Pesticide Residue Levels in Water and Sediment from River Thiba, Kirinyaga County, Kenya.

The study assessed the physicochemical parameters and pesticides residue levels in water and sediment samples from Thiba River Catchment area, Mwea Irrigation Scheme, Kirinyaga County, Kenya. The samples were collected from six selected sampling sites in February and May 2021 representing dry and wet seasons respectively, by the meteorological department and based on the human activities and crops grown. The physicochemical parameters determined were pH, total dissolved solids (TDS), total suspended solids (TSS) and electrical conductivity (EC) in water samples and pH and total organic carbon content in the sediment samples. The pH of water samples ranged from 6.12 ± 0.12- 6.91± 0.17, as Total dissolved solids were at 10.03 ± 2.04 mg/L – 100.04 ± 1.10 mg/L while Total Suspendeed Solid values were 103.01 ±3.51mg/L -316.01 ± 6.81 mg/L, and Eelectrical Conductivity was in the range of 30.01± 1.21 µscm-1–200.13 ± 2.01 µscm-1. In the sediment samples, the pH ranged from 5.42 ± 0.12 – 7.52 ± 0.11 and the organic matter content ranged from 27.72± 0.62 – 49.07± 0.78. The farmers applied 12 different pesticides, under different trade names and active ingredient. Chlorpyrifos was the highest used pesticide by 72 % of the farmers followed by Lambda-cyhalothrin at 64 %. The pesticides obtained from the samples were Metalaxyl, chlorpyrifos, and lambda cyhalothrin. In the water samples, chlorpyrifos residue levels ranged from 3.60±0.92 µg/L – 88.24 ±65.97 µg/L, with Metalaxyl from < 0.05 µg/L – 4.24 ± 1.75 µg/L, while Lambda cyhalothrin were in the range of < 0.005 – 3.75±0.72 µg/l in both the dry and wet seasons. Generally, the Chlorpyrifos residue levels were higher in wet season than the dry season while Metalaxyl and Lambda-cyhalothrin values were higher in the dry than wet season. Metalaxyl residue levels in sediment samples in the dry ranged from < 0.05 µg/kg – 1.27±0.90 µg/kg for wet seasons, while Chlorpyrifos ranged from 4.77±17 µg/kg – 47.78±7.12 µg/kg and Lambda-cyhalothrin values were from < 0.005-0.67±0.18 µg/kg in both the dry and wet seasons respectively. Chlorpyrifos pesticide residue levels were the highest in the sample matrices, due to its widespread use to control ticks and fleas from the livestock in the study area. For the chlorpyrifos pesticide levels, all the sites in the dry season were within the acceptable limits of 30 µg/L except site 2. However, the levels were higher than the limit except in sites 1 and 3. For the Metalaxyl and lambda-cyhalothrin pesticides, all the sites had levels that were below the set limits for drinking water by WHO of 10 µg/L.