Hydraulic and pollutant retention capacity of selected permeable pavements
In principle rainwater is polluted to some extend by atmospheric contaminants. Due to the contact with the areas drained, like roofs, yards and roads, the surface runoff takes up additional pollutants. As a result of these pollutants the stormwater infiltration by subsoil infiltration facilities is not approved in quite a lot of federal states of Germany. Because of the high building density the free space required for topsoil infiltration facilities is often not sufficient. Additionally, approximately 40 % of the entire urban area is covered by traffic areas. Therefore it is recommended to restore the contact between the underlying soil and the atmosphere by unsealing road surfaces. A case study conducted by the Bavarian Environmental Agency indicates for permeable pavements a higher pollutant retention capacity than for other infiltration facilities.
Because permeable pavements are restricted by the German guideline RStO to streets of type V and VI, four different block pavements and two porous asphalt pavements were investigated with regard to the pollutant retention capacity. At first, the maximum adsorption capacity of the building materials was examined in batch tests. In order to determine the impact of the specific surface and the reaction time on the adsorption process, physical models of the surface course pavements were investigated additionally. The surface runoff concentrations for different runoff areas were known from a literature review done before. With that it was possible to calculate the time to exhaustion by a simple balancing approach.
Due to the ideal test conditions, concerning the permanent mixing of dilution and adsorbent as well as the test duration adapted to the adsorption process, the adsorption capacity determined in batch tests can differ from reality. Therefore the pollutant retention capacity of the pavements was also examined in half scale tests. The half scale test was conducted in a test stand especially developed for this purpose. In the test the different pavements were irrigated with heavy-metal-containing water for four days and the seepage water was sampled.
Long-time simulations over 50 years helped to compare the concentrations of the seepage water and of the building materials with the corresponding limit values. The hydraulic parameters were calibrated on the seepage hydrograph of the half scale tests. The parameters describing the pollutant retention were derived from the batch tests. In the model the runoff from roof areas and traffic areas was infiltrated by permeable pavements. The heavy metal concentrations in the runoff were set in accordance to the event mean concentrations published in literature.
From the test and simulation results can be concluded that porous asphalt pavements should not be used for draining runoff from metal roofs. Due to the well PAH retention it is obvious to use porous asphalt pavements for draining road runoff with minor heavy metal concentrations. Block pavements are characterized by their excellent heavy metal retention capabilities. Concurrent block pavements show a poor PAH retention. Altogether the block pavement with porous concrete blocks proved to be the road construction with the highest pollutant retention capacity. In order to use the effect of runoff reduction and runoff retention, when infiltrating stormwater by permeable pavements, the German guidelines have to take road constructions as infiltration facilities more into account. In a research project conducted at the University of Essen permeable pavements showed a better pollutant retention capacity than infiltration swales and conventional infiltration trenches.