Mass balance of nitrogen and potassium in urban groundwater in Central Africa, Yaounde/Cameroon

- Predicting TIN by EC measurement in urban groundwater in a ferralsol aquifer
- Anthropogenic groundwater types dominated by NO3−, NH4+ and urine related salts
- Incomplete nitrification under acidic pH conditions
- Indirect evidence for very little denitrification
- Nutrient turnover (TIN & K) is very high compared to national fertilizer imports.

Mass flow of nutrients from innumerous latrines and septic tanks was assessed to best describe the groundwater quality situation in the urban environment of Yaounde. 37 groundwater samples were taken at the end of dry season 2012 and analysed for nutrient related (NO3−, NH4+, NO2−, K+, Cl−, HPO42 − and TOC) and physico-chemical ambient parameters. A survey on waste water discharge close to water points constrained point sources from sanitation. The results showed that the median of nitrate concentration exceeds the WHO limit. We realized that EC increases from the geogenic background to very high levels in the urban area within short distance, suggesting anthropogenic input. Dug wells showed nitrate and ammonium in equivalent concentrations, indicating incomplete nitrification and mandating their inclusion into water type classification. The mass turnover of nutrients in urban groundwater scales high in comparison to national statistical figures on fertilizer import for 2012. A mass N,K balance for infiltration water overestimates observed concentrations by a factor of 4.5. The marked balance gap is attributed to dynamic non-equilibrium between input and output. Unresolved questions like a) urban sanitation, b) hygiene & health and c) environmental protection urgently call for closing the nutrient cycle. In the light of Cameroonian strategies on rural development, tackling the groundwater nutrient, urban agriculture, food - NEXUS might partially restore urban and periurban ecosystem services under economical constraints and thus improve living conditions.

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