Origins and processes of groundwater salinization in the urban coastal aquifers of Recife (Pernambuco, Brazil): A multi-isotope approach

Combined effects of ammonia-N exposure and salinity changes on hematological and serum biochemical factors and thyroid hormones in Nile tilapia (Oreochromis niloticus)

The aim of this research was to evaluate the interaction effects of ammonia-N levels and salinity on hematological and serum biochemical parameters in Nile tilapia (Oreochromis niloticus). The fish were randomly divided into 12 treatments including the levels of salinity (0, 4, 8 and 12 ppt) and 0, 50% of LC50-96 h of ammonia-N and 30% of LC50-96 h of ammonia-N in a factorial design (4 salinity levels x 3 ammonia levels). Hemoglobin value in all treatments, except for salinity treatments, namely 2, 3, 4, showed a significant decrease than the control (0 ppt and no poisoning). Also, red blood cells in treatment ammonia-N levels were significantly less than the control. Serum protein concentration, in treatments 9 (50% of LC50-96 h of ammonia-N) and 5 and also with increasing salinity (treatments 2, 3 and 4) had a significant decrease compared to the control. There is a significant increase in serum glucose, cortisol, ammonia and urea levels in 50% and 30% of LC50-96 h of ammonia-N treatments compared to the control, meanwhile these parameters were significantly increased with increasing salinity. Serum thyroid stimulating hormone (TSH), T3 and T4 levels in acute and sub-acute ammonia-N treatments were significantly lower than the control. Moreover, with increasing salinity in 50% and 30% of LC50-96 h of ammonia-N treatments, TSH showed a decreasing pattern. According to the results, fluctuations in blood biochemical factors, increase of stress and decrease of thyroid hormones show that the salinity, ammonia, and their interaction caused adverse effects on fish health during the 96 h of testing.

Effects of global and climate change on the freshwater-seawater interface movement in a Mediterranean karst aquifer of Mallorca Island

Karst aquifers are globally prized freshwater sources, posing a significant preservation challenge. These aquifers typically exhibit dual or even triple porosities, encompassing matrix, fractures-fissures and conduits, rendering them highly responsive to variations in chemical characteristics and hydraulic head. In coastal regions, these aquifers often possess extensive subsurface conduit networks intricately linked to the rock matrix, facilitating groundwater discharge into the sea. Therefore, they display acute sensitivity to seawater intrusion, swiftly reacting to changes in precipitation and pumping regimes. This makes them exceptionally vulnerable to short-term meteorological fluctuations and long-term climate change. Their high heterogeneity leads to uneven penetration of the freshwater-seawater interface, causing rapid seawater intrusion inland over significant distances. The Mediterranean region, characterized by water deficit and water stress, faces strong impacts from climate change, featuring a warming atmospheric trend exceeding the global average, along with diminished rainfall exacerbating water scarcity. Increasing water demands for agriculture, urban development, and the growing tourism industry, because of global change, are worsening water stress. Our primary research objectives were analyzing the environmental consequences of global and climate change on seawater intrusion in Mediterranean coastal karst aquifers, with a focus on the role of the double-flow model, thus contributing to the understanding of the processes involved. To achieve this, we selected a study region on Mallorca Island in the western Mediterranean, where a karst aquifer system discharges into the sea. We employed various study methods, notably hydrochemical techniques and multi-isotopic analysis, encompassing the examination of 2H and 18O isotopes in water, 87Sr/86Sr ratio, Sr and B concentrations, and δ11B in water. A key finding is the rebound effect, wherein aquifers recontaminate due to solute molecular back-diffusion following cessation of extractions and the retreat of marine intrusion, providing insight into the impact of climate and global change on Mediterranean karst aquifers.

Modeling stochastic saline groundwater occurrence in coastal aquifers

The issue of freshwater salinization in coastal areas has grown in importance with the increase of the demand of groundwater supply and the more frequent droughts. However, the spatial patterns of salinity contamination are not easy to be understood, as well as their numerical modeling is subject to various kinds of uncertainty. This paper offers a robust, flexible, and reliable geostatistical methodology to provide a stochastic assessment of salinity distribution in alluvial coastal areas. The methodology is applied to a coastal aquifer in Campania (Italy), where 83 monitoring wells provided depth-averaged salinity data. A Monte Carlo (MC) framework was implemented to simulate depth-averaged groundwater salinity fields. Both MC stochastic fields and the mean across MC simulations enabled to the delineation of which areas are subject to high salinity. Then, a probabilistic approach was developed setting up salinity thresholds for agricultural use to delineate the areas with unsuitable groundwater for irrigation purposes. Furthermore, steady spatial patterns of saline wedge lengths were unveiled through uncertainty estimates of seawater ingression at the Volturno River mouth. The results were compared versus a calibrated numerical model with remarkable model fit (R²=0.96) and versus an analytical solution, obtaining similar wedge lengths. The results pointed out that the high groundwater salinities found inland (more than 2 km from the coastline) could be ascribed to trapped paleo-seawater rather than to actual seawater intrusion. In fact, the inland high salinities were in correspondence of thick peaty layers, which can store trapped saline waters because of their high porosity and low permeability. Furthermore, these results are consistent with the recognition of depositional environments and the position of ancient lagoon alluvial sediments, located in the same areas where are the highest (simulated) salinity fields. This robust probabilistic approach could be applied to similar alluvial coastal areas to understand spatial patterns of present salinization, to disentangle actual from paleo-seawater intrusion, and more in general to delineate zones with unsuitable salinity for irrigation purposes.

Hydrochemical evolution processes of multiple-water quality interfaces (fresh/saline water, saline water/brine) on muddy coast under pumping conditions

A thorough understanding of the evolution of coastal brine-bearing groundwater systems under the influence of human activities contributes to the sustainable use of coastal groundwater resources. Therefore, this study comprehensively investigated the processes associated with hydrochemical changes in groundwater during brine exploitation based on long-term monitoring data. The dataset comprised 102 samples (including groundwater, seawater, rain and river samples) collected from 1966 to 2021 to capture the hydrochemical variability. Significant changes in the brine water table and concentration between the pre-overexploitation period (1965-2000) and the exploitation period (2000-2021) are observed. From the relationship between stable isotope (δ¹⁸O and δ²H) values and Cl^- contents, shallow saline water (SSW) near the drawdown cone (with δ¹⁸O and δ²H values of -4.66 ‰ ~ -3.57 ‰ and - 42.1 ‰ ~ -32.8 ‰, respectively) is similar geochemically to the brine inside the drawdown cone (with values of -4.30 ‰ ~ -3.10 ‰ and - 39.0 ‰ ~ -32.3 ‰, respectively), indicating that the SSW has a recharge effect on the underground brine. The delta values of major cations were calculated to analyze the hydrochemical processes at different water quality interfaces (saline/freshwater interface, salinewater/brine interface). The results demonstrated that the SSW and deep saline water (DSW) at the offshore brine/saline water interface undergo seawater intrusion, where SSW undergoes a salinization process with cation exchange (Na⁺ is exchanged for Ca²⁺ and Mg²⁺ in clay), while DSW undergoes a freshening process. The SSW at the saline/freshwater interface is affected by freshwater pumping, and the DSW undergoes a salinization process influenced by brine intrusion presenting the common salinization process with cation exchange. The hydrochemical model shows that the brine concentration recovered during the restricted exploitation stage due to the dissolution of gypsum and halite. If no measures are taken to restrict exploitation, then brine will eventually be replaced by saline water.

Geophysical investigation for seawater intrusion in the high-quality coastal aquifers of India: a review

Around the globe, seawater intrusion in the coastal aquifer is a significant problem. Excessive groundwater extraction because of population growth, industrialization, tourism, and other anthropogenic activities and geogenic processes initiates and accelerates this problem. The contaminated groundwater impacts the health, economic activities, and social and cultural development of coastal regions. This work aims to explore the current status and a holistic comprehending review of geophysical studies applied to delineate the seawater intrusion in the high-quality coastal aquifers in India, as well as its origin and causes, mitigation strategies, and recent advancements in geophysical techniques to access the qualitative and quantitative properties of the complex aquifer system. In the future, it is recommended to do a detailed subsurface imaging of the entire coastal belt of India to decipher the lateral and vertical variation of the lithological conditions and seawater intrusion in space and time with improved/advanced geophysical techniques, which can lead toward sustainable development.

