Hydrochemical, Isotopic, and Geophysical Studies Applied to the Evaluation of Groundwater Salinization Processes in Quaternary Beach Ridges in a Semiarid Coastal Area of Northern Patagonia, Argentina

Quaternary sea level fluctuations have led to the development of beach ridges on many South Atlantic coasts. The objective of this paper was to asses from lithological, hydrochemical, iso-topic, and geophysical studies the salinization processes affecting groundwater stored in Pleistocene and Holocene beach ridges of the northern Patagonian coast. A hydrogeomorphological characterization of the area was performed using digital elevation models, the interpretation of satellite images, and field studies. Vertical electrical soundings were performed on transects running perpendicular to beach ridges in order to define variations in the freshwater-saltwater interface position. The salinity, chemistry, and stable isotopes of the groundwater were analyzed. The results demonstrated that the groundwater salinization of Pleistocene ridges responds to processes associated with the geological-geomorphological evolution of the area. The cementation of these surface sediments limits rainwater infiltration, which consequently prevents the development of freshwater lenses. This suggests that saline water is the result of ancient marine ingressions. Freshwater lenses develop in Holocene beach ridges; however, slight water salinization is detectable in the most populated areas as a result of intensive exploitation. The data provided are useful for freshwater resource prospection along the arid coast of Patagonia, where beach ridge deposits abound and populations experience serious drinking water supply problems.Fil: Carol, Eleonora Silvina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Perdomo, Santiago. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos Aires. Universidad Nacional del Noroeste de la Provincia de Buenos Aires. Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos Aires; ArgentinaFil: Alvarez, Maria del Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto Patagónico para el Estudio de los Ecosistemas Continentales; ArgentinaFil: Tanjal, Carolina Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Bouza, Pablo Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Instituto Patagónico para el Estudio de los Ecosistemas Continentales; Argentin

Groundwater quality index as a hydrochemical tool for monitoring saltwater intrusion into coastal freshwater aquifer of Eastern Dahomey Basin, Southwestern Nigeria

Saltwater intrusion into coastal freshwater aquifers is a threat to groundwater quality globally. This study aims to determine the extent of saltwater intrusion into the coastal freshwater aquifer of the Eastern Dahomey Basin (EDB), Nigeria. Groundwater chemistry was sampled and analysed for ionic ratios and interpreted using a hydrochemical facie evolution diagram (HFE-D), the saltwater mixing index (SMI) and the Groundwater quality index for saltwater intrusion (GQIswi). High EC and TDS and the concentration of dissolved ions showed increased salinity as a result of seawater intrusion in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. Correlation of ions in the wet season also suggests higher salinities which originate partly from industrial and municipal effluents especially from wells which are close to river channels, while dry season groundwater shows the dominant influence of seawater intrusion. HFE-D revealed that mixed groundwater of Na + Ca–HCO3, Na–Cl and Ca–Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi shows areas within 3 km from the coastline that are more sensitive to saltwater intrusion based on abstraction rate and depth of the wells. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin

Contribuição para a gestão integrada do sistema aquífero Mexilhoeira Grande, Portimão

Dissertação de Mestrado, Geomática, Faculdade de Ciências e Tecnologia, Instituto Superior de Engenharia, Universidade do Algarve, 2016Este estágio pretendeu atualizar a informação sobre o Sistema Aquífero da Mexilhoeira Grande – Portimão. Recolheram-se dados relativos à quantidade e à qualidade desta massa de água subterrânea, fazendo um estudo da sua evolução temporal nos últimos trinta anos. A componente prática realizou-se na APA-ARHAlgarve, durante cerca de quatro meses. Foram georreferenciadas 485 captações de água no aquífero e identificados os usos do solo na região, recorrendo à informação recolhida nos processos de licenciamento e à fotointerpretação. Determinaram-se os volumes de água extraídos para os diferentes usos do solo, os níveis piezométricos e o balanço hídrico do sistema aquífero. Recolheram-se dados relativos aos cloretos e à condutividade, no sentido de se verificar a existência de fenómenos de intrusão salina. Utilizaram-se as concentrações de nitratos como indicadoras da pressão agrícola. Os resultados foram representados em ambiente SIG em mapas. A análise da informação recolhida evidenciou um balanço hídrico positivo neste sistema aquífero, considerando-se que atualmente se encontra em bom estado quantitativo. Nos dados relativos à qualidade da água, nos últimos anos, parece ter-se verificado uma melhoria não havendo evidências de situações graves de intrusão salina. Relativamente à pressão agrícola, só em casos pontuais, foram detetados elevados valores de nitratos no aquífero. Consideraram-se três tipos de cenários futuros, com diferentes possibilidades de consumos de água, associados ao aumento das áreas cultivadas com citrinos e ao aumento dos espaços verdes urbanos. Chegou-se à conclusão que, toda a área classificada como Reserva Agrícola Nacional poderia ser utilizada para produção de citrinos, e que os espaços não ocupados na área urbana Portimão poderiam ser utilizados para espaços verdes regados, sem que isso pusesse em causa a sustentabilidade do sistema aquífero. Portanto, atendendo aos dados analisados neste Estágio, considera-se que atualmente se pode aumentar o volume de água extraído deste aquífero sem pôr em causa a sua sustentabilidade. No entanto, é fundamental que este sistema aquífero seja monitorizado periodicamente, de forma a atualizar-se o seu Balanço Hídrico e o estado da qualidade da água, assegurando que os volumes extraídos são adequados de forma sustentável à procura de água e que se contribui para o desenvolvimento económico da região

Geochemistry of groundwater of shallow coastal aquifers of Eastern Dahomey Basin, Southwestern Nigeria

Coastal basins offer valuable land, water and economic resources and have high economic contribution to national and global development; hence the high population of coastal cities around the world due to their commercial, industrial and agricultural significance. Some of the cities and towns situated in coastal parts of Africa rely on the freshwater of the coastal aquifers to meet the vast shortage in water supply resulting from infrastructural deficiency and decay. Complexity of groundwater occurrence and distribution in basement aquifers lead most of the developing countries to depending on basins to meet their daily water demand as these basins are known for high freshwater potential. The Eastern Dahomey Basin (EDB) is not an exemption. The high rate of population growth, unpredictable rainfall patterns, rise in sea-levels, coupled with over abstraction and seawater intrusion, land use activities, climate conditions, and the geological setting have significant influence on groundwater chemistry and quality. Recently, groundwater resources management has become preferentially higher in the agenda of the sustainable development goal of the United Nations with much attention on developing countries where groundwater remains their source of water demand for various usages. As data and information on groundwater is critical to its sustainable management, especially at a basin scale. This study presents a comprehensive groundwater geochemistry of the Eastern Dahomey basin to contribute to the amount of knowledge available to better increase the effective framework of integrated water resources management in Sub-Saharan Africa. A total of 230 water samples were collected between May 2017 and April 2018, a period which spanned through wet and dry seasons, from the shallow boreholes and hand-dug wells. These 230 groundwater samples (97 in wet season and 133 in dry season) were analysed for essential water quality parameters such as pH, Electrical Conductivity (EC), Total Dissolved solid (TDS), Total Hardness (TH), Ca, Mg, Na, K, HCO3, Cl, SO4, NO3, F and the trace metals As, Cd, Fe, Mn, Pb and Si and stable Environmental Isotopes of δ18O and δ2H. In addition to these, stable isotopes of δ18O and δ2H in precipitation data from three selected GNIP stations, Douala, Cotonou and Kano within the West Africa, were collected for comparative analysis. These data were subjected to evaluation for different quality indices such as GQIswi, SMI, GWQI and ionic ratios, while modelling, plotting and statistical were carried out using analysis using ArcGIS, MATLAB, Geochemist Workbench, SPSS, Origin pro and Microsoft Excel. Electrical resistivity prospecting and borehole logging were carried out in locations with enhanced electrical conductivity around the coastal communities. Three traverses A-B, C-D and E-F were selected along which ERT and IP were carried out in directions perpendicular and parallel to the coastline and correlated with borehole logs. Higher salinities above 1000 μS/cm were observed in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. HFE-D revealed that mixed groundwater of Na+Ca-HCO3, Na-Cl and Ca-Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi show areas within 3 km from the coastline that are more sensitive vulnerable to seawater intrusion. This result therefore guided our selection of areas for electrical resistivity geophysical investigation. Electrical resistivity tomography (ERT) and induced polarisation (IP) indicated a saline water-saturated layer of fine-grained sand and silty clay which is overlaid by the unconsolidated unconfined freshwater aquifer in area around Lekki and Okun Ajjah while low resistivity of clay lenses were found at Igbokoda, Ugbonla and Ode-Mahin. Correlation of selected ERT results with borehole logs further affirmed the suspected lithology from the sections. Results of the hydrochemical model revealed Ca-HCO3 and Na-Cl as dominant water types with other mixing water types such as Ca-SO4, Ca-Cl, Na-SO4 and K-Mg-HCO3 which characterised early stage of groundwater transformation in the shallow coastal basin. In addition, a comparison of the δ18O and δ2H isotopic compositions of groundwater and precipitation in the three selected stations, with their respective D-excess values established low evapotranspiration induced isotope enrichment, which could be due to higher precipitation and humidity in the region resulting in low isotope fractionation, hence, little effect of seasonal variations. Assessing the future use of groundwater for irrigation suggests some parts of the aquifer may have unsuitable quality based on the percentage sodium (%Na), Kelly’s ratio (KR), magnesium ratio (MR) and total hardness. Results of GQWI revealed 44.8, 22.9 and 12.5% of water samples during the wet season fell in the class of excellent, good and moderate quality respectively, while 8.3 %, 1.0 and 10.4 % fell in a class of poor, very poor and nonpotable water quality. Correlating the spatial distribution of the GWQI with the land use pattern map of the area revealed the least potable water is clustered around settlement areas, indicating groundwater quality has been impacted by municipal, industrial and agricultural waste. The linear regression modelling established significant relationships between SWL, SO42−, NO3−, Fe, and Eh for both wet and dry seasons with the p-value falling between 75% and 95%, which can also be seen in the plots of Eh/ORP against Fe2+, Mn2+, SO42−, and NO3−. This study has identified groundwater quality deterioration within the shallow coastal aquifers of the eastern Dahomey basin. Hydrochemical and geophysical approach established saltwater intrusion into freshwater aquifer occurred both as seawater intrusion, dissolution of evaporites within the clay lenses and sea spraying within the close proximity to the sea. Hydrochemical and environmental Isotopes classified as young from a meteoric source, which is still in the early stage of geochemical evolution with short residence time. A combination of groundwater quality indices (GWQI) and biogeochemical (redox) analysis also established the influence of natural and anthropogenic effect of climate change, urbanisation, industrialisation and agriculture on the groundwater quality of the basin. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin. There is a need for waste management policy review and enforcement to support sustainable groundwater resource management and sustainable development goal number 6 (SDG6). Keywords: Groundwater; Saltwater intrusion; Groundwater quality index; freshwater; Coastal aquifer; Clay lenses; Saline water; Freshwater; Geoelectrical Layers; Hydrogeochemistry; mineral dissolution; Saturation Index; Stable Isotopes; Environmental Isotopes; Precipitation; Groundwater Origin; Residence time; Climate change; Biogeochemical Processes; Redox and Metals mobilisation; land use; seasonal effects; Anthropogenic; water resource management.Coastal basins offer valuable land, water and economic resources and have high economic contribution to national and global development; hence the high population of coastal cities around the world due to their commercial, industrial and agricultural significance. Some of the cities and towns situated in coastal parts of Africa rely on the freshwater of the coastal aquifers to meet the vast shortage in water supply resulting from infrastructural deficiency and decay. Complexity of groundwater occurrence and distribution in basement aquifers lead most of the developing countries to depending on basins to meet their daily water demand as these basins are known for high freshwater potential. The Eastern Dahomey Basin (EDB) is not an exemption. The high rate of population growth, unpredictable rainfall patterns, rise in sea-levels, coupled with over abstraction and seawater intrusion, land use activities, climate conditions, and the geological setting have significant influence on groundwater chemistry and quality. Recently, groundwater resources management has become preferentially higher in the agenda of the sustainable development goal of the United Nations with much attention on developing countries where groundwater remains their source of water demand for various usages. As data and information on groundwater is critical to its sustainable management, especially at a basin scale. This study presents a comprehensive groundwater geochemistry of the Eastern Dahomey basin to contribute to the amount of knowledge available to better increase the effective framework of integrated water resources management in Sub-Saharan Africa. A total of 230 water samples were collected between May 2017 and April 2018, a period which spanned through wet and dry seasons, from the shallow boreholes and hand-dug wells. These 230 groundwater samples (97 in wet season and 133 in dry season) were analysed for essential water quality parameters such as pH, Electrical Conductivity (EC), Total Dissolved solid (TDS), Total Hardness (TH), Ca, Mg, Na, K, HCO3, Cl, SO4, NO3, F and the trace metals As, Cd, Fe, Mn, Pb and Si and stable Environmental Isotopes of δ18O and δ2H. In addition to these, stable isotopes of δ18O and δ2H in precipitation data from three selected GNIP stations, Douala, Cotonou and Kano within the West Africa, were collected for comparative analysis. These data were subjected to evaluation for different quality indices such as GQIswi, SMI, GWQI and ionic ratios, while modelling, plotting and statistical were carried out using analysis using ArcGIS, MATLAB, Geochemist Workbench, SPSS, Origin pro and Microsoft Excel. Electrical resistivity prospecting and borehole logging were carried out in locations with enhanced electrical conductivity around the coastal communities. Three traverses A-B, C-D and E-F were selected along which ERT and IP were carried out in directions perpendicular and parallel to the coastline and correlated with borehole logs. Higher salinities above 1000 μS/cm were observed in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. HFE-D revealed that mixed groundwater of Na+Ca-HCO3, Na-Cl and Ca-Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi show areas within 3 km from the coastline that are more sensitive vulnerable to seawater intrusion. This result therefore guided our selection of areas for electrical resistivity geophysical investigation. Electrical resistivity tomography (ERT) and induced polarisation (IP) indicated a saline water-saturated layer of fine-grained sand and silty clay which is overlaid by the unconsolidated unconfined freshwater aquifer in area around Lekki and Okun Ajjah while low resistivity of clay lenses were found at Igbokoda, Ugbonla and Ode-Mahin. Correlation of selected ERT results with borehole logs further affirmed the suspected lithology from the sections. Results of the hydrochemical model revealed Ca-HCO3 and Na-Cl as dominant water types with other mixing water types such as Ca-SO4, Ca-Cl, Na-SO4 and K-Mg-HCO3 which characterised early stage of groundwater transformation in the shallow coastal basin. In addition, a comparison of the δ18O and δ2H isotopic compositions of groundwater and precipitation in the three selected stations, with their respective D-excess values established low evapotranspiration induced isotope enrichment, which could be due to higher precipitation and humidity in the region resulting in low isotope fractionation, hence, little effect of seasonal variations. Assessing the future use of groundwater for irrigation suggests some parts of the aquifer may have unsuitable quality based on the percentage sodium (%Na), Kelly’s ratio (KR), magnesium ratio (MR) and total hardness. Results of GQWI revealed 44.8, 22.9 and 12.5% of water samples during the wet season fell in the class of excellent, good and moderate quality respectively, while 8.3 %, 1.0 and 10.4 % fell in a class of poor, very poor and nonpotable water quality. Correlating the spatial distribution of the GWQI with the land use pattern map of the area revealed the least potable water is clustered around settlement areas, indicating groundwater quality has been impacted by municipal, industrial and agricultural waste. The linear regression modelling established significant relationships between SWL, SO42−, NO3−, Fe, and Eh for both wet and dry seasons with the p-value falling between 75% and 95%, which can also be seen in the plots of Eh/ORP against Fe2+, Mn2+, SO42−, and NO3−. This study has identified groundwater quality deterioration within the shallow coastal aquifers of the eastern Dahomey basin. Hydrochemical and geophysical approach established saltwater intrusion into freshwater aquifer occurred both as seawater intrusion, dissolution of evaporites within the clay lenses and sea spraying within the close proximity to the sea. Hydrochemical and environmental Isotopes classified as young from a meteoric source, which is still in the early stage of geochemical evolution with short residence time. A combination of groundwater quality indices (GWQI) and biogeochemical (redox) analysis also established the influence of natural and anthropogenic effect of climate change, urbanisation, industrialisation and agriculture on the groundwater quality of the basin. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin. There is a need for waste management policy review and enforcement to support sustainable groundwater resource management and sustainable development goal number 6 (SDG6). Keywords: Groundwater; Saltwater intrusion; Groundwater quality index; freshwater; Coastal aquifer; Clay lenses; Saline water; Freshwater; Geoelectrical Layers; Hydrogeochemistry; mineral dissolution; Saturation Index; Stable Isotopes; Environmental Isotopes; Precipitation; Groundwater Origin; Residence time; Climate change; Biogeochemical Processes; Redox and Metals mobilisation; land use; seasonal effects; Anthropogenic; water resource management

Development of a groundwater quality index for seawater intrusion in coastal aquifers

Coastal aquifers are increasingly threatened by seawater intrusion due to increased urbanization, groundwater exploitation, and global sea-level rise. Pattern diagrams, which constitute the outcome of several hydro-geochemical processes, have traditionally been used to characterize vulnerability to seawater intrusion. However, the formats of such diagrams do not facilitate the geospatial analysis of groundwater quality, thus limiting the ability of spatio-temporal mapping and monitoring. This raises the need to transform the information from current pattern diagrams into a format that can be readily used under a GIS framework to define vulnerable areas prone to seawater intrusion. In this study, a groundwater quality index specific to seawater intrusion (GQISWI) was developed for the purpose of aggregating data into a comprehensible format that allows spatial analysis. The index was evaluated with data from various coastal regions worldwide and then applied at a pilot karstic aquifer along the eastern coast of the Mediterranean Sea

Monitoring seawater intrusion into the fractured UK Chalk aquifer using measurements of self-potential (SP)

Using laboratory, numerical and field experiments this study investigated whether borehole measurements of self-potential (SP) can be used to monitor seawater intrusion into the fractured UK Chalk aquifer. The SP, a natural voltage, arises in water saturated fractured porous media due to gradients in pressure (electrokinetic (EK) potential) and concentration (exclusion-diffusion (EED) potential), both features of seawater intrusion. An electrode array was installed in a monitoring borehole c.1.7 km from the coast, in Saltdean, East Sussex, and c.1.3 km from an active abstraction borehole. Head fluctuations in the monitoring borehole were controlled by tidal processes and seasonal changes in inland head. SP monitoring over 1.5 years revealed tidal SP signals. The fluctuations (c.600 μV) were two orders of magnitude larger than those observed at an inland site in the same aquifer, near Reading in Berkshire. Numerical simulation, supported by laboratory measurements, of the coupled hydrodynamic and electrical processes in the coastal aquifer suggested that the EK potential generated by tidal processes was one order of magnitude too small to be responsible for the tidal SP fluctuations. Instead, SP was caused by the EED potential that arose due to the concentration gradient between groundwater and seawater across the saline front (i.e. the 1000 mg/l isoline) some distance from the borehole. The saline front moved through a fracture at the base of the borehole in response to tides. A vertical SP gradient (c.0.22 mV/m), only present in the coastal borehole, was also observed. Modelling suggested that the gradient was due to the close proximity of the saline front (c.4 m) below the borehole and was caused by the EED potential. In August 2013 and 2014, tides and a decline in inland head caused saline water to enter the borehole. Fluid electrical conductivity logging showed that entry was via the fracture. Prior to each occurrence of saline breakthrough, an increase in the SP of c.300 μV was observed, commencing c.7 days before saline water was detected in the borehole. Although this study focused on a monitoring borehole, SP arrays could be installed in abstraction boreholes. The results suggest that SP monitoring may be used to provide early warning of saline water breakthrough, allowing for improved management of groundwater resources in coastal aquifers.Open Acces