samples is assessed considering the application of different conventional water quality indicators, particularly, electrical conductivity (EC), sodium absorption ratio (SAR), soluble sodium percentage (SSP), magnesium adsorption ratio (MAR), Kelly ratio (KR), and permeability index (PI). The results obtained indicate a variability for EC: 3.06 to 14.98 mS.cm^-1; SAR: 4.08 to 19.30; SSP: 35.78 to 71.53%; MAR: 34.19 to 56.01; PI: 38.47 to 72.74; and KR: 0.56 to 2.47. These results suggest that groundwater from Menzel Habib aquifer system is classified between excellent to unsuitable according to the applied water quality indices. Furthermore, the groundwater samples are also plotted in the Richards diagram classification system, based on the relation between SAR and EC, suggesting that almost groundwater samples present a harmful quality. Moreover, fuzzy logic model has been proposed and created to assess groundwater quality for irrigation. The membership functions are constructed for six significant parameters such as EC, SAR, SSP, MAR, KR, and PI and the rules are, then, fired to get a simple Fuzzy Irrigation Water Quality Index (FIWQI). The obtained groundwater quality results suggest that 3% of the samples from Menzel Habib region are considered as "good" for irrigation, 3% are classified as "good to permissible", 33% with a "permissible" quality, 36% "permissible to unsuitable", while 25% of groundwater present an "unsuitable" quality. Thus, the use of fuzzy logic techniques has more reliable and robust results by overcoming the uncertainties in the decision-making attributed to the conventional methods by the creation of new classes (excellent to good, good to permissible, and permissible to unsuitable) in addition to the classes proposed by Richards diagram classification (excellent, good, permissible, and unsuitable) to assess the groundwater quality suitability for irrigation purposes.

Assessment of natural and anthropogenic contamination sources in a Mediterranean aquifer by combining hydrochemical and stable isotope techniques

The Atalanti basin is an intensively cultivated area in central Greece, facing groundwater quality deterioration threats due to natural and anthropogenic-related contamination sources. A combination of statistical and hydrogeochemical techniques, and stable isotope compositions (δ²H-H₂O and δ¹⁸Ο-Η₂Ο, δ¹⁵Ν-ΝΟ₃^- and δ¹⁸Ο-ΝΟ₃^-, δ³⁴S-SO₄^2- and δ¹⁸O-SO₄^2-) were applied to elucidate the origin of salinity and nitrate contamination, and shed light on the potential associations between geogenic Cr(VI) and NO₃^- sources and transformations. Nitrate and Cr(VI) concentrations reached up to 337 mg L^-1 and 76.1 μg L^-1, respectively, exceeding WHO threshold values in places. The cluster of samples with the high salinity was mostly influenced by irrigation return flow and marine aerosols, and less by seawater intrusion, as evidenced by the ionic ratios (e.g., Na⁺/Cl^-) and the stable isotopes of oxygen and hydrogen in water, and sulphur and oxygen in sulphates. The δ¹⁵Ν-ΝΟ₃^- and δ¹⁸O-NO₃^- values ranged from +2.0 ‰ to +14.5 ‰ and + 0.3 ‰ to +11.0 ‰, respectively. We found that the dominant sources of NO₃^- in groundwater were fertilizers in the central part of the area and sewage waste in the northern part around the residential area of Livanates. The occurrence of denitrification was evident in the northern part of the basin, where the DO levels were lowest (≤ 2.2 mg L^-1), whereas nitrification of NH₄⁺-fertilizers prevailed in the central part. Elevated Cr(VI) values (≥ 20 μg/l) were associated with the lowest deviation of the measured from the theoretical nitrification δ¹⁸Ο-NO₃^- values, whereas the lowest Cr(VI) values were observed in the denitrified water samples. Our isotope findings revealed the strong influence of redox conditions on the biogeochemical transformations of N species and the mobilization of Cr(VI) that will help improve the understanding of the fate of these contaminants from the unsaturated zone to the groundwater in areas of agricultural and urban land use.

Combined effects of seawater intrusion and nitrate contamination on groundwater in coastal agricultural areas: A case from the Plain of the El-Nil River (North-Eastern Algeria)

This study focuses on coastal aquifers subject to uncontrolled land use development by investigating the combined effects of seawater intrusion and nitrate contamination. The research is undertaken in a Mediterranean coastal agricultural area (Plain of the El-Nil River, Algeria), where water resources are heavily impacted by anthropogenic activities. A multi-tracer approach, integrating hydrogeochemical and isotopic tracers (δ²H_H2O, δ¹⁸O_H2O, δ¹⁵N_NO3 and δ¹⁸O_NO3), is combined with a hydrochemical facies evolution diagram, and a Bayesian isotope mixing model (MixSIAR) to assess seawater contamination with its inland intrusion, and distinguish the nitrate sources and their apportionment. Results show that seawater intrusion is circumscribed to the sector neighboring the Mediterranean Sea, with two influencing functions including classic inland intrusion through the aquifer, and upstream seawater impact through the river mouth connected to the Mediterranean Sea. Groundwater and surface water samples reveal nitrate concentrations above the natural baseline threshold, suggesting anthropogenic influence. Results from nitrate isotopic composition, NO₃ and Cl concentrations, and the MixSIAR model show that nitrate concentrations chiefly originate from sewage and manure sources. Nitrate derived from the sewage is related to wastewater discharge, whereas nitrate derived from the manure is attributed to an excessive use of animal manure to fertilise agricultural areas. The dual negative impact of seawater intrusion and nitrate contamination degrades water quality over a large proportion of the study area. The outcomes of this study are expected to contribute to effective and sustainable water resources management in the Mediterranean coastal area. Furthermore, this study may improve scientists' ability to predict the combined effect of various anthropogenic stressors on coastal environments and help decision-makers elsewhere to prepare suitable environmental strategies for other regions currently undergoing an early stage of water resources deterioration.

