Optimal Evaluation of Ecological Risk of Heavy Metals in Urban Road Sediments

Abstract

Heavy metals in urban road sediments can pose ecological risks to receiving water bodies under the erosion of precipitation runoff. Based on the total amount, toxicity and chemical composition of heavy metals, single-factor pollution index method (Pi), ecological risk index method (RI), and risk coding evaluation method (RAC) were used to analyze the sediments in 11 main roads in Laoshan District, Qingdao City. The results showed that the evaluation methods were quite different. A new fuzzy comprehensive evaluation system was established to optimize these three evaluation methods. The results are as follows: (1) Cu, Zn, Pb, Cd were all accumulated in road sediments. The main sources of the four heavy metals were traffic activities, atmosphere deposition and road marking paint. The fraction of heavy metals was unevenly distributed. Cu was mainly in the oxidation fraction, Zn was mainly in the weak acid extraction fraction, Pb was mainly in the residue fraction, and Cd was mainly in weak acid extraction, reducible and oxidizable fractions. (2) The results of the Pi evaluation showed that the Pi values of Cu, Zn, Pb, and Cd were 3.80, 27.61, 1.21, 8.33, respectively. Zn and Cd showed heavy pollution level, Cu showed moderate pollution level, and Pb showed light pollution level. The result of RI comprehensive evaluation was high risk, among which Cd was high risk, and the other 3 metals were low risk. The result of RAC evaluation was Zn (40.44%)>Pb (28.24%)>Cd (25.37%)>Cu (3.80%). Zn presented high ecological risk, Pb and Cd presented medium ecological risk, and Cu presented low ecological risk. The reason for the different results of the three evaluation methods is that they respectively focus on the total amount, toxicity and fraction of heavy metals. (3) A new fuzzy comprehensive evaluation system was established and a new evaluation standard was set to accurately evaluate the pollution status of heavy metals. The optimization evaluation results showed that Cd (87.4)>Zn (72.04)>Pb (53.96)>Cu (53.56). This indicated that the toxicity of heavy metals has a greater impact on the ecological risk of heavy metals than the total amount and form of heavy metals, and the total amount will also affect the evaluation results to a certain extent. In actual work, when the results obtained by various evaluation methods are different, the fuzzy comprehensive evaluation system is conducive to more accurate grasp of heavy metal pollution and ecological risks.