Spatial distribution and pollution assessment of nitrogen, phosphorus, and heavy metals in the surface sediments of Xiaonanhai Lake, Hubei Province

Abstract Located in Nanhai Town, Songzi City, Hubei Province, Xiaonanhai Lake is the largest natural lake in Songzi. It was once severely polluted due to the discharge of urban and rural domestic sewage, disorderly development of agricultural planting, unregulated aquaculture, and poultry farming. However, relevant estimations of the pollutant content in its sediment have not been carried out. This study analyzed the spatial patterns of heavy metal pollution and eutrophication at 36 water sampling sites in the Xiaonanhai Lake area, focusing on eight heavy metals: Cd, Cr, Cu, Ni, As, Pb, Hg, and Zn. The nutrient status of the lake area was evaluated using the nitrogen‐phosphorus comprehensive pollution index, and heavy metal pollution status of the lake area was evaluated using geo‐accumulation and the potential ecological risk index. Spatial autocorrelation analysis revealed the spatial correlation and aggregation of eutrophication levels in Xiaonanhai Lake. The results showed that the overall trophic state of the Xiaonanhai Lake area was moderate eutrophication, with a gradually decreasing eutrophication level from north to south. The Chengnan Wastewater Treatment Plant in the northern part of the lake area and surface source pollution from aquaculture were the main nitrogen and phosphorus sources. The overall ecological risk index of heavy metal pollution was medium and gradually weakened from north to south, consistent with the thickness of the bottom mud. The heavy metal pollution load was mainly precipitated from the bottom mud in the lake area. The eutrophication and heavy metal pollution levels in the lake area showed significant positive spatial autocorrelation, the influence range of the regional eutrophication level was small, and the spatial heterogeneity of the eutrophication and heavy metal pollution levels in Xiaonanhai Lake was relatively high. The northern part of the lake was a hotspot (high/high aggregation) of eutrophication ( p < 0.01) while the southern part was a cold spot (low/low concentration; p < 0.05). The middle and northern part of the lake area was the hot spot (high/high concentration) of heavy metal pollution level ( p < 0.1) while the southern part was the cold spot (low/low concentration; p < 0.1). Therefore, when carrying out water environment management in Xiaonanhai Lake, the northern area and the middle area should be prioritized for eutrophication prevention and control and dredging.