Local infiltration devices at parking sites - experimental assessment of temporal changes in hydraulic and contaminant removal capacity
Background and aims:
Runoff of storm water originating from paved surfaces such as streets or parking areas are often infiltrated on site. In some countries such infiltration practice is even mandatory as long as hydrologic conditions allow the onsite infiltration.
Regulations such as ÖNORM-B2506-1 (2000), ATV-DVWK A138 (2002) and ÖWAV-Arbeitsblatt 35 (2003) deal with the design of the devices. It is common practice to derive the design rules predominately from hydraulic considerations dealing with the permeability of the soil material. However, runoff from parking lots is frequently subject to significant loads of heavy metals and/or organic compounds. These loads result from traffic, air entrainment or leakage from vehicles. Thus, infiltration devices have to ensure both, drainage of the storm water and retention of the associated pollutant loads in order to avoid groundwater contamination.
In infiltration devices the runoff water is commonly drained via an active topsoil layer (humus, humus-sand mixture, compost-sand mixture). The passage through the topsoil enables the retention of particulate and absorptive substances and the microbiological degradation of organic compounds. Design rules required usually a topsoil thickness of app. 30 cm. Although the performance of infiltration devices for highway runoff are well covered in the literature (e.g. (Dierkes and Geiger, 1999; Mikkelsen et al., 1996), devices at parking areas are not. Thus the focus of this work is put on infiltration devices located at parking lots. Types of pollutants from street and highway runoffs (Sansalone and Buchberger, 1995) are comparable to the current investigation, where design of devices and magnitude of pollutant loading is not.
Numerous infiltration devices at parking lots have been constructed and operated in the past years. The life expectancy of the topsoil is commonly assumed as 15 years. Over that period the hydraulic capacity as well as the biodegradation is assumed to be sufficient. The investigation attempts to verify this assumptions.
The key aspect of the research is to identify the relation between the pollutant retention capacity of the devices and the hydraulic permeability given. Comparing sites with variable ages enables to demonstrate the temporal changes in the performance. Main boundary conditions to be considered are the age of the device and the mean daily traffic load given at the parking lot.
Methods and Results:
In total seven infiltration devices are chosen for experimental investigations, located at parking lots of a supermarket chain in the region of Tyrol. The sites differ with regard to size of the paved surface connected, the age of the infiltration devices, the mean daily traffic load and geographical (and climatic) location.
Figure 1, Schematic on soil sampling points.
At each site soil samples are taken for testing for hydrocarbons and heavy metals (lead, cadmium and caesium) being the most relevant substances. Samples are taken at both the side and mid of the infiltration swale in the upper 5 cm for considering the surface flow path. For assessing the magnitude of the pollutant concentration with the depth, samples at 10 and 25 cm below surface are taken. Concentration levels are expected to decrease with the infiltrated depth, thus majority of substances are retained in the top part near the surface. Concentrations levels are expected also be less at the side of the swale compared to the mid.
Hydraulic permeability is measured onsite at each location as well. Due to continuous operation during several years, the swale may be silted up and thus no longer meet the required hydraulic capacity for infiltration. A comparison of the actual against the design permeability (used in a planning stage) will indicate the (hydraulic) life expectancy of the device.
While the sites have already been chosen and prepared for the investigation the sampling still has to be finished. The final paper will present the results analysed as outlined in the above.
Literature:
ATV-DVWK-A138 (2002). Arbeitsblatt ATV-DVWK-A138 - Planung, Bau und Betrieb von Anlagen zur Versickerung von Niederschlagswasser. Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall, Hennef, Deutschland.
Dierkes C. and Geiger W. F. (1999). Pollution retention capabilities of roadside soils. Water Science and Technology, 39 (2),201-208.
Mikkelsen P. S., Hafliger M., Ochs M., Tjell J. C., Jacobsen P. and Boller M. (1996). Experimental assessment of soil and groundwater contamination from two old infiltration systems for road run-off in Switzerland. Science of The Total Environment, 189-190,341-347.
ÖNORM-B2506-1 (2000). Regenwasser-Sickeranlagen für Abläufe von Dachflächen und befestigten Flächen (Soakaways for rain water from roof gutters and reinforced surfaces - Application, hydraulic dimensioning, construction and operation). Österreichisches Normungsinstitut, Wien, Österreich.
ÖWAV-Arbeitsblatt-35 (2003). Behandlung von Niederschlagswässern. Österreichischer Wasser- und Abfallwirtschaftsverband, Wien, Österreich.
Sansalone J. J. and Buchberger S. G. (1995). An infiltration device as a best management practice for immobilizing heavy metals in urban highway runoff. Water Science and Technology, 32 (1),119-125.
