Land Use and Water Pollution along the Altitudinal Gradient of the Likii River, Laikipia County, Kenya

Abstract

The study aimed at documenting the key land cover and land use types along the Likii River from the upstream to downstream and establishing the presence or absence of heavy metals (arsenic, mercury, lead) and pesticide residue (atrazine, β-endosulfan-isomer, mirex) along the altitudinal gradient. Land cover and land use analysis were undertaken through field missions conducted within the 2km zone on both sides of the river while water quality was based on surface water samples from 32 water sampling points along the altitudinal gradient from 2014 m to 1852 m. Heavy metal detection was done through Inductively Coupled Plasma Mass Spectrometry (ICP-MS) while the pesticide analysis was undertaken through the Solid-Phase Microextraction (SPME) and Gas Chromatography-Mass Spectrometry (GCMS). The land cover and land use analysis established that the river system was characterized by over ten zones along the altitudinal gradient. Arsenic was detected in the river between 1893-1938 m, with the highest concentration of 1.23 µg As/L at 1910 m, near the Likii low-income residential area. The mean lead concentration was 2.72 mgPb/L with the highest mercury concentration at 7.1µgHg/L between 1938 m and 1893 m in the Likii low-income residential area. The presence of atrazine was detected from below 2018 m near the Kariki Flower Farm after which the level increased downstream to a maximum of 76 µg/L at 1864 m. The maximum concentration of β-endosulfan-isomer at 56.7 µg/L was well above the WHO tentative limit of 20 µg/L in most sections of the river below 2015 m. The mean level of Mirex was 49.7 µg/L with two distinguishable peaks near the Kariki Flower Farm and the Likii low-income residential area. The findings indicated that the water in the Likii River was largely unsafe for human consumption below 2018 m because of the presence of heavy metals (below 1910 m for lead and mercury) and pesticide residue (1934 m for atrazine, 1938 m -2018 m for mirex and 2015 m for β-endosulfan-isomer).