TRANSPORT OF FECAL BACTERIA FROM PONDS TO AQUIFERS IN RURAL BANGLADESH: THE ROLE OF ADJACENT SEDIMENT GRAIN SIZE

Widespread fecal contamination observed in shallow aquifers in Bangladesh is suspected to contribute to high rates of diarrheal disease. In Bangladesh and other developing countries, ponds that receive untreated latrine effluent represent potential point sources of fecal contamination to drinking water wells emplaced within shallow aquifers. This may be especially important during the monsoon when ponds tend to rapidly fill with contaminated runoff water and then drain into the ground. To test the hypotheses that ponds are sources of fecal bacteria to groundwater, nine transects of monitoring wells radiating away from four ponds were installed in a shallow sandy aquifer underlying a village in rural Bangladesh. Sediment coring revealed that two of these ponds were emplaced within fine-grained sand, one was in fine, silty sand and one was in silt. Eight of the nine transects were sampled monthly for cultured E. coli from September 2008 through October 2009. E. coli was detected only at very low concentrations (<10 MPN/100 mL) in the adjacent aquifers during the dry season. During the early monsoon, however, high concentrations of E. coli (>800 MPN/100 mL) and molecular E. coli and Bacteroidales (>100,000 copies/100 mL) were found in monitoring wells in the aquifer adjacent to the two ponds emplaced within clean sand. In June of 2009, water levels in all four ponds were artificially raised by 16 to 63 cm to simulate early monsoon flooding conditions and fecal indicator bacteria (FIB) were monitored at least once a week in four transects, one adjacent to each pond. Increases in FIB concentrations up to 7 meters away from two ponds were accompanied by increases in lateral hydraulic gradient. This only occurred adjacent to ponds emplaced within clean sand, whereas FIB concentrations were very low or not-detected next to ponds emplaced within silty sand or silt. Thus, whether ponds are point sources of fecal contamination depends sensitively on the grain size of sediments immediately adjacent to the ponds.