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.

Abstract: Abstract In the Manas River basin (MRB), groundwater salinization has become a major concern, impeding groundwater use considerably. Isotopic and hydrogeochemical characteristics of 73 groundwater and 11 surface water samples from the basin were analysed to determine the salinization process and potential sources of salinity. Groundwater salinity ranged from 0.2 to 11.91 g/L, and high salinities were generally located in the discharge area, arable land irrigated by groundwater, and depression cone area. The quantitative contributions of the evaporation effect were calculated, and the various groundwater contributions of transpiration, mineral dissolution, and agricultural irrigation were identified using hydrogeochemical diagrams and δD and δ18O compositions of the groundwater and surface water samples. The average evaporation contribution ratios to salinity were 5.87% and 32.7% in groundwater and surface water, respectively. From the piedmont plain to the desert plain, the average groundwater loss by evaporation increased from 7% to 29%. However, the increases in salinity by evaporation were small according to the deuterium excess signals. Mineral dissolution, transpiration, and agricultural irrigation activities were the major causes of groundwater salinization. Isotopic information revealed that river leakage quickly infiltrated into aquifers in the piedmont area with weak evaporation effects. The recharge water interacted with the sediments and dissolved minerals and subsequently increased the salinity along the flow path. In the irrigation land, shallow groundwater salinity and Cl− concentrations increased but not δ18O, suggesting that both the leaching of soil salts due to irrigation and transpiration effect dominated in controlling the hydrogeochemistry. Depleted δ18O and high Cl− concentrations in the middle and deep groundwater revealed the combined effects of mixing with paleo-water and mineral dissolution with a long residence time. These results could contribute to the management of groundwater sources and future utilization programs in the MRB and similar areas.