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.