Long and short-term cation exchange linked to a negative hydraulic barrier in a coastal aquifer

Saltwater extraction in coastal aquifers generates a negative hydraulic barrier that prevents marine intrusion and produces a general freshening landward from this barrier. In the Andarax delta aquifer, SE Spain, two instances of saltwater extraction were performed and their effect on the aquifer hydrochemistry was studied. ¹⁴C groundwater dating, together with chemical analysis, reflects the presence of waters with different infiltration ages. Old marine groundwater (~10 ky) must be the remains of marine intrusion generated during the Holocene transgression at the same time the delta was formed. The freshening induced by the saltwater extraction triggers cation exchange between the aquifer substratum and groundwater. Unlike what is described in other examples of cation exchange in coastal aquifers, in the Andarax delta the freshening causes an exchange between Mg, which is released into the groundwater, and Na, which is held in the clay mineral structural unit. This process is reverted the moment the hydraulic barrier stops acting. Short saltwater pumping-stopping cycles generate fast inversions in this exchange chemical reaction. At the same time, a clear excess of Ca ion can be seen in all the groundwater samples. This excess is attributed to the release of this ion resulting from the overall marine intrusion in this area during the Holocene transgression. Contrasting what occurs with the Na-Mg exchange, the Na-Ca exchange process is more long-lasting in time.

Assessing the Influences of Land Use Change on Groundwater Hydrochemistry in an Oasis-Desert Region of Central Asia

Land use change greatly affects groundwater hydrochemical cycling and thereby food and ecosystem security in arid regions. Spatiotemporal distribution of groundwater hydrochemistry is vital to understand groundwater water-salt migration processes in the context of land use change, while it is not well known in the oasis-desert region of arid inland basins. Here, to investigate the influences of land use change on groundwater hydrochemistry and suggest sustainable management, 67 water samples were obtained in the Luntai Oasis, a typical oasis desert of Central Asia. Stable isotopes and chemical components of samples were analyzed. Piper and Gibbs plots were used to elaborate the chemical type and major mechanisms controlling water chemistry, respectively. The results showed that cultivated land area has markedly expanded in the Luntai Oasis over the last 20 years (increasing by 121.8%). Groundwater seasonal dynamics and groundwater–surface water interaction were altered dramatically by farmland expansion and groundwater exploitation. Specifically, the spatial heterogeneity and seasonal variability of groundwater hydrochemistry were significant. Compared with the desert area, the δ18O and TDS of river water and shallow groundwater in the oasis cropland exhibited lower values but greater seasonal variation. Higher TDS was observed in autumn for river water, and in spring for shallow groundwater. The chemical evolution of phreatic water was mainly controlled by the evaporation-crystallization process and rock dominance, with a chemical type of Cl-SO4-Na-Mg. Significant spatiotemporal heterogeneity of groundwater hydrochemistry demonstrated the influence of climatic, hydrogeological, land use, and anthropogenic conditions. Groundwater overexploitation would cause phreatic water leakage into confined water, promoting groundwater quality deterioration due to fresh saltwater mixing. Improving agricultural drainage ditches in conjunction with restricting farmland expansion and groundwater extraction is an effective way to alleviate groundwater environment deterioration and maintain oasis-desert ecosystems in arid regions.

Review of drivers and threats to coastal groundwater quality in China

With rapid socio-economic development, China's coastal areas are among the fastest growing and most economically dynamic regions in the world. Under the influence of climate change and human activities, protecting the quality of coastal groundwater has emerged as one of the key environmental and resource management issues for these areas. This paper reviews (for the first time) groundwater quality data for the coastal basins of China, where over 600 million people live, focussing on key inorganic indicators/pollutants; groundwater salinity, nitrate, fluoride, and arsenic. These pollutants present major water quality issues and are also valuable as indicators of wider processes and influences impacting coastal groundwater quality - e.g. saltwater intrusion, agricultural pollution and release of geo-genic contaminants. We discuss the major drivers causing water quality problems in different regions and assess future trajectories and challenges for controlling changes in coastal groundwater quality in China. Multiple processes, including modern and palaeo seawater/brine migration, groundwater pumping for agricultural irrigation, pollution from agrochemical application, rapid development of aquaculture, urban growth, and water transfer projects, may all be responsible (to different degrees) for changes observed in coastal groundwater quality, and associated long-term health and ecological effects. We discuss implications for sustainable coastal aquifer management in China, arguing that groundwater monitoring and contamination control measures require urgent improvement. The evolution and treatment of coastal groundwater quality problems in China will serve as an important warning and example for other countries facing similar pressures, due to climate change, coastal development, and intensification of anthropogenic activity in coming decades.

Delineating groundwater contamination risks in southern coastal metropoles through implementation of geochemical and socio-environmental data in decision-tree and geographical information system

Due to global warming and local anthropogenic pressures, sustainable groundwater resource exploitation in coastal cities is increasingly threatened. For example, the fifth largest Brazilian city, Recife, is considered as a representative hot spot for these issues and illustrates the great challenges facing many urban areas in the southern hemisphere. There, recharge as well as surface water and groundwater quality are altered by frequent droughts and poorly planned environmental management since decades. To maintain access to water, thousands of private wells were dug in order to pump water from the multi-layered aquifer system found under the city. This massive exploitation is causing a chronic lowering of the water levels, as well as seawater intrusion and contaminations by wastewater or polluted surface waters. Through hydrochemical characterization, mainly Cl/Br ratio and Cl concentrations, of wells sampled throughout the metropole, this study first characterizes the main environmental impacts on the resource, i.e. waste waters and seawater. Combining this evaluation with lithological, land-use and socio-environmental data, it was then possible to build decision trees identifying combinations of multiple factors possibly having an impact on contamination types. The well and population densities, the waste and sewage management, as well as the absence of sanitary facilities in houses appeared as critical parameters to target in order to reduce the risk of contamination of the water resource and ensure its preservation. Based on these factors, we created a risk map for contamination types that should help in identifying areas where groundwater resource may present an environmental (and then health) issue for people. Besides, this study shows that the combination of hydrochemical, geomorphological and socio-environmental characterizations of these urban systems featuring very contrasted situations between neighborhoods is a relevant tool to propose further groundwater management strategies.

Seawater intrusion: an appraisal of taxa at most risk and safe salinity levels

Seawater intrusion into low-lying coastal ecosystems carries environmental risks. Salinity levels at these coastal ecosystems may vary substantially, causing ecological effects from mortality to several sublethal endpoints, such as depression of rates of feeding, somatic growth, or reproduction. This review attempts to establish safe salinity levels for both terrestrial and freshwater temperate ecosystems by integrating data available in the literature. We have four specific objectives: (i) to identify the most sensitive ecological taxa to seawater intrusion; (ii) to establish maximum acceptable concentrations-environmental quality standards (MAC-EQSs) for sea water (SW) from species sensitivity distributions (SSDs); (iii) to compile from the literature examples of saline intrusion [to be used as predicted environmental concentrations (PECs)] and to compute risk quotients for the temperate zone; and (iv) to assess whether sodium chloride (NaCl) is an appropriate surrogate for SW in ecological risk assessments by comparing SSD-derived values for NaCl and SW and by comparing these with field data. Zooplankton, early life stages of amphibians and freshwater mussels were the most sensitive ecological receptors for the freshwater compartment, while soil invertebrates were the most sensitive ecological receptors for the terrestrial compartment. Hazard concentration 5% (HC₅ ) values, defined as the concentration (herein measured as conductivity) that affects (causes lethal or sublethal effects) 5% of the species in a distribution, computed for SW were over 22 and 40 times lower than the conductivity of natural SW (≈ 52 mS/cm) for the freshwater and soil compartment, respectively. This sensitivity of both compartments means that small increments in salinity levels or small SW intrusions might represent severe risks for low-lying coastal ecosystems. Furthermore, the proximity between HC₅ values for the soil and freshwater compartments suggests that salinized soils might represent an additional risk for nearby freshwater systems. This sensitivity was corroborated by the derivation of risk quotients using real saline intrusion examples (PECs) collected from the literature: risk was >1 in 34 out of 37 examples. By contrast, comparisons of HC₅ values obtained from SSDs in field surveys or mesocosm studies suggest that natural communities are more resilient to salinization than expected. Finally, NaCl was found to be slightly more toxic than SW, at both lethal and sublethal levels, and, thus, is suggested to be an acceptable surrogate for use in risk assessment.

