Impact of high-strength wastewater infiltration on groundwater quality and drinking water supply: the case of Sana'a, Yemen

Abstract

In Sana'a, the capital of the Yemen Arab Republic, a major well inventory was compiled in 1995 during which samples were analysed for major cations and anions. Five years later the opportunity was taken to repeat the exercise on a sub-set of the original wells. The results showed that groundwater in the urban area was characterised by high concentrations of almost all major cations and anions due to the continuous infiltration of wastewater into the aquifers via cesspits. The dominant watertype appeared to be CaCl2. The Cl−-concentration ranged from 3 to 10mmol/l and NO3−-concentration ranged from 1 to 3mmol/l while NH4+ was absent in all samples. It is concluded that cation exchange has taken place. Ca2+ in groundwater has been enriched, while Na+, K+ and NH4+ have been depleted. Groundwater affected by wastewater had pH values of 0.5–1 unit lower than groundwater not affected by wastewater, indicating that acidification has taken place. Over the period between the two surveys, concentrations of almost all major anions and cations increased, while pH decreased, both owing to the continuous infiltration of wastewater. An exploratory one-dimensional transport model of a 200m column of the aquifer underlying Sana'a showed that, over a 15-year period of continuous wastewater infiltration, a quarter of the NH4+ present in raw sewage would oxidise to NO3− thereby producing acidity and some 60% would be adsorbed. The model indicates that after 50 years of wastewater infiltration, exchange of NH4+ has become limited due to the limited cation exchange capacity (CEC) of the soil. Therefore more NH4+ will be oxidised to NO3− and [NO3−] in groundwater will rise. At the same time, groundwater in the zone of NH4+ oxidation will become very acid due to a lack of buffering minerals. The modelling studies, together with the results from the surveys, tend to indicate that up to 12% of the current population of the city could be dependent on contaminated groundwater for their drinking water supply.