Spatial risk profiling and strategic chlorination for cholera control in Urban drinking water supply. A case study from Karachi, Pakistan.

A risk assessment of Vibrio cholerae contamination in Karachi's drinking water revealed alarming levels of bacterial presence across five districts, based on 1,468 analyzed samples. Total coliforms (TC) were detected in 62% to 92.3% and fecal contamination, indicated by E. coli, were found in 36.7% to 74% of samples with the highest levels in district Korangi. Most concerning was the presence of V. cholerae, the cholera-causing bacterium, in 8% to 24% of the samples. Notably, all samples prior to the disinfection activities lacked free residual chlorine (FRC), a critical disinfectant, underscoring the heightened risk of pathogen transmission. To mitigate this crisis, secondary chlorination using sodium hypochlorite (NaOCl) was introduced at 16 water pumping stations and one reservoir in high-risk areas. The intervention targeted FRC levels of 1.0-2.5 mg/L at the source and 0.1-0.5 mg/L at the consumer end to ensure effective disinfection. Post-intervention analysis showed a dramatic decrease in contamination. All samples displayed the presence of FRC and tested negative for V. cholerae. Pre-chlorination, contamination was highest in Korangi (total coliforms median: 1236 cfu/100 mL, E. coli median: 611.5 cfu/100 mL) and Malir (total coliforms median: 1154 cfu/100 mL, E. coli median: 40 cfu/100 mL). Post-chlorination, bacterial levels dropped to near zero across all districts. These findings underscore the critical role of chlorination in reducing cholera risks and safeguarding Karachi's drinking water, providing a scalable model for urban water safety interventions.