Hydrochemistry in coastal aquifer of southwest Bangladesh: origin of salinity

Socio-hydrogeological Potential for Managed Aquifer Recharge in the Fresh-saline Aquifers of Southwestern Bangladesh

“…Parts of the groundwater are described to consist of connate groundwater, with the salinity being determined by the moment at which the aquifers became sealed off . Some limited present-day processes have been proposed to occur in areas with a relatively thin clayey top layer, such as infiltration of saline water from aquaculture ponds (Paul et al, 2011), evaporite salts redissolving in the soil during the wet season (Sarker et al, 2018) and infiltration of brackish or saline water from tidal rivers , as well as infiltration of freshwater from rain or freshwater ponds . However, much salinity variation had not been explained, and the exact controlling processes were unclear.…”

“…The protocol for measuring EC from the tube wells included the purging of the stagnant water from the tube well by pumping each tube well for at least a minute. We expect the measurements collected over the span of several years to be comparable, as seasonal and inter-annual salinity variation is expected to be low because of the stagnancy of the connate-groundwater-dominated hydrological system Sarker et al, 2018;. This low-cost method made it possible to collect 1998 new EC groundwater measurements at tube wells.…”

“…Most previous studies in southwestern Bangladesh focussed on understanding the groundwater salinity variation by identifying the controlling hydrological processes in small, local study areas Sarker et al, 2018;. These studies have found much of the groundwater to be connate groundwater due to the subsurface lithology being characterized by thick covering clay layers and the landscape being flat leading to a stagnant hydrological system.…”

“…These studies have found much of the groundwater to be connate groundwater due to the subsurface lithology being characterized by thick covering clay layers and the landscape being flat leading to a stagnant hydrological system. The occurrence and thickness of surface clay layers with low infiltration capacity is described to explain how isolated the groundwater is and, therefore, whether the groundwater consists of solely connate water formed under paleo conditions or whether the groundwater salinity has also been influenced by active processes Sarker et al, 2018;. Previous studies proposed several active processes that vary depending on the present-day landscape features.…”

“…Freshwater recharge from rain or rain-fed pond water has been described in areas of higher elevation in southwestern Bangladesh , as well as in other coastal regions . Salinization in low-lying areas has been attributed to three processes in southwestern Bangladesh: tidally influenced flooding , recharge by rainwater that has dissolved evaporite salts in the soil (Sarker et al, 2018), and recharge from aquaculture ponds (Paul and Vogl, 2011;. At a regional scale, the relevance of these hydrological processes is unexplored and, accordingly, the regional groundwater salinity variation remains poorly understood: Only some rough regional salinity gradients have been delineated, indicating a trend for salinity to increase from north to south (Bangladesh Water Development Board, 2013;Zahid et al, 2018).…”