Nitrate pollution of groundwater by pit latrines in developing countries

Abstract

Pit latrines are one of the most common forms of onsite sanitation facilities in many developing countries. These latrines are suitable as a means of isolating human waste, however, conditions within pits often lead to nitrification of the contained waste. In areas with a near-surface aquifer, the potential for nitrate pollution arising from pit latrines cannot be ignored. In this study, site visits were made to three densely populated, peri-urban areas near three West African cities (Dakar, Abidjan, Abomey-Calavi) to gather relevant information about the latrines in use and the soil and groundwater underneath the sites. Modelling was then conducted to demonstrate the potential for nitrate pollution of the groundwater from the latrines in such settings. The depth from the bottom of the pits to the water table was considered as 5, 10 or 30 m, to represent the range of aquifer depths at the study sites. Nitrate half-lives ranging from 500 to 1500 days were considered, and time scales from 6 months to several years were modelled. The results highlighted the high likelihood of nitrate pollution of groundwater reaching levels exceeding the World Health Organization guideline value for nitrate in drinking water of 50 mg/L after as short a period as two years for the aquifer situated 5 m below the pits, when considering moderate to long nitrate half-lives in the subsurface. Careful siting of latrines away from high water table areas, more frequent pit emptying, or switching to urine diversion toilets may be effective solutions to reduce nitrate passage from pit latrines into groundwater, although these solutions may not always be applicable, because of social, technical and economic constraints. The study highlights the need for more reliable data on the typical nitrate concentrations in pit latrines and the nitrate half-life in different subsurface conditions.