SOURCES OF SALINITY IN SURFACE AND GROUND WATER IN A POLDER IN SOUTHWESTERN BANGLADESH

To identify the cause of salinization in a polder in the coastal zone of Bangladesh we collected, during the dry (May 2012) and wet (October 2012) seasons, surface water samples from freshwater ponds (n=27), rice paddies (13), saltwater ponds for brine shrimp aquaculture (11), and tidal channels (12), and shallow groundwater samples from tubewells (54). We analyzed for sulfur isotopes, Dissolved Organic Carbon (DOC), cations using ICP, and anions using ion chromatography. Groundwater often exceeds safe limits for salinity and arsenic, while surface waters often exceed safe drinking water limits for salinity. Even in the wet season 23% of rice paddy samples were too saline for farmed crops.

Shallow groundwater has high spatial and compositional variability, but little or no seasonal variation, suggesting low recharge rates and flow velocities. Drill cuttings, exposures in pits and regional studies show a ~5m thick mud cap atop a shallow aquifer composed of fine sand and interbedded silts and clays. Adjacent saline and freshwater ponds suggest mud-rich surface deposits have low permeability. Inversions of electromagnetic survey data (GEM-2) show potential recharge areas where the mud cap pinches out.

Water compositions show that shrimp ponds are sourced from tidal channel water in the dry season, but have higher DOC. Principal components analysis suggests that surface waters are mixtures of meteoric water and salts from soil, while groundwaters are mixtures of seawater and meteoric water, but with high DOC possibly sourced from shrimp ponds. High DOC likely causes Fe oxyhydroxide reduction in shallow aquifers, which adds As to groundwater but causes sulfide precipitation. Salts may be introduced to surface water bodies and soil by shrimp farming in the dry season and by inundation during monsoons, but are not introduced by use of groundwater for irrigation.