This study aimed to develop a geochemical database by thoroughly analyzing groundwater and sediments from coastal aquifers of southwest Bangladesh. Moreover, we investigated the source of sediment deposition and the mechanisms behind the presence of arsenic and salinity in groundwater. The seasonal distribution patterns of arsenic among the shallow and deep coastal aquifers were found to be 45.12 µg/l and 20.65 µg/l during dry and wet seasons, respectively. Moreover, the groundwater salinity distribution ranged from 3262.88 mg/l to 1930.88 mg/l during the dry and wet seasons. Cored sediment samples showed fine to medium sands of 92%, with silt and clay particles. The petrographic study of authigenic and heavy minerals revealed that the mineral grains were subangular to angular, indicating their textural immaturity of coastal sediments. The reactivity of goethite (FeOOH) and siderite (FeCO₃) minerals suggests that the aquifers were subjected to slightly oxidized to moderately reducing conditions, with ORP values ranging from +50.40 mv to −149.5 mv. Such redox conditions could potentially result in the enrichment and mobility of arsenic in the groundwater. Although arsenic concentrations in deep aquifers are relatively low, higher salinity values are found in both shallow and intermediate coastal aquifers.

A synchronization study among hydrochemistry, hydrochemical facies evaluation, EC observation, salinity distribution and groundwater flow direction has been addressed to assess river water–groundwater–seawater interactions in the coastal delta of southern Bangladesh. The findings show that river water, shallow groundwater and deep groundwater interact with seawater at various intensities within the complex dynamics of hydrochemical facies evaluation. Deep groundwater is intensively influenced by seawater, where shallow groundwater is moderately affected and river water is very negligibly affected. Major cation and anion have been plotted in the Piper diagrams and hydrochemical facies diagrams (HFE-D) to clarify the result. More than 60% of the water samples of the river lie on the Ca-HCO₃ (or Mg-HCO₃) facies quadrant, and more than 70% of the shallow groundwater samples and more than 95% of the deep groundwater samples lie on the Na-Cl facies quadrant of the HFE-D diagram. River water types are dissimilar, and approximately 82% of facies are characterized by freshening phases and 18% by intrusion phases. Mixed water types with predominate of Na-Cl were observed in shallow groundwater where the hydrochemical facies are characterized by 53 percent freshening phases and 47 percent intrusion phases. Deep groundwater hydrochemistry clearly indicates the dominant Na-Cl type of water in the study area where only four hydrochemical facies are observed and 78 percent correspond to the intrusion phases and 22 percent to the freshening phases. Both direct and reverse cation exchange reactions take place in shallow groundwater, where deep groundwater is predominantly characterized by reverse cation exchange reactions. Two end members: seawater of Bay of Bengal and freshwater, contribute to the exchange reactions in the coastal aquifer of the study area. In terms of nitrate contamination, river waters are affected by negligible to low concentrations, shallow groundwater is affected by moderate to high concentrations and deep groundwater is affected by moderate to very high nitrate concentrations. Dissimilarity in electrical conductivity (EC) values, variation of salinity distribution maps and groundwater flow direction suggest the possible interconnections among river water, groundwater and aquifer sediments. Significant concentrations of Na⁺ and Cl⁻ ions lead to seawater contamination in groundwater, and HCO₃⁻ along with Na⁺, Ca²⁺, Mg²⁺ in river water suggests mixing of freshwater and seawater, which could have adverse effects both in coastal delta aquatic life and in agriculture.

