Hydrogeologic investigation of options for improving drinking water security in a selected polder

Abstract

Bangladesh has made substantial achievement in drinking water supply as per the Millennium Development Goals (MDG); however, achieving the Sustainable Development Goal (SDG) of safely managed drinking water is considered a development challenge. The situation is more complex in the southwest coastal region due to complex hydrogeology and adverse water quality. In this study, the state of drinking water security is investigated and options for improving coverage is analyzed in Polder-29, a small geographical area of southwest coastal region, using a mixed method approach, including hydrogeologic mapping, water audit of tubewells along with testing for salinity, household survey data, participatory methods such as focus group discussions and Key Informant Interviews, spatial multi-criteria analysis, and groundwater salinity modeling. The study reveals that high degree of hydrogeologic heterogeneity and high spatial variability of groundwater salinity contribute to high levels of spatial inequalities of groundwater security risks, with substantial implications for achieving SDG 6.1 targets. Despite hydrogeologic and salinity constraints, tubewells are the main sources of drinking water (72%) in the study area, with the number of tubewells having grown exponentially, and quadrupling in the last decade. The growth has been more for shallow tubewells, mostly used for household purposes, while deep tubewells are used mainly for drinking purpose. Majority of the drinking tubewells are privately owned, without any monitoring, which raises substantial concerns. Alarmingly, salinity concentrations in 50% of the drinking tube-wells exceeded the drinking water standard, which include 41% of drinking DTWs and 85% of drinking STWs. The apparent high drinking water coverage by tube-wells without considering salinity levels becomes substantially low, when salinity constraints are considered, with reduction being particularly high in hydro-geologically difficult southern parts of the study area. The national assessments fail to capture these realities, as the findings sharp contrast with the national assessment of 80-100% of tubewell coverage for the same area. This clearly demonstrates that although tube-wells grew, households are at greater risks than what the numbers suggest, indicating greater SDG challenges in an area with higher drinking water coverage by tubewells compared to other acutely water stressed areas. Considerable concerns were found with alternative options (e.g. pond sand filter, rainwater harvesting, and vended water) used by households in hydro-geologically difficult areas in relation to SDG relevant dimensions of accessibility and reliability. Nearly 15% of households have very low access to water source (from > 30 minutes to several hours travel time), while about 75% of households use more than one water source round the year. Salinity intrusion modeling revealed increased groundwater salinity risk, especially in the shallow aquifer, in the future. For a two time increase in river salinity, salinity is likely to increase (from 3100-5500 to 3500-7000 mg/l) in considerable areas in the south as well as in the middle of the polder where STWs are used for drinking water. Potentials for such enhanced salinity risks have implications for SDG 6.1 target of the government to expand groundwater-based drinking water supplies, in terms of determining locations for such expansion schemes. Salinity being the dominant water quality constraint in the southern coastal area thus needs to be considered adequately as an important parameter along with arsenic and E. Coli in the safe drinking water agenda and hence in any regional survey. It warrants systematic monitoring and management of groundwater salinity risks via proper regulatory framework or protocol and development of human resource in both private and public sectors. Better hydrogeologic delineation is a priori for the government’s plan to expand the groundwater-based drinking water services. This study demonstrates that spatial analysis using a multicriteria approach has potential to help identify new locations for expanding groundwater-based supplies via tubewells, specifically targeting the poor.