Assessment Of Salinity And Sodicity Of Water Used For Irrigation In Three Sub-Catchments In The Upper Athi River Basin, Kenya

ABSTRACT

The erratic pattern of rainfall over the years coupled with increasing population has left no option but to cultivate more farmlands through irrigation to meet food demand. Irrigated Agriculture as essential as it is if not done with care through good practices and use of quality water can lead to soil salinization and ecological unsustainability. In Kenya a lot of irrigated lands have been abandoned after few years of cultivation due to salinization. This study was carried out to assess the salinity and sodicity in water used for irrigation in three sub catchments in the Upper Athi River Basin of Kenya. Specific objectives included determining concentrations of major salt ions in water sources, assessment of salinity and sodicity hazards or degree of restriction on use of the water sources and also the potential effect of the water sources on soil infiltration. Water samples from twenty-six (26) irrigation water sources made up of four (4) rivers, nine (9) streams, two (2) dams, two (2) ponds, two (2) water pans, five (5) shallow wells and two (2) boreholes were collected and analysed for major cations (Na+ , Ca2+, Mg2+and K+ ) and anions (HCO3 - , CO3 2- , Cl- , SO4 2- and NO3 - ) and also for alkalinity in the chemistry laboratory, Kenyatta University. Standard methods were used for all the laboratory analysis; pH, electrical conductivity (EC) and Temperature of water samples were measured in the field. The laboratory analysis results were analysed and used to compute irrigation water quality parameters such as Total Dissolve Salts, Sodium Adsorption Ratio and Residual Sodium Carbonate. The results were then compared with FAO (1985) irrigation water standards and classifications and NEMA (2006) standards. Temperature, pH, and alkalinity were in the ranges 21.1 - 26.1oC, 4.82 - 6.84 and 10 - 144 mg/L respectively and within FAO permissible limits for irrigation purposes. The major salt ions except CO3 - , NO3 - and K + were within acceptable limits for all samples. Nitrate was high in all groundwater sources and majority of surface waters as a result farming activities in close proximity to those water sources. The high concentration of potassium in some samples is also attributed to the same. EC, SAR, RSC and both TDS’, values range from 309.43 to 895.56μS/cm, 0.29 to 1.35meq/L, -3.48 to 0.46meq/L, 198.04 to 573.16mg/L and 194.63 to 459.42mg/L respectively. Results further shows no significant variation between surface and groundwater sources in relation to TDS, EC, SAR and RSC. TDS also has a positive correlation with all the other parameters except calcium and RSC. Categorizing the water source into FAO classification in terms of its effect on soil infiltration, all the water sources have slight to moderate restriction except one which has no restriction. The study therefore recommends that, farming in close proximity (less than 20 meters) of water sources in the study area should be avoided or discouraged through the creation of buffer around them to avoid nitrate and potassium pollution; frequent and constant irrigation water monitoring should be carried out during dry seasons when irrigation is intensive; more boreholes and shallows wells should be dug and use for irrigation during the dry season; irrigators should be educated on salinity issues and appropriate management practices to overcome potential infiltration hazards and commercial farmers with large farm sizes should be encourage to construct subsurface drains to transport excess water from root zones. These approaches will protect soil and land and ensure sustainable irrigated agriculture in the sub-catchments.