Comprehensive assessment of heavy metal contamination in forested and agricultural soils: Distribution patterns, source apportionment, environmental and human health risks

Soil contamination by heavy metals (HMs) is a growing concern in Sakarya Province, where migration and land-use changes increasingly threaten forested and agricultural areas. To overcome to limitations of conventional surface-only studies, this study introduces a novel dual sampling strategy that integrates 1317 surface soil and 745 drill core samples, to assess HM contamination and associated ecological and human health risks in the Kamışlı (Geyve) region, an important agricultural zone in northwestern Türkiye. This combined approach allows a more accurate distinction between geogenic versus anthropogenic sources, improving source apportionment and pollution characterization. Maximum HMs concentrations in the soils reached 70 ppm (As), 56 ppm (Co), 175 ppm (Cu), 896 ppm (Ni), 41 ppm (Pb), and 156 ppm (Zn). Some soils exceeded not only local rock concentrations but also geological reference values. Litho-geochemical correlations between soils and rocks highlight geogenic contributions, while multivariate statistics reveal overlapping anthropogenic influences. Geoaccumulation and pollution load indexes indicate variable contamination levels, and ecological risk assessment emphasizes Ni and As as the most critical pollutants. Human health risk analysis shows that both adults and children may face carcinogenic and non-carcinogenic risks primarily from Ni, while As and Pb pose tolerable and low cancer risks. By integrating geological and environmental data, our sampling framework provides clearer evidence of contamination pathways, improves the accuracy of source apportionment, and suggests a stronger basis for sustainable land-use planning. Effective control requires identifying pollution sources and integrating soil contamination assessments into early-stage environmental planning, especially in rapidly transforming rural areas.