Heavy metal(loid) contamination in forest fire affected soil and surface water: pollution indices and human health risk assessment

Forest fires, whether natural or anthropogenic, release and mobilize heavy metal(loids) (HM). Following intense rainfall events, soil-bound HM are transported from soil to surface water through surface runoff, leading to water quality deterioration. Pollution and ecological risk indices are effective tools for assessing HM contamination. Most forest fire-affected soils and surface water exhibited a degree of contamination greater than 3 and 8 (high and moderate pollution), with associated high and extremely high ecological risks (165 and 2389, respectively). Pollution indices revealed that soils were highly contaminated with Ni, Cu, Cr, and Pb, while Ni, Cu, Hg, Cd, and As posed significant ecological risks. Surface water was heavily contaminated with Pb, Mn, Al, and Fe, with Ni and V contributing to extremely high ecological risks. This study highlights that trace HM also requires substantial removal efforts to make water potable, with removal efficiencies needed for Sb (94.49%), Be (85.83%), Ba (70.75%), V (68.19%), and Se (65.51%). Fire-affected surface water poses an elevated cancer risk to both children (0.18 and 4.5 × 10^–3) and adults (0.39 and 1.53 × 10^–3) through oral and dermal exposure, respectively. Children are more vulnerable to dermal cancer and noncancer risks compared to adults. Low-cost treatment methods, such as the application of immobilizing agents combined with compost, straw mulching, and seeding, can be implemented to control soil erosion in forest areas, thereby reducing the transport of soil-bound HM to surface water. These findings can aid government agencies in developing new soil and water quality standards and implementing effective treatment measures.