Cost/Benefit Issues   Porous paving may cost 10% more than standard paving material, but since it absorbs rain near where it falls, it can sometimes save money in expensive drainage systems, making it 12 to 38% cheaper overall (Thompson and Sorvig, 2000). Even if the porous pavement does not negate the need for expensive drainage systems, its many benefits may still outweigh the additional costs. Porous pavement can serve to treat water by removing pollutants, improve road safety because of better skid resistance, and improve recharge to local aquifers (EPA, 1999). When using porous paving, there is some chance that pollutants might not be filtered out and could lead to groundwater contamination (see Construction and Siting Issues). According to Thompson and Sorvig, porous pavement can reduce both runoff and the concentration of overland flow, thus slowing downstream erosion and siltation (2000).       Porous paving can sometimes reduce maintenance costs, especially with snow removal. Snow that falls on porous pavement often melts fast and drains into the pores. But porous pavement requires maintenance itself; the EPA recommends that it should be vacuum swept at least four times a year, followed by high-pressure hosing to get rid of clogs. Estimated costs of this maintenance for a porous pavement parking lot are $200 per acre per year (EPA, 1999).