Table 1.
Variations in the association between recent rainfall and fecal contamination of shallow wells by well type, local hydrology, and the presence of fecal contamination hazards
| No. of wells | Unadjusted | Adjusted‡ | |||||
|---|---|---|---|---|---|---|---|
| δ* | 95% CI | P value† | δ* | 95% CI | P value† | ||
| Overall | 3.82 | (1.57–9.30) | 2.79 | (1.17–6.67) | |||
| Well type | |||||||
| Bucket | 18 | 2.52 | (0.45–14.01) | 0.892 | |||
| Mechanical | 41 | 2.89 | (1.04–8.06) | ||||
| Latrine near well§ | |||||||
| No | 29 | 4.98 | (1.37–18.17) | 0.571 | 4.44 | (1.31–15.00) | 0.269 |
| Yes | 30 | 2.98 | (0.84–10.56) | 1.70 | (0.49–5.92) | ||
| Chickens near well§ | |||||||
| No | 21 | 2.66 | (0.60–11.77) | 0.492 | 2.49 | (0.59–10.50) | 0.756 |
| Yes | 38 | 5.03 | (1.68–15.03) | 3.30 | (1.08–10.14) | ||
| Pigs near well§ | |||||||
| No | 43 | 2.25 | (0.80–6.31) | 0.062 | 1.74 | (0.64–4.71) | 0.069 |
| Yes | 16 | 14.78 | (2.72–80.42) | 9.93 | (1.94–50.87) | ||
| Dogs near well§ | |||||||
| No | 24 | 2.05 | (0.50–8.38) | 0.249 | 1.57 | (0.41–5.96) | 0.244 |
| Yes | 35 | 5.96 | (1.84–19.24) | 4.31 | (1.40–13.29) | ||
| Specific catchment area‖ | |||||||
| Small | 29 | 2.38 | (0.67–8.46) | 0.274 | 1.44 | (0.43–4.90) | 0.126 |
| Large | 30 | 6.33 | (1.84–21.86) | 5.22 | (1.64–16.64) | ||
CI = confidence interval.
δ is the ratio of Escherichia coli concentration at wells where recent rainfall (> 1 mm in a 24-hour period) had occurred versus had not occurred estimated using linear regression. Interaction models were fit for each variable of interest and used to estimate δ for each value of the conditions listed in the left-hand column.
We tested the hypothesis that the variable of interest modifies the association between rainfall and E. coli concentrations by examining the significance of the interaction term in each model.
Adjusted models include well type as a covariate.
Observed within 10 m of the well. The presence of cats, cows, and horses were also recorded for each well, but because these domestic animals were rarely observed within 10 m of a well (7%, 2%, and 0% of wells, respectively), they were not included in the analysis.
Wells lying in a grid cell not expected to receive drainage from additional upslope cells were considered to have a small catchment area and all others were considered to have a large catchment area.