Multi-tracer approach to understand nitrate contamination and groundwater-surface water interactions in the Mediterranean coastal area of Guerbes-Senhadja, Algeria

Implementing sustainable groundwater resources management in coastal areas is challenging due to the negative impacts of anthropogenic stressors and various interactions between groundwater and surface water. This study focuses on nitrate contamination and transport via groundwater-surface water exchange in a Mediterranean coastal area (Guerbes-Senhadja region, Algeria) that is heavily affected by anthropogenic activities. A multi-tracer approach, integrating hydrogeochemical and isotopic tracers (δ²H_H2O, δ¹⁸O_H2O, ³H, δ¹⁵N_NO3 and δ¹⁸O_NO3), is combined with a Bayesian isotope mixing model (MixSIAR) to (i) elucidate the nitrate sources and their apportionments in water systems, and (ii) describe potential interactions between groundwater and surface water. Results from nitrate isotopic composition and the MixSIAR model show that nitrate concentrations mainly originate from sewage and manure sources. Nitrate derived from the sewage is attributed to urban and rural wastewater discharge, whereas nitrate derived from the manure is related to animal manure used to fertilise agricultural areas. High apportionments of nitrate-based atmospheric precipitation are identified in groundwater and surface water; a finding that is specific to this study. The multi-origin stresses combined with evidence of interactions between surface water and groundwater contribute to negatively impacting large parts of the study coastal area. The outcomes of this study are expected to contribute to sustainable management of coastal ecosystems by drawing more attention towards groundwater use and protection. Furthermore, this study may improve scientists' ability to predict the behavior of anthropogenically impacted coastal ecosystems and help decision-makers elsewhere to prepare suitable environmental strategies for other coastal ecosystems currently undergoing an early stage of groundwater resources deterioration.

Groundwater quality and vulnerability in farms from agricultural-dairy basin of the Argentine Pampas

Agricultural and livestock activities strongly influence groundwater quality and conditioning its use as water supply in rural areas. The aim was to determine the quality and suitability of the groundwater supply used in dairy farms of an agricultural area of Pampa plain of Córdoba (Argentina). Piper's diagram showed that the groundwater types were sodium bicarbonate, sodium bicarbonate-chloride, sodium chloride-sulfate, and sodium sulfate. Physicochemical parameters revealed that cations and anions showed a high and significant correlation in water samples, indicating a strong water-rock interaction. Nitrate (NO₃^-) content was significantly correlated with pH, water well depth, and distance from contamination sources. A high positive correlation between arsenic (As) and bicarbonate, sulfate, sodium, and chloride (p < 0.05) indicates a similar origin. Among the pesticides monitored, 2,4-D was detected in 25% of groundwater samples (0.4 to 0.8 µg/L) coinciding with the ordinary application practices. In general, most of the groundwater samples did not comply with national and international regulations for drinking water and dairy hygiene, due to the high content of As, NO₃^-, bacteria, and the presence of 2,4-D herbicide. However, the quality of water was suitable for livestock drink. The data obtained in this study contribute to a better understanding of the contamination processes taking place and improve the agricultural and livestock management for an efficient use of this resource by dairy farmers.

Assessment of Groundwater Quality and the Main Controls on Its Hydrochemistry in a Changing Climate in Morocco (Essaouira Basin)

Groundwater is essential for both water supply and environmental conservation, especially in semi-arid and desert areas. Managing groundwater resources requires a thorough understanding of groundwater characteristics and dynamics. The hydrogeochemical properties and evolution of groundwater in the Essaouira synclinal basin in northwest Morocco were studied in this research, with 105 water samples collected in 2009, 2017, 2018, and 2019. The Water Quality Index (WQI) and Irrigation Water Quality Index (IWQI) were developed to determine groundwater quality for consumption and irrigation purposes. The chemical evolution of groundwater is mainly dominated by evaporite, mineral carbonate dissolutions, and cation exchange. Contamination by nitrates is particularly severe in agricultural and tourist areas. The WQI of the 2019 campaign showed that 6.7% of groundwater samples are unsuitable for drinking; 76.7% are poor quality water; and 13.3% are very poor-quality water; while only 3.3% are drinkable. According to IWQI, the total study area has been split into 50% (good), 43.3% (bad), and 6.6% (unfit), respectively, and no excellent groundwater areas have been identified. Therefore, the water is suitable for agriculture but must be treated for drinking. The presence of evaporation and maritime intrusion and the contribution of recent precipitations to aquifer recharging were demonstrated by stable isotope content.