Effects Of Seawater Intrusion Research Articles

Influences of seawater intrusion and anthropogenic activities on shallow coastal aquifers in Sri Lanka: evidence from hydrogeochemical and stable isotope data.

Water supplies in coastal aquifers throughout the world are often threatened by salinization due to seawater intrusion and anthropogenic activities. In the Kalpitiya Peninsula in Sri Lanka, agricultural and domestic water supplies entirely depend on groundwater resources extracted from unconfined Holocene sandy aquifers. To differentiate the effects of seawater intrusion and agriculture on the coastal aquifers of this 160km2 peninsula, 43 groundwater samples were collected. These samples were analyzed for major ions, trace elements, and stable isotopes of water (δ18O and δ2H). The solute compositions were dominated by Cl-, [Formula: see text], and [Formula: see text], which were mostly balanced by Ca2+, Na+, and Mg2+. Among the four main water types, Na+-Cl- and Ca2+-[Formula: see text] classifications were predominant in the investigated aquifers. Modifications of the groundwater due to evaporation during irrigation activities, but also due to seawater intrusion seem most plausible as indicated by the correlation of δ18O with δ2H (δ2H = 5.51*δ18O-3.08, r = 0.93) deviating from the local meteoric water line. Particularly in the southern part of the peninsula, Mg2+/Ca2+ ratios and stable isotopes of water attributed salinization of groundwater to agricultural activities. However, especially in the north, seawater intrusions were also evident. Established mass balance calculations revealed that local groundwater had seawater admixtures of up to 12%. Our results indicate that integrated water management is essential and water resources should critically monitor in the Kalpitiya Peninsula in order to avoid over-exploitation and further seawater inflows.

Chemical Characteristics of the Surface Water around Ras El-Bar Island, Damietta Governorate, Egypt

The present study represents an attempt to classify and evaluate the surface water quality around Ras El-Bar Island by investigating the surface water physical and chemical parameters near Damietta Branch Outlet, Damietta Port Canal and Mediterranean Sea. The measured pH values show a tendency towards alkaline side aquatic system. The variations in the total dissolved solids and the electrical conductivity values show relatively high values in the Sea water compared with that in the Damietta Branch and the Damietta Port Canal. The observed increasing in the dissolved solids and electrical conductivity values seaward may be related to the effect of sea water intrusion. However, the observed lower values of the dissolved solids and electrical conductivity in Damietta Branch water may be related to the effect of receiving many waste-water resulted from domestic sewage, industrial activities and the extensive boating activities at Ezbt El-borg. The sodium and chloride values increase downward, through Damietta Branch toward its outlet to the Sea water and through the site of connection with the Damietta Port Canal toward the sea. The observed increasing in sodium and chloride values seaward may be related to the effect of sea water intrusion.The potassium, calcium and bicarbonate values decrease downward, through Damietta Branch toward its outlet to the sea water and through the site of connection with the Damietta Port Canal toward the sea. The observed decreasing in Potassium, Calcium and Bicarbonate values seaward may be related to the effect of sea water intrusion. The alkalis (Sodium and potassium) exceed alkaline earths (calcium and magnesium) and strong acids (Cl+SO4) exceed weak acids (C03+HCO3) indicating secondary salinity. High contents of Lead and Cadmium values are observed in the in the study surface water around Ras El-Bar Island. Cadmium and Lead fall in the category “high”. The category “high” is usually observed near the main shipping routes to the ports.The studied surface water samples show non-permissible water quality for drinking, domestic and irrigation purposes.

