Background and Objectives: Landfill centers, as places for accumulation of various wastes, can become sources of soil and groundwater pollution. Waste leachate generated from municipal landfills constitutes a highly toxic and hazardous form of wastewater, recognized as a potential source of environmental contamination. Heavy metals are among the most important pollutants present in these centers, which, due to their high toxicity and long persistence in the environment, pose a serious threat to human health and the ecosystem. Currently, over 90% of waste generated in Mazandaran Province is managed through traditional disposal methods, which include either burial or surface piling at waste disposal centres. Accumulation of waste in the forest ecosystems leads to elevated concentrations of heavy metals, including lead, cadmium, selenium, zinc, and arsenic. Additionally, various pollutants infiltrate the soil and compromise the natural moisture regulation mechanisms, resulting in toxicity. Certain tree species exhibit high sensitivity to sewage leachate and may exhibit rapid desiccation. The present study aims to investigate the concentration of such heavy metals as chromium, lead, nickel, and zinc in landfill leachate located in the Semskandeh Sari landfill center.
Materials and Methods: The Semeskandeh habitat is situated along the southern edge of the historic Sari-Neka road within the Mazandaran Province, encompassing an area of over 1041 hectares. Predominantly, this region is characterized by diverse forest ecosystems. The area supports a range of tree and shrub species, including oak, Azad (almond), Angelica, maple, alder, larch, milkweed, hornbeam, black and white ash, oleander, wild plum. For this purpose, 6 sampling stations were considered along a waterway that extends from the landfill center into the forest. At each station, three sampling points were considered. The first sampling point was soil sampling at a depth of 10 cm from the bottom of the water channel where the leachate flows, the second point was less than 10 meters from the channel, and the third point was more than 10 meters from the center of the channel, and a sample was taken from a depth of 10 cm. To investigate the trend of changes in leachate composition, soil sampling was carried out to examine the concentration of 4 heavy elements (lead, nickel, zinc, chromium).
Results: Results of the investigations showed that the highest concentration of all 4 studied elements, except for zinc, was obtained at station number 2, with chromium at 55.14 ppm, lead at 51.33 ppm, nickel at 32.55 ppm, and zinc at 12.82 ppm. Gradually, the concentrations decreased with distance from the landfill center. Also, the highest concentration at the three sampling points was obtained at the bottom of the channel, with chromium at 57.59 ppm, lead at 28.93 ppm, nickel at 49.67 ppm, and zinc at 99.08 ppm. Also, the area located within 10 meters from the canal exhibited the highest concentrations of pollutants in the second place, while samples from areas beyond 10 meters showed significantly lower levels. The highest concentrations of elements were recorded in the canal bottom samples, with a gradual decline in element concentrations observed as the distance from the channel increased.

Conclusion: The findings of this study demonstrate elevated concentrations of all analysed elements at various distances from the landfill site. Given that forest lands in this city have been utilized for landfilling, this practice poses significant harmful effects. These effects encompass environmental pollution from various hazardous substances detrimental to the health of all living organisms, air quality degradation, and particularly groundwater contamination. The latter is especially concerning as it serves as the primary water supply for residential areas surrounding these forests.