Investigating Fecal Contamination Pathways to An Unconfined Sandy Aquifer in Bangladesh

In a village underlain by an unconfined shallow sandy aquifer in Bangladesh, culturable E. coli was detected in 10-45% of wells shallower than 20 m sampled since January of 2008. The village is an area of enhanced local recharge due to the particularly sandy nature of surface soils and, therefore, particularly low levels of arsenic in shallow aquifers. Occurrence of fecal contamination varied both spatially and temporally, although wells with previous fecal contamination had a greater likelihood of subsequent fecal occurrence. Likely sources of fecal contamination of the groundwater include both humans and livestock, especially cattle and chickens which roam throughout the village. Human sewage is disposed of through shallow (~ 1 m deep) infiltration pits located beneath or beside individual latrines, or by discharge from multiple latrines to 3-5 m deep ponds (typically 10 – 15 m in diameter) which are common throughout the village. The ponds tend to fill completely during the summer monsoon and then water levels slowly decline throughout the dry season and even during periods of no precipitation they receive runoff from domestic washing areas adjacent to the individual water supply wells. We hypothesize that these ponds are the primary source of fecal contamination to the aquifer and that fecal input to the aquifer from the ponds will vary seasonally due to the effect of the monsoon. Fecal wastes in the ponds can directly enter the saturated aquifer, unlike the latrine pits which are typically located in the vadose zone (at least during the dry season). To test this hypothesis we are setting up transects of monitoring wells adjacent to several ponds to measure vertical and lateral hydraulic gradients and occurrence of fecal bacteria, including E. coli, Bacteroides and pathogens in the ponds and the aquifer.