Investigation of the source of pollution of heavy metals and petroleum hydrocarbons in agricultural lands south of Tehran

As a result of human development and population growth, there is a corresponding need for essential resources of human. Industrial and agricultural activities have greatly polluted most agricultural lands. Petroleum compounds and heavy metals, both are the common pollutants of soils that have been irrigated by untreated wastewater, which pose a potential threat to the environment. Soil pollution of the agricultural lands will lead to a decline in cultivation, and finally decrease food production. Agricultural lands in the southern part of Tehran are being irrigated with untreated wastewater for more than 30 years to produce a variety of vegetables, legumes, cereals.

In this study the concentration of heavy metals and petroleum compounds were determined in 83 sampling points at two depths (0 to 30 and 30 to 60 cm). The study area was divided into two separate zones, in which 44 points were located in zone 2 and 39 points were located in zone 1. Petroleum hydrocarbons and heavy metals, As, Cd, Co, Cr, Cu, Ni, Pb, V, and Zn, were measured at top and subsoil by using MOOPAM method, ICP-AES method, respectively. ArcGIS and R software were applied to create distribution maps of the pollutants and some statistical analyses.

The results showed that the soil of agricultural land in the area is highly polluted, as the concentration of Cr, Pb, Co and Ni has exceeded the standard level e,g. 0.22 mg/kg for Cd and 620 mg/kg for Pb. The concentration of petroleum compounds in wastewater irrigated lands in both top and subsoil were higher than that of ground water irrigated lands. High level of As, Cr, Cu, Pb and Zn were observed in ground water irrigated lands compare with wastewater irrigated lands. Cr and Pb were almost 89 and 8 times higher than the standard limit of agricultural lands of Iran, respectively. The spatial distribution map of petroleum compounds showed that only the southeast of the area fall into heavily contaminated class. The distribution map of heavy metals also revealed that most parts of the studied area fall into the heavily contaminated class. Soil organic matter has more concentration in topsoil.

Overall, the south of the studied area has been more affected by wastewater irrigation, agrochemicals and ground water pollution in terms of heavy metals and petroleum compounds. Our study revealed various anthropogenic pollution sources, more effectively by wastewater irrigation and application of agrochemicals. Therefore, a management plan should be applied to the agricultural lands of this region to control and reduce the level of contamination.