The alien octocoral Carijoa riisei is a biogenic substrate multiplier in artificial Brazilian shipwrecks

Despite the obvious negative effects caused by invasive species, some recent studies have shown that the impacts at local scale are diverse and not necessarily negative. Arborescent benthic organisms such as octocorals form three-dimensional structures capable of increasing the amount of substrate available and providing shelter for epibiont species. We investigated the role of the alien octocoral Carijoa riisei on the diversity of benthic communities in three shipwrecks on the north-eastern coast of Brazil. We expected that (a) the fauna associated with the octocoral are richer and more diverse compared to the adjacent; (b) some species are exclusively associated with C. riisei; (c) the species that are present both in the areas with and without C. riisei have a greater abundance when associated with the octocoral. For this, we compared the macrobenthic communities associated with C. riisei to those found in adjacent areas where the octocoral was absent. Our study showed that the communities associated with the octocoral were 1.5 times richer and 10 times more abundant than adjacent communities, with 29 exclusive taxa. The dominant taxa were the amphipods Ericthonius brasiliensis and Podocerus brasiliensis and polychaetes of the family Syllidae. These taxa were present in areas with presence and absence of C. riisei, but their abundance was significantly greater where the octocoral was present. Our results reinforce the idea that Carijoa riisei acts as an ecosystem engineer in coastal reefs, creating new habitats and increasing diversity at a local scale, even though it is an alien species.

Evaluating the genesis and dominant processes of groundwater salinization by using hydrochemistry and multiple isotopes in a mining city

The increasing salinization of groundwater renders it challenging to maintain the water quality. Moreover, knowledge regarding the characteristics and mechanism of groundwater salinization in mining areas remains limited. This study represents the first attempt of combining the hydrochemical, isotope (δD, δ¹⁸O, δ³⁷Cl, and ⁸⁷Sr/⁸⁶Sr) and multivariate statistical analysis methods to explore the origin, control, and influence of fluoride enrichment in mining cities. The TDS content of groundwater ranged from 275.9 mg/L to 2452.0 mg/L, and 54% of the groundwater samples were classified as class IV water according to China's groundwater quality standards (GB/T 14848-2017), indicating a decline in the water quality of the study area. The results of the groundwater ion ratio and isotope discrimination analysis showed that dissolution and evaporation involving water-rock interactions and halite were the main driving processes for groundwater salinization in the study area. In addition to the hydrogeological and climatic conditions, mine drainage inputs exacerbated the increasing salinity of the groundwater in local areas. The mineral dissolution, cation exchange, and evaporation promoted the F^- enrichment, while excessive evaporation and salinity inhibited the F^- enrichment. Gangue accumulation and infiltration likely led to considerable F^- enrichment in individual groundwater regions. Extensive changes in the groundwater salinity indicated differences in the geochemical processes that controlled the groundwater salinization. Given the particularity of the study area, the enrichment of salinization and fluoride triggered by mining activities cannot be ignored.

Integrated hydrochemical and ERT approach for seawater intrusion study in a coastal aquifer: a case study from Jafrabad Town, Gujarat State, India

Hydrochemical and geophysical approach has been adopted to evaluate the seawater intrusion (SWI) in coastal aquifers of Jafrabad Town, Gujarat State, India. Electrical Resistivity Tomography (no. 9) was carried out with spread length of 160-400 m which provided penetration depth of about 23 to 76 m. Very low resistivity zone (0-3 Ω-m range) has been observed in the ERT profiles conducted in the study area. Parameters, namely, TDS, Na, and Cl, have been considered to examine the signature of SWI. The results obtained from ionic ratios, Piper plot, and Chaddha's diagram also confirm the influence of saline water within aquifer. The very low resistivity signature is correlated with the high TDS values in the nearby wells. SWI has been observed up to 9 km from the coast, and it is observed at a depth of 20-22 m in the existing limestone mines near the coast.

Variations of Groundwater Dynamics in Alluvial Aquifers with Reclaimed Water Restoring the Overlying River, Beijing, China

Some of the rivers in northern China are dried, and reclaimed water (RW) is used to restore these degraded river ecosystems, during which the RW could recharge the aquifer by river bank infiltration. From 2007 to 2018, 2.78 × 108 m3 of RW has been replenished to the dried Chaobai River (Shunyi reach), Beijing, China, which is located on the edge of one depression cone in groundwater caused by groundwater over-pumping. The groundwater hydrodynamic variations and the flow path of the RW were identified by eight-year hydrological, hydrochemical, and stable isotopic data, together with multivariate statistical analysis. The RW infiltration drastically impacts the groundwater dynamics with a spatiotemporal variation. The 30-m depth groundwater levels at Perennial intake reach increased quickly around 3 m after 2007, which indicated that they were dominated by RW infiltration. Other 30-m depth groundwater levels were controlled by precipitation recharge from 2007 to 2011, showing significant seasonal variations. In 2012, with more RW transferred to the river, the hydrodynamic impact of the RW on 30-m depth aquifer expanded downstream. However, the 50-m and 80-m depth groundwater levels showed decreasing trend with seasonal variations, due to groundwater pumping. The 30-m depth aquifer was mainly recharged by RW, being evidenced by the enriched δ2H and δ18O. The depleted δ2H and δ18O of the 50-m and 80-m depth groundwater indicated that they were dominated by regional groundwater with meteoric origin. The heterogenous properties of the multi-layer alluvial aquifer offer the preferential flow path for RW transport in the aquifers. The proportion of the RW in the aquifers decreases with depth that was calculated by the chloride conservative mixing model. The increased lateral hydraulic gradient (0.43%) contributes to the RW transport in the 30-m depth aquifer. RW usage changed 30-m depth groundwater type from Ca·Mg-HCO3 to Na·Ca·Mg-HCO3·Cl. RW preferentially recharged the 50-m and 80-m depth aquifers by vertical leakage.

Combining multi-isotopic and molecular source tracking methods to identify nitrate pollution sources in surface and groundwater

Nitrate (NO₃^-) pollution adversely impacts surface and groundwater quality. In recent decades, many countries have implemented measures to control and reduce anthropogenic nitrate pollution in water resources. However, to effectively implement mitigation measures at the origin of pollution,the source of nitrate must first be identified. The stable nitrogen and oxygen isotopes of NO₃^- (ẟ¹⁵N and ẟ¹⁸O) have been widely used to identify NO₃^- sources in water, and their combination with other stable isotopes such as boron (ẟ¹¹B) has further improved nitrate source identification. However, the use of these datasets has been limited due to their overlapping isotopic ranges, mixing between sources, and/or isotopic fractionation related to physicochemical processes. To overcome these limitations, we combined a multi-isotopic analysis with fecal indicator bacteria (FIB) and microbial source tracking (MST) techniques to improve nitrate origin identification. We applied this novel approach on 149 groundwater and 39 surface water samples distributed across Catalonia (NE Spain). A further 18 wastewater treatment plant (WWTP) effluents were also isotopically and biologically characterized. The groundwater and surface water results confirm that isotopes and MST analyses were complementary and provided more reliable information on the source of nitrate contamination. The isotope and MST data agreed or partially agreed in most of the samples evaluated (79 %). This approach was especially useful for nitrate pollution tracing in surface water but was also effective in groundwater samples influenced by organic nitrate pollution. Furthermore, the findings from the WWTP effluents suggest that the use of literature values to define the isotopic ranges of anthropogenic sources can constrain interpretations. We therefore recommend that local sources be isotopically characterized for accurate interpretations. For instance, the detection of MST inferred animal influence in some WWTP effluents, but the ẟ¹¹B values were higher than those reported in the literature for wastewater. The results of this study have been used by local water authorities to review uncertain cases and identify new vulnerable zones in Catalonia according to the European Nitrate Directive (91/676/CEE).

