Ecological risk assessment and source tracing of heavy metals in surface sediments of a hilly riverine reservoir in Chongqing, China

Background This work conducted quantitative and qualitative analyses on the heavy metals in 32 surface sediment samples collected from eight sampling sites across four seasons in the Yutan reservoir, a typical mountainous hilly riverine reservoir in Southwest China. Results Nine heavy metals were identified: (Cr, Cd, Pb, Cu, Zn, Hg, As, Ni, and Co). The concentrations varied within the ranges of 23.16 similar to 34.62, 0.53 similar to 1.09, 31.88 similar to 59.04, 51.94 similar to 85.84, 106.76 similar to 227.69, 0.08 similar to 0.15, 12.57 similar to 25.60, 12.06 similar to 27.64 and 11.04 similar to 14.56 mg/kg, respectively, following a decreased concentration order of Zn > Cu > Pb > Cr > Ni > As > Co > Cd > Hg. Except for Cd, which accumulated in winter, and Hg, which reduced in spring. The concentration of heavy metals showed no significant seasonal variation and generally had higher contents at the river mouth and lower concentrations in open water areas and branches. The geoaccumulation index (I-geo) assessment also indicated that the reservoir was uncontaminated by Cr and Ni (I-geo < 0), slightly polluted by Pb, Cu, Zn, and Co (0 < I-geo < 1), moderately polluted by Hg and As (1 < I-geo < 2), and heavily polluted by Cd (1.44 < I-geo < 2.48). Notably, the most polluted sites were at the river mouth, followed by the sediments in branches with slight pollution. Source tracing analysis revealed that Cr, Ni, Cd, Pb, Hg, As, organic matter, total nitrogen, and total phosphorus were primarily attributed to non-point sources. In contrast, Fe was linked to the hardware industry. Moreover, Al originated from sewage and drinking water treatment processes. Cu and Zn were discharged from three components, indicating complex sources. Conclusions The findings underscored that non-point sources were the primary contributors to the increased risk of heavy metal contamination in the reservior's sediment. In addition, to effectively manage the risk and enhance the aquatic environment, greater focus should be placed on the inner load of heavy metals in the sediment of the mountainous hilly riverine reservoirs, particularly after controlling external pollution sources.