Assessment of Nitrate and Phosphorus Losses via Surface Runoff in Enkare-Narok River Basin, Kenya

Abstract

The excessive nutrient enrichment of water bodies, primarily from agricultural runoff, is a leading cause of eutrophication worldwide. The Enkare-Narok River in Kenya faces severe nutrient pollution; however, a quantitative assessment of specific nutrient losses is critically needed to inform effective mitigation strategies. This study aimed to (1) quantify the concentrations of nitrate (NO₃⁻) and phosphorus (P) during the single rainy season (April–May 2025); (2) analyze their spatial and temporal variations at the selected points. A sampling campaign collected 200 water samples from two points: Pumping Station and downstream RGS 2K03. A total of 200 water samples were collected: 100 from the Pumping Station and 100 from the RGS 2K03 site. At each site, 50 samples were allocated for nitrate (N) analysis and 50 for phosphorus (P) analysis. The selection of the two sampling points, Pumping Station and RGS 2K03, was strategic and guided by the GIS-based land use analysis to ensure they captured the dominant pollution pathways and cumulative impacts within the Enkare-Narok River Basin. The primary criteria for selection were representativeness of key land uses, position along the pollution gradient, and practical feasibility. The N and P concentrations were analyzed in triplicate using a calibrated HACH DR900 spectrophotometer following USEPA methods. A two-way Analysis of Variance (ANOVA) tested the main effects of sampling location and time on nutrient concentrations, and linear regression quantified the relationship between water level, nitrate and Phosphorus. The results exceed regulatory thresholds, with nitrate averaging 13.9 mg/L and phosphorus maximum at 2.5 mg/L. These levels pose serious risks: the mean nitrate concentration exceeds the WHO (10 mg/L) and KEBS (10 mg/L) drinking water thresholds, elevating health risks for vulnerable populations like infants (methemoglobinemia). The extreme total phosphorus level (2.5 mg/L), which is 25 times the WHO-recommended 0.1 mg/L eutrophication threshold, signals a high potential for toxic cyanobacterial proliferation and aquatic habitat degradation through hypoxia. Statistical analyses confirmed significant spatial and temporal variations, with nitrate concentrations significantly higher downstream and spiking sharply following rainfall events (p < 0.001). Regression showed a positive correlation (R² = 0.58, p < 0.001) between water level and nitrate, confirming that storm runoff is a primary driver of nutrient pulses. The significantly higher nitrate levels at the Pumping Station align with its position receiving cumulative runoff from upstream agricultural and urban anthropogenic activities from the city. This study, though based on a two-month rainy season snapshot, conclusively identifies agricultural runoff and urban discharges as the primary drivers of nutrient pollution threatening both ecosystem integrity and public health. This study underscores the critical need for targeted strategies to mitigate non-point source pollution and protect surface water quality. These include implementing riparian buffers to intercept subsurface flow, a key pathway delivering nitrate to surface water, and promoting farmer-led agroforestry to reduce surface erosion and phosphorus runoff.