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Hydrochemical and stable isotope (δ 18 O, δ 2 H) data were used to identify the recharge sources of major springs and the hydraulic interconnection between the volcanic aquifer and springs in the Gilgel Abay catchment and adjacent areas. The hydrochemical data analysis showed that all water samples of springs and shallow wells have freshwater chemistry, Ca HCO 3 to Ca Mg HCO 3 types. This is mainly controlled by dissolution/hydrolysis of silicate minerals. The analyzed stable isotope data indicate that springs water, except Dengel Mesk, Kurt Bahir and Bility springs, and well waters, except Dangila well, fall close to the LMWL. This clearly shows that the infiltrated rainwater did not undergo much evaporation and δ 18 O values for spring water and groundwater are nearly equal to the value of Ethiopian summer rainfall, which is −2.5‰. Therefore, generally both stable isotope and hydrochemical data show the recharge source to springs and shallow groundwater is primarily from precipitation. Furthermore, data suggest that rock-water interaction has remained relatively limited, pointing to relatively short residence times, and local recharge rather than regional recharge. [ABSTRACT FROM AUTHOR]

People in the southwestern (SW) coastal part of Bangladesh are suffering from a severe freshwater crisis due to saline groundwater at a shallow depth. Fresh groundwater below a 200 m depth is an option, but it is costly to construct deep tubewells for the local inhabitants. The processes of salinization and freshening were previously identified using conventional methods. In this study, we brought new insight into these processes by analyzing existing datasets using multivariate statistics to identify the factors affecting groundwater chemistry. Cluster analysis (CA) revealed three major clusters. Cluster A corresponded to saline (NaCl-type) water. Cluster B was also saline (NaCl-type) water but showed mixing effects. Cluster C was fresh groundwater (NaHCO3-type) and isolated. The hydrochemical characteristics of clusters A, B and C compared remarkably well with the groundwaters from the upper shallow aquifer (USA), lower shallow aquifer (LSA) and deep aquifer (DA), respectively. Factor analysis (FA) showed that 75% of the total variance was influenced by evaporate dissolution, carbonate dissolution/precipitation, cation exchange and anthropogenic pollution to some extent. Therefore, the integrated approach showed the validity of applying multivariate statistical techniques to infer the dominant hydrochemistry and to characterize and understand a complicated hydrogeological system. [ABSTRACT FROM AUTHOR]

Several studies conducted on occurrences of evaporitic salts in various parts of the world often interpret the formation of such salts as the result of the evaporation of subsurface brines, saline lake waters and sea water-contaminated groundwater, on one hand, and the deposition of sea sprays and wind-transported salts, on the other hand. This study seeks to understand processes governing the formation of evaporitic salts in a humid tropical context and to establish a link between the occurrence of these salts and the different processes controlling the hydrogeochemical evolution of groundwater in Bugesera region, in north-eastern Burundi. Unlike previous studies, salt occurrences in the study area occur in a continental and humid tropical environment where chemical analyses of water samples from several small lakes reveal rather fresh water. While salt incrustations reported in the western Burundi occur in a sedimentary basin which is made up of a mixture of lacustrine and fluviatile sediments, the evaporitic salts in the depression of Bugesera appear in the upper layers of the weathered overburden developed on Precambrian basement rocks. These facts suggest that the lithological nature and setting of the area may not be the key factor controlling the formation of these salt deposits. X-ray diffraction (XRD) analyses performed on saline soils from two quarries in Bugesera region confirm the presence of salt minerals including particularly gypsum (CaSO4·2H2O), halite (NaCl) and thenardite (Na2SO4). Other minerals identified such as goethite, kaolinite, muscovite and quartz may be part of the soil matrix. Chemical analyses on leachates of soil samples from the 2 quarries in Bugesera region confirm their saline nature as evidenced by high TDS (1826-17,679 mg/l) and pH values (8.3-8.5). The predominance of Na+, Mg2+, Ca2+, K+, SO42−, Cl− in the chemical composition of the dissolved salts clearly reflects their mineralogical composition. This study clearly demonstrates that these evaporitic salts originate from the combination of three processes, namely the dissolution of aluminosilicate minerals which enrich groundwater in different salt-forming ions, the capillary rise which lifts the shallow groundwater and the dissolved ions up to the reach of evaporation and ultimately the evapoconcentration which precipitates the salts following well-established pathways. These findings are well supported by the results of the simulation of the evapoconcentration process with the PHREEQC hydrochemical model. [ABSTRACT FROM AUTHOR]

