Modeling the Effects of Projected Sea Level Rise on Water Quality in Coastal Bangladesh

Abstract

Climate models show that Bangladesh will experience high rates of change in temperature, precipitation, runoff, and local sea level rise rates by 2100. Sea level rise will result in the degradation of groundwater quality by causing vertical infiltration of brackish water from tidal channels and lateral intrusion of seawater into aquifers from the ocean. It is also likely that sea level rise will cause an increase in pH and ionic concentrations of the groundwater, which may result in the mobilization of arsenic into the water as a result of ionic competition for sorbing sites. Sea level rise maps were produced using Shuttle Radar Topography Mission data for sea level rise scenarios at 1 meter, 2 meters, and 5 meters and results indicate that populations of 28 million, 39 million, and 71 million people living in inundated zones will be affected, respectively. An examination of water quality data from the Bangladesh Water Development Board (BWDB) and British Geological Survey (BGS) wells show that the water type is highly variable, with Na-Ca-Mg-HCO3 as the dominant water type and Na-Cl type in some wells close to the Bay of Bengal and tidal channels. Surface water salinity shows that all locations have a higher salinity in the dry season than in the wet season; Regional Climate Models (RCMs) and Global Climate Models (GCMs) for the region suggest that the dry seasons will become drier and wet seasons will become wetter, which is likely to exacerbate the problems of water salinization during dry seasons or droughts. Groundwater salinity distribution maps were produced for both shallow and deep wells and indicate that the shallow aquifer has a higher mean (4517 mg/l) and wider range (96 to 25,422 mg/l) of TDS concentrations than the mean (1213 mg/l) and range (123 to 8814 mg/l) of the main aquifer, suggesting widespread vertical infiltration of brackish water from tidal channels. The main aquifer has some areas with TDS concentrations under the drinking water limit, located mostly in the north and east of coastal Bangladesh. The shallow aquifer has fewer areas with TDS concentrations under the drinking water limit, and none of these areas are located in the coastal districts. Lateral saltwater intrusion models were constructed for the sea level rise scenarios and results show a saltwater wedge consistent with the shape predicted by the Ghyben-Herzberg relation. Sensitivity analysis for these models shows that saltwater intrusion can be limited by an increase in the hydraulic gradients of fresh groundwater in the southern (downgradient) direction or by the presence of a confining clay layer in the coastal region. Vertical saltwater infiltration models show that small tidal channels have a local effect with a infiltration of saline surface water into the shallow layers, whereas larger tidal channels affect a larger area and can reach the deeper layers and main aquifer. Sensitivity analysis for these models shows that the presence of a confining clay layer restricts intrusion of saline water into the deeper layers, but can cause a larger zone of diffusion in both shallow and deeper strata. Arsenic concentrations were compared to Cl- concentrations for all BWDB wells and showed no positive correlation. While the pH or salinity effect may cause additional mobilization of arsenic in this area, the pH of wells included in this study have a mean of 7.75, which is below the arsenic desorption threshold of 8.5. It is likely that other mechanisms (e.g., bacterial reduction of iron oxides, sulfate reduction, etc.) may also affect arsenic mobilization, but the pH effect could become problematic in the future should seawater intrusion continue to drive up pH and salinity in the aquifers.