Salinity causes a hostile environmental impact throughout the year in the coastal region of Bangladesh, and its severity increases day by day. Because of upstream freshwater flow reduction and massive groundwater extraction, salinity has increased substantially over the last three decades. Moreover, arsenic contamination in shallow groundwater makes the groundwater unsuitable for potable use. Consequently, the coastal area suffers from acute storage of safe water supply. Salinity also negatively impacts human activities, livelihood, agricultural production, and the aquatic ecosystem. Though the shallow aquifer contains high salinity and a small amount of Arsenic (As), the very shallow aquifer (within 3m to 8m) contains fresh water in many areas in the rainy season due to the direct recharge of rainwater. However, rainfall recharge varies significantly depending on the geological and hydrogeological settings. Specifically, up to 50% of annual rainfall is stored in shallow aquifers of Quaternary sands through direct infiltration. The research’s principal objective is to identify the safe and sustainable drinking water source in the arsenic and saline-prone coastal region. Groundwater samples were collected from the different locations of the study area during both dry and wet seasons and examined seasonal variations in groundwater table and salinity levels. The chemical analyses and Physico-chemical parameters indicate that the groundwater samples are suitable for drinking. Except for some groundwater samples from the wet season, the salinity of all samples was under the allowable limit for Bangladesh (<2000 µS/cm), and the targeted aquifer was almost arsenic (50 µg/l) free. Therefore, a comprehensive analysis has been made to accomplish the study goals. Particularly, the groundwater’s electrical conductivity (EC) values of most samples were measured within the limit of fresh or brackish water (<2000 μS/cm). Overall, the results indicate the prospect of a very shallow aquifer as a source of freshwater for drinking purposes throughout the year, considering both arsenic and salinity, which effectively solve the freshwater shortage, especially in the saline-arsenic prone area.

This study assessed the extent of potential toxic element (PTE) contamination and its associated health risks in groundwater from the Tista Floodplain, Rangpur, northern Bangladesh. A total of 130 groundwater samples (65 from the wet season and 65 from the dry season) were analyzed for six PTEs: As, Al, Cu, Mn, Cr, and B using Atomic Absorption Spectrophotometry (AAS) and VIS Spectrophotometry. The mean concentrations of PTEs followed a decreasing order of B > Mn > Cu > Al > As > Cr in the wet season and Mn > B > Al > Cu > As > Cr in the dry season. Metal evaluation index and degree of contamination indicated minor contamination, while the Nemerow pollution index showed moderate to severe contamination, especially in Nilphamari district. Contamination levels were higher in the dry season compared to the wet season. No individual PTEs posed non-carcinogenic health risks, but combined PTEs presented potential health threats, particularly to infants, with higher severity during the dry season. No dermal health risks were identified. Despite the relatively low contamination levels, monitoring policies for new and existing tubewells should be implemented to ensure sustainable groundwater management and long-term water quality.

This study evaluates the water quality and hydrogeological assessment for domestic use, irrigation, and drinking near the Barapukuria coal mine area. The groundwater table data from 2010 to 2021 revealed that it reached its lowest point (8.0 to 11.0 m) pre-monsoon and its highest (3.0 to 6.0 m) post-monsoon. The concentrations of major anions and cations, and the values of EC, DO, TDS, pH, and salinity, were found within the acceptable and allowable limits for all purposes, except for a small amount of Fe²⁻ and HCO₃⁻, when conducting field investigation and laboratory analysis. Numerous noteworthy analyses were carried out, including Piper's trilinear diagram, US salinity chart, Durov diagram, and Wilcox diagram. The weathering of silicates, specifically crystalline dolomitic limestone and Ca-Mg silicates, is responsible for the dominance of ions in the sequence Na⁺ > Mg²⁺ > Ca²⁺ > K⁺. Alkali-earth metals and weakly acidic anions are present, like early groundwater reserves, as the dominance of Na-K-HCO₃ and Mg-HCO₃ and less recently recharged water of Ca-HCO₃, which has a mixed anion and cation dominance and temporary hardness. The computed values of TDS, EC, SAR, TH, PI, and CAI indicate excellent to good quality water and are acceptable for all purposes with a few anomalies in Na% (64.99 meq/L), MH (59.02 meq/L), and RSC (144.11 meq/L). Changes in groundwater quality and level are gradually lowering as mining continues, falling below the suction limit (> 7.5 m) for hand tube wells (HTWs) and eventually because groundwater is being used for power plants. Regular surface and groundwater quality and level monitoring is recommended around the mine area.