M. Huber

Critical review of heavy metal pollution of traffic area runoff: Occurrence, influencing factors, and partitioning.

A dataset of 294 monitored sites from six continents (Africa, Asia, Australia, Europe, North and South America) was compiled and evaluated to characterize the occurrence and fate of heavy metals in eight traffic area categories (parking lots, bridges, and three types each of both roads and highways). In addition, site-specific (fixed and climatic) and method-specific (related to sample collection, preparation, and analysis) factors that influence the results of the studies are summarized. These factors should be considered in site descriptions, conducting monitoring programs, and implementing a database for further research. Historical trends for Pb show a sharp decrease during recent decades, and the median total Pb concentrations of the 21st century for North America and Europe are approximately 15 μg/L. No historical trend is detected for Zn. Zn concentrations are very variable in traffic area runoff compared with other heavy metals because of its presence in galvanized structures and crumbs of car tire rubber. Heavy metal runoff concentrations of parking lots differ widely according to their use (e.g., employee, supermarket, rest areas for trucks). Bridge deck runoff can contain high Zn concentrations from safety fences and galvanizing elements. Roads with more than 5000 vehicles per day are often more polluted than highways because of other site-specific factors such as traffic signals. Four relevant heavy metals (Zn, Cu, Ni, and Cd) can occur in the dissolved phase. Knowledge of metal partitioning is important to optimize stormwater treatment strategies and prevent toxic effects to organisms in receiving waters.

Evaluation of site-specific factors influencing heavy metal contents in the topsoil of vegetated infiltration swales

Stormwater runoff of traffic areas is usually polluted by organic and inorganic substances and must be treated prior to discharge into groundwater. One widely used treatment method is infiltrating the runoff over the topsoil of vegetated swales. The aim of this study was to evaluate the factors influencing the heavy metal contents in such topsoil layers of vegetated infiltration swales near highways, roads, and parking lots. In total, 262 topsoil samples were taken from 35 sampling sites, which varied in age, traffic volume, road design, driving style, and site-specific conditions. In the evaluation of all soil samples, the median heavy metal values of cadmium, chromium, copper, lead, and zinc were yielding 0.36 (mean: 1.21) mg/kg DM, 37.0 (mean: 44.5) mg/kg DM, 28.0 (mean: 61.5) mg/kg DM, 27.0 (mean: 71.9) mg/kg DM, and 120 (mean: 257) mg/kg DM, respectively. The main purpose was to evaluate the site-specific data (i.e., surrounding land use characteristics, traffic area site data, and operational characteristics). In general, heavy metal contents increased with increasing traffic volumes. However, other factors also had a notable impact. Factors such as road design (e.g., curves, crossings, and roundabouts) and grade of congestion significantly influenced the heavy metal contents. High heavy metal contents were detected for stop-and-go areas, roundabouts, crossings, and sites with traffic lights, signs, and guardrails. Findings of this study can be used to identify highly polluted traffic areas and to verify or improve standards regarding the treatment of runoff from traffic areas.

A novel test method to determine the filter material service life of decentralized systems treating runoff from traffic areas

In recent years, there has been a significant increase in the development and application of technical decentralized filter systems for the treatment of runoff from traffic areas. However, there are still many uncertainties regarding the service life and the performance of filter materials that are employed in decentralized treatment systems. These filter media are designed to prevent the transport of pollutants into the environment. A novel pilot-scale test method was developed to determine - within a few days - the service lives and long-term removal efficiencies for dissolved heavy metals in stormwater treatment systems. The proposed method consists of several steps including preloading the filter media in a pilot-scale model with copper and zinc by a load of n-1 years of the estimated service life (n). Subsequently, three representative rain events are simulated to evaluate the long-term performance by dissolved copper and zinc during the last year of application. The presented results, which verified the applicability of this method, were obtained for three filter channel systems and six filter shaft systems. The performance of the evaluated systems varied largely for both tested heavy metals and during all three simulated rain events. A validation of the pilot-scale assessment method with field measurements was also performed for two systems. Findings of this study suggest that this novel method does provide a standardized and accurate estimation of service intervals of decentralized treatment systems employing various filter materials. The method also provides regulatory authorities, designers, and operators with an objective basis for performance assessment and supports stormwater managers to make decisions for the installation of such decentralized treatment systems.