A review of the distribution, sources, genesis, and environmental concerns of salinity in groundwater

Awareness concerning the degradation of groundwater quality and their exacerbating adverse effects due to salinization processes is gaining traction, raising for adequate understanding of the distribution, sources, genesis, and environmental concerns of salinity in groundwater. Saline groundwater is widely distributed all over the world, with an area of 24 million km² (16% of the total land area on earth) and 1.1 billion people living in the affected areas, especially the arid/semi-arid areas in developing countries. These large-scale groundwater salinization problems are sourced from two major ways: natural and anthropogenic. The natural sources are diversified from connate saline groundwater, seawater intrusion, evaporation, dissolution of soluble salts, membrane filtration process to geothermal origin. The anthropogenic sources include irrigation return flow, road deicing salts, industrial and agricultural wastewater, and gas and oil production activities. The integrated approach of geochemical tracers and multiple isotopes (δ¹⁸O_H2O, δ²H_H2O, δ¹¹B, δ³⁶Cl, δ³⁴S_sulfate, ⁸⁷Sr/⁸⁶Sr, and δ⁷Li) is proved to be useful in the constraints of the origin and transport of solutes in groundwater. Groundwater salinization is often associated with high levels of some toxic elements like arsenic, fluoride, selenium, and boron. Four "triggers" lead to this association: salt effect, competing adsorption, microbial processes, and cation exchange.

Evolution of groundwater chemistry in coastal aquifers of the Jiangsu, east China: Insights from a multi-isotope (δ2H, δ18O, 87Sr/86Sr, and δ11B) approach

Groundwater salinization is currently a very serious and challenging issue in many parts of the world. With an increasing demographic pressure and remarkable changes of water and land uses over the last decades, the multilayer coastal aquifer system of Jiangsu province, east China, was affected by increasing salinization. In this study, we investigate the groundwater salinization process and the salinity sources of the aquifer system in Nantong area (southern part of the Jiangsu coastal plain) using a multi-isotope (δ²H, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ¹¹B) approach. The results show that the TDS (total dissolved solids) values in most deep groundwater samples are generally lower than those of the shallow groundwater samples. The TDS of both shallow and deep groundwater increase from western Nantong (inland) to the eastern coastal region of the Yellow Sea. The chemical types transform from Ca-Mg-HCO₃ or Mg-Ca-HCO₃ to NaCl. The stable hydrogen and oxygen isotopes signatures of the groundwater samples indicate that local precipitation likely acts as the main recharge source of both the shallow and deep confined groundwater systems. The deep groundwater shows more depleted isotopes, suggesting recharging by the precipitation under a cold climate before the Holocene period. The shallow groundwater features heavier water isotopes, indicating recharging source from recent precipitation under a warm climate. The variations in δ¹¹B and ⁸⁷Sr/⁸⁶Sr of groundwater samples can be explained by the changes of solute sources. In the inland region (western Nantong), shallow groundwater with higher TDS is mainly caused by evaporation-induced concentration, whereas in coastal areas, seawater intrusion exerts a major influence on the chemical composition of the shallow groundwater. Our results show that that seawater intrusion mainly occurs in eastern and southeastern Nantong area. We also find that hydraulic connection between shallow and deep groundwater is strengthened by continuous overexploitation, and deep groundwater is mixed with shallow groundwater at some points. The mixing between upper saline water and deep freshwater, together with water-rock interactions, likely explain the observed low salinity in deep groundwater in coastal areas. Overall, with growing observations of salty seawater intrusion in the estuary region of the Yangtze River, future efforts are needed to prevent further seawater intrusion as sea level rises and groundwater table declines. In this context, our findings provide key information for groundwater management in other coastal aquifers, east China.

Impact of human activities on coastal groundwater pollution in the Yang-Dai River plain, northern China

Overexploitation of groundwater has resulted in seawater intrusion in many semiarid and arid coastal areas. This study illustrates the origin of groundwater salinity and assesses seawater intrusion/extrusion process in the Yang-Dai River plain aquifer, by analyzing hydrochemical and stable isotopic compositions of surface water, groundwater, geothermal water, and seawater. A cone of depression in groundwater is caused by intensive groundwater pumping formed in the late 1980s in the alluvial Yang-Dai River plain. In the northern part, groundwater exploitation has caused seawater intrusion identified by Ca-Cl type water. However, the widely distributed silty clay prevented the seawater intrusion in the southern part, evidenced by Ca-HCO₃ type water with depleted δ²H (-60 to -46‰) and δ¹⁸O (-8.9 to -4.7‰). Anthropogenic pollution also plays a significant role in groundwater salinization. The positive correlation between Cl and NO₃^- for most groundwater and the extremely high nitrate concentrations (up to 652.7 mg/L) indicate that fertilizer from agricultural activities has greatly influenced groundwater quality. Irrigation return flow evaporation during agricultural activities also accounts for groundwater salinity. Besides the intensive fertilizer usage, seawater intrusion and the established anti-tide dams reduced the surface water and groundwater discharge to the sea and then resulted in the extremely high nitrate concentration. This study may improve the understanding of the groundwater salinization processes in a complex coastal aquifer, which is greatly influenced by anthropogenic activities.

Isotope and chemical assessment of natural water in the Jaffna Peninsula in northern Sri Lanka for groundwater development aspects

The demand for fresh groundwater in the Jaffna peninsula in northern Sri Lanka, has been increasing as the only water source for domestic and agricultural needs. Isotopic and hydro-chemical assessment on groundwater recharge and quality was done with the objective of assisting groundwater management plans. Water samples were collected in pre and post monsoon periods for isotope analyses (²H, ¹⁸O and ³H) and major ions. Isotope data provide evidence that the groundwater is predominantly recharged by inter-monsoon (convectional) and North-East monsoon rains. Different residence times of shallow groundwater were found. The study revealed that the groundwater quality is mainly deteriorated due to dissolution of geogenic elements. However, in some instances, seawater intrusion and evaporation have caused lowering of groundwater quality. More saline groundwater is found in deeper levels in the karstic limestone aquifer system. Fresh water is available in coastal sandy aquifers in the eastern part of the peninsula. The impact of rainfall variations and agricultural activities are contributory factors for producing variations in the quality of groundwater. The current study suggests artificial groundwater recharge for the development and utilization of groundwater resources through small scale tanks/reservoirs or ponds in the areas where direct rain recharge occurs.

Hydrogeochemical characteristics of a multi-layered coastal aquifer system in the Mekong Delta, Vietnam

Groundwater is a primary freshwater source for various domestic, industrial and agricultural purposes, especially in coastal regions where there are lacking surface water supply. However, groundwater quality in coastal regions is often threatened by seawater intrusion and contamination due to both anthropogenic activities and natural processes. Therefore, insights into groundwater geochemistry and occurrences are necessary for sustainable groundwater management in coastal regions. The main aim of this study is to investigate the hydrogeochemical characteristics and their influencing factors in a coastal area of the Mekong Delta, Vietnam (MD). A total of 286 groundwater samples were taken from shallow and deep aquifers for analyzing major ions and stable isotopes. The results show that deep groundwater is dominated by Ca-HCO[Formula: see text], Ca-Na-HCO[Formula: see text], Ca-Mg-Cl, and Na-HCO[Formula: see text] while shallow groundwater is dominated by the Na-Cl water type. In this region, the main geochemical processes controlling groundwater chemistry are ion exchanges, mineralization and evaporation. Groundwater salinization in coastal aquifers of the Mekong Delta is caused by (1) paleo-seawater intrusion and evaporation occurring in the Holocene and Pleistocene aquifers, (2) dissolution of salt sediment/rock and leakage of saline from upper to lower aquifers due to excessive groundwater exploitation and hydraulic connection. High nitrate concentrations in both shallow and deep aquifers are related to human activities. These results imply that groundwater extraction may exacerbate groundwater quality-related problems and suitable solutions for sustainable groundwater management in the coastal area of the Mekong Delta are needed.