Use of 1-D subsurface thermal profiles to characterize the groundwater flow in the Central Nile Delta region

Borehole temperature logs of twelve wells were analyzed to clarify the connection among groundwater and the subsurface thermal conditions in the central part of the Nile Delta as an irrigated land in an arid area. The annual average surface air temperature amid the interval from 1997 to 2017 was 22.74 °C, and the annual increment is 0.0629 °C year−1. This surface warming influenced the subsurface thermal system. Thermal gradient of the conducted temperature–depth (T–D) profiles varies from − 0.009 °C m−1 (well 4) to 0.0098 °C m−1 (well 12). The vertical change in the thermal gradient values lower than zero was detected in wells 1, 3, 4, 5, 6, 7, 9, and 11 while wells that have gradient higher than zero were wells 2, 8, 10, and 12. The T–D logs were numerically examined and the outcomes compared with standard curves to predict the vertical groundwater stream direction and speeds. The logged wells could be divided into three groups. Wells were characterized by downward groundwater flow as wells 1, 4, 7, 8, and 10. Wells were of upward groundwater flow as in wells 2, 3, 5, 9, 11, and 12. Wells were characterized with upward and downward groundwater flows as well 6. Vertical downward groundwater flow velocities in the study area ranged from 11.53 cm year−1 in well 10 at a depth of 200–236 m to 69.20 cm year−1 in well 1 at a depth of 32 m. The upward groundwater flow velocities varied from 498.26 cm year−1 in well 11 at a depth below 60 m to 10.65 cm year−1 in well 12 at a depth below 115 m. Seawater intrusion effect was estimated from the upward groundwater movement in the northern part (well 11), and the downward movement of the freshwater was reflected from well 10 in the southern part of the study area. Downward flow noticed in wells 7 and 6 reflects the recharge effect of Damietta and Rosetta Nile branches, respectively. The groundwater flow velocity and flow direction change with depth could be related to the aquifer heterogeneity, groundwater pumping, and surface water–groundwater interaction.

Characterizing microbial diversity and community composition of groundwater in a salt-freshwater transition zone

A salt-freshwater transition zone due to seawater intrusion to groundwater promotes changes in microbial diversity and community composition in a coastal aquifer. The main purpose of this study is to explore the effect of seawater intrusion on the groundwater quality in a salt-freshwater transition zone and identify the microbial fingerprints of seawater intrusion. The changes in microbial community diversity response to the seawater intrusion were characterized by comparing the community structures of the microbes in fresh groundwater, seawater, and salty groundwater from various monitoring wells at different depths using the high throughput 16S rDNA gene sequencing. Results show that seawater had the lowest taxon richness and evenness, and the irrigation water had the highest richness and evenness. Statistical analysis showed that DO%, ORP, and Cl− affected microbial distribution in the groundwater; while DO% was a main environmental factor influencing microbial community diversity. The analysis of microbial community structures indicates that the order Oceanospirillales and the family Alteromonadaceae could be used as indicators of seawater intrusion.

The effect of pumping saline groundwater for desalination on the fresh–saline water interface dynamics

Over the past few decades, seawater desalination has become a necessity for freshwater supply in many countries worldwide, particularly in arid and semi-arid regions. One potentially high-quality feed water for desalination is saline groundwater (SGW) from coastal aquifers, which has lower fouling propensity than seawater. This study examines the effect of pumping SGW from a phreatic coastal aquifer on fresh groundwater, particularly on the dynamics of the fresh–saline water interface (FSI). Initially, we constructed a 3D finite-element model of a phreatic coastal aquifer by using the FEFLOW software, which solves the coupled variable density groundwater flow and solute transport equations. Then, we compared and validated the results of the model to those of a field-scale pumping test. The model indicates that pumping SGW from a coastal aquifer freshens the aquifer and rehabilitates parts that were salinized due to seawater intrusion – an effect that increases with increasing pumping rate. In addition, when simultaneously pumping fresh groundwater further inland and SGW from below the FSI, the freshening effect is less pronounced and the salinity of the aquifer is more stable. In line with the results of the model, the field experiment revealed that salinity in the observation well decreases over the course of pumping. Taken together, our findings demonstrate that, in addition to providing a high-quality source feed water for desalination, pumping SGW does not salinize the aquifer and even rehabilitates it by negating the effect of seawater intrusion. These findings are important for planning shoreline desalination facilities and for managing arid coastal regions with lack of water supply and over exploited aquifers.

