Based on different investigations in Switzerland and other countries, it can be concluded that a great extent of the pollutants from road surfaces are deposited along the road sides by spraying and air drift (on roads with heavy traffic up to 80%). Consequently, if topographically feasible, the discharge of road runoff over the shoulder has been proposed. There is not much information available on permissible pollutant loads and mass transfer and transport phenomena in road shoulders and the concomitant risks for soil and groundwater pollution.
The research is aiming at the following goals:
- Determination of the accumulation rates of various pollutants in the road shoulder as a function of the distance from the road side,
- Determination of the pollutant content in plants,
- Qualitative and quantitative influence of the infiltration water on groundwater quality,
- The research will serve as a basis for the renewal of the SN 640 354/ Discharge of road runoff over the shoulder.
The following stages are provided:
- Collating the specific literature, publications, guidelines, other research, particularly of that of EPFL Dr. Parriaux "Retention of Polluants des eaux de Chaussés selan de systems" infiltration on the benches.
- Installation of the measuring equipment (slope lysimeter), measurements and laboratory analyses; Analysis of results and conclusions;
- Interim Reports and coordination sessions;
- Final Report;
- Proposals for the standard 640 354 / drainage over the banquet.
The results of this research project are based on a two years sampling scheme of a pilot plant, which is located at a road between Burgdorf and the highway junction Kirchberg. The most important results are summarised as follows.
- The vegetated road shoulder will be capable to retain PAH and heavy metals for many years or even decades. However, the leaching potential will increase in parallel with ongoing operating time and with increasing accumulation of the pollutants.
- The properties of the top layer of the vegetated road shoulder is well suited for the retention of PAH and heavy metals. As recommendation for other road shoulders, a thickness of the top layer (A-horizon) should be > 30 cm with a cationic exchange capacity (CAC) of > 100 mmol
- According to the control targets, a recommendation of Negleitung Bodenaushub," PAH are the crucial parameters for the assessment, not heavy metals. The concentration of PAH in the soil exceeds the control targets until a distance of 4 m to the road and a depth of 50 cm.
- The loading of the soil of the vegetated road shoulder with heavy metals until a distance of 2 m to the road can be explained with the input loads of road runoff, road spray and local deposition (< 25 m distance to the road).
- Therefore, with respect to the drainage of heavily travelled roads, the width of the impermeable area along the road is fundamental. The larger the width, the higher is the amount of road runoff which can be collected and drained; and as a consequence the greater the percentage of pollutants, which can be collected and treated.
- If, as in the case of the Schachenstrasse, only the road runoff would be drained; only 20% of the total pollutants load could be collected. As a result, the accumulation rate of pollutants in the soil of the vegetated road shoulder would not be significantly decreased. Therefore, in similar situations collection of road runoff should be avoided.