Abstract: In arid and semi-arid areas, the groundwater is the main source of water supply and agricultural activity. Overexploitation of coastal aquifers and pollution vulnerability are among the main problems related to groundwater resources assessment and management in these zones. In fact, in the last decades, these resources have been threatened by a degradation of their quality and quantity that furthers natural and anthropic effects, such as climate change, seawater intrusion and overexploitation. However, the protection and management of these resources requires knowledge of the origin of their mineralization. In this study, the Essaouira basin is selected as a typical example. Stable isotopes (18O and 2H) together with geochemical data were used to identify the groundwater salinization origin in the coastal aquifers of the Essaouira basin. The results of both the approaches show that the groundwater mineralization is due to: (1) the dissolution of salt minerals, (2) the ion exchange phenomena, (3) seawater intrusion, and (4) sulphate reduction. Also, the recharge is supported by fast infiltration of oceanic precipitation without significant evaporation.

Abstract: 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.