Abstract:
Groundwater, a critical resource for drinking, agriculture, and industry in Bangladesh, faces significant challenges to its quality and sustainability due to spatial, depth-related, and seasonal variability. This study evaluates the seasonal and spatial variability of hydrochemical characteristics of groundwater across 11 physiographic regions of Bangladesh. Groundwater data from over 900 wells, covering shallow (<50m), intermediate depth (50 – 200m), and deep (>200m) groundwater, were collected during both wet (August – October, 2020) and dry (March – May, 2021) seasons. Water quality index (WQI) and Irrigation Water Quality Index (IWQI), coupled with statistical tests and hydrochemical analysis, were used to assess variations in water quality and underlying geochemical processes. For hydrochemical analysis, Piper diagram, Gibbs diagram, Gaillardet diagram, and USSL diagram were prepared for each physiographic region. Results indicate silicate weathering is the dominant geochemical process nationwide, with carbonate dissolution, seawater mixing, and redox processes contributing to region-specific variations. Coastal regions, particularly the Delta (Tidal) tract, exhibit poor water quality due to seawater intrusion, resulting in Na-Cl/Na-SO4 facies with high salinity and sodicity, rendering water largely unsuitable for drinking and irrigation. Central and southern floodplains show depth-stratified quality, with shallow groundwater heavily degraded by salinization, arsenic, Fe2+, and Mn2+ mobilization, and over half classified as “unsuitable” by the WQI. Intermediate depth groundwater shows mixed quality, vulnerable to seasonal salinity spikes, while deep groundwater generally offers better potable water with stable Ca-Mg-HCO3 facies. Seasonally, dry periods exacerbate concentrations of EC, TDS, major ions, and trace metals due to reduced recharge and evapotranspiration, deteriorating drinking water quality. However, IWQI remains relatively stable across seasons and shows slight improvement in the dry season due to ion flushing and cation enrichment. These findings highlight the need for targeted management, including enhanced monitoring in coastal zones and shallow depths.