Seasonal and Spatial Variations of Heavy Metals Levels in Water and Sediments from the Ondiri Wetland and Its Outflows

Purpose: The purpose of this study is to assess pollution levels, identify contamination sources, and evaluate ecological and human health risks to the community around and far from the study area. Methodology: The study was carried out in Ondiri wetland, which is located 200m South of Kikuyu town in Kikuyu ward, Kikuyu Sub-County in Kiambu County and 20km West of Nairobi City. Strategically sampling points were chosen across spatial gradients representing the swamp interior/source, active agricultural fringes, and major outflow points downstream. Aliquots of 500ml Water samples were collected approximately 20cm below the surface. Portions of 5 drops of preservative (HNO3) were added to each sampled water were sealed. The sediments were collected from all study areas; SP1, SP2, SP3, SP4, SP5, and SP6. Two samples of sediments were taken from each sampoling point. Sediments were scooped into a zipable polythen bags with lebles, using a trowel from the same points where the water was sampled. All collected samples were stored in a cooler box and transported to the laboratory where they were refrigerated at 4 0C until analysis time. An atomic absorption spectrophotometer (AAS) was used to analyze heavy metals (Lead, Cadmium, Copper, and Zinc) in sediments and water samples. Findings: The study found that in both seasons, the sediments contained high levels of the heavy metals compared to the water samples. Metal concentrations in the sediments were marginally greater than those in the water. This indicates that even when water tests appear to be safer, sediments serve as a significant trap for contaminants, posing a long-term threat to the local ecology and human health. Unique Contribution to Theory, Practice and Policy: The study recommends reducing chemical input into the environment, focusing on improving monitoring, creating vegetated buffer zones, and managing land better to reduce sediment runoff around the Ondiri wetland.