Salinization of reservoirs in regions with exposed evaporites. The unique case of Upper Gotvand Dam, Iran

Construction of the great Upper Gotvand Dam in the Karun River, Iran, with an impoundment capacity of 4.5 billion cubic meters, has resulted in an environmental disaster: accumulation of 66.5 million metric tonnes of dissolved salt in the reservoir and a dramatic increase in the salinity of the reservoir water up to 200 g/L. This paper aims to identify and assess the main sources of the salinization of the reservoir water integrating multiple data: (1) geological and geomorphological evidence; (2) continuous vertical records of electrical conductivity at 11 stations along the reservoir; (3) total dissolved solids and major ion concentrations of 108 water samples at 58 sampling stations; (4) δ¹⁸O, δ²H in 35 water samples and δ³⁷Cl in 16 water samples from the reservoir and surrounding rivers and springs. Geological and geomorphological evidence, hydrogeochemical data and isotopic signature reveal that halite dissolution is the main cause of salinization in the Gotvand Reservoir. The results show that salt dissolution in the Gachsaran Formation (especially at Anbar Ridge), which was under-estimated in the study phase of the project, has provided about 86 per cent of the dissolved salt accumulated in the reservoir over a 2-year period since the impoundment of the reservoir in 2011.

Origin and formation mechanism of salty water in Zuli River catchment of the Yellow River

The Zuli River is one of the branches of the upper Yellow River, as an inland catchment with semiarid climate in northwestern China, and the formation, evolution, and development of brackish water at such a large scale have remained unclear. This study aims to find clues about the origin and formation mechanism of salty water through multiple methods of hydrochemistry and isotope hydrology. The results show that groundwater is dominantly recharged by precipitation, and the river water was mainly recharged by groundwater discharge. The relatively high tritium content of groundwater (>5.0 TU) clearly suggests the occurrence of a modern recharge and rapid circulation. The dissolution of evaporate minerals, followed by incongruent dissolution of carbonate minerals (dolomite), constituted the main processes controlling groundwater salinization. In addition, the intense evaporation and unreasonable use of fertilizers further increase the TDS of the river, which should be the primary external mechanism of water salinization. PRACTITIONER POINTS: The authors aimed to find clues about the formation mechanism of salty water in an inland catchment of the Yellow River. The results of this research shows that the dissolution of dissolved minerals constituted the main processes controlling groundwater salinization. In addition, the intense evaporation and unreasonable use of fertilizers, which should be the primary external mechanism of water salinization. This work would provide a theoretical basis for government to develop rational utilization of brackish water resources in the study area, which is also significant for understanding the mechanism of water salinization in an inland mountain watershed and even in similar inland watersheds around the world.

Lithium and its isotopes as tracers of groundwater salinization: A study in the southern coastal plain of Laizhou Bay, China

In the southern coastal plain of Laizhou Bay, due to intensive exploitation of groundwater since the early 1970s, the shallow aquifer has been severely influenced by saltwater intrusion, which causes the extraction to shift from shallow to deeper aquifer changing the hydrogeological condition greatly. This study was conducted to investigate the groundwater salinization using hydrochemistry and H, O and Li isotope data. Dissolved Li shows a linear correlation with Cl and Br in seawater, brine and saline groundwater indicating the marine Li source, whereas the enrichment of Li in surface water, brackish and fresh groundwater is impacted by dissolution of silicate minerals. The analyses of hydrochemistry and isotopes (H, O and Li) indicate that brine originated from seawater evaporation, followed by mixing processes and some water-rock interactions; shallow saline groundwater originated from brine diluted with seawater and fresh groundwater; deep saline groundwater originated from seawater intrusion. The negative correlation of δ⁷Li and Li/Na in surface water, brackish and fresh groundwater is contrary to the general conclusion, indicating the slow weathering of silicate minerals and hydraulic interaction between surface water and shallow groundwater in this area. The analyses of hydrochemistry and isotopes (Li, H and O) can well identify the salinity sources and isotope fractionation in groundwater flow and mixing, especially groundwater with high TDS. As both mixing with saltwater and isotope fractionation can explain the combination of high δ⁷Li and low TDS in brackish groundwater, isotope fractionation may limit their use in recognizing salinity sources of groundwater with low TDS.

Regulations are needed to protect freshwater ecosystems from salinization

Anthropogenic activities such as mining, agriculture and industrial wastes have increased the rate of salinization of freshwater ecosystems around the world. Despite the known and probable consequences of freshwater salinization, few consequential regulatory standards and management procedures exist. Current regulations are generally inadequate because they are regionally inconsistent, lack legal consequences and have few ion-specific standards. The lack of ion-specific standards is problematic, because each anthropogenic source of freshwater salinization is associated with a distinct set of ions that can present unique social and economic costs. Additionally, the environmental and toxicological consequences of freshwater salinization are often dependent on the occurrence, concentration and ratios of specific ions. Therefore, to protect fresh waters from continued salinization, discrete, ion-specific management and regulatory strategies should be considered for each source of freshwater salinization, using data from standardized, ion-specific monitoring practices. To develop comprehensive monitoring, regulatory, and management guidelines, we recommend the use of co-adaptive, multi-stakeholder approaches that balance environmental, social, and economic costs and benefits associated with freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Are boron isotopes a reliable tracer of anthropogenic inputs to rivers over time?

This study aims at determining how the boron signal of the Seine River evolved in terms of concentration and isotopic signatures over eighteen years (1994-95 and 2006-12) and if boron isotopes can reliably trace anthropogenic inputs over time. In the anthropised Seine River watershed, boron is widely released by human activities, and even if boron concentrations ([B]) are below the potability limit, our study confirms the potential of boron isotopes (δ¹¹B) to trace urban anthropogenic contaminations. Between 1994 and 2012, [B] have decreased across the anthropised part of the Seine River basin (and by a factor of two in Paris) while δ¹¹B has increased. This means either that urban inputs have been reduced or that the boron signature of urban inputs has changed over time. Both hypotheses are in agreement with the decrease of perborate consumption in Europe over 15years and are not mutually exclusive. Results of a thorough analysis of urban effluents from the sewage network of Paris conurbation that are in fine released to the Seine River suggest a shift of the urban δ¹¹B from -10‰ in 1994 to 1.5±2.0‰ in 2012, in agreement with our second hypothesis. We attribute this change to the removal of perborates from detergents rather than to the modernisation of wastewater treatment network, because it does not significantly impact the wastewater boron signatures. Eighteen years after the first assessment and despite the decreased use of perborates, geochemical and isotopic mass budgets confirm, that boron in the Seine River basin is mainly released from urban activities (60-100%), especially in Paris and the downstream part of the basin. Contrastingly, in headwaters and/or tributaries with low urbanisation, the relative boron input to river from agricultural practices and rains increased, up to 10% and by 10 to 30%, respectively.

