Assessment of Phosphate and Nitrogen Loading from Agricultural Runoff into the Osun River, Osun State

This study assesses the loading of phosphate and nitrogen from agricultural runoff into the Osun River in Osun State, South West Nigeria. Agricultural intensification in the Osun River catchment, particularly the increased use of synthetic fertilisers for vegetable and cash crop production, has raised concern about non-point source nutrient pollution that contributes to downstream eutrophication and degrades the river's use for drinking water abstraction and fisheries. Water quality sampling was conducted at twelve stations along a 45-kilometre river reach downstream of the principal agricultural zone during three rainfall events of different intensity and in a dry season baseflow period. Total phosphorus, soluble reactive phosphorus, total nitrogen, nitrate, and ammonium were measured for each sample. Event mean concentrations during rainfall periods were significantly higher than baseflow concentrations at all stations downstream of the main cropped area. During the highest intensity rainfall event, total phosphorus concentrations reached 0.62 milligrams per litre, exceeding the eutrophication threshold. Nitrate concentrations at some stations approach the WHO drinking water guideline of 50 milligrams per litre during peak runoff. A nutrient flux calculation using measured discharge and concentration data estimates annual phosphorus export at 2.4 tonnes per kilometre of river length. The study identifies the main tributaries draining intensively cropped areas as the primary phosphorus loading points. Recommendations include promoting buffer strip schemes along watercourses and encouraging controlled-release fertiliser use in the catchment. Keywords: agricultural runoff, nutrient loading, phosphorus, Osun River, non-point source pollution.