Suitability assessment of effluents from Mwea irrigation scheme for reuse in irrigation for rice production, Kirinyaga County, Kenya
Onderi, Josephine Nyabonyi
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Declining quantity and quality of irrigation water are serious challenges facing rice production in Mwea irrigation scheme. As such the aim of this study was to assess the suitability of effluents from the scheme for recycling for the same irrigation purpose within the scheme and areas down stream. Water from River Thiba intake (point 1) and waste water from Kiruara drain (point 2) and Thiba main drain (point 3) were sampled and analyzed for quality parameters thus:- pH, Electrical conductivity (EC), Total Dissolved Solids (TDS), Total Suspended Solids (TSS), Calcium, Magnesium, Sodium, Potassium, Bicarbonates and Nitrates. The results were used to compute Sodium Adsorption Ratio (SAR) and further compared to FAO irrigation water quality standards. Also soil samples from three fields adjacent to the water sampling points were analyzed for pH, EC, total organic carbon, total nitrogen, sodium, phosphorus, potassium, magnesium and calcium. In addition, a survey was conducted to obtain the socio-economic aspects of the rice farmers of the scheme. The results indicated that water and wastewater from all the three study sites were suitable for irrigated rice production based on FAO recommended standards of irrigation water. Wastewater recorded a positive progressive gain in all the parameters tested as point 1< point 2< point 3 for Ca, EC, TDS, TSS, Na, K and HCO3 which were statistically significant (p<0.05). 88.3% of Mwea Irrigation Scheme farmers experienced water shortage during paddy rice production. It was also observed that already 51.5% of Mwea Irrigation Scheme farmers recycled wastewater/effluents from paddy fields and 50% of those who had not used wastewater said it was not available. The highest production was obtained from Karaba section with a mean of 27.9 bags of paddy rice /acre and farmers attributed this to the use of “enriched irrigation water.” Zinc and potassium were found to be too low in all the three soil samples tested but soil samples from the wastewater reuse site recorded gains in nitrogen and phosphorus indicating a deposition via wastewater. Though Nitrate concentrations in the three study sites were not significantly different (p>0.05), they were above 5mg/l which may cause damage to N sensitive plants and eutrophication in the receiving water masses. Also soil phosphorous levels of 30ppm at site 3 implies that farmers using wastewater at and beyond the Prison farm can do one rice season without applying P fertilizers hence a saving for them. Therefore, there is need for alternative disposal of these nutrient rich effluents and the best way is by recycling so as to; save the water masses downstream from eutrophication and growth of aquatic weeds, reduce cost of N fertilizers and obtain more water for expanding rice fields to increase rice outputs and reduce imports.