Massive arrival of desalinated seawater in a regional urban water cycle: A multi-isotope study (B, S, O, H)

"Man-made" or unconventional freshwater, like desalinated seawater or reclaimed effluents, is increasingly introduced into regional water cycles in arid or semi-arid countries. We show that the breakthrough of reverse osmosis-derived freshwater in the largely engineered water cycle of the greater Tel Aviv region (Dan Region) has profoundly changed previous isotope fingerprints. This new component can be traced throughout the system, from the drinking water supply, through sewage, treated effluents, and artificially recharged groundwater at the largest Soil-Aquifer Treatment system in the Middle East (Shafdan) collecting all the Dan region sewage. The arrival of the new water type (desalinated seawater) in 2007 and its predominance since 2010 constitutes an unplanned, large-scale, long-term tracer test and the monitoring of the breakthrough of desalination-specific fingerprints in the aquifer system of Shafdan allowed to get new insights on the water and solute flow and behavior in engineered groundwater systems. Our approach provides an investigation tool for the urban water cycle, allowing estimating the contribution of diverse freshwater sources, and an environmental tracing method for better constraining the long-term behavior and confinement of aquifer systems with managed recharge.

Multi-layered water resources, management, and uses under the impacts of global changes in a southern coastal metropolis: When will it be already too late? Crossed analysis in Recife, NE Brazil

Coastal water resources are a worldwide key socio-environmental issue considering the increasing concentration of population in these areas. Here, we propose an integrative transdisciplinary approach of water resource, water management and water access in Recife (NE Brazil). The present-day water situation is conceptualized as an imbricated multi-layered system: a multi-layered water resource, managed by a multi-layered governance system and used by a multi-layered social population. This allows identifying processes of quantitative, qualitative, and sanitary conflicts between governance and population strategies regarding water supply, as well as the institutional and individual denials of these conflicts. Based on this model, we anticipate future water-related problematic fates. Concerning the water resource system, the rapid groundwater level decrease due to unsustainable water predatory strategies, and the very low recharge rate have drastically modified the aquifer system functioning, inducing hydraulic connection between shallow groundwater (contaminated and locally salty) and deep ones (mostly fresh, with local inherited salinity), threatening the deep strategic water resource. Concerning the water governance system, the investments to increase the capacity storage of surface water, the water regulation agencies and the public/private partnership should shortly improve the water supply and wastewater issue. Nevertheless, the water situation will remain highly fragile due to the expected water demand increase, the precipitation decrease and the sea-level increase. Concerning the water access system, the population variably perceives these current and further effects and the possible mitigation policies, and develops alternative individual strategies. Authorities, policymakers and water managers will have to implement a well-balanced water governance, taking into account the specificities of the PPP, public and private groundwater users, and with a strong political willingness for a sustainable water management to ensure water supply for all the population. In other words, an anticipatory and integrated vision is necessary to reduce the discrepancies in this complex system.

Evolution of groundwater chemistry in coastal aquifers of the south-eastern White Sea area (NW Russia) using 14C and 234U-238U dating

The specific objectives of the study are to clarify the sources and characteristics of groundwater in the aquifers along the coast of the White Sea in northwestern Russia, and on this basis to perform a broad ¹⁴C and ²³⁴U/²³⁸U dating of all their types, taking into account the mixing processes. Investigation of an evolution of the groundwater chemistry revealed that the main evolutionary trends are the following: (1) Mixing Late Pleistocene brackish water end member (brackish1) and Mikulino seawater end member with strongly brackish and salty water in the Vpd aquifer (salty Vpd) formation. Groundwater dating showed the "brackish1" residence time in the aquifer of 32.96±2.3ka. Recharge of "brackish1" could have occurred in MIS 3. (2) Mixing Late Pleistocene freshwater end member (fresh LP) and "salty Vpd" end member with brackish water (brackish2) formation. Groundwater dating showed the "brackish2" residence time in the aquifer from 25.1±0.7 to 39.2±6.3ka. Recharge of "fresh LP" could have occurred ~ in MIS 3 also. (3) Mixing Middle Pleistocene-Holocene freshwater of melting glaciers (fresh MP-H) end member and brine end member with the strongly brackish and salty water in Vmz aquifer (salty Vmz) formation. Recharge of "fresh MP-H" could have occurred in Middle Pleistocene-Holocene during MIS 12-MIS 1. As a result of intensive and rapid recharge after the glacial melting, glacial fluids have penetrated at depth to >200m. The results of this study provide a better understanding of the interrelationship of various groundwater flows near the coasts and contribute to a more justified and efficient use of them for drinking water supply in large cities, balneological treatment and industrial extraction of iodine waters. They also allow assessment of the risks of dumping saline drainage water into the environment.

Piezometric level and electrical conductivity spatiotemporal monitoring as an instrument to design further managed aquifer recharge strategies in a complex estuarial system under anthropogenic pressure

Recife Metropolitan Region (RMR, NE Brazil) lies over a multi-layered aquifer system located in an estuarial area. The region has experienced fast population growth and repeated droughts in the last three decades, which led to unprecedented anthropogenic pressure on groundwater resources because of intense water pumping. Accordingly, scientific and stakeholder communities have been challenged to ensure the maintenance of sustainable groundwater resource by managing all water cycle. Because controlling pumping rates is difficult due to the large number of illegal wells, the Managed Aquifer Recharge (MAR) strategies are now under consideration. The RMR presents a tropical climate and an annual average rainfall rate of approximately 2450 mm year^-1, providing great potential volumes of water to be used for piezometric level recovery. However, MAR implementation requires a detailed and in-depth knowledge of the human-impact on the hydrogeological behavior of the resource over the long-term, in order to find out the most appropriate recharge strategy. Therefore, the present study illustrates how routine data monitoring, i.e., piezometric level and electrical conductivity (EC), in combination with the geological knowledge, may allow proposing further MAR strategies. Two contrasted behaviors were observed in RMR: (i) groundwater level decrease and stable EC in the North and Southernmost areas of Recife; and (ii) stable groundwater level and high/varying EC values next to the estuarial zone. Although aquifers are undergoing over-abstraction, this spatiotemporal heterogeneity suggests that a recharge is possibly locally favored next to the estuarial area of the RMR thanks to hydraulic connections between surface and deep aquifers throughout extended paleo-channels. Thus, based on this typology, MAR implementation through controlled infiltration close to the estuarial area seems to be more appropriated, whereas the direct deep injection appears to be more relevant in more distant zones.

Sediment Source Fingerprinting of the Lake Urmia Sand Dunes

Aeolian sand dunes are continuously being discovered in inner dry lands and coastal areas, most of which have been formed over the Last Glacial Maximum. Presently, due to some natural and anthropogenic implications on earth, newly-born sand dunes are quickly emerging. Lake Urmia, the world’s second largest permanent hypersaline lake, has started shrinking, vast lands comprising sand dunes over the western shore of the lake have appeared and one question has been playing on the minds of nearby dwellers: where are these sand dunes coming from, What there was not 15 years ago!! In the present study, the determination of the source of the Lake Urmia sand dunes in terms of the quantifying relative contribution of each upstream geomorphological/lithological unit has been performed using geochemical fingerprinting techniques. The findings demonstrate that the alluvial and the fluvial sediments of the western upstream catchment have been transported by water erosion and they accumulated in the lower reaches of the Kahriz River. Wind erosion, as a secondary agent, have carried the aeolian sand-sized sediments to the sand dune area. Hence, the Lake Urmia sand dunes have been originating from simultaneous and joint actions of alluvial, fluvial and aeolian processes.

Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isotopes in groundwater

The La Paz aquifer system (Baja California Sur, Mexico) is under severe anthropogenic pressure because of high groundwater abstraction for urban supply (city of La Paz, around 222,000 inhabitants) and irrigated agriculture (1900ha). In consequence, seawater has infiltrated the aquifer, forcing the abandonment of wells with increased salinity. The objective of this study was to assess seawater intrusion, understand the hydrogeochemical processes involved and estimate the contribution of seawater in the wells tested. The aquifer comprises mainly the alluvial filling and marine sediments of a tectonic graben oriented north-south, in contact with the Gulf of California. Groundwater samples were collected in 47 locations and analyzed for major constituents. A subset of 23 samples was analyzed for strontium and boron concentrations and isotopic signatures (⁸⁷Sr/⁸⁶Sr and δ¹¹B). Results were interpreted using standard hydrochemical plots along with ad hoc plots including isotopic data. Seawater intrusion was confirmed by several hydrogeochemical indicators, such as the high salinity in areas of intense pumping or the Na⁺-Ca²⁺ exchange occurring in sediments that were previously in chemical equilibrium with fresh water. However, seawater contribution was not sufficient to explain the observed concentrations and isotopic signatures of Sr and B. According to the isotopic data, desorption processes triggered by a modification in chemical equilibrium and an increase in ionic strength by seawater intrusion significantly increased Sr and probably B concentrations in groundwater. From a calculation of seawater contribution to the wells, it was estimated that one-third of the sampled abstraction wells were significantly affected by seawater intrusion, reaching concentrations that would limit their use for human supply or even irrigated agriculture. In addition, significant agricultural pollution (nitrates) was detected. Planned management of the aquifer and corrective measures are needed in order to invert the salinization process before it severely affects water resources in the long term.

Strontium isotopes as tracers of water-rocks interactions, mixing processes and residence time indicator of groundwater within the granite-carbonate coastal aquifer of Bonifacio (Corsica, France)

This study aims at identifying the water-rock interactions and mixing rates within a complex granite-carbonate coastal aquifer under high touristic pressure. Investigations have been carried out within the coastal aquifer of Bonifacio (southern Corsica, France) mainly composed of continental granitic weathering products and marine calcarenite sediments filling a granitic depression. A multi-tracer approach combining physico-chemical parameters, major ions, selected trace elements, stable isotopes of the water molecule and ⁸⁷Sr/⁸⁶Sr ratios measurements is undertaken for 20 groundwater samples during the low water period in November 2014. 5 rock samples of the sedimentary deposits and surrounding granites are also analysed. First, the water-rock interactions processes governing the groundwater mineralization are described in order to fix the hydrogeochemical background. Secondly, the flow conditions are refined through the quantification of inter aquifer levels mixing, and thirdly, the kinetics of water-rock interaction based on groundwater residence time from a previous study using CFCs and SF₆ are quantified for the two main flow lines. A regional contrast in the groundwater recharge altitude allowed the oxygene-18 to be useful combined with the ⁸⁷Sr/⁸⁶Sr ratios to differentiate the groundwater origins and to compute the mixing rates, revealing the real extension of the watershed and the availability of the resource. The results also highlight a very good correlation between the groundwater residence time and the spatial evolution of ⁸⁷Sr/⁸⁶Sr ratios, allowing water-rock interaction kinetics to be defined empirically for the two main flow lines through the calcarenites. These results demonstrate the efficiency of strontium isotopes as tracers of water-rock interaction kinetics and by extension their relevance as a proxy of groundwater residence time, fundamental parameter documenting the long term sustainability of the hydrosystem.

Glacial recharge, salinisation and anthropogenic contamination in the coastal aquifers of Recife (Brazil)

Implying large residence times and complex water origins deep coastal aquifers are of particular interest as they are remarkable markers of climate, water use and land use changes. Over the last decades, the Metropolitan Region of Recife (Brazil) went through extensive environmental changes increasing the pressure on water resources and giving rise to numerous environmental consequences on the coastal groundwater systems. We analysed the groundwater of the deep aquifers Cabo and Beberibe that are increasingly exploited. The processes potentially affecting groundwater residence times and flow paths have been studied using a multi-tracer approach (CFCs, SF6, noble gases, 14C, 2H and 18O). The main findings of these investigations show that: (1) Groundwaters of the Cabo and Beberibe aquifers have long residence times and were recharged about 20,000years ago. (2) Within these old groundwaters we can find palaeo-climate evidences from the last glacial period at the tropics with lower temperatures and dryer conditions than the present climate. (3) Recently, the natural slow dynamic of these groundwater systems was significantly affected by mixing processes with contaminated modern groundwater coming from the shallow unconfined Boa Viagem aquifer. (4) The large exploitation of these aquifers leads to a modification of the flow directions and causes the intrusion through palaeo-channels of saline water probably coming from the Capibaribe River and from the last transgression episodes. These observations indicate that the current exploitation of the Cabo and Beberibe aquifers is unsustainable regarding the long renewal times of these groundwater systems as well as their ongoing contamination and salinisation. The groundwater cycle being much slower than the human development rhythm, it is essential to integrate the magnitude and rapidity of anthropogenic impacts on this extremely slow cycle to the water management concepts.

Coastal groundwater salinization: Focus on the vertical variability in a multi-layered aquifer through a multi-isotope fingerprinting (Roussillon Basin, France)

The Roussillon sedimentary Basin (South France) is a complex multi-layered aquifer, close to the Mediterranean Sea facing seasonally increases of water abstraction and salinization issues. We report geochemical and isotopic vertical variability in this aquifer using groundwater sampled with a Westbay System® at two coastal monitoring sites: Barcarès and Canet. The Westbay sampling allows pointing out and explaining the variation of water quality along vertical profiles, both in productive layers and in the less permeable ones where most of the chemical processes are susceptible to take place. The aquifer layers are not equally impacted by salinization, with electrical conductivity ranging from 460 to 43,000μS·cm(-1). The δ(2)H-δ(18)O signatures show mixing between seawater and freshwater components with long water residence time as evidenced by the lack of contribution from modern water using (3)H, (14)C and CFCs/SF6. S(SO4) isotopes also evidence seawater contribution but some signatures can be related to oxidation of pyrite and/or organically bounded S. In the upper layers (87)Sr/(86)Sr ratios are close to that of seawater and then increase with depth, reflecting water-rock interaction with argillaceous formations while punctual low values reflect interaction with carbonate. Boron isotopes highlight secondary processes such as adsorption/desorption onto clays in addition to mixings. At the Barcarès site (120m deep), the high salinity in some layers appear to be related neither to present day seawater intrusion, nor to Salses-Leucate lagoonwater intrusion. Groundwater chemical composition thus highlights binary mixing between fresh groundwater and inherited salty water together with cation exchange processes, water-rock interactions and, locally, sedimentary organic matter mineralisation probably enhanced by pyrite oxidation. Finally, combining the results of this study and those of Caballero and Ladouche (2015), we discuss the possible future evolution of this aquifer system under global change, as well as the potential management strategies needed to preserve quantitatively and qualitatively this water resource.

Groundwater contamination in coastal urban areas: Anthropogenic pressure and natural attenuation processes. Example of Recife (PE State, NE Brazil)

In a context of increasing land use pressure (over-exploitation, surface-water contamination) and repeated droughts, identifying the processes affecting groundwater quality in coastal megacities of the tropical and arid countries will condition their long-term social and environmental sustainability. The present study focuses on the Brazilian Recife Metropolitan Region (RMR), which is a highly urbanized area (3,743,854 inhabitants in 2010) on the Atlantic coast located next to an estuarial zone and overlying a multi-layered sedimentary system featured by a variable sediment texture and organic content. It investigates the contamination and redox status patterns conditioning potential attenuation within the shallow aquifers that constitute the interface between the city and the strategic deeper semi-confined aquifers. These latter are increasingly exploited, leading to high drawdown in potenciometric levels of 20-30m and up to 70m in some high well density places, and potentially connected to the surface through leakage. From a multi-tracer approach (major ions, major gases, δ(11)B, δ(18)O-SO4, δ(34)S-SO4) carried out during two field campaigns in September 2012 and March 2013 (sampling of 19 wells and 3 surface waters), it has been possible to assess the contamination sources and the redox processes. The increasing trend for mineralization from inland to coastal and estuarial wells (from 119 to around 10,000μS/cm) is at first attributed to water-rock interactions combined with natural and human-induced potentiometric gradients. Secondly, along with this trend, one finds an environmental pressure gradient related to sewage and/or surface-channel network impacts (typically depleted δ(11)B within the range of 10-15‰) that are purveyors of chloride, nitrate, ammonium and sulfate. Nitrate, ammonium and sulfate (ranging from 0 to 1.70mmol/L, from 0 to 0,65mmol/L, from 0.03 to 3.91mmol/L respectively are also potentially produced or consumed through various redox processes (pyrite oxidation, denitrification, dissimilatory nitrate reduction to ammonium) within the system, as is apparent within a patchwork of biogeochemical reactors. Furthermore, intensive pumping in the coastal area with its high well density punctually leads to temporary well salinization ([Cl] reaching temporarily 79mmol/L). Our results, summarized as a conceptual scheme based on environmental conditions, is a suitable basis for implementing sustainable management in coastal sedimentary hydrosystems influenced by highly urbanized conditions.

