Metals in European roadside soils and soil solution – A review

Highlights

• Summary of studies analysing metals in soils and soil solution at European roadsides.

• Metal concentrations in topsoil 5 m beside the road are influenced strongly by traffic.

• Solute concentrations of metals are mostly independent from soil concentrations.

• High percolation rates lead to high annual loadings directly beside the road.

Abstract

This review provides a summary of studies analysing metal concentrations in soils and soil solution at European roadsides. The data collected during 27 studies covering a total of 64 sites across a number of European countries were summarised. Highest median values of Cr, Cu, Ni, Pb, and Zn were determined in the top soil layer at the first 5 m beside the road. Generally, the influence of traffic on soil contamination decreased with increasing soil depth and distance to the road. The concentration patterns of metals in soil solution were independent from concentrations in the soil matrix. At 10-m distance, elevated soil metal concentrations, low pH, and low percolation rates led to high solute concentrations. Directly beside the road, high percolation rates lead to high annual loadings although solute concentrations are comparatively low. These loadings might be problematic, especially in regions with acidic sandy soils and a high groundwater table.

Introduction

The contamination of roadside environments of heavily trafficked main roads and highways has been a subject of investigation for more than forty years. Although regulations like the ban on leaded petrol have reduced emissions from single vehicles, this effect is negated by the increasing worldwide traffic (Monks et al., 2009). Emissions from major highways contain different kinds of contaminants such as metals, polycyclic aromatic hydrocarbons and road salts which can occur in both particulate and dissolved form. In particular, metals are of a great concern because they cannot be decomposed by micro-organisms (persistence) and have a long term toxicity for plants, animals and humans. The most recognised and examined metals in roadside environments are Cd, Cr, Cu, Pb, Ni, and Zn (Münch, 1993, Folkeson et al., 2009, Kayhanian et al., 2012). But also increased concentrations of other metals like As, Co, Sb, Se, Sr, and V and many others occur near heavily trafficked roads (Thorpe and Harrison, 2008). Due to its presence in vehicular brake pads especially Sb has attained much attention as a roadside contaminant in various recent publications (e.g. Hjortenkrans et al., 2008, Ceriotti and Amarasiriwardena, 2009, Kadi, 2009).

The metals introduced into the environment by traffic derive from many different sources. Emissions resulting from traffic are caused mainly by tire wear off, break lining, wear of individual vehicular components such as the car body, clutch or motor parts, and exhaust (Lindgren, 1996). Pollutants released by motor vehicles may also originate from the residues from incomplete fuel combustion, oil leaking from engine and hydraulic systems, and fuel additives. Road abrasion, pavement leaching, traffic control device corrosion, and road maintenance, i.e. de-icing activities are also relevant sources of pollutants (Lindgren, 1996, Hjortenkrans et al., 2007, Kluge and Wessolek, 2012). Folkeson et al. (2009) classified road traffic pollution sources into five groups: traffic & cargo, pavement & embankment material, road equipment, maintenance & operation and external sources.

A quantification of the release of individual building components is difficult because the composition varies widely, depending on the manufacturer. Nevertheless, some studies have been carried out on the release and deposition rates of particulate pollutants from motor components and road degradation (Revitt et al., 1990, Muschak, 1990, Lindgren, 1996). Furthermore, traffic related factors like road design, types of fuel used, volume of light and heavy traffic, intersections, driving speed, and driving behaviour influence the emission quality. Therefore, a very complex mixture of substances is released in roadside environments. The most widely recognised pollutants are carbon monoxide, nitrogen oxides, hydrocarbons, sulphur dioxide, methane, metals, and organic pollutants (Wessolek et al., 2011).

Particulate or dissolved pollutants are transferred into the surrounding environment via aerial transport or the infiltration of road runoff and spray water (Legret and Pagotto, 2006, Bakirdere and Yaman, 2008). Fig. 1 shows a typical view of a road with pathways of dispersion by dry and wet depositions into the roadside environment.

The pollutants are transported across the road surface with the runoff and then deposited as suspended or dissolved particles. Depending on the type of road and the inclination of the slope, spray and road runoff water can be transported as far as 10 m across the adjacent roadside area (Golwer, 1991). With the additional influence of wind and airflow, very fine particulate matter can be transported and deposited up to a distance of 250 m (Zechmeister et al., 2005).

