Ecological risk of heavy metal hotspots in topsoils in the Province of Golestan, Iran

• Golestan Province, Iran showed Cd, Cu, Ni, Pb and Zn contaminations to some degree.
• There were three pollution hotspots groups of heavy metals in Golestan Province.
• Metal concentrations in the topsoil have both natural and anthropogenic origin.
• Anthropic heavy metal input is related with mining, farming and vehicle emissions.
• Moderate and high potential ecological risk covered about 90% of Golestan Province.

Human activities, such as agriculture or mining, are a continuous source of risk for heavy metal pollution that seriously disturbs the soil environment. Massive efforts are being made to identify the tools to determine indicators of soil quality condition. This study characterises and evaluates the heavy metal (Cd, Cu, Ni, Pb and Zn) contents in the Province of Golestan (northern Iran). Pollution was assessed using the pollution index (PI) and the integrated pollution index (IPI). The potential harmful effects of these heavy metals were evaluated by the Potential Ecological Risk Index (PERI) Method. Kernel density estimation (KDE) and Local Moran's I were used for the hotspot analysis of soil pollution from a set of observed hazard occurrences. In all, 346 topsoils were examined, which represent three areas, approximately including the middle-south, west and north-east areas in this region. The heavy metal concentrations in the analysed samples did not generally present high values, despite anthropic heavy metal input. However, the potential ecological risk indexes (RI) indicated that approximately 68% and 5% of the study samples had medium and high pollution levels, respectively. Multiple hotspots for the above five heavy metals were located in the middle-south and west study areas. This anthropic heavy metal input is related to mining, agricultural practices and vehicle emissions. It was concluded that a moderate and high potential ecological risk covered about 90% of this province. In contrast, the natural origin input became more marked on a long spatial scale.