Date Published: April 12, 2018
Publisher: The American Society of Tropical Medicine and Hygiene
Author(s): Abu Mohd. Naser, Eilidh M. Higgins, Shaila Arman, Ayse Ercumen, Sania Ashraf, Kishor K. Das, Mahbubur Rahman, Stephen P. Luby, Leanne Unicomb.
We assessed the ability of sodium dichloroisocyanurate (NaDCC) to provide adequate chlorine residual when used to treat groundwater with variable iron concentration. We randomly selected 654 tube wells from nine subdistricts in central Bangladesh to measure groundwater iron concentration and corresponding residual-free chlorine after treating 10 L of groundwater with a 33-mg-NaDCC tablet. We assessed geographical variations of iron concentration using the Kruskal–Wallis test and examined the relationships between the iron concentrations and chlorine residual by quantile regression. We also assessed whether user-reported iron taste in water and staining of storage vessels can capture the presence of iron greater than 3 mg/L (the World Health Organization threshold). The median iron concentration among measured wells was 0.91 (interquartile range [IQR]: 0.36–2.01) mg/L and free residual chlorine was 1.3 (IQR: 0.6–1.7) mg/L. The groundwater iron content varied even within small geographical regions. The median free residual chlorine decreased by 0.29 mg/L (95% confidence interval: 0.27, 0.33, P < 0.001) for every 1 mg/L increase in iron concentration. Owner-reported iron staining of the storage vessel had a sensitivity of 92%, specificity of 75%, positive predictive value of 41%, and negative predictive value of 98% for detecting > 3 mg/L iron in water. Similar findings were observed for user-reported iron taste in water. Our findings reconfirm that chlorination of groundwater that contains iron may result in low-level or no residual. User reports of no iron taste or no staining of storage containers can be used to identify low-iron tube wells suitable for chlorination. Furthermore, research is needed to develop a color-graded visual scale for iron staining that corresponds to different iron concentrations in water.
Household chlorination is one of the most cost-effective point-of-use (POU) water treatment interventions in resource-limited settings.1 Chlorine treatment inactivates the vast majority of human enteric pathogens.2,3 Chlorine-based disinfectants leave free chlorine residual in treated water, which provides protection against further introduced micro-organisms.4 The Centers for Disease Control and Prevention (CDC) recommends residual-free chlorine levels between 0.2 and 2 mg/L to ensure adequate disinfection and residual protection.5
The median iron concentration of the 654 tested wells was 0.91 mg/L (IQR: 0.36–2.01 mg/L; Table 1), and 18% (117/654) of wells had iron concentrations greater than 3 mg/L (range: 7–38% at the subdistrict level; Table 1). Among 22 unions in the small-scale study, 15 unions had at least one well with iron concentrations greater than 3 mg/L. Median iron concentration of tube wells varied within small geographical areas with significant variation between villages in the same union (P = 0.008) and between unions (P = 0.0001). In the Kaliganj subdistrict, the union-level median iron concentration varied from 1.3 to 4.3 mg/L; in Kapasia from 0.4 to 3.0 mg/L, and in Sreepur from 0.6 to 1.8 mg/L (Figure 3, Table 2). When we considered a village as a cluster, the intra-cluster correlation coefficient within a union ranged from 0 to 0.54 (Table 2). In the larger-scale study, the subdistrict level median iron concentration varied from 0.31 to 1.94 mg/L (P = 0.0001; Table 1). The median tubewell depth was 150 ft (range: 12–550 ft). Iron concentration was inversely associated with the reported depth of tube wells—there was a 0.20 mg/L (95% confidence interval [CI]: −0.30, −0.10, P < 0.001) decrease in median iron concentration for each 100 feet increase in well depth. Iron concentration was not associated with year-round presence of water in tube wells (Table 3). The region selected for this study was reported by the BGS in 2001 to have low average iron concentration; we detected a similar median iron concentration (0.9 mg/L) in this study conducted approximately 10 years later, surveying a larger number of tube wells within selected sites (N = 654) than the BGS (N = 84). Even though the sites were considered to have a low average iron concentration, we found variation across small geographical units (i.e., villages) and large geographical units. In our large-scale study, we found that between 4% and 38% of wells had an iron concentration greater than 3 mg/L. Based on these findings, we did not recommend the Mirzapur subdistrict as a potential site for the WASH Benefits Bangladesh study as 38% of wells in this subdistrict had an iron concentration greater than 3 mg/L. The variability in iron concentration was similar to that found in a study conducted in a smaller area of northwestern Bangladesh.23 Local variability can be explained by geochemical properties of groundwater aquifers.28 The source of dissolved ions in groundwater includes mineral accumulations in rocks near the land surface;11 thus iron-containing materials are usually present in shallow aquifers. Moreover, dissolved organic matter in shallow aquifers in Bangladesh can act as a substrate for iron-reducing bacteria, enhancing iron concentration in superficial layers. These factors may explain the inverse relationship between the iron concentration and estimated tubewell depth.23,29 Source: http://doi.org/10.4269/ajtmh.16-0954