Unsealed tubewells lead to increased fecal contamination of drinking water

Abstract

Bangladesh is underlain by shallow aquifers in which millions of drinking water wells are emplaced without annular seals. Fecal contamination has been widely detected in private tubewells. To evaluate the impact of well construction on microbial water quality 35 private tubewells (11 with intact cement platforms, 19 without) and 17 monitoring wells (11 with the annulus sealed with cement, 6 unsealed) were monitored for cultured E. coli over 18 months. Additionally, two “snap shot” sampling events were performed on a subset of wells during late-dry and early-wet seasons, wherein the fecal indicator bacteria (FIB) E. coli, Bacteroidales and the pathogenicity genes eltA (ETEC E. coli), ipaH (Shigella) and 40/41 hexon (adenovirus) were detected using qPCR. No difference in E. coli detection frequency was found between tubewells with and without platforms. Unsealed private wells, however, contained cultured E. coli more frequently and higher concentrations of FIB than sealed monitoring wells (p<0.05), suggestive of rapid downward flow along unsealed annuli. As a group the pathogens ETEC, Shigella and adenovirus were detected more frequently (10/22) during the wet season than the dry season (2/20). This suggests proper sealing of private tubewell annuli may lead to substantial improvements in microbial drinking water quality.

Arsenic Aquifer Sealing Technology in Wells: A Sustainable Mitigation Option

“…Meanwhile, wells have higher piezometric head than the lower ones (like Fig. 1a and as explained by Knappett et al (2012). Furthermore, pile vial field tests showed no microbial contaminations in the sealed bore wells, which is a very contrasting result to the study case conducted by DWSS/UNICEF in Nepal identifying microbiological contamination in tubewells where over 33 % of tube wells-deep (56) and 44 % shallow tubewells (total 160) were found contaminated with coliforms (WSSD 2011).…”

“…Although wells are considered an "improved" drinking water source by WHO (WHO 2000), these wells still do not have adequate protection from annular flow and are considered unsealed (Knappett et al 2012). Proper sealing of the well annulus is required to prevent contamination of the ground water.…”

“…Proper sealing of the well annulus is required to prevent contamination of the ground water. Adequate sealing prevents the well annulus from serving as a conduit for transport of either microbiological contaminant like fecal indicator bacteria (FIB) Escherichia Coli (E. Coli), Bacteroidales and the pathogenicity geneseltA (enterotoxigenic E. coli; ETEC), ipaH (Shigella) and 40/41 hexon (adenovirus) (Knappett et al 2012), or mineralogical contaminant like arsenic or other harmful heavy metals/chemicals. The unsealed wells have been found to contain culturable E. Coli, more frequent and higher concentrations of FIB than sealed monitoring wells.…”

“…The unsealed wells have been found to contain culturable E. Coli, more frequent and higher concentrations of FIB than sealed monitoring wells. The bacteriological contaminations have been observed to occur via downward flow along unsealed well annulus which is enhanced during wet seasons or monsoons (Boutt et al 2001;Knappett et al 2012). …”

“…This rise is due to the construction of large number of borewells (unprotected/unsealed) in order to meet increasing water demand of rapidly growing urbanization. However, it is increasing at a local level due to easily accessible lowcost drilling mechanisms for shallow wells, which is unorganized and follows unscientific practices, in this part of the sub-continent (Knappett et al 2012). In general, during well construction in the multi-aquifer hydrogeology of Nepal and South Asia, after drilling, the water mains (pipes) are lowered locating screen/ multi-screens at particular depths intercepting deeper aquifer/s and the well-annulus (annular space around the water extracting mains) are entirely filled with gravel pack from bottom to the top ground level (GL) followed by compaction and ultimately well development is carried on using compressors (Danert et al 2010).…”