Abstract

Declining quantity and quality of irrigation water coupled with increasing water demand are serious challenges facing paddy rice production in the world. Reuse of agricultural waste water is increasingly popular in paddy systems but few studies have been done with regards to its quality and effects on soil productivity and environmental risks. The aim of this study was to assess the suitability of effluents from the Mwea Irrigation scheme in Kenya for recycling within the scheme and for reuse in new areas downstream. Water was sampled from River Thiba intake (point 1, control) and waste water from farmers plots, Kiruara drain (point 2) and Thiba main drain (point 3). Corresponding adjacent soils were sampled and analyzed for important physico-chemical quality parameters. Results showed that the EC, TDS, TSS, Ca²⁺, Mg²⁺, K⁺, Na^+, HCO₃^-, NO₃^- and SO₄^2- significantly (p<0.05) increased in wastewater as point 1< point 2< point 3. Whereas, 88.3% of Mwea rice farmers experienced water shortage during peak demand, 51.5% of them recycled wastewater from paddy fields. The soil total N and available P from the wastewater reuse sites increased by 48.4 and 400% respectively to amounts that could save fertilizer P application once every 3 seasons. The wastewater NO₃^- concentrations increased above 8 mg/L likely to cause damage to N sensitive plants and eutrophication in the receiving water masses. The fresh water and wastewater in the Mwea scheme were suitable for irrigated rice production since all the nutrient parameters were within critical limits as recommended by FAO standards.

Key words: Irrigation, recycling, rice production, wastewater, effluent, physico-chemical, Water quality, precision agriculture.