SUITMA 2005 Cairo - Water and contaminant fluxes in paved urban soils
SUITMA 2005 Cairo
| This abstract is about one of the papers of the Consumed mining and military areas theme of the SUITMA 2005 symposium. |
| Thomas Nehls,i | Heiner Stoffregen,i |
| Gerd Wessolek.i |
- i - Department of Soil Conservation, Technical University of Berlin, Berlin, Germany.
Sealing is one of the most profound anthropogenic alterations of the hydraulic cycle in urban areas. A higher runoff, less infiltration, less evaporation and subsequently the urban heat effect are negative consequences. One of the positive effects is the reduced input and transport of contaminants compared to non-sealed soils.
However, ecological motivated urban planners want to reduce the sealing rates in urban areas and thereby increase the infiltration by substituting sealing by partly sealing. Semi-pervious pavements are seen as good technological measures to reach higher infiltration and evaporation by keeping convenience for the pedestrians. Does that imply a contamination risk for the soils and the groundwater ?
Urban areas are always sources of potential pollutions for the soil and the groundwater, at least for traffic borne contaminants like heavy metals. Furthermore, cities were often founded in river basins where coarse materials are found - low in retention capacity - and the groundwater level is high - like Berlin or Warsaw.
To assess the above mentioned contamination risk, we investigated several pavement systems for the retention capacities of the construction materials as well as for flow path patterns and their preferential flow susceptibility. Furthermore, we installed lysimeter and analyzed soil water for heavy metals.
By investigating the retention capacity of pavement construction material, we separated the dark seam material, which developed by urban inputs of organic carbon like diesel soot, for the first time. We discovered that it has a much higher retention capacity compared to the unchanged, original construction material: "urban dirt cleans urban contaminated runoff". The average CEC of seam material is 2.3 cmol + /kg soil, while it is only 0.5 for the underlying construction sand. Heavy metal adsorption isotherms affirm this trend: the Freundlich - coefficient for Ni for instance is 72 for the seam material and only 8 for the underlying sand. By introducing the material characteristics into the Hydrus-2D model, we could assess the effect of this increased retention of this only a few cm deep materials.
Our dye tracer experiments on partly sealed urban soils revealed their high susceptibility for preferential flow. Due to that, the effective retention capacity of the whole pavement system is further reduced. To assess that reduction, we introduced the preferential flow characteristics into Hydrus-2D.
Following, we calculated different scenarios to assess the influence of the different processes. In the presentation, we show the synopsis of our work: modeled water fluxes and contamination loads are compared to own measurements. Advantages and disadvantages of semi pervious pavements in urban areas are discussed and we can give suggestions from our work, which could be: increasing the organic Carbon content of pavement construction materials for retention capacity increasing, introducing coarse gravel layers, which destroy preferential flow path patterns and introducing street trees and other plants like grass which keep the water in the upper soil. Such could keep the city cool and the soils clean.
