Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 S U M M A R Y Bridge Stormwater Runoff Analysis and Treatment Options Studies have been conducted regarding the design, operation, construction and effective ness of best management practices (BMPs) for the control of highway runoff water quality but few have investigated BMPs specifically for bridge deck runoff. While pollutant loads from highways are similar to those from bridge decks, highway pollutant loads can be more easily treated or sequestered, whereas loads from bridge decks are transported directly to receiving waters via dry deposition or stormwater runoff. Studies reviewed on the impact of bridge deck runoff on receiving waters found little evidence of either water quality or eco system degradation, leaving open the question of what stormwater controls, if any, are appropriate for the practitioner to apply for new and reconstruction bridge projects in the event that stormwater mitigation must be included. This guide was developed to provide the practitioner with a stepwise approach to select the best combination of source control, operational and treatment control BMPs for a bridge crossing a perennial, intermittent or ephemeral stream, river, lake or estuary, for virtually any span length. Runoff from a bridge deck may contribute to receiving water quality impair ment in areas where the pollutants in the receiving water are elevated due to urbanization or a nonÂpoint source. Solutions to managing this contribution to pollution have a range of costs. The practitioner must be the steward of public funding and the environment, balanc ing the objectives of each to ensure sustainability. The information in this guide provides a practical approach to assist the practitioner in this regard and to develop clear documentation of the decision process. Assessment Framework The assessment framework begins at the project environmental documentation stage. Supporting technical studies for the National Environmental Policy Act (NEPA) and com panion state environmental assessment documents will generally follow National Pollutant Discharge Elimination System (NPDES) regulations. However, the detail in environmental documents and supporting technical studies may vary, and there may be no discussion of bridge deck runoff requirements. The practitioner may be engaged to provide technical infor mation during the environmental documentation phase, and the information in this Guide can assist in providing a supportable technical analysis. The guide framework defines two general cases for bridge assessment: rural and urban. The rural case (category) is defined as a bridge in any location that is not covered by an NPDES permit or in a location that is outside of an urbanized area and that would not nor mally require treatment of runoff. The Bureau of the Census determines urbanized areas by applying a detailed set of published criteria (see 55 FR 42592, October 22, 1990).
2The urban category of the guide assessment framework is defined as a bridge crossing located in an area subject to NPDES Permit coverage based on census urbanized area crite ria. The NPDES Permit may likely have requirements for BMP application to all roadways, including bridges, which are part of a project. Bridges in urban areas are no more likely to cause receiving water pollution than rural areas, but they may contribute to pollution that is already present. Note also that average annual daily traffic has been determined to be a significant variable in the magnitude of pollutant load from highways. Chapter 4 of the guide describes BMPs that should be considered, as applicable, for all bridge projects. These are source control and maintenance and operational BMPs that may only apply to certain material types (such as exposed zinc surfaces) or in specific locations (such as where deicers are used), but should be evaluated as the most environmentally rel evant and costÂeffective method of pollution control. This chapter also discusses bridge inspection as a tool to reduce the discharge of pollutants from bridges. Bridge crossings will also likely be subject to one or more resource agency permits. Nation ally, the U.S. Army Corps of Engineers (USACE) administers the Section 404 permit system of the Clean Water Act under the authority of the US Environmental Protection Agency (EPA). A Section 404 permit is required to deposit or remove dredge material from waters of the United States. A companion state permit may be required for work in waters of the state. A Section 404 permit requires a Section 401 certification from the state (or the USEPA if the state is nonÂdelegated). The 404 permit, or 401 water quality certification, may include BMP requirements as mitigation for receiving water impacts. The BMP requirements in resource permits should provide environmental benefits consistent with whole life costs. The practi tioner can assist the regulatory agencies in making this determination, if needed, by develop ing a numeric evaluation of potential impacts to the receiving water from the bridge runoff and assessing the costs of BMPs. The guide provides two assessment procedures to assist the practitioner in determining if the project will cause, or could contribute to, receiving water pollution. The simple assessment is a generic dilutionÂbased procedure that provides an estimate of the pollutant contribution from bridge runoff relative to the receiving water. In many cases, this simple procedure can demonstrate that the contribution of the bridge deck runoff to the receiving water is de minimis. For instances when a postÂconstruction, refined estimate of receiving water pollutant concentration(s) is needed, the complex assessment procedure should be used. The com plex assessment procedure is also dilution based, but follows a more rigorous mathematical approach to arrive at a conservative estimate of pollutant concentration in the receiving water based on the critical stream discharge rate. The guide provides a spreadsheet tool that can assist the practitioner in estimating BMP performance and costs. Treatment BMPs and Bridges The use of treatment BMPs for bridge deck runoff is problematic because there are few viable options for treatment of runoff on the deck, and routine maintenance on the deck is difficult and dangerous due to the lack of workspace. Additional BMPs can be considered if stormwater is conveyed back to the bridge abutment. For some projects, the use of treatment BMPs may be mandated or prudent from an environmental perspective. Treatment BMPs that the practitioner can consider when treatment of deck runoff is required are described in Chapter 5. Treatment of runoff from a comparable section of highway on land is preferable to treatment of runoff from the bridge deck for two reasons. First, it is difficult (expensive) to convey bridge deck runoff to the abutment for treatment. Bridge deck conveyance systems are costly to design, construct, and maintain. Second, it is not as effective to treat runoff from bridge decks as it is to treat runoff from a terrestrial highway section, particularly in
3 an urban area, since pollutants are dispersed from the bridge deck on a continuous basis and cannot be captured later for treatment. Chapter 2 and Appendix A of this guide discuss the deposition and transport of pollut ants on a bridge deck. Traffic movement and ambient wind constantly remove particulates from the roadway and deposit them in adjacent, low energy areas. In the case of a bridge, the adjacent area is the receiving water, and pollutants are deposited directly on the surface. In the case of a comparable terrestrial highway section, pollutants are deposited on the paved or unpaved shoulder areas and accumulate there until the next rain event, where they may be transported by the drainage system (absent treatment controls) to the receiving water. Treatment from roadside vegetation or other forms of terrestrial sequestration may remove deposited highway pollutants, which are forms of pollutant removal unavailable to runoff from a bridge deck. Thus, a portion of dry deposition to receiving waters from bridges cannot be effectively treated as in a comparable terrestrial highway system. To maximize environmental benefit, the practitioner should prioritize treatment of runoff from the bridge approach roadway, or at a similar location in the watershed, over treatment of runoff from the bridge deck. BMP Selection Evaluation Tool This guide includes a spreadsheetÂbased BMP Selection Evaluation Tool (referred to as the tool) to assist the practitioner in evaluation and/or to optimize BMP selection if the practitioner will be installing treatment BMPs for bridge deck runoff. The tool allows the practitioner to maximize pollutant removal effectiveness while minimizing whole life costs for the given physical constraints. The tool may be used to compare BMP choices for treat ing runoff at a terrestrial highway section or treating runoff from the bridge deck at the abutment. The tool allows the practitioner to compare the cost advantages of all BMPs, the bridge deck conveyance system cost, as well as the treatment benefit that can be realized. Tool inputs can be customized by the practitioner, including runoