Study of groundwater pollution from onsite sanitation

Abstract

Groundwater is considered as a reliable source of domestic water supply in Bangladesh. But in recent days, it has become a major concern that groundwater may be contaminated by faecal pollutants from onsite sanitation especially from pit latrines. In this backdrop, the current study was conducted at some suitable locations in southern coastal area, northern upland and lower Meghna river flood plain area of the country. A pit latrine was chosen as study latrine at each study locations and boreholes were constructed at different lateral distances to study the transport of microbial and chemical contaminants from pit latrine to groundwater. Field test results of water samples collected from three study areas showed varied FC counts at different vertical distances of borehole for each variation of lateral distances. Presence of FCs in water samples indicates that there exists potential risk of faecal contamination of groundwater from pit latrines. Considering guideline values of FCs for drinking water supply, safe vertical separation distances of tubewell screen from existing ground level were suggested for each variation of the chosen lateral distances. Water samples were also analyzed for chemical contaminants from pit latrine and concentration of chemical pollutants were found to be well below the guideline values for drinking water. Hydrogeological formations of top 4/5 m layer of the study locations are of silUclay or very fine sand where most bacterial attenuation could be achieved. But, relatively higher water table in the study areas cause reduced subsurface attenuation of microorganisms. Based on the study findings and review of earlier studies on this issue, indicative guideline for avoiding water contamination by pit latrine pollutants is to keep 3 m lateral separation distance between pit latrine and tubewell, if the tubewell screen is placed at depths more than 30 m from EGL. If the intake screen of tubewell is placed between 25 m and 30 m depths from EGL, 6 to 9 m lateral separation distances between pit latrine and tubewell may be considered safe. If tubewell screen is to be placed at very shallow depth «15 m), the tubewell should be installed at more than 12 m lateral distance from pit latrine. The recommendations made by this study are specific for three areas that were studied. Further studies are needed for different hydrogeological conditions and at smaller intervals of lateral and vertical distances in order to confirm safe distance criteria between pit latrines and water points.