Assessing the pollution risk of a groundwater source field at western Laizhou Bay under seawater intrusion

Coastal areas have great significance for human living, economy and society development in the world. With the rapid increase of pressures from human activities and climate change, the safety of groundwater resource is under the threat of seawater intrusion in coastal areas. The area of Laizhou Bay is one of the most serious seawater intruded areas in China, since seawater intrusion phenomenon was firstly recognized in the middle of 1970s. This study assessed the pollution risk of a groundwater source filed of western Laizhou Bay area by inferring the probability distribution of groundwater Cl(-) concentration. The numerical model of seawater intrusion process is built by using SEAWAT4. The parameter uncertainty of this model is evaluated by Markov Chain Monte Carlo (MCMC) simulation, and DREAM(ZS) is used as sampling algorithm. Then, the predictive distribution of Cl(-) concentration at groundwater source field is inferred by using the samples of model parameters obtained from MCMC. After that, the pollution risk of groundwater source filed is assessed by the predictive quantiles of Cl(-) concentration. The results of model calibration and verification demonstrate that the DREAM(ZS) based MCMC is efficient and reliable to estimate model parameters under current observation. Under the condition of 95% confidence level, the groundwater source point will not be polluted by seawater intrusion in future five years (2015-2019). In addition, the 2.5% and 97.5% predictive quantiles show that the Cl(-) concentration of groundwater source field always vary between 175mg/l and 200mg/l.

Investigating the salinization and freshening processes of coastal groundwater resources in Urmia aquifer, NW Iran

This paper presents the results of an assessment about interaction between Urmia Lake (UL) and coastal groundwater in the Urmia aquifer (UA). This aquifer is the most significant contributor to the freshwater supply of the coastal areas. The use of hydrochemical facies can be very useful to identify the saltwater encroachment or freshening phases in the coastal aquifers. In this study, the analysis of salinization/freshening processes was carried out through the saturation index (SI), ionic deltas (Δ), binary diagrams, and hydrochemical facies evolution (HFE) diagram. Based on the Gibbs plot, the behavior of the major ions showed that the changes in the chemical composition of the groundwater are mainly controlled by the water-soil/rock interaction zone and few samples are relatively controlled by evaporation. A possible explanation for this phenomenon is that the deposited chloride and sulfate particles can form the minor salinity source in some coastal areas when washed down by precipitation. The SI calculations showed that all groundwater samples, collected in these periods, show negative saturation indices, which indicate undersaturation with respect to anhydrite, gypsum, and halite. In addition, except in a few cases, all other samples showed the undersaturation with respect to the carbonate minerals such as aragonite, calcite, and dolomite. Therefore, these minerals are susceptible to dissolution. In the dry season, the SI calculations showed more positive values with respect to dolomite, especially in the northern part of UA, which indicated a higher potential for precipitation and deposition of dolomite. The percentage of saltwater in the groundwater samples of Urmia plain was very low, ranging between 0.001 and 0.79 % in the wet season and 0.0004 and 0.81 % in the dry season. The results of HFE diagram, which was taken to find whether the aquifer was in the saltwater encroachment phase or in the freshening phase, indicated that except for a few wells near the coast, there is very little hydraulic interaction between UA and UL. In this coastal area, most of the samples that were collected repeatedly in both wet and dry seasons showed the same hydrochemical facies, which suggested that the seasonal groundwater fluctuations cannot significantly change the chemical composition of groundwater.

Marine water from mid-Holocene sea level highstand trapped in a coastal aquifer: Evidence from groundwater isotopes, and environmental significance

A multi-layered coastal aquifer in southeast Australia was assessed using environmental isotopes, to identify the origins of salinity and its links to palaeo-environmental setting. Spatial distribution of groundwater salinity (electrical conductivity values ranging from 0.395 to 56.1 mS/cm) was examined along the coastline along with geological, isotopic and chemical data. This allowed assessment of different salinity sources and emplacement mechanisms. Molar chloride/bromide ratios range from 619 to 1070 (621 to 705 in samples with EC >15 mS/cm), indicating salts are predominantly marine. Two distinct vertical salinity profiles were observed, one with increasing salinity with depth and another with saline shallow water overlying fresh groundwater. The saline shallow groundwater (EC=45.4 to 55.7 mS/cm) has somewhat marine-like stable isotope ratios (δ(18)O=-2.4 to -1.9 ‰) and radiocarbon activities indicative of middle Holocene emplacement (47.4 to 60.4pMC). This overlies fresher groundwater with late Pleistocene radiocarbon ages and meteoric stable isotopes (δ(18)O=-5.5 to -4.6‰). The configuration suggests surface inundation of the upper sediments by marine water during the mid-Holocene (c. 2-8 kyr BP), when sea level was 1-2m above today's level. Profiles of chloride, stable isotopes, and radiocarbon indicate mixing between this pre-modern marine water and fresh meteoric groundwater to varying degrees around the coastline. Mixing calculations using chloride and stable isotopes show that in addition to fresh-marine water mixing, some salinity is derived from transpiration by halophytic vegetation (e.g. mangroves). The δ(13)C ratios in saline water (-17.6 to -18.4‰) also have vegetation/organic matter signatures, consistent with emplacement by surface inundation and extensive interaction between vegetation and recharging groundwater. Saline shallow groundwater is preserved only in areas where low permeability sediments have slowed subsequent downwards propagation. The configuration is unlikely to be stable long-term due to fluid density; this may be exacerbated by pumping the underlying aquifer.

Residence times of groundwater and nitrate transport in coastal aquifer systems: Daweijia area, northeastern China

Groundwater within the coastal aquifer systems of the Daweijia area in northeastern China is characterized by a large of variations (33-521mg/L) in NO3(-) concentrations. Elevated nitrate concentrations, in addition to seawater intrusion in the Daweijia well field, both attributable to anthropogenic activities, may impact future water-management practices. Chemical and stable isotopic (δ(18)O, δ(2)H) analysis, (3)H and CFCs methods were applied to provide a better understanding of the relationship between the distribution of groundwater mean residence time (MRT) and nitrate transport, and to identify sources of nitrate concentrations in the complex coastal aquifer systems. There is a relatively narrow range of isotopic composition (ranging from -8.5 to -7.0‰) in most groundwater. Generally higher tritium contents observed in the wet season relative to the dry season may result from rapid groundwater circulation in response to the rainfall through the preferential flow paths. In the well field, the relatively increased nitrate concentrations of groundwater, accompanied by the higher tritium contents in the wet season, indicate the nitrate pollution can be attributed to domestic wastes. The binary exponential and piston-flow mixing model (BEP) yielded feasible age distributions based on the conceptual model. The good inverse relationship between groundwater MRTs (92-467years) and the NO3(-) concentrations in the shallow Quaternary aquifers indicates that elevated nitrate concentrations are attributable to more recent recharge for shallow groundwater. However, there is no significant relationship between the MRTs (8-411years) and the NO3(-) concentrations existing in the carbonate aquifer system, due to the complex hydrogeological conditions, groundwater age distributions and the range of contaminant source areas. Nitrate in the groundwater system without denitrification effects could accumulate and be transported for tens of years, through the complex carbonate aquifer matrix and the successive inputs of nitrogen from various sources.