Assessment of sources and transformation of nitrate in groundwater on the slopes of Mount Meru, Tanzania

The stable isotope compositions of nitrogen-nitrate (¹⁵N-NO₃) and oxygen-nitrate (¹⁸O-NO₃), and concentration of nutrients (NO₃ ⁻, NH₄ ⁺, NO₂ ⁻, PO₄ ³⁻) for water samples collected from springs, shallow wells and boreholes during dry and wet seasons were used to investigate sources and biogeochemical transformation of NO₃ ⁻ in groundwater along the slopes of Mount Meru. About 80 % of all water sources had nitrate concentration higher than background concentration of 10 mg/l during both seasons, while NH₄ ⁺ and NO₂ ⁻ concentrations were very low probably due to nitrification. Concentrations of NO₃ ⁻ above 50 mg/l were observed in some water sources. Concentrations of PO₄ ³⁻ in all groundwater sources were very low during the two seasons owing to dilution and adsorption. The δ¹⁵N-NO₃ ⁻ for boreholes waters averaged +11.6 ± 2.1 and +10.7 ± 2.1 ‰ during dry and wet seasons, respectively. Similarly, the δ¹⁸O-NO₃ ⁻ of borehole waters for the wet and dry seasons averaged +5.2 ± 1.3 and +4.6 ± 1.9 ‰, respectively. With regard to dug wells, the δ¹⁵N-NO₃ ⁻ of well water averaged +13.3 ± 2.5 and +12.5 ± 2.3 ‰ during dry and wet seasons, respectively, while the δ¹⁸O-NO₃ ⁻ for the wet and dry seasons averaged +7 ± 2.3 and +6.4 ± 2.1 ‰, respectively. The δ¹⁵N-NO₃ ⁻, for springs averaged +11.2 ± 2.2 ‰ during dry season and +11.7 ± 3.5 ‰ during wet season whereas the δ¹⁸O-NO₃ ⁻ for the wet and dry seasons averaged +7.3 ± 3.6 and +5.9 ± 2.6 ‰, respectively. The isotopic data suggested that the source of NO₃ ⁻ in the water sources are dominated by sewage and/or animal manure and to less extent, soil organic N. Few samples collected in close proximity of manure heaps and sanitary facilities had δ¹⁵N values between 16 and 20 ‰ suggesting occurrence of denitrification.