Heavy Metal Transfer Dynamics and Human Health Risk Assessment in Vegetables Grown on Sewage Sludge-Amended Soils in Nyeri County, Kenya

Sewage sludge is increasingly used as an organic fertilizer in Kenyan peri-urban agriculture, yet concerns persist regarding heavy metal transfer to edible crops and associated health risks. This study evaluated the magnitude of cadmium (Cd), chromium (Cr), and lead (Pb) transfer from sewage sludge-amended soil to kale (Brassica oleracea) leaves and assessed non-carcinogenic health risks to consumers in Nyeri County, Kenya. A Repeated Measures Longitudinal Design was employed with paired soil and kale sampling from 50 plots across wet and dry seasons. Each plot served as its own control, and linear mixed models with random intercepts for plot were used to quantify metal-specific transfer efficiencies. Results showed that soil concentrations significantly exceeded leaf concentrations for all metals in both seasons (p < 0.001). The Linear Mixed Model (LMM) revealed a significant Soil × Metal interaction (χ2 (2) = 14.8, p < 0.001), with estimated soil-to-leaf transfer slopes of 0.183 (Pb), 0.235 (Cr), and 0.271 (Cd). Mean Target Hazard Quotient values were highest for Cr, followed by Cd, and lowest for Pb. Hazard Index values were significantly higher in the dry season (0.00568 ± 0.00422) than in the wet season (0.00383 ± 0.00285; p < 0.001) yet remained substantially below the regulatory threshold of 1.0. These findings indicate negligible non-carcinogenic health risk from the consumption of kale grown on sewage sludge-amended soils under the assessed exposure scenario. However, the elevated transfer efficiency of Cd warrants continued monitoring. This study underscores the need for Kenya to develop national guidelines for the safe agricultural use of sewage sludge, drawing on international benchmarks to ensure consumer safety while supporting sustainable soil amendment practices.