Depth Variation in Copper, Lead, and Zinc Concentrations and Mass Enrichment Ratios in Soils of an Urban Watershed

Abstract

Previous studies in the Manoa watershed (Hawaii) have indicated significant anthropogenic enhancement of trace metals in road deposited sediment and fluvial bed sediments. This study was conducted to examine trace metal loadings with depth in soils of Manoa watershed. A total of 78 roadside (disturbed) and 10 background (control) soils were sampled at two depths for selected soil properties and for total and HCl‐extractable (labile) concentrations of Al, Cu, Pb, and Zn. Based on sample proximity to roads, typically <2 m, the automobile is considered the primary source of inputs. Concentration, mass loading, and mass per area enrichment ratio (MAER) data indicate that Pb was the most significantly enhanced trace metal. Labile Pb concentrations were log‐normally distributed with maximum values of 2870 and 3560 mg kg −1 in topsoil and subsoil samples, respectively. Roadside labile Pb was four to five times higher than in background soils. Mass loading and MAER data indicate that subsoils were statistically more enriched in labile Cu and Pb, while Zn had similar values with depth. More than 50% of the subsoils were classified as having significant to extreme labile Pb anthropogenic signals. Trace metals in this study have undergone significant translocation, possibly through processes of colloidal transport and/or preferential flow. The environmental significance of this research is discussed in terms of potential influence of Pb mobility on ground and surface waters, and the continued contribution of high levels of labile Pb to road surfaces with erosion of exposed subsoil layers.