| dc.description.abstract | This project attempts to identify the source of surface water and groundwater salinity observed in a polder in southwest Bangladesh through geochemical analyses. All surface water samples are mixtures of tidal channel water and meteoric water concentrated by evaporation; B, K, Mg, Na, Sr, Cl, Br, and S behave conservatively. Surface water types include rice paddies, freshwater ponds, shrimp ponds, and tidal channels. Thirty percent of rice paddy samples exceed the upper salinity limit for rice crops of 1.56 parts per thousand (ppt). Although compositionally similar to rice paddy water, field observations show that freshwater ponds are sourced from wet season tidal channels and then concentrated by evaporation. Evidence indicates that salts in freshwater ponds were not added by inundation of brackish water following Cyclone Aila. Brine shrimp ponds are sourced from dry season tidal channels, which contain water that is significantly more saline than in the wet season.
Groundwater is also identified as tidal channel water mixed with meteoric water, but is chemically unique from surface waters in that B and S do not behave conservatively, dissolved organic carbon concentrations are higher (except for shrimp ponds) and oxidation-reduction potential values (Eh in millivolts (mV)) are lower. Groundwater compositions show high spatial variability, suggesting low flow velocities. Groundwater conservative element concentrations are 30-50% higher in the dry season, suggesting limited recharge in the wet season. Nearly 50% of groundwater samples surpass the Bangladesh drinking water limit for arsenic of 50 micrograms per liter (µg/L). Sixty one percent of groundwater samples exceed the salinity limit of 3.25 ppt for safe consumption. Tidal channel water is the primary source of salinity in both surface water and groundwater. | |