An analysis of studies on major roads and highways determined three different areas of pollution for roadside environments (Golwer, 1991). These are the area of 0–2 m, which is dominated by runoff water from the road and spray water; the area of 2–10 m, which is dominated by splash water and partly influenced by runoff water, depending on the inclination of the slope; and the area 10–50 m, which is affected mainly by airborne pollutant transport.

The composition of the road runoff and the dry deposition is affected by the traffic related factors mentioned previously as well as by environmental factors such as wind direction, amount and intensity of precipitation, previous dry periods and vegetation cover (Sansalone and Buchberger, 1997, Van Bohemen and Van de Laak, 2003). Dry depositions affected by traffic have shown higher concentrations of metals and many organic contaminants than comparable areas in rural environments. Wet depositions in urban areas in the form of street runoff and spray water, contain high concentrations of pollutants compared to normal precipitation (Harrison et al., 1985, Makepeace et al., 1995, Wigington et al., 1986). The quality of road runoff and street dust has been studied intensively and is well documented in many publications e.g. Sternbeck et al., 2002, Harrison et al., 2003, Göbel et al., 2007 and Kayhanian et al. (2012).

The construction, use, and maintenance of roads changes the original physical, biological and chemical properties of the soil in the adjacent area (Fig. 1). Often the topsoil was taken away during road construction or left buried beneath the base course at depths of >1 m. Roadside soils often contain up to 30% technogenic materials and stones. The presence of these materials, among other factors, like alkaline deposition from road surface, cause an increase of soil pH even higher than 7. The embankment, built during the course of the road construction, often measures up to 5 m and is located directly along the road edge (Fig. 1). In many cases this area also contains slopes and ditches to drain and infiltrate the runoff from the road. At the distance 5–10 m the soils often are compacted and disturbed with little to no vegetation. After this distance the influence of the road slowly decreases and after 10–15 m predominantly original soil profiles occur (for further information see Wessolek et al., 2011). Roadside soils are one of the main targets for pollutants emitted from roads, and many investigations have been carried out to determine metal concentrations along European main roads and highways (Fig. 2). A growing awareness of the environment in the mid-1970s led to an increase of investigations in roadside environments.

The studies of Lagerwerff and Specht, 1970, Laxen and Harrison, 1977 and Warren and Birch (1987) in the 1970s and '80s are among the first studies to systematically investigate road traffic induced soil contamination. Lagerwerff and Specht (1970) focussed on the influence of airborne metals and their impact on roadside soils. They determined elevated metal concentrations in the soils adjacent to the road and decreasing concentrations with increasing distance and soil depth. This was confirmed in many further studies (e.g. Garcia et al., 1996, Turer et al., 2001, Hjortenkrans et al., 2008).

The metal concentrations in roadside soils and vegetation are also correlated positively to traffic intensity as the investigations of e.g. Rodriguez-Flores and Roddriguez-Castellon (1982) and Arslan and Gizir (2006) showed. Other authors could not find a direct correlation between traffic intensity and metal concentrations in roadside soil (Perez et al., 2008). Laxen and Harrison (1977) investigated the influence of highway traffic on lead pollution of water resources. They showed that the high concentration of lead in the road runoff (up to 100 times higher than background concentrations) could be immobilised effectively by the first 10 cm of roadside soils. Other authors showed that easily mobilised trace elements like Cd and Zn can be transferred to deeper soil layers (e.g. Legret and Pagotto, 2006, Kluge and Wessolek, 2012).

A large part of the metals that are transferred to roadside soils are bound to particles. These particles are retained to a large extent by physical mechanisms immediately after they enter the soil and therefore mostly remain in the top horizon (Boivin et al., 2008).

The mobility of metals in roadside soils is influenced strongly by soil pH and organic matter as many authors have demonstrated (e.g. Ramakrishna and Viraraghavan, 2005, Turer and Maynard, 2003, Kocher et al., 2005, Kluge and Wessolek, 2012). Turer and Maynard (2003) found a strong positive correlation between soil organic matter and certain metal concentrations. They also could show that a large fraction of metals is bound to an insoluble form of organic matter that is probably of anthropogenic origin.

