The Effect of Irrigation Source on Arsenic and Salt Concentrations in Soil in Southwest Bangladesh

Abstract

Soil (n=45) and water (n=111) samples were collected in SW Bangladesh in the wet (May) and dry (November) seasons in 2016 to investigate the effects of irrigation by tidal channel water and groundwater on concentrations of arsenic and salts in soil. Between 1.4% and 2.1% of arsenic in soils is soluble, indicating that most arsenic is bound to soil minerals. Bulk soil arsenic is positively correlated with arsenic in irrigation water, suggesting that tube well irrigation adds arsenic to soils. Soluble As in rice paddy soil extracts is positively correlated with concentrations of sulfur and organic carbon in extracts and negatively correlated with S concentration in bulk soil, suggesting that As is released from soil by sulfide dissolution and possibly complexes with organic carbon in solution. Bulk soil arsenic is higher in rice paddies during the wet season, suggesting that reducing conditions present in waterlogged soils cause As-rich sulfide precipitation. Rice paddy soils irrigated with As-rich tube well water do not have higher bulk or soluble arsenic concentrations than paddies irrigated by tidal channel water. Salts in water are concentrated during the dry season due to decreased precipitation and increased evaporation rates. Soluble log median Na soil concentrations fall between 158 ppm and 1,259 ppm, while log median Na concentrations in bulk soils range between 6,310 ppm and 10,000 ppm, indicating the binding of Na to clay minerals. Comparison of soil/sediment samples from rice paddies, tidal channels, and the Sundarbans suggests that cultivation reduces dry season soluble salt and As concentrations in rice paddy soils.