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Tracking Sources and Fate of Groundwater Nitrate in Kisumu City and Kano Plains, Kenya

1
Isotope Bioscience Laboratory-ISOFYS, Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Gent, Belgium
2
Department of Land Resource Management and Agricultural Technology, University of Nairobi, P.O. Box 29053-00625 Nairobi, Kenya
*
Author to whom correspondence should be addressed.
Water 2020, 12(2), 401; https://doi.org/10.3390/w12020401
Received: 27 November 2019 / Revised: 27 January 2020 / Accepted: 30 January 2020 / Published: 2 February 2020
(This article belongs to the Special Issue Rainfall Interception Processes and Urban Hydrology)
Groundwater nitrate (NO3) pollution sources and in situ attenuation were investigated in Kisumu city and Kano plains. Samples from 62 groundwater wells consisting of shallow wells (hand dug, depth <10 m) and boreholes (machine drilled, depth >15 m) were obtained during wet (May–July 2017) and dry (February 2018) seasons and analyzed for physicochemical and isotopic (δ15N-NO3, δ18O-NO3, and δ11B) parameters. Groundwater NO3 concentrations ranged from <0.04 to 90.6 mg L−1. Boreholes in Ahero town showed significantly higher NO3 (20.0–70.0 mg L−1) than boreholes in the Kano plains (<10.0 mg L−1). Shallow wells in Kisumu gave significantly higher NO3 (11.4–90.6 mg L−1) than those in the Kano plains (<10.0 mg L−1). About 63% of the boreholes and 75% of the shallow wells exceeded the drinking water WHO threshold for NO3 and NO2 (nitrite) during the study period. Mean δ15N-NO3 values of 14.8‰ ± 7.0‰ and 20.7‰ ± 11.1‰, and δ18O-NO3 values of 10.2‰ ± 5.2‰ and 13.2‰ ± 6.0‰ in wet and dry seasons, respectively, indicated manure and/or sewage as main sources of groundwater NO3. However, a concurrent enrichment of δ15N and δ18O was observed, especially in the dry season, with a corresponding NO3 decrease, indicating in situ denitrification. In addition, partial nitrification of mostly sewage derived NH4+ appeared to be responsible for increased NO2 concentrations observed in the dry season. Specifically, targeted δ11B data indicated that sewage was the main source of groundwater NO3 pollution in shallow wells within Kisumu informal settlements, boreholes in Ahero, and public institutions in populated neighborhoods of Kano; while manure was the main source of NO3 in boreholes and shallow wells in the Kano and planned estates around Kisumu. Waste-water sanitation systems in the region should be urgently improved to avoid further deterioration of groundwater sources. View Full-Text
Keywords: Kenya; Kisumu; groundwater; nitrate; hydrochemistry; denitrification Kenya; Kisumu; groundwater; nitrate; hydrochemistry; denitrification
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Nyilitya, B.; Mureithi, S.; Boeckx, P. Tracking Sources and Fate of Groundwater Nitrate in Kisumu City and Kano Plains, Kenya. Water 2020, 12, 401.

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