The Contribution of Soil Permeability to Pesticide Aquifer Vulnerability Along the Shores of Lake Naivasha, Kenya

Lake Naivasha has a farming system that is well expanded in the riparian zone. Bordering the lake are some of the biggest flower farms in the world, making it the most important area for cut flowers in Kenya. Agricultural products, especially the ones produced for export have to match a high-quality standard. To achieve these quality standards, it is necessary to have a good program of weed control and pest management. The use of pesticides is one of the most used tools to achieve it. Increasing use of pesticides threatens the quality of surface and ground waters by contamination. Various approaches have been used or proposed for assessing groundwater vulnerability occurring in the vadose zone and groundwater regime, to models that weight critical factors affecting vulnerability through either statistical methods or expert judgment. Soil permeability measures how fast water can move downward through a particular soil. Water moves quickly through soils with high permeability, losing dissolved chemicals with the percolating water. Therefore, the soil's permeability should be considered when applying pesticides. This study used the permeability of soils in the study area to calculate the value of aquifer vulnerability from pesticides used along the shores of Lake Naivasha, Kenya. Soil samples were collected from 19 field sites around Lake Naivasha, and their permeabilities determined, using empirical methods based on grain size distribution. The results showed that all the 19 sites where soils were collected for permeability determination had medium permeability (90 to 841 µms<sup>-1</sup>) and thus only one zone of low vulnerability was identified throughout the aquifer around Lake Naivasha. The results therefore, resulted in an aquifer vulnerability of 6.78% being determined along the shores of Lake Naivasha, considering pesticide transport to groundwater determined from soil permeability alone. It was concluded that this aquifer vulnerability arising from pesticide mobility was low and groundwater in the area therefore, not at risk of pesticide contamination based on soil permeability alone. Further studies to determine a combined aquifer vulnerably index taking into consideration other contributors is recommended in order to make a decision on the safety of groundwater for domestic use in the study area.