Tracer Dyes and Microbial Indicators Suggest Mobilization of Fecal Waste Flows from Latrines Following Rainfall in Kibera, Kenya

Malfunctioning sanitation systems can increase fecal contamination and exposure risks during rainfall, yet rapid methods for assessing system performance remain limited. This study evaluated sanitation system performance in low-income urban areas of Nairobi, Kenya, usingEscherichia coli as a fecal indicator and fluorescent tracer dye to assess excreta flow. We collected 144 soil and 48 open drain water samples before and after rainfall and tested them forE. coli A fluorescent dye was introduced into sanitation systems to track leakage into surrounding environments.E. coli was detected in 75% of samples, with mean concentrations of 47 Most Probable Number per gram (MPN/g) in facility entrance soil, 45 MPN/g at household entryways, 44 MPN/g in compound soil, and 90 MPN/100 mL in open drains. Concentrations were lower immediately after rainfall, although the difference was not statistically significant (p = 0.097). Dye was detected outside sanitation systems in 56% of cases, rising to 78% following rainfall. Detection varied by system type: 83% of pit latrines, 50% of septic systems, and all direct discharge systems showed evidence of leakage. These findings reveal frequent containment failures and highlight the value of tracer dyes for identifying contamination pathways and informing targeted public health interventions.