Effluents in irrigation water: a case study of Asa River on farmlands in Ilorin, Nigeria

Asa River is the most important river in Ilorin, serving, among other uses as source of water for irrigation of farmlands. On the other hand, wastes (likely containing salts) from industrial, domestic and agricultural origins are dumped into this river; therefore, the need arises to determine its salinity status and long term use implications on soil structural stability. During late rainy season (late October) of 2014 a study which comprised of two (2) factors laid out in a Randomised Complete Block Design (RCBD) was conducted. Factors were source of irrigation water in Asa River and irrigation history (duration) of plots. Four (4) locations were selected. The irrigation history was 0, 10, 20 and 30 years. In each location four farmers plots with same irrigation history were selected and each replicated thrice. Results indicated that the electrical conductivity of the water used by various farmers for irrigation (0.069 – 0.20 dS/m) was significantly lower (P < 0.05) only at point 600 m downstream with value of 0.069 dS/m which falls within the non-saline range for river water. Varying the source of irrigation water resulted in significantly higher soil chemical (electrical conductivity, sodium absorption ratio and exchangeable sodium percentage) and structural (mean weight diameter, total porosity, bulk density and saturated hydraulic conductivity) degradation in locations 200 m and 400 m downstream. Soil chemical and structural properties were significantly affected by irrigation history, its deterioration been higher in plots with irrigation history from 10-20 years. The results therefore suggest that the observed chemical and structural degradation is due to prolonged usage, especially during drier seasons when dilution effect is reduced.