The global industrial revolution has released the exceptional value of toxic elements (contaminants) in the environment. The majority of pollutant sources are characterized by rapid urban sprawl and increased growth in population (Dhaliwal et al., 2020; Shammi et al., 2021). This exponential growth of the world population has trained phenomenal rise in municipal solid wastes worldwide (Karak et al., 2012). In developing countries like Iran, non-engineered landfills are the only waste management technique. They are considered an economical and convenient option to dispose of municipal solid waste (MSW) (Alam et al., 2020; Rubinos and Spagnoli, 2018). However, this convenient option has some disadvantages, including solid waste mismanagement and landfill leachate, which causes soil pollution, and surface and groundwater contamination, especially from the harmful and toxic heavy metals (Oziegbe et al., 2021; Ju et al., 2020; Tsai et al., 2020; Hussein et al., 2020).
Despite the employed safeguards, leachate production is the inevitable consequence of the operation of landfills which are regarded as one of the main polluters of the soil and water environment and should be adequately addressed (Wdowczyk and Szymańska-Pulikowska, 2021; Hussein et al., 2019; Manikanda Bharath et al., 2021). They arise from excessive rainwater seeping through waste layers (Gupta et al., 2014; Kumari et al., 2019). Leachate accumulates significant environmental contaminants containing heavy metals, turbidity, ammonium nitrogen (NH3-N), suspended solids, chemical oxygen demand (COD), biochemical oxygen demand (BOD), color, as well as other substances (Jayanthi et al., 2017; Yang et al., 2017). Heavy metal is a significant parameter of concern in landfill leachate due to its cumulative, persistent, and toxicity. Pollution resulting from heavy metals has been a worrying concern even in the ocean environment for a substantial period (Patel et al., 2018; Adamcová et al., 2017). Unlike organic matter in leachate, which may decrease the methane production from solid waste (Yusof et al., 2009), trace elements can remain within the landfills for about 150 years if they are leached at a rate of 400 (mm/year) (Adelopo et al., 2018).
Soils can accumulate multiple contaminants, including Pb, zinc (Zn), Cr, nickel (Ni), Selenium (Se), Cd, As, Hg, and copper (Cu). These heavy metals have been classified by the United States Environmental Protection Agency (USEPA) as priority control pollutants because of their toxicity, bioaccumulation, and low degradability (Chen et al., 2016; Zhou et al., 2019). Also, arsenic (As), cadmium (Cd), chromium (Cr), and nickel (Ni) are categorized as group 1 carcinogens based on the International Agency for Research on Cancer (IARC, 2018), which are also toxic in the environment. In addition, HMs enrichment can result in intense ecological risks by absorbing various aquatic organisms, consequently entering into a complex food chain (Dash et al., 2019). The human central nervous system tends to be negatively impacted by long-term exposure to heavy metal soil pollution (Khanam et al., 2019; Tseng et al., 2019). Moreover, previous investigations have claimed that extended exposure to HMs such as Cr, Cd, and Ni can lead to allergy, asthma, dermatitis, diarrhea, or even lung cancer (Bhattacharya et al., 2015; Moreira et al., 2018; Shen et al., 2019). However, the mechanisms of HMs dynamics can be better understood by investigating the metal fractionations, mobility, and potential bioavailability (Gujre et al., 2021).
Although many studies have been conducted about the management and treatment of landfill leachates in Iran, such as Moradian et al. (2020), Reshadi et al. (2021; 2020), Bakhshoodeh et al. (2020), and Torkashvand et al. (2021), only limited attention has been paid to the effect of the landfill and its leachate on the geochemical properties of the soils. However, several kinds of research (Hussein et al., 2020; Alam et al., 2019; Borba et al., 2020; Afolagboye et al., 2020; Gujre et al., 2021) have been conducted to evaluate the pollution of HMs in the soil around a municipal waste dumpsite all over the world. For instance, Hussein et al. (2020) investigated the impact of leachate on soils and natural soils of several landfills in Malaysia. They reported that the impacted soils, compared to background values, and natural soil around the dumps, had high contents of HMs, particularly at non-sanitary unlined landfills. Alam et al. (2019) evaluated the HM pollution in water, soil, and plant nearby the open landfill site Mogla Bazar in Sylhet, Bangladesh. Their results showed that the accumulation level of HM in soil was slightly higher than the standard level, which pointed out that it would be hazardous to humans and the vicinity of the environment. Borba et al. (2020) examined the concentration of HMs in soil applied as landfills in Southern Brazil. Despite the collection depth, results showed variations in the concentration of cadmium and chromium. In addition, the presence of iron oxides/hydroxides and 1:1 low CEC kaolinite clay in the soil samples could have contributed to the displacement of these elements to the underground environment. Afolagboye et al. (2020) studied the HMs concentration and pollution status in soils nearby a municipal waste dumpsite in Nigeria. They reported that the soils in and nearby the landfill are contaminated due to the dump site. Gujre et al. (2021) studied the extent of soil pollution and potential ecological and health risks related to the dump near a Ramsar site in Assam, India. Their results indicated that 16 of the 20 sampling sites were heavily polluted for Cr, while others near the metal segregation units were strong to highly contaminated. In a few areas, considerable enrichment was examined for Zn, and Mn was minimal to moderate enrichment. Health risk assessment results showed that Cr posed a higher risk to human health through ingestion.
In Iran, Azizpour et al. (2020) evaluated the impact of HMs concentration on soils nearby a landfill in the Tonekabon region, NW of Iran. Results indicated that the HM pollution ranged from uncontaminated to highly contaminated, which showed significant HM pollution in the study area associated with MSW disposal in Tonekabon. Jawed Pazhmaan et al. (2021) examined the HM contamination and the spatial distribution of soil pollution nearby a landfill site in the southwest of Gorgan, Iran. Their study showed that soil in the landfill area had a moderate ecological risk index and the central parts of the study area had elevated concentrations of Cd and Pb than the southern parts because of the slope and runoff of the waste leachate. Additionally, the concentration of Cd and Pb in the sampling sites was higher than in the control site. Seyed Asri et al. (2019) examined the HM pollution trend in soils nearby a landfill in Tonekabon, Iran. Their result indicated that the leachate ultimately impacted the HMs in soil from the landfill site due to a similar trend and slope in HMs in leachate and soils. Additionally, compared to world and Iranian environmental standards, the value of all HMs except for Pb, Cr, and Zn were higher than allowable limits.
Therefore, in this study, we investigated soil chemical characteristics at specific MSW landfill sites in Kazerun. The aims of the survey can be declared as follows: (i) to evaluate the concentration of As, Cd, Co, Cu, Cr, Mo, Ni, and Zn in the soils polluted with MSW, (ii) to evaluate the pollution level and ecological risks based on several pollution indices and factors, (iii) to assess the human health risk assessment models to analyze whether the exposure to HMs of any dose could result in an adverse effect to human health.