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, six unsealed) were monitored for culturable Escherichia coli over 18 months. Additionally, two 'snapshot' 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 (enterotoxigenic E. coli; ETEC), ipaH (Shigella) and 40/41 hexon (adenovirus) were detected using quantitative polymerase chain reaction (qPCR). No difference in E. coli detection frequency was found between tubewells with and without platforms. Unsealed private wells, however, contained culturable 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.

Figure 1

Locations of 35 unsealed private tubewells (triangles), 6 unsealed (circles) and 11 sealed monitoring wells (squares) within Char Para (Site K). The 6 unsealed monitoring wells are contained within two multilevel piezometer nests. Image produced in Google Earth©. The inset country map is from www.mapresources.com. The scale bar in the bottom left corner represents 200 m.

Figure 2

Detection Frequencies of E. coli in monthly monitored private (P), unsealed monitoring (M), and sealed monitoring (MS) wells from April 2008 through November 2009. For the Well Type plots (a–c) the number of wells with at least five months of monthly data in each season were 33, 6 and 11 for P, M and MS respectively. There were a total of 12 possible wet season sampling events and 6 dry season months. In the Platform Presence plots (d–f), only private wells are presented here since no monitoring wells had cement platforms. A reduced number of private wells (n=30) was available due to missing information.

Figure 3

Comparing cultured E. coli prevalence in wells with rainfall and water table levels. (a) Weekly precipitation (vertical grey bars) for Matlab located 50 Km south of Site K (left-axis). Manual groundwater levels are displayed at Site K (black line with grad symbols) from 04/01/08 through 11/1/09 (right-axis). Months assigned to the wet season are indicated by boxes outlined by dashed lines. (b) Monthly proportion of private (P) (n=35) and sealed monitoring (MS) (n=11) wells testing positive for cultured E. coli (left-axis). 75^th percentile cultured E. coli concentrations (MPN/100 mL) for both P (dashed grey line) and MS (solid grey line) wells (right-axis).

Figure 4

Concentrations of mE. coli and Bacteroidales DNA in private (P) and sealed monitoring wells (MS) during dry season and wet season snap shot sampling events. Sample size for each group is indicated in parentheses. Unsealed monitoring wells (M) were not included in this analysis due to low sample numbers (n<5).

Figure 5

Inter-seasonal comparisons of FIB and pathogens detected in unsealed private wells (P), unsealed monitoring wells (M) and sealed monitoring wells (MS). (a) Measured FIB marker gene concentrations in wells from during the wet and dry season snap shot sampling events (n=14). Only the equation for the line of best fit for Bacteroidales is displayed since the fit was very poor for mE. coli. (b) Wet season concentrations of Bacteroidales 16S genes and pathogen presence/absence plotted against cultured E. coli detection frequency for all months (n=22). Samples where no pathogens were detected are indicated by black symbols. Samples positive for Shigella and Adenovirus are indicated by red and blue, respectively.

Figure 6

Comparison of FIB concentrations and pathogen presence/absence during wet (n=22) and dry season (n=20) snap shot sampling events. (a) Comparison of synoptic measurements of cultured E. coli (MPN/100 mL) and mE. coli (copies/100 mL) from the wet season snap shot sampling event. The cultured E. coli method detection limit (MDL) was 0.5 MPN/100 mL. (b) Comparison of Bacteroidales and mE. coli concentrations during wet season snap shot. (c) Comparison of Bacteroidales and mE. coli during dry season snap shot. Cultured E. coli was not measured synoptically during the dry season snap shot sampling event. Samples where no pathogens were detected are indicated by black symbols. Samples positive for Shigella, EltA (ETEC E. coli) and Adenovirus are indicated by red, green and blue, respectively.