Distribution and risk assessment of heavy metals in overlying water, porewater, and sediments of Yongding River in a coal mine brownfield

Heavy metals from coal mines migrate into rivers and cause a severe impact on the health of local residents utilizing these rivers as a water source. However, current studies on coal mine brownfields have yet to thoroughly investigate river-associated heavy metals produced by coal mining. Therefore, this study aims to explore the distribution and risk assessment of heavy metals, namely Cr, Cu, Mn, Ni, Pb, and Zn, in overlying water, porewater, and sediments of the Yongding River, China. Overlying water, porewater, and sediments were sampled at 11 sites along the Yongding River. Heavy metal concentrations were determined through ICP-MS, and sediments were subjected to a three-step sequential extraction procedure to verify the behavior of heavy metals through principal component analysis (PCA). Sediments were separated into seven size fractions (< 4, 4-8, 8-16, 16-75, 75-150, 150-250, and 250-840 mu m) with sedimentation method based on Stokes' law. The characteristics of heavy metal distribution in different particle sizes were also explored using the Tucker 3 method. Interstitial water criteria toxic units (IWCTUs) were used to assess porewater heavy metal pollution. Risk assessment code (RAC), sediment quality guidelines (SQGs), and pollution load index (PLI) were applied to evaluate the ecological risk of heavy metals in sediments. Heavy metal concentrations increase in the following sequence: overlying water < porewater < sediment. Lead and Zn in porewater are higher than the criteria maximum concentrations (CMCs). The IWCTUs of Pb and Zn indicate their possible risk of toxicity. Principal component analysis reveals that Mn in sediments is preferentially associated with the exchangeable fraction, Cu and Zn are mainly associated with the reducible fraction, and Cr, Ni, and Pb are strongly related to the residual fractions. Heavy metals in sediments mainly accumulate in the fine fractions (< 4, 4-8, and 8-16 mu m). Nickel, Pb, and Zn in sediments occasionally cause adverse biological effects. Overall, PLIs of sediments at sites 1, 4, and 10 in the Yongding River show moderate pollution due to Cr, Cu, Mn, Ni, Pb, and Zn. Different heavy metals possess unique partition coefficients between overlying water and sediments because these metals exhibit various properties and mobility. The chemical fractions of heavy metals also confirm the partition coefficients. Heavy metals in fine sediment fractions also enhance their migration and occasionally cause severe biological effects. Therefore, drinking water security in these areas should be ensured.