Heavy metal accumulation from zinc smelters in a carbonate rock region in Hezhang County, Guizhou Province, China

In this study, four sites (Sites A, B, C and D, which were associated with varying durations of zinc smelting) namely two slag piles (Profile 1, Profile 2) at Sites B and C and two water channels at Sites A and D in Hezhang County, Guizhou Province, Southwestern China, were investigated. Significantly elevated mean Pb, Zn and Cd concentrations, 11,192 (+/- 8,991), 20,563 (+/- 16,727) and 137 (+/- 112) mg kg(-1) in slag Profile 1(n = 10), and 12,535(+/- 4,408), 26,189 (+/- 6,209) and 47.9 (+/- 30.1) mg kg-1 in slag Profile 2 (n = 12) were detected, respectively. Both profiles had high pH values (7.72 +/- 0.37 in Profile 1 and 8.33 +/- 0.49 in Profile 2). In addition, high mean Pb, Zn and Cd in soils (14,945 (+/- 9,696), 17,059 (+/- 12,534) and 61.9 (+/- 33.6) mg kg-1) and in stream sediments (11,450 (+/- 7,097), 20,768 (+/- 11,404) and 66.8 (+/- 25.5) mg kg-1) were found at Site D. A Composite Pollution Index (CPI) determined for each study Site confirms the anthropogenic origins of the observed Pb, Zn and Cd contamination. Small portions (< 3% of the total) of exchangeable Pb and Zn fractions in samples were extracted by using a sequential extraction technique. In contrast, 37.7% (+/- 7.69%), 21.3% (+/- 7.75%) and 22.7% (+/- 13.2%) of total Pb, Zn and Cd was associated with the carbonate bound fraction. This is indicative of the alkaline pH associated with the carbonate based-ores of this region. Further, total metal concentrations were positively and significantly correlated with soil Fe contents. This may in large part be due to the co-precipitation of Pb, Zn and Cd with Fe minerals in the surface weathering environment. However, a potential threat still exists with regards to the environment release of Pb, Zn and Cd as a result of an alteration of the pH environment, such as in the acidic rhizosphere environment of cultivated crops or acid deposition in this region. It is therefore suggested that immediate remediation measures are implemented to minimize potential risks.