Highway traffic generates heavy metals and particulate matter through various vehicular and tire-pavement abrasion mechanisms. These abraded materials are deposited, they accumulate, and they are transported by storm water. Soils subject to years of such loading can serve as a sink and a potential source for heavy metals. The results of geotechnical analyses, heavy metal distributions, drainage influences, and correlations to geotechnical indices for surficial (0 to 15 cm) glacial till samples recovered from two transects along a heavily traveled urban interstate highway were compared with a control site subjected to only urban atmospheric deposition. This investigation indicated, for this site, that heavy metal accretion in the surficial soils is a function of depth, surface drainage patterns, distance from the pavement edge, and soil indices. Particulate-bound heavy metal deposition and accretion or export were a function of surface flow conditions such as velocity, flow depth, and surface cover. Results indicated that heavy metal accretion rapidly decreases as a function of distance from the traveled roadway. Along the longitudinal transect, correlations between heavy metals and soil organic content were statistically significant, particularly for copper. Along the transverse transect, correlations between soil plasticity, organic content, and heavy metals were statistically significant. Although there is little control of traffic levels and past accretion, indices such as soil organic content and plasticity index, as well as pavement runoff surface drainage patterns, can provide information about whether highway soils might act as a sink or source of heavy metals and, consequently, if best management practices may be justified.
Surface area is a primary factor in determining many physical and chemical properties of solids, especially particles. In urban and highway runoff, solids can mediate the partitioning between the dissolved and particulate-bound phases of metal elements and organic compounds. Solids are also capable of adversely affecting roadway drainage appurtenances through sedimentation and clogging. Solids characteristics of primary importance for both solute adsorption and clogging and sedimentation are particle size distributions (PSDs), specific surface areas (SSAs), and mass loadings. PSD and SSA results are presented for rainfall and snowmelt solids from a heavily traveled urban roadway in Cincinnati. Integration of the PSD and SSA results indicates that particle surface area is greatest for the midrange (> 100 μm) to the coarser end (<2000 μm) of the gradation. SSA results determined using the assumption of smooth spherical particles are indicated to grossly underestimate actual SSA values.
Urban roadway drainage often contains high concentrations of anthropogenic metal elements, solids, and organic compounds. Metal elements and solids accumulate on the pavement surface between precipitation events and are transported to surficial soils, receiving waters, and ground-water by pavement drainage. A report is provided on the design and performance of a bench-scale and prototype in situ control strategy, called a partial exfiltration trench (PET). The PET serves as a modification to the current practice of pavement underdrainage, providing a water quality function, in addition to water quantity control. The water quality modification functions by immobilizing infiltrated metal elements within the PET. The PET utilizes sand modified with an oxide coating (OCS). Bench-scale PET simulations were utilized to evaluate PET feasibility and breakthrough capacity. Results indicate OCS capacity for zinc, cadmium, lead, and copper was significantly improved as the pH increased from 6.5 to 8.0. Based on characteristic metal loadings from urban Cincinnati pavement drainage, bench-scale PET performance indicates that the design life of a PET may exceed 15 years in a humid climate. Performance of a prototype PET installed along an urban Cincinnati highway indicates metal element mass removal efficiency is generally greater than 80 percent after nearly 1 year of pavement drainage loadings.
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