Effects of Agricultural Nutrients Influx on Water Quality in Thiba River basin, a sub-catchment of Tana River Basin in Kirinyaga County, Kenya

Excessive enrichment of waters with nutrients and the associated adverse biological effects leads to eutrophication, which is one of the major environmental problems across the world. Various studies have revealed the overuse of inorganic fertilizers to increase agricultural productivity in Kenya contributes to pollution of water bodies. In order to meet increasing demand for clean water, sustainable use and conservation of available water resources is therefore paramount. This study was done to find out effects of agricultural nutrient pollution in Thiba River, a sub-catchment of Tana River Basin, located in Kirinyaga County, Kenya. The study area was divided into four distinct agro-ecological zones based on different anthropogenic activities. Ecological survey design was used in the study. Sampling was done during the wet and dry season. Water samples were analysed for temperature, transparency, pH, electrical conductivity (EC), salinity, total dissolved solids (TDS), dissolved oxygen (DO), biological oxygen demand (BOD), phosphates, nitrates, nitrites, ammonia, and toxic microalgae. All parameters showed both spatial and temporal variations with statistically significant differences (p˂0.01). Temperature of the river ranged from 14.57 0C to 28.08 0C due to climatic changes along the agro-ecological zones. The pH ranged from 7.02 to 8.51. The DO values decreased from the highlands to lowland within the range of 9.00 mg/l at the reference site to 5.43 mg/l at the most polluted site. EC ranged from 20.54 µScm-1 at the reference site during the wet season to 251.2 µScm-1 at the rice irrigation scheme site during dry season. The TDS ranged from 16.9 ppm at the reference site to 167.05 ppm at the most polluted site. Salinity also had a high variation with a range of 0.01 ppt at the reference site to 0.07 ppt at the most polluted site. The highest values of BOD (3.49 mg/l) were recorded at the rice irrigation scheme during the wet season and the lowest (0.22 mg/l) at the forest edge reference site. The lowest levels of NH4, P, NO2 and NO3 were recorded at the forest edge reference site at 1.088 µg/l, 1.177 µg/l, 0.217 µg/l and 0.148 µg/l respectively during the dry season while the highest values for the same nutrients were recorded at the rice irrigation site at 11.439 µg/l, 4.933 µg/l, 1.518 µg/l and 2.721 µg/l in the same order. There was a high peak of all nutrient levels at the rice irrigation scheme zone which was attributed to the extensive use of inorganic fertilizers. Dam water samples were analysed for members of Cyanobacteria group of microalgae which are bio-indicators of eutrophic waters. Out of the seven members of this group that were identified four were toxic genera. These were Nostoc, Oscillatoria, Anabaena and Microcystis. During the wet season Nostoc had the highest population followed by Oscillatoria and Anabaena. No Microcystis was observed during the wet season. There was a steady increase of all the genera during the dry season with Microcystis making appearance. Microcystis and Oscillatoria had a very high correlation. This study concludes that various anthropogenic activities especially agriculture along the study site are the main factors of Thiba River pollution hence a major threat to human, livestock and aquatic organisms. Environmental protection laws should be enforced by the government