Streets are the largest single source of nonpoint-source pollution under the direct control of National Pollutant Discharge Elimination System (NPDES) permittees. It therefore seems reasonable to consider pavement cleaning as a best management practice (BMP) to fulfill permit requirements, particularly the key provision “Thou shalt implement BMPs to the maximum extent practicable” (MEP). I believe that pavement cleaning must be part of a permittee’s BMP program. But it must be done in the most effective–and cost-effective–manner to comply with the MEP provision. Consider that street pavement constitutes 10% or more of an urban watershed; with parking lots, pavement constitutes 20% to 40% of commercial areas. Furthermore, greater unit loadings occur from pavement than from landscapes and roofs. Pollutant loadings from pavement constitute a disproportionate fraction of the total urban loading for many if not most pollutants but in particular for metals and hydrocarbons. Pavement is also a significant source of pollutants not directly associated with transportation, such as nutrients and pesticides, due to wash-on from landscapes and atmospheric deposition.
I distinguish between pavement sweeping and pavement cleaning. The former refers to what is generally done currently, the removal of litter and related material for aesthetic improvement using sweepers whose pickup capabilities are modest. In contrast, pavement cleaning maximizes the reduction of nonpoint-source pollution loading to receiving waters, employing the most effective sweepers used at the most cost-effective frequency, which varies with road type, and properly trained and committed operators.
It is not a question of whether communities should sweep. Most do. Rather the question is optimal sweeping: getting the most pollutant-loading reduction for the dollars spent: i.e., pavement cleaning. It is likely few permittees have met this criterion and therefore few are meeting the MEP stipulation. The first step is to use more effective sweepers. About 90% of the sweepers on the road today are mechanical/brush models. These models are generally only half as effective as vacuum sweepers with respect to pollutant-loading reduction per curb-mile swept. Vacuum sweepers are much more effective at removing fine sediments, silts, and clays, where much of the pollution resides. As the small material is more easily carried off by stormwater, its removal by pavement cleaning likely has a pronounced effect on stormwater quality.
Informal discussions between the author and state and EPA NPDES regulators indicates that some do not believe credit should be given for sweeping because it predates the issuance of the permits. Stated differently, stormwater quality already reflects the loading reduction provided by sweeping. However, as noted above, we are not sweeping optimally. Credit should be given for the incremental improvement: i.e., shifting from sweeping to true pavement cleaning. It also should be noted that current sweeping practices may in fact degrade stormwater quality. Early studies of the effect of sweeping on stormwater quality found that pollutant concentrations increased in stormwater from swept areas. How is this possible? It appears that mechanical/brush sweepers used at that time were only removing the larger sediments. These sediments had been covering the finer sediments, much like armoring of streams by gravel. Their removal from the street gutter exposed the fine sediments to stormwater, resulting in the fine sediments’ removal by gutter flows.
Many in our profession question the effectiveness of pavement sweeping. This view likely stems from the early studies noted above that found sweeping was ineffective in improving stormwater quality. But conditions have changed, particularly with respect to the capability of sweepers. Also, differing views on this question are perhaps due to differing definitions of effectiveness. I use two terms: effectiveness and cost-effectiveness. Effectiveness is defined as the total pounds of pollutants prevented from entering the stormwater due to their removal from exposure to stormwater. Cost-effectiveness is defined as dollars per pound of pollutant removed. Some BMPs may have low effectiveness (i.e., pounds removed from exposure) but are cost-effective and therefore should be implemented to comply with the MEP provision.
Whether pavement cleaning measurably improves the water quality of a nearby water body is not the proper criterion by which to judge its efficacy. Rather, the proper criterion by which to judge any BMP is whether it reduces nonpoint-source pollution cost-effectively. Any reduction of nonpoint-source pollution loading improves stormwater quality, however small. It simply may not be discernable depending on the point of measurement, the variability of stormwater quality, and the many other sources of pollution.
The difference between effectiveness and cost-effectiveness can be illustrated with an example: pavement cleaning versus homeowner education to reduce the loading of nutrients and pesticides. It is likely that the loading reduction of these pollutants will be far greater with pavement cleaning, given the amount of pavement in the community. However, homeowner education should not be discontinued if pavement cleaning is implemented. While homeowner education may not be as effective as pavement cleaning, it is likely as cost-effective: i.e., dollars per pound of pollutant prevented from entering stormwater. Education also provides the wider benefit of citizen awareness of the problem of nonpoint-source pollution and the community’s obligation with respect to the NPDES permit.
Pavement cleaning is particularly important for older areas of a community where stormwater treatment devices do not exist and are unlikely to exist in the near future, given the cost to retrofit such devices. Clearly, cost-effective sweeping is critical and absolutely essential to move us toward meaningful reductions in pollutant loading in these areas.
What about areas of the community that have end-of-pipe treatment? A combination of pavement cleaning and treatment is likely the most effective and cost-effective. Pavement cleaning reduces influent concentrations and loadings to the treatment system. With most treatment systems, effluent concentrations generally rise with influent concentrations, following first-order removal kinetics. Hence, reducing influent concentrations by pavement cleaning should reduce median effluent concentrations. One study has shown that even minimal cleaning frequency reduces the number of extreme concentration events.
Reduction in loadings extends the maintenance cycle of treatment systems: e.g., clogging of filters and filling of forebays in ponds and small manufactured wet vaults such as swirl concentrators. It is likely cheaper to remove sediments from the streets than from the treatment device. The same can likely be said for litter control. What a community needs to know is the optimal cost relationship between cleaning frequency and the maintenance of treatment facilities. The optimal solution will differ with the sweeper model and street type–freeway, arterial, residential.
The cost to remove sediment from streets is on the order of $2 to $5 per pound, on the order of the cost to clean sediment or litter from structural treatment devices and inlet inserts.
Many consulting firms and academic experts can assist communities. One firm, Pacific Water Resources, provides freely a simple procedure a community’s NPDES permit coordinator can use to provide an initial estimate of pollutant loading reduction by the community’s current sweeping program. The procedure can be downloaded at http://pacificwr.com. The author also recommends the Web site www.worldsweeper.com, an excellent source for the latest information on sweeper technology including the experiences of municipalities.
Needed is a better database of sediment/pollutant accumulation rates as affected by street type (e.g., freeway, arterial), pavement type and condition (asphalt, concrete), and adjacent and regional land uses (e.g., agriculture). These data are necessary to ascertain the optimal cleaning frequency for each street type and to provide reasonably accurate predictions of loading reduction as a function of program structure.What about private parking lots? A community should first establish its street cleaning program, learning from this experience. In time it should establish an educational program for parking lot owners and companies that provide sweeping services. It first should conduct a survey to determine which property owners sweep and at what frequency. The community should interview companies that sweep to determine the extent of their service in the community, equipment used, and what they believe are the barriers to the broader application of their services. The community should also gather information on the unit pollutant loadings from various types of parking lots and the acreage of each: fast food, shopping malls, mini-malls, etc. The community’s education program should be shaped by this information, identifying priorities. Once we have a full understanding of the relative contribution of pollutant loadings by the type of parking lot and the cost-effective cleaning frequency for each, the community should consider the incentives to encourage cleaning at the proper frequency. Eventually regulations should be considered.