Salinization and arsenic contamination of surface water in southwest Bangladesh

Abstract

To identify the causes of salinization and arsenic contamination of surface water on an embanked island (i.e., polder) in the tidal delta plain of SW Bangladesh we collected and analyzed water samples in the dry (May) and wet (October) seasons in 2012–2013. Samples were collected from rice paddies (wet season), saltwater ponds used for brine shrimp aquaculture (dry season), freshwater ponds and tidal channels (both wet and dry season), and rainwater collectors. Continuous measurements of salinity from March 2012 to February 2013 show that tidal channel water increases from ~0.15 ppt in the wet season up to ~20 ppt in the dry season. On the polder, surface water exceeds the World Health Organization drinking water guideline of 10 μg As/L in 78% of shrimp ponds and 27% of rice paddies, raising concerns that produced shrimp and rice could have unsafe levels of As. Drinking water sources also often have unsafe As levels, with 83% of tubewell and 43% of freshwater pond samples having >10 μg As/L. Water compositions and field observations are consistent with shrimp pond water being sourced from tidal channels during the dry season, rather than the locally saline groundwater from tubewells. Irrigation water for rice paddies is also obtained from the tidal channels, but during the wet season when surface waters are fresh. Salts become concentrated in irrigation water through evaporation, with average salinity increasing from 0.43 ppt in the tidal channel source to 0.91 ppt in the rice paddies. Our observations suggest that the practice of seasonally alternating rice and shrimp farming in a field has a negligible effect on rice paddy water salinity. Also, shrimp ponds do not significantly affect the salinity of adjacent surface water bodies or subjacent groundwater because impermeable shallow surface deposits of silt and clay mostly isolate surface water bodies from each other and from the shallow groundwater aquifer. Bivariate plots of conservative element concentrations show that all surface water types lie on mixing lines between dry season tidal channel water and rainwater, i.e., all are related by varying degrees of salinization. High As concentrations in dry season tidal channel water and shrimp ponds likely result from groundwater exfiltration and upstream irrigation in the dry season. Arsenic is transferred from tidal channels to rice paddies through irrigation. Including groundwater samples from the same area (Ayers et al. in Geochem Trans 17:1–22, 2016), principal components analysis and correlation analysis reveal that salinization explains most variation in surface water compositions, whereas progressive reduction of buried surface water by dissolved organic carbon is responsible for the nonconservative behavior of S, Fe, and As and changes in Eh and alkalinity of groundwater.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-017-0042-3) contains supplementary material, which is available to authorized users.

Preliminary Evidence of Transport‐Limited Chemical Weathering and Element Immobility in the Ganges Tidal Delta Plain of Bangladesh

“…For water samples collected in the same season (dry season in May and wet season in November), and extracts prepared from solid samples collected in the same season, element concentrations were most often lognormally distributed, as was reported for water samples from the same region by Ayers et al (2017). All statistical tests and plots therefore use log 10 values of concentrations.…”

“…The six sites in the Ganges tidal floodplain chosen for this study span a wide range of surface water salinity and irrigation practices (Patton, 2018). The sites were located near Polder 32 and the Sundarbans, an area that has been the focus of numerous complementary field studies (Auerbach et al, 2015;Ayers et al, 2016Ayers et al, , 2017Benneyworth et al, 2016;Hale et al, 2019;Peters & Hornberger, 2019). Five sites were primarily used for agriculture, while one located in the Sundarbans mangrove forest represents natural background (Table 1 and Figure 1).…”

Observations and scaling of tidal mass transport across the lower Ganges–Brahmaputra delta plain: implications for delta management and sustainability

“…3), but on average remained well above those of the dry season. For comparison, the mean annual SSC of the main-stem Ganges-Brahmaputra River is ∼ 1 g L −1 , and depth-averaged values in the main estuary mouth and on the inner shelf commonly range 2-5 g L −1 during high river discharge (Barua et al, 1994;Ali et al, 2007). In total, SSC values well in excess of 1 g L −1 are regularly observed during the wet season from the main-stem river to the inner shelf and into the tidal channels of the lower delta plain.…”

Seasonal Variability of Forces Controlling Sedimentation in the Sundarbans National Forest, Bangladesh

“…The majority of freshwater and sediment are delivered to the region by inland advection from the Bay of Bengal during the southwest monsoon season (July to September; e.g., Barua, 1990;Barua et al, 1994;Allison and Kepple, 2001;Rogers et al, 2013;Shaha and Cho, 2016;Hale et al, 2019). As a result, the entire SNF is characterized by fresh water (∼0 PSU) from July to late November, and brackish to saline waters (∼20-30 PSU) for the remainder of the year (Shaha and Cho, 2016;Ayers et al, 2017). Accompanying the fresh water is a substantial change in local suspended sediment concentration (SSC), which varies by a factor of five or more from dry-season minima (∼0.2 g/l) in June to Monsoon maxima (>1.5 g/l) in September (Hale et al, 2019).…”