Recommended urban storm water infiltration devices for different types of run-off under varying hydrogeological conditions

Background, aim, and scope The decentralised near-natural infiltration of storm water in urban areas over a long-term period can cause local pollution of soil, seepage water and groundwater due to heavy metals (e.g. Pb, Zn, Cu), polycyclic aromatic hydrocarbons (PAH), petroleum hydrocarbons and readily soluble salts, which compounds are partly classified as hazardous. The aim of this paper is to present a recommendation matrix for suitable storm water infiltration devices. The scope is limited to eight different run-off types and two different infiltration devices (swales and trenches with three different trench-filling materials) under two different hydrogeological conditions (high adsorbing soil with low permeability, low adsorbing soil with high permeability). The examined run-off types are sub-divided as follows: run-off from unpaved areas (gardens, grassed areas, cultivated land); green roofs, aluminium roofs; roofs without zinc gutters and down-pipes; roofs with zinc gutters and down-pipes; copper roofs; zinc roofs and trafficked areas (cycle and pedestrian ways, yards, car parks and residential roads). The recommendation matrix should assist decision-makers such as city planners, architects and private house builders. Materials and methods The potential for storm water infiltration to pollute soil, seepage water and groundwater is investigated with long-term 3-D numerical water flow and chemical transport modelling in unsaturated and saturated zones over 50 years, which were already presented by Zimmermann et al. (Water Sci Technol 51(2):11-19, 2005). The recommendation is based on a comparison between modelling results and several guideline values prepared by several German authorities. The evaluation process leads to four hazard levels regarding the impact on topsoil (i.e. first 20 cm of the soil), on seepage water (1 m below the infiltration device) and on groundwater (at the unsaturated-saturated boundary). Results The recommendation matrix consists of 56 individual statements. Relating to dissolved organic substances like phenanthrene and fluoranthene, the infiltration of trafficked areas run-off is critical. The infiltration of metalliferous run-off has a high hazard accumulation potential. Here the storm water infiltration via sub-ground of low permeability and high adsorbing soil material is critical for seepage water in any case; the infiltration of zinc roofs run-off via trench infiltration devices is even critical for groundwater at 4 m depth. Sub-ground of low permeability and high adsorbing soil material has a lower potential hazard in terms of storm water infiltration from roof run-off. The storm water infiltration via swales effects a very large accumulation of heavy metals in the topsoil. The storm water infiltration via trenches leads to the accumulation of hazardous substances in the deeper sub-ground, particularly where the trench-filling material has low adsorbing capacity and high permeability. Discussion The transferability of the results to other sites depends particularly on the hydrogeological conditions. Before using the recommendation matrix, details of the hydrogeological conditions should be collected. The long-term simulation process is simplified by several impact factors such as non-constant rainfall, soils heterogeneity, macro-porous flow, particle-bounded transport and microbiological decomposition. Conclusions Based on the scale of risks to soils, seepage water and groundwater, the matrix should be used in the selection of the roof construction materials and appropriate storm water infiltration devices so that the environmental risks can be minimised. If the sub-ground has a high permeability and low adsorption capacities, the infiltration of metalliferous roof run-off water is, in general, not advisable without putting treatment facilities in place upstream. Thus, architects need to realise that the choice of a suitable infiltration device depends, on the one hand, on the type of run-off and, on the other hand, on the hydrogeological condition and the building materials. Recommendations and perspectives Replacement of the topsoil in swale infiltration devices is recommended because, in particular, heavy metal (zinc) in run-off from roofs with zinc gutters and down-pipes accumulates in the soil matrix. The replacement interval depends on the hydrogeological conditions and, for this run-off example, lies between 10 and 20 years. If infiltration is essential, constructing special treatment facilities upstream can be an alternative. The existing numerical model could be adapted to suit other site-specific materials and be enhanced regarding several complex impact factors.