Mixed method analysis of anthropogenic groundwater contamination of drinking water sources in Malawi.

Groundwater contamination poses significant challenges to public health and sustainable development in Malawi, where approximately 80 % of the population relies on groundwater sources for drinking water. This study investigates the presence and drivers of nitrate and E. coli contamination in groundwater used for drinking. Analysis was conducted on results from 3388 boreholes/tube wells for nitrate contamination and 2418 drinking water sources drawn from groundwater for E. coli contamination. Overall, 6.11 % and 57.2 % of water-points did not meet WHO guidelines for safe drinking water quality for nitrate and E. coli contamination, respectively. Through a mixed-method approach, utilizing generalised linear mixed models and random forest regression modelling, the study identifies factors relating to sanitation usage as critical drivers of both nitrate and E coli contamination. Pit-latrine usage was identified as a particularly important factor in contamination; accounting for pit latrine density specifically, rather than population density, resulted in better model prediction for both nitrate and high E. coli contamination indicating that consideration of the specific type of sanitation is important in predicting water quality. In addition, a stable isotope tracer analysis method to validate predictions and monitor nitrate in drinking water was piloted, identifying human waste as a likely source of nitrate contamination. Overall, this study underscores the urgency of addressing sanitation-related contamination of drinking water sources to ensure access to safe drinking water in low-income settings.