Heavy metal removal mechanisms of sorptive filter materials for road runoff treatment and remobilization under de-icing salt applications

The objective of this research study was to elucidate the removal and remobilization behaviors of five heavy metals (i.e., Cd, Cu, Ni, Pb, and Zn) that had been fixed onto sorptive filter materials used in decentralized stormwater treatment systems receiving traffic area runoff. Six filter materials (i.e., granular activated carbon, a mixture of granular activated alumina and porous concrete, granular activated lignite, half-burnt dolomite, and two granular ferric hydroxides) were evaluated in column experiments. First, a simultaneous preloading with the heavy metals was performed for each filter material. Subsequently, the remobilization effect was tested by three de-icing salt experiments in duplicate using pure NaCl, a mixture of NaCl and CaCl2, and a mixture of NaCl and MgCl2. Three layers of each column were separated to specify the attenuation of heavy metals as a function of depth. Cu and Pb were retained best by most of the selected filter materials, and Cu was often released the least of all metals by the three de-icing salts. The mixture of NaCl and CaCl2 resulted in a stronger effect upon remobilization than the other two de-icing salts. For the material with the highest retention, the effect of the preloading level upon remobilization was measured. The removal mechanisms of all filter materials were determined by advanced laboratory methods. For example, the different intrusions of heavy metals into the particles were determined. Findings of this study can result in improved filter materials used in decentralized stormwater treatment systems.

Determination of Heavy Metals in a Highly Porous Sorptive Filter Material of Road Runoff Treatment Systems with LA-ICP-MS

To remove heavy metal contaminations from road runoff prior to discharge into surface water or groundwater, highly porous sorptive filter materials are used. One effective material is a technical product based on granular ferric hydroxide. To specify the removal mechanisms, lab-scale column experiments were performed preloading the material with Cd, Cu, Ni, Pb, and Zn. To identify removal mechanisms and the distribution of heavy metals in the material, investigations with scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDX) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were performed. With SEM/EDX, only Pb was detectable; all other heavy metals were not represented on the surface of the particles. To determine the intrusion of the metals, LA-ICP-MS was used. Thereby, the high porosity necessitated a special data evaluation that enabled the determination of the heavy metal removal and the leaching of other elements from the filter material as a function of depth. The measured depth profiles varied for each element and correlated with the metal mobility.

Development of a Laboratory Method for the Comparison of Settling Processes of Road-Deposited Sediments with Artificial Test Material

Sediments deposited on road surfaces are contaminated with pollutants; the load of pollution increases from coarse to fine particles. When it rains, different fractions of the road-deposited sediments are washed off depending on the rain intensity, the slope of the catchment, and other site-specific factors. This road runoff is often treated using settling processes implemented in different types of manufactured treatment devices. These devices can be tested with well-defined artificial test materials to determine the removal efficiencies of particulate matter in a reproducible manner. However, the suitability of the currently deployed artificial test materials to represent the settling behavior of real runoff particle collectives is largely unknown. In this study, a laboratory method to measure and compare the settling behavior of artificial and real particle collectives with a reproducible particle size composition was developed. The particle collectives were obtained from different road surfaces, fractionated into sieve classes, and then recomposed into a defined particle size distribution that represented the road runoff. The settling velocity was analyzed in a modified settling column setup under constant conditions. The resulting data form a cumulative curve of the settling velocities for both artificial and real particle collectives. The main result from this work is that the tested artificial material and the recomposed real particle collectives have comparable settling behaviors despite different losses on ignition and densities.