**1. Introduction**

In the world, in many study areas, most of the heavy metal (HM) content in the samples did not exceed the local standards, which can be mentioned as the study of [1] conducted a determination of soil samples of HM components in the North-Western area of Thessaloniki, North Greece near the insanitary landfill. Soil samples were taken at a depth of 2.5–17.5 m. Heavy metals Cd, Cr, Cu, Ni, Pb and Zn were analysed, however, although this area has a high degree of industrialisation, the soil is not contaminated by HMs. Research by Agamuthu and Fauziah [2] conducted sampling of typical soil at different locations in two landfills of Panchang Bedena and Kelana Jaya (Malaysia) to analyse HM content. Soil sampling depth is 2 m–35 m. The analysis results of samples at the Panchang Bedena landfill showed that all the analysed HMs have lower concentrations than the Dutch standard [3]. Opaluwa et al. [4] studied HMs in the soil at a depth of 0–15 cm and leaves on the campus of the Agriculture Faculty of National Polytechnic University, Nasarawa State, Nigeria and the areas near landfills and got the soil concentration of metals As, Cd, Co, Cu, Fe, Ni, Pb and Zn lower than the levels allowed by the World Health Organisation. Similarly, the translocation of HMs from the polluted soil to the aboveground parts of plants and lichens leads to a 1.5- to 5-fold increase in the content of HMs in all species, which fortunately neither exceed the toxicity threshold and nor prevent their growth in the experimental plots as by the work of Lyanguzova et al. [5]. According to Piyada and Suksaman [6], the landfill on the edge of Nai Muang Phichai district in Uttaradit Province, Thailand is one of the most polluted landfills in the world, leachate has a high content of HMs. Samples of soil and groundwater from the landfill and in the vicinity of the landfill are collected and analysed for HM content. Fortunately, the concentrations of Cd, Pb, Cu, Zn and Fe are all low, within the limits of soil quality standards. Research by Siti et al. [7], in Selangor state, Malaysia, there are 20 landfills, including the Ampar Tenang landfill closed in January 2010. However, the landfill is not covered with protective soil according to operational design standards, and before that the garbage is dumped directly onto the ground without any insulating material. Surface soil is relatively polluted by As, Pb, Fe, Cu and Al. As and Pb concentrations are greater than the allowable levels which are 5.90 mg/kg and 31.0 mg/kg, respectively. In addition, only Cu tends to decrease concentration with depth. Kamarudin et al. [8] studied the distribution of HMs in underground aquifers in the solid waste treatment area in Taiping, Perak, Malaysia. Soil samples were taken in 6 boreholes at a depth of 6 m–30 m, and a sampling distance with depth was 1 m. HMs Pb, Mn, Cr, Fe, Zn and Cd were analysed by ICP-MS. The concentrations of Pb, Mn, Fe and Zn are quite high, exceeding the allowable concentrations in the drinking water standard.

It can be seen that most of the HM contents in the soil environment of the landfill do not exceed the allowable levels. In cases where the content of HMs exceeds the standards, there will be an insanitary landfill that does not operate properly with waste burial techniques.

From the above review of several studies, it can be seen that untreated leachate, containing high levels of HMs, is a clear pollution source. Best. Leachate from landfills is capable of polluting the soil and groundwater environment if there are no measures for collection and treatment and to prevent the release of leachate to the surrounding environment. Most studies show that soil at the depth of 2 m and greater is not contaminated with HMs at a level greater than the allowable levels for agricultural land. However, the transport of HMs in the upper soil layer is extremely slow, especially thanks to the effect of adsorption.