Drivers, patterns and velocity of saltwater intrusion in a stressed aquifer of the East African coast: Joint analysis of groundwater and geophysical data in southern Kenya

Fresh groundwater resources in coastal East Africa are crucial for the region's socio-economic development but are under threat of salinization caused by changes in recharge patterns and increasing abstraction. With the aim of establishing the drivers behind saltwater intrusion and its current spatial extent, we studied the Kenyan South Coast aquifer, a representative, strategic aquifer under increased pressure. Investigations included electrical resistivity tomography (ERT) surveys and in-situ groundwater measurements (water table and basic quality) together with the analysis of available long-term climatic and borehole monitoring data. Over the last 40 years, groundwater electrical conductivity values at the well field increased by about three times and groundwater levels declined by 1–3 m over the last decade. When put in perspective with the long-term climate (rainfall, temperature) and abstraction records, these trends in groundwater appear to be primarily driven by increased borehole abstraction (+400 m3/day per year in average), whereas observed increasing temperature (+0.02 °C per year) and decreasing rainfall (−0.8 mm per year) could potentially act as a secondary control through reduced recharge. However the low statistical significance obtained for both rainfall and temperature trends over the observation period suggests that no clear conclusion can be made with regards to long-term climate impact on groundwater. Groundwater quality mapping showed that proximity to the ocean, presence of abstraction well-fields and regional geology control groundwater salinity patterns at regional scale. Locally, geophysical data showed that, saltwater intrusion spatial patterns are controlled by local aquifer lithology, groundwater abstraction and freshwater recharge in floodplains. Comparison with previous (1984) resistivity data showed that the saltwater front has advanced toward the well-field by up to 2 km and rose by up to 80 m over the last 30 years, which corresponds to a maximal velocity of about 60 m/y horizontally and 2 m/y vertically. Implementation of groundwater management strategies such as sustainable groundwater exploitation, sourced alternative water supply, and managed aquifer recharge are required to mitigate the effects of seawater intrusion along the East African coastal strip.

Analysis of Saline Intrusion into a Coastal Aquifer: A Case History of Legacy Issues and Challenges to Water Security

Coastal aquifers are subject to seawater intrusion. Therefore, managing freshwater aquifers in coastal areas remain challenging. At present, determining safe yields from the coastal aquifers to prevent seawater intrusion is primarily based on the use of numerical simulation-optimization models or by the use of analytical models based on the Ghyben-Herzberg principle. This study examines the cause and effects of seawater intrusion into a coastal aquifer, Lincoln Basin in southern Eyre Peninsula, South Australia and shows that application of simple techniques would have prevented seawater intrusion. Three freshwater lenses, Lincoln A, B, and C of the Lincoln Basin, located about 13 km southwest of Port Lincoln township, have been developed as a town water supply source in 1960. The capacity of the basin has been assessed by three long-term pumping tests. Based on pump tests results, three areas were developed to supply 2×106 m3 per year distributed across three lenses as lens A : four wells to supply 0.84×106 m3, lens B: four wells to supply 0.5×106 m3 and lens C: four wells to supply 0.66 ×106 m3. Neither recharge to the freshwater lenses nor a water balance had been assessed, and a precautionary approach to groundwater extraction was not followed. The apparent driver for managing the basin was demand for the township. In this study, we assessed the recharge using two methods; water-table fluctuation (WTF) and the conventional chloride mass balance (CMB) method. Total recharge to the freshwater lenses is estimated at 1.6×106 m3 per year which is less than the average annual groundwater extraction from the basin during the 1961-1977 periods (average 2.14×106 m3). As a result mining of the groundwater storage has occurred in the basin leading to saline intrusion, upconing and lateral flow of brackish water into wellfield areas. The total volume extracted from the basin was 35×106 m3, which exceeded the average recharge over the 15 year period, 24×106 m3. Using analytical methods, the seawater/freshwater interface movement from its original position was estimated to be 35 m in lens A, 337 m in lens B and 188 m in lens C. For each pumping well at maximum discharge rate, the transient interface location directly underneath the well was calculated. This results in interface rises under pumping wells in lens A of 3.8 m, lens B of 0.5 m, and in lens C about 0.7 m. According to the risk-based groundwater allocation method, maximum extraction would have been as a proportion of 25% of the annual recharge. Thus, maximum annual abstraction limits for lens A, B and C would have been 210×103 m3, 72×103 m3 and 130×103 m3, totaling 412×103 m3.