The soil pH near roads is influenced strongly by traffic activities. Especially road abrasion, which is transferred to the soil adjacent to the road, changes the pH value over time to neutral or even above neutral. The elevated soil pH in turn enhances metal retention (Kocher et al., 2005). Barbosa and Hvitved-Jacobsen (1999) pointed out that beside soil texture and composition, a high resistance to desorption at low pH positively influences metal mobility in roadside soils.

Roulier et al. (2008) studied the impact of preferential flow path in roadside soils. Preferential flow is a physical process in soils in which a fast transport of water and other compounds takes place only in a small portion of the pore system. The latter could show that an increased transport of Pb via preferential flow and DOM takes place after intensive rainfall events (21.9 mm in 3 h).

It is well-known that Pb and certain other metals form very stable complexes with functional groups of humic substances and Fe and Mn oxides and oxyhydroxides because of the high adsorption capacities of these compounds (Jordan et al., 1997, Hassellöv and von der Kammer, 2008). Kretzschmar and Sticher (1997) could show that humic-coated Fe oxide colloids facilitate the transport of Pb²⁺ and Cu²⁺ in the presence of high Ca²⁺ concentrations, which often occurs in road runoff.

In several studies (e.g. Legret and Pagotto, 2006, Hjortenkrans et al., 2008), the mobile fraction of soil metals was determined. The latter could show that more than 40% of the total concentrations for Cd, Cu, Ni, Pb, and Zn can be counted as part of the mobile fraction. This metal fraction is mobilised easily if soil is disturbed or extreme weather conditions like dry periods (oxidation) or long intensive rain periods (reducing condition) occur. Legret and Pagotto (2006) determined a high risk of transfer to plants and groundwater from polluted soils especially for Pb. Othman et al. (1997) could show that the Pb level of plants growing close to roads was higher than the natural level. The metal uptake of plants is affected by element mobility in soil, essentiality for plants, and plant species. Metal levels in plants decreased with increasing distance from the road (Zechmeister et al., 2005, Modlingerova et al., 2012).

The use of de-icing agents for road maintenance promotes a high dispersion and leaching of organic matter in roadside soils. Under these conditions, Arnhein et al. (1992) concluded from soil column experiments that dispersion is the dominant mechanism of metal mobilisation. Arnhein et al. (1992) also showed that NaCl from de-icing salts could promote colloid assisted transport of metals. Bäckstrom et al. (2004) observed the influence of de-icing salts on metal mobility in field lysimeter. They also determined a clear positive relation between the application of de-icing agents and Cd, Cu, Pb, and Zn concentrations in soil solution. Although mobilisation mechanisms were diverse and sometimes even counteracting.

Another aspect of road maintenance is to ensure road de-watering. In some European countries e.g. Germany it is common to remove the top layer of the embankment (2–5 cm) about every 5–12 years. As a result of this procedure a considerable amount of metals is relocated or removed from the roadside environment (Kocher et al., 2008).

There are only few studies that examined the metal concentrations in soil solution near highly trafficked roads (Reinirkens, 1996, Dierkes and Geiger, 1999, Bäckström et al., 2004, Kocher et al., 2005, Kluge and Wessolek, 2012). The relationship between metals in soil solution and roadside soils was analysed by Reinirkens (1996) with field lysimeter. He concluded that the displacement of the different metals is variable and that the compounds of metals emitted by road traffic are fixed mainly in the soil.

In studies performed on different roadside locations, Kocher et al. (2005) showed that high concentrations in the soil matrix do not necessarily lead to increased soil solution concentrations.

Nonetheless high percolation rates near the road edge could cause a remarkable transfer of metal loadings and increase the risk of groundwater pollution. Furthermore the study of Kluge and Wessolek (2012) showed that soil solution concentrations increase with distance due to decreasing percolation rates and lower soil pH.

There are multiple studies regarding the impact of highways on the environment, but at present no literature study has systematically gathered and compared the quantitative data of metal concentrations in roadside soils and soil solutions of European road networks. Therefore this study focuses on the following objectives:

• A comparison of metal concentrations in the soils and soil solutions of European roadside environments.

• Analysing the data with regards to various impact factors like (I) distance and depth, (II) soil characteristics such as pH and loss on ignition (LOI), and (III) traffic intensity.

• An analysis and discussion of soil and groundwater pollution risk in roadside environments.