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

Abstract

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 agroecological 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 agroecological 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.