Semianalytical solutions for contaminant transport under variable velocity field in a coastal aquifer

Existing closed-form solutions of contaminant transport problems are limited by the mathematically convenient assumption of uniform flow. These solutions cannot be used to investigate contaminant transport in coastal aquifers where seawater intrusion induces a variable velocity field. An adaptation of the Fourier-Galerkin method is introduced to obtain semi-analytical solutions for contaminant transport in a confined coastal aquifer in which the saltwater wedge is in equilibrium with a freshwater discharge flow. Two scenarios dealing with contaminant leakage from the aquifer top surface and contaminant migration from a source at the landward boundary are considered. Robust implementation of the Fourier-Galerkin method is developed to efficiently solve the coupled flow, salt and contaminant transport equations. Various illustrative examples are generated and the semi-analytical solutions are compared against an in-house numerical code. The Fourier series are used to evaluate relevant metrics characterizing contaminant transport such as the discharge flux to the sea, amount of contaminant persisting in the groundwater and solute flux from the source. These metrics represent quantitative data for numerical code validation and are relevant to understand the effect of seawater intrusion on contaminant transport. It is observed that, for the surface contamination scenario, seawater intrusion limits the spread of the contaminant but intensifies the contaminant discharge to the sea. For the landward contamination scenario, moderate seawater intrusion affects only the spatial distribution of the contaminant plume while extreme seawater intrusion can increase the contaminant discharge to the sea. The developed semi-analytical solution presents an efficient tool for the verification of numerical models. It provides a clear interpretation of the contaminant transport processes in coastal aquifers subject to seawater intrusion. For practical usage in further studies, the full open source semi-analytical codes are made available at the website https://lhyges.unistra.fr/FAHS-Marwan.

Assessing the impact of sea level rise due to climate change on seawater intrusion in Mekong Delta, Vietnam.

In the context of climate change, salinity intrusion into rivers has been, and will be, one of the most important issues for coastal water resources management. A combination of changes, including increased temperature, change in regional rainfall, especially sea level rise (SLR) related to climate change, will have significant impacts on this phenomenon. This paper presents the outcomes of a study conducted in the Mekong Delta of Vietnam (MKD) for evaluating the effect of sea water intrusion under a new SLR scenario. Salinity intrusion was simulated by one-dimensional (1D) modeling. The relative sea level projection was constructed corresponding to the RCP 6.0 emission scenario for MKD based on the statistical downscaling method. The sea level in 2050 is projected to increase from 25 cm to 30 cm compared to the baseline period (in 2000). Furthermore, the simulated results suggested that salinity greater than 4 g/l, which affects rice yield, will intrude up to 50-60 km into the river. Approximately 30,000 ha of agricultural area will be affected if the sea level rise is 30 cm.

Hydrological Studies of the Dniester Estuary in 2012-2017

Purpose. The study of the features of long-term changes of the Dniester Estuary hydrological regime main characteristics in 2012-2017 based on the results of annual surveys performed by Odessa National I. I. Mechnikov University. Methods. Measuring of the water transparency, temperature and electrical conductivity in the surface and near-bottom layers were carried out using standard techniques with Secchi disk and portable HACH analyser with temperature and conductivity sensors. Results. Based on the studies performed, almost two times decrease in water transparency values in summer of 2012-2017 has been established compared with the same period of 2003-2011. It was shown that the maximum transparency values were observed in the southern part of the estuary, where the presence of seawater is almost constant. Analysis of electrical conductivity spatial distribution has revealed practically constant influence effect of seawater intrusion in the southern part of the estuary, especially in the bottom layer, and periodical influence effect in the middle and northern parts. Significant increasing of the range of water temperature and electrical conductivity variations in the Dniester Estuary has been recorded in summer 2012-2017 comparison with summer 2003-2011. Conclusions. The established peculiarities of hydrological characteristics’ spatial distribution indicate that river discharge and intrusion of marine water are the most important factors determining the Dniester Estuary hydrological regime. Increase of temperature and conductivity of the Dniester Estuary water in summer 2012-2017 in comparison with 2003-2011 was registered. An anomalous penetration of marine waters into most part of the estuary was detected in July 2016, which in the past was observed for the last time in 2011.

Geoelectrical and hydrochemical study for the assessment of seawater intrusion evolution in coastal aquifers of Oualidia, Morocco

This study aims to assess the evolution of seawater intrusion between 1992 and 2011 periods in the coastal aquifers of Oualidia. To achieve this objective, the combination of geoelectrical and hydrochemical methods was adopted. Apparent resistivity maps, using 74 Vertical Electrical Sounding (VES) performed on 1992, allowed to distinguish two different zones. The conductive one, with apparent resistivity ranging between 4 and 86Ω·m, is limited to 1km with respect to the ocean. Meanwhile, the resistant one is much farther from the coastline. Besides, results of Electrical Resistivity Tomography (ERT) profiles performed during 2011 are in good agreement with those obtained by apparent resistivity maps. The ERT profiles show a conductive level characterized by low resistivity below 30Ω·m assigned to seawater intrusion effect. Moreover, hydrochemical analysis, performed on 19 wells during three different periods, on June, December 2010 and May 2011, indicates that the most affected part with marine waters was at that time localized in the first kilometer from the ocean, with high amounts of Na+ and Cl− ions. Beyond this fringe, mineralization becomes very weak. Overall, the comparison of old VES with recent ERT coupled with hydrochemical results suggest no important evolution of the salt wedge since 1992.

Feasibility of adopting smart water meters in aquifer management: An integrated hydro-economic analysis

The feasibility of groundwater monitoring was investigated by a case study on adopting smart water meters to measure groundwater extraction at individual farms and a centralized online information management system to measure collective aquifer water extraction. Benefits of optimal groundwater management was estimated using hydro-economic models that simulate, for a 70-year period, private and social optimality, taking into account the effects of seawater intrusion on groundwater salinity. A Bayesian inference system was used as an interface between a dynamic programming model and MODFLOW groundwater simulation model. The case study’s cost data were scaled-up to the aquifer level and compared to the incremental benefits between private and socially optimal water extraction. The results showed that the Net Present Value of measuring and monitoring groundwater extraction using smart water meters as $790 million ($1332/ha/year) with an Internal Rate of Return of 93%. The sustainable use of the aquifer results to a reduction of the cropped area by 10%, a reduction of the groundwater extraction by 20%, a change in the crop mix, and 42% of the least-efficient farms exiting farming. The exiting farmers could convert farm lands to other land uses such as residential, urban, industrial land use, with adequate facilitation provided by the government. The impact of change of groundwater management strategy on the arid ecology with reduction in tree cover is noted.

Trace elements (Li, B, Mn and Ba) as sensitive indicators for salinization and freshening events in coastal aquifers

The current global intrusion of seawater into coastal aquifers causes salinization of groundwater and thus significant degradation of its quality. This study quantified the effect of seawater intrusion and freshening events in coastal aquifers on trace elements (Li, B, Mn and Ba) across the fresh-saline water interface (FSI) and their possible use as indicators for these events. This was done by combining field data and column experiments simulating these events. The experiments enabled quantification of the processes affecting the trace element composition and examination of whether salinization and freshening events are geochemically reversible, which has been seldom investigated.The dominant process affecting trace element composition during salinization and freshening is ion exchange. The results of the experiments show that the concentrations of major cations and Li+ were reversible during salinization and freshening, whereas B, Mn2+ and Ba2+ were not. During salinization, Li+ and B were depleted due to sorption by 10 and 100μmol·L−1, respectively, to about half of their expected conservative concentrations. The relative depletion of Li+ increased with distance from the shore, representing the propagation of salinization. Ba2+ and Mn2+ were desorbed from the sediment during salinization and enriched by tenfold in the aqueous phase compared to their concentration in seawater (~0.1 μeq·L−1). During freshening both were depleted by almost tenfold compared to their concentration in fresh groundwater (~0.7 μeq·L−1). The depletion of Mn2+ is a sensitive marker for freshening because Mn2+ has a strong affinity to the solid phase. Moreover, this study shows that both Mn2+ and Ba2+ can be used as sensitive hydrogeochemical tools to distinguish between salinization and freshening events in the FSI zone in coastal aquifers.

Monitoring and Management of Karstic Coastal Groundwater in a Changing Environment (Southern Italy): A Review of a Regional Experience

The population concentration in coastal areas and the increase of groundwater discharge in combination with the peculiarities of karstic coastal aquifers constitute a huge worldwide problem, which is particularly relevant for coastal aquifers of the Mediterranean basin. This paper offers a review of scientific activities realized to pursue the optimal utilization of Apulian coastal groundwater. Apulia, with a coastline extending for over 800 km, is the Italian region with the largest coastal karst aquifers. Apulian aquifers have suffered both in terms of water quality and quantity. Some regional regulations were implemented from the 1970s with the purpose of controlling the number of wells, well locations, and well discharge. The practical effects of these management criteria, the temporal and spatial trend of recharge, groundwater quality, and seawater intrusion effects are discussed based on long-term monitoring. The efficacy of existing management tools and the development of predictive scenarios to identify the best way to reconcile irrigation and demands for high-quality drinking water have been pursued in a selected area. The Salento peninsula was selected as the Apulian aquifer portion exposed to the highest risk of quality degradation due to seawater intrusion. The capability of large-scale numerical models in groundwater management was tested, particularly for achieving forecast scenarios to evaluate the impacts of climate change on groundwater resources. The results show qualitative and quantitative groundwater trends from 1930 to 2060 and emphasize the substantial decrease of the piezometric level and a serious worsening of groundwater salinization due to seawater intrusion.

Assessing the Seawater Intrusion Due to Beach Wells in the Desalination Plant

Short-Term Low Volume (STLV) Sea Water Desalination Plant of 6000 m3/d is under construction in the middle area of Gaza Strip. The plant will provide desalinated water for 75,000 inhabitants in regions in Khanyounis and Rafah. The intake of desalination plant will be indirectly from four beach wells. This article aims at providing the environmental impacts of these wells on the aquifer and the mitigation measures in case of negative impacts. In order to study the impacts of beach wells on the aquifer, a prediction groundwater three-dimensional model for the beach wells area, starting from the year 2000 until year 2030, was used. MODFLOW software was used for modeling the groundwater flow and SEAWAT software was used to model the seawater intrusion effect. The aquifer parameters were set as if they were in the transient model. The long term seasonal recharge rate for the summer and winter is considered to represent the seasonal differences in recharge through each year. The study showed that the steady four-meter drawdown in the beach wells will force the flow from the eastern direction to the sea. This will have positive impacts on the aquifer since it will decrease the seawater intrusion to the aquifer. The beach wells will pump water with Cl concentration equal to 18,000 mg/l. This means that the beach wells will accelerate the flow from the aquifer to the sea direction but still the pumped water is considered as seawater. This indicates the positive impacts on the groundwater aquifer since it will decrease the seawater intrusion in the beach wells area (Gaza Strip Middle area). In conclusion, these beach wells in this desalination plant (small capacity) are safe for the groundwater aquifer and it will decrease the effect of seawater intrusion on the aquifer.

The effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from column experiments under aerobic and anaerobic conditions

Summary This study experimentally quantified the effect of seawater intrusion (salinization) and freshening events in coastal aquifers on nutrient (N, P and DSi) dynamics across the fresh–saline groundwater interface. Laboratory column experiments were conducted under aerobic and anaerobic conditions in order to simulate the processes occurring in the fresh–saline interface. They were performed with aquifer sediments, simulating the natural conditions during alterations of natural fresh groundwater to seawater and vice versa. The salinization and freshening experiments showed that NH4+ and PO43− and DSi were affected mainly by ion exchange processes while microbial activity controlled the nitrogen species NO3− and NO2−. Due to the cation exchange, salinization generated enrichment (above the expected conservative behavior) of NH4+, up to 80 μmol L−1 (an order of magnitude higher than in seawater or fresh groundwater). Under anaerobic conditions NO3− was removed by denitrification, as demonstrated by the decrease in NO3− concentrations, the increase in NO2− concentrations, and the increase in δ15N by 15–25‰. Clear evidence was shown for anion exchange of PO43−, which competes with HCO3− and boron on adsorption sites. DSi seems to take part in the exchange process, similar to PO43−.

The effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from field data

Summary The effect of seawater intrusion and freshening events in coastal aquifers on nutrient (dissolved inorganic nitrogen species, phosphate and silica) dynamics across the fresh–saline groundwater interface (FSI) were quantified using field data. Seasonal vertical profiles revealed a clear transition between nutrient species across the FSI, which is also an oxycline. In view of the results of our experimental simulations, it is clear that the major process controlling the nutrient dynamics at the FSI, besides the mixing that takes place between the two different water bodies, is the seasonal variation between seawater intrusion (salinization) in summer and flushing of the aquifer (freshening) in winter. Aquifer salinization during the summer shifts the FSI and the anaerobic depth-location upwards and leads to the enrichment of NH4+, PO43− and DSi (dissolved silica) in the saline groundwater. NH4+ and PO43− are enriched due to ion exchange, and DSi is enriched either by ion exchange (as PO43−) or as a result of dissolution of biogenic silica. Denitrification occurs at the base of the FSI, as indicated by the slight NO3− depletion and the enrichment in δ15N of NO3−. Aquifer freshening during the winter shifts the FSI downward and the water becomes suboxic with the influence of the oxic fresh groundwater. This leads to nitrification of the NH4+, enrichment of NO2− and depletion of 15N in the residual NO3− in the FSI. These cyclic processes generate a certain depletion of N and enrichment of P in the saline groundwater. Circulation of the saline groundwater below the FSI back to the sea can serve as a partial counterbalance to the high anthropogenic load of N impacting the coastal groundwater system.

Modelling and management of a Mediterranean karstic coastal aquifer under the effects of seawater intrusion and climate change

The study and management of the groundwater resources of a large, deep, coastal, karstic aquifer represent a very complex hydrogeological problem. Here, this problem is successfully approached by using an equivalent porous continuous medium (EPCM) to represent a karstic Apulian aquifer (southern Italy). This aquifer, which is located on a peninsula and extends to hundreds of metres depth, is the sole local source of high-quality water resources. These resources are at risk due to overexploitation, climate change and seawater intrusion. The model was based on MODFLOW and SEAWAT codes. Piezometric and salinity variations from 1930 to 2060 were simulated under three past scenarios (up to 1999) and three future scenarios that consider climate change, different types of discharge, and changes in sea level and salinity. The model was validated using surveyed piezometric and salinity data. An evident piezometric drop was confirmed for the past period (until 1999); a similar dramatic drop appears to be likely in the future. The lateral intrusion and upconing effects of seawater intrusion were non-negligible in the past and will be considerable in the future. All phenomena considered here, including sea level and sea salinity, showed non-negligible effects on coastal groundwater.

Estimating the effects of seawater intrusion on an estuarine nitrogen cycle by comparative network analysis

Nitrogen (N) removal from estuaries is driven in part by sedimentary microbial processes. The processes of denitrification and anaerobic ammonium oxidation (anammox) remove N from estuaries by producing di-nitrogen gas, and each can be coupled to N recycling pathways such as nitrification and dissimilatory nitrate reduction to ammonium (DNRA). Environmental conditions in estuaries influence sedimentary N cycling processes; therefore, seawater intrusion may affect the coupling of N cycling processes in the freshwater portions of estuaries. This study investigated the potential effects of seawater intrusion on these process couplings through a comparative modeling approach. We applied environ analysis, a form of ecological network analysis, to two N cycling mass-balance network models constructed at freshwater (oligohaline) and saltwater (polyhaline) sites in the Cape Fear River Estuary, North Carolina. We used a space-for-time substitution to predict the effects of seawater intrusion on the sedimentary N cycle. Further, we conducted an uncertainty analysis using linear inverse modeling to evaluate the effects of parameterization uncertainty on model results. Nitrification coupled to both denitrification and anammox was 2.5 times greater in the oligohaline model, while DNRA coupled to anammox was 2.7 times greater in the polyhaline model. However, the total amount of N2 gas produced relative to the nitrogen inputs to each network was 4.7% and 4.6% at the oligohaline and polyhaline sites, respectively. These findings suggest that changes in water chemistry from seawater intrusion may favor direct over coupled nitrogen removal, but may not substantially change the N removal capacity of the sedimentary microbial processes.

Effects of seasonal change and seawater intrusion on water quality for drinking and irrigation purposes, in coastal aquifers of Dar es Salaam, Tanzania

Groundwater is the major source to meet domestic, industrial and agricultural needs in the city of Dar es Salaam, Tanzania. However, population growth, increasing urbanization, industrialization and tourism, and climatic changes have caused an intensive exploitation of groundwater resources leading the aquifers become more vulnerable to seawater intrusion. The aim of this study is to examine the variations of groundwater chemistry (as resulting from natural and anthropogenic inputs) depending on seasonal changes, in order to evaluate water quality for drinking and irrigation purposes. Physical and chemical data come from the analysis of groundwater samples, collected from 72 wells, used for the evaluation of water quality parameters, during a year of monitoring. Pattern diagrams, geochemical modeling techniques and Principal Component Analysis (PCA) have been used to identify the main factors influencing groundwater composition. Based on the hydrochemistry, the groundwater was classified into three types: (a) Na–Cl, (b) Ca–Cl, (c) mixed Ca–Na–HCO3–Cl (d) mixed Ca–Mg–Cl–SO4. The geochemical modeling results show that groundwater chemistry is mainly influenced by evaporation process, as it is suggested by the increase of Na and Cl ions concentrations. According to irrigation water quality assessment diagrams of USDA, most water samples from dry and rainy seasons, distributed in category C2–S1, C3–S1, C3–S2, C4–S2 highlighting medium to very high salinity hazard and low to medium sodium content class. PCA evidenced the role of seawater intrusion, evaporation process and anthropogenic pollution (i.e. high NO3 levels due to agricultural activities), as the major factors that influenced the water chemistry, and hence the water quality. Based on Pearson correlation matrix, the presence of high correlations (>0.8) among Na, Cl, Mg and SO4, in association with EC, were interpreted as the seawater intrusion effects. In this area groundwater quality is generally low, and often exceeds permissible limits of standard guideline values of WHO and FAO, referred to EC and chloride values. The high salinity and the groundwater level depletion create serious problems for current use of water supplies as well as future exploitation.