Nitrogen contamination of groundwater by on-site sanitation
Creators: name: Hammoud, Acile Sybella
Publisher: name: Imperial College London
Year: 2018

description: Pit latrines are one of the most common forms of onsite sanitation facilities in many developing countries. Although these latrines are suitable as a means of isolating human waste, conditions within the pits often lead to nitrification of the contained waste. Leaching of nitrogen-containing compounds (e.g., ammonia, nitrate) from pit latrines and seepage tanks into groundwater may pose health risks that cannot be ignored, given that groundwater is a significant source of drinking water in many low-income countries. Most research to-date on the pollution of groundwater by pit latrines has focused on microbiological contamination and, at the start of this research, there was a lack of evidence-based advice for minimising nitrogen pollution. An advection-dispersion-reaction transport model was created at the early stage of this research to estimate the potential for nitrate pollution of groundwater from latrines under high uncertainty, i.e. where only limited hydrogeological data is available (which is often the case in low-income settings). The results highlighted the common strong likelihood of nitrate pollution of groundwater reaching levels exceeding the World Health Organization guideline value for nitrate in drinking water of 50 mg∙L-1, e.g. after as short a period as only two years for cases when the aquifer is situated 5 meters below the pits and when considering moderate to long nitrate half-lives in the subsurface. Next, the issue of nitrogen contamination of wells situated near to latrines was explored via data collection in three communities within Kathmandu Valley, Nepal (Manohara, Kupondole, and Lokanthali). Well water samples were analysed over two seasons (wet and dry) for their nitrogen content, dissolved oxygen (DO), chemical oxidation demand (COD) and oxidation-reduction potential (ORP) and samples collected from within the nearby pits were also analysed to determine the nitrogen content and COD. Hand dug wells were found to be more likely receptors of contamination than tube wells, with inter-well variations related to the relative redox conditions in the wells. Increased pit-emptying frequency was related to lower levels of nitrogen in the latrines and in the nearest wells, all else being equal, indicating that this may be an effective strategy for reducing the risks of groundwater contamination in such settings, as long as emptying can be done safely and there are sufficient facilities available nearby for proper management of the emptied latrine contents. Finally, a more comprehensive, source-receptor type of numerical model was created using PHREEQC and PHAST, which was verified using the Nepal fieldwork data, in order to develop a better understanding of the most relevant factors dictating the fate and transport of nitrogen from latrines to aquifers through the subsurface. The modelling approach was comprised of three parts: (i) one part was created to estimate the hydraulic load (and hence nitrogen load) migrating from the bottom of the pit latrine; (ii) a hydrological model was used to identify the most sensitive parameters influencing the nitrogen transport in the groundwater; and (iii) geochemical relationships were incorporated to represent the fate of leaching nitrogen from pit latrines, accounting for processes such as sorption of nitrogen-containing species onto different soil types. The load of nitrogen migrating in the subsurface was most influenced by the population density of the latrines and the decomposition rate of the sludge inside the pit. Again, more frequent pit emptying was shown to limit the migration of the nitrogen, and new recommendations for latrine emptying and siting are made based on the observed important factors, such as population density and soil type. Overall, the research has significantly improved our understanding of the migration of nitrogen from pit latrines into groundwater and has provided new modelling tools and recommendations for practitioners constructing and maintaining (i.e. emptying) on-site sanitation facilities in low-income countries, with different population densities, soil types, and aquifer characteristics.