Investigating the Risk of Heavy Metal Contamination in Suspended Sediments and Surface Soil of Laniz Watershed, Karaj

Water and soil play an important role in all human environmental activities and controling physical, chemical and biological processes in the environment. The existence of limitations of water resources, especially the fresh water of rivers and dams, makes it necessary to pay more attention to them against pollution. Contamination of water and soil with heavy metals are serious environmental pollutants that enter the environment in many different ways with natural and human origin and cause serious damage. Karaj Dam is one of the most vital sources of drinking water supply in the metropolis of Tehran, which faces a serious risk of pollution due to the development of human activities. This study investigated the concentrations of heavy metals (Zn, V, Ti, Pb, Ni, Mn, Cu, Cr, and As) in 40 soil samples and sediments, as well as five suspended sediment samples, in the river of the Laniz watershed in Karaj, Iran. Due to the construction activities in the watershed including the construction of the Tehran-North Freeway, the natural condition has been affected over the past decade and therefore need to use and new inventory method to separate the effects of natural geochemical from humane effects. The single-element indices of contamination factor (CF), geoaccumulation index (Igeo), normalized enrichment factor (EF), and ecological risk potential factor (ER), as well as two multi-element indices of pollution load index (PLI) and ecological risk index (RI), were employed. At first the elements and samples were grouped by principal component analysis (PCA) and cluster analysis. FA distinguished two groups of elements; called natural and human-natural origins. The cluster analysis divided 50% of the freeway-affected samples into two distinct clusters. Based on the CF index, the As contamination was extreme in the suspended sediment samples and soil samples related to the Freeway while the Pb and Mn carried out the next rank. Also, the single Igeo index revealed the high As contamination. Moreover, based on the Igeo, most suspended sediment samples related to the Freeway put in the extreme contamination class for all of the elements. However, the Mn-modified enrichment factor showed only one freeway-affected and two freeway-unaffected samples exhibited extreme enrichment, and the enrichment of other samples was low or moderate. Unlike the single-element indices, PLI and RI showed that the suspended sediment and soil samples related to the freeway had low pollution load and ecological risk. Compare of single and multiple contamination indices revealed an inconsistency of pollution risk that can be addressed into the background value (in single indices) and the toxicity response factor of the metals (in multiple indices). Results indicated that due to spatial-temporal changes of human effects, it would be better to use different values for both background and elements toxicity response. Therefore, to reach a reality image of contamination risk, more researches are needed to elucidate the mentioned standard value based on the regional variations.