Determination of Nitrate and Phosphate Levels in Agricultural Runoff Entering River Benue, Benue State

Excessive input of nitrate and phosphate from agricultural fields into surface water bodies drives eutrophication, which degrades aquatic ecosystems and impairs the quality of water used for domestic and irrigation purposes. This study determined nitrate and phosphate concentrations in agricultural runoff entering the River Benue from farming zones in Makurdi, Agatu, and Gwer-West Local Government Areas of Benue State. Water samples were collected from drainage channels and river entry points during the rainy season, when runoff transport of nutrients is most intense. A total of sixty samples were collected and analysed using spectrophotometric methods. Nitrate was measured by the cadmium reduction method, and phosphate was determined using the ammonium molybdate ascorbic acid method. Nitrate concentrations in runoff samples ranged from 8.4 to 47.6 mg/L, with 68 percent of samples exceeding the WHO drinking water guideline of 10 mg/L NO3-N. Phosphate concentrations ranged from 1.8 to 12.4 mg/L, which are above the ecological threshold of 0.1 mg/L associated with eutrophication risk. River entry point samples showed significantly elevated levels compared to upstream control points. Seasonal comparison confirmed substantially higher loadings during peak rainy-season months. Algal blooms consistent with eutrophication were visually observed at two sampling stations. These findings demonstrate significant nutrient pollution risk to River Benue from agricultural activities in Benue State and highlight the need for riparian buffer zones, precision fertiliser application, and nutrient management planning as mitigation strategies.

Keywords: nitrate, phosphate, agricultural runoff, eutrophication, River Benue