In the coastal region of Bangladesh, groundwater is mainly used for domestic and agricultural purposes, but salinization of many groundwater resources limits its suitability for human consumption and practical application. This paper reports the results of a study that has mapped the salinity distribution in different aquifer layers up to a depth of 300 m in a region bordering the Bay of Bengal based on the main hydrochemistry and has investigated the origin of the salinity using Cl/Br ratios of the samples. The subsurface consists of a sequence of deltaic sediments with an alternation of more sandy and clayey sections in which several aquifer layers can be recognized. The main hydrochemistry shows different main water types in the different aquifers, indicating varying stages of freshening or salinization processes. The most freshwater, soft NaHCO3-type water with Cl concentrations mostly below 100 mg/l, is found in the deepest aquifer at 200-300 m below ground level (b.g.l.), in which the fresh/saltwater interface is pushed far to the south. Salinity is a main problem in the shallow aquifer systems, where Cl concentrations rise to nearly 8000 mg/l and the groundwater is mostly brackish NaCl water. Investigation of the Cl/Br ratios has shown that the source of the salinity in the deep aquifer is mixing with old connate seawater and that the saline waters in the more shallow aquifers do not originate from old connate water or direct seawater intrusion, but are derived from the dissolution of evaporite salts. These must have been formed in a tidal flat under influence of a strong seasonal precipitation pattern. Long dry seasons with high evaporation rates have evaporated seawater from inundated gullies and depressions, leading to salt precipitation, while subsequent heavy monsoon rains have dissolved the formed salts, and the solution has infiltrated in the subsoil, recharging groundwater. [ABSTRACT FROM AUTHOR]

This study investigates hydrogeochemical processes controlling groundwater evolution in a Precambrian basement aquifer situated in northeastern Burundi, in Bugesera region. The hydrogeochemistry has been evaluated based on the analytical results of 143 water samples comprising 66 samples from shallow wells, 61 samples from springs, and 16 samples from the shallow lakes. While the stability diagrams, the evolutionary sequence of groundwater inferred from the Piper diagram and bivariate plots highlight the weathering of aluminosilicates as the primary process controlling groundwater chemistry in this aquifer, the increasing trend of chloride levels between the highlands (recharge area) and the depression of Bugesera (discharge area) and the random distribution of high nitrate content even in water samples from springs suggest that additional processes such as evaporative concentration and anthropogenic pollution play a secondary role in the hydrogeochemical evolution of groundwater. The spatial distribution of different hydrogeochemical parameters shows a general increasing trend from the highlands where water samples, mainly from springs, show low mineralisation (TDS = 26–152 mg/l) and low pH (field pH = 4.6–6.6), towards the depression of Bugesera where higher values of mineralisation (TDS = 44–3229 mg/l) and pH (field pH = 5.6–7.7) are observed. The overriding control of aluminosilicates weathering on the hydrogeochemical evolution of groundwater in Bugesera region is clearly demonstrated by the predominance of NaHCO3 and CaHCO3 water types and the cross-plots which show an increasing trend of alkaline and alkaline earth cations (Na + + K + + Ca 2 + + Mg 2 + ) released from the weathering of aluminosilicates along the flow line. Moreover, the poor correlation between the concentrations of SiO 2 and Na + , Ca 2 + , Mg 2 + , K + , HCO 3 – and TDS reflects the incongruent character of the weathering of aluminosilicates which causes part of the silica released from the weathering of silicates to be taken up into the neoformation of clay minerals. The anthropogenic pollution is reflected by the occurrence of NaNO 3 , CaNO 3 , KNO 3 and NaSO 4 types even in spring water. [ABSTRACT FROM AUTHOR]

Groundwater is globally important for human consumption, and changes in quality can have serious consequences. The study area is within a coastal aquifer where groundwater quality is influenced by various potential sources of salinity that determine the composition of water extracted from wells. Groundwater chemistry data from the aquifer have been acquired to determine the geochemical conditions and processes that occur in this area and assess their implications for aquifer susceptibility. Analysis of groundwater samples shows that the dominant watertype is mostly NaCl with pH < 7 in both aquifers (i.e. upper and lower) except for the shallow wells where CaHCO 3 prevails with pH ⩾ 7, and boreholes located near the Indian Ocean, where coral reef limestone deposits are located and the watertype evolves towards CaHCO 3 . In the lower aquifer, Cl − is higher than in the upper aquifer. The origin of salinity in the area is strongly influenced by groundwater ascending from deep marine Miocene Spatangid Shales through faults, seawater incursion on the border of the Indian Ocean, and throughout, there is some salinity within the Quaternary aquifer, especially in intercalated deltaic clays in the fluviatile deposits, showing some marine influences. The seawater intrusion is linked to the strongly increasing groundwater exploitation since 1997. Another process that plays a major role to the concentration of major ions in the groundwater is calcite dissolution. Next to geogenic salinity and seawater intrusion, anthropogenic pollution as well is affecting groundwater quality in the aquifer. An important result of this study is the observation of high nitrate concentrations, that call for improved sanitation in the area, where domestic sewage with on-site sanitation (mainly pit latrines) also threatens the groundwater resource. [ABSTRACT FROM AUTHOR]