Construction and Siting Issues   Permeable paving allows for water and air to flow through it, allowing more options than traditional paving for placement onsite. Permeable paving can be used for parking lots, paths, courtyards, patios, etc. It is not yet widely used for areas with high vehicular traffic. Site preparation for permeable paving systems is much like that for conventional paving (see Components section). Since porous paving is still an emerging technology, many contractors lack experience with constructing it, and some building codes may not allow for its installation.       Maintenance should always be planned for with permeable pavements as the void spaces between the pavers and/or the pores in the paving can become blocked. Jet-washing, commercial street sweeping or vacuuming may be required.       There is some risk of groundwater contamination when using porous paving. Pollutants such as nitrates and chlorides may not be filtered out by the pavement or the soil, and as a result might contaminate the groundwater. Therefore, porous pavement should not be used near groundwater drinking supplies. However, current knowledge of these effects is not conclusive. Some initial studies in Rockville, MD and Prince William, VA suggest that porous pavement may have high removal efficiencies for sediment, phosphorus, nitrogen, zinc, lead, and chemical oxygen demand (EPA, 1999).         Porous pavement has traditionally had a high rate of failure. In most cases, failure was probably caused by poor design, bad construction techniques, low permeability soils, heavy vehicular traffic, and resurfacing with non-porous materials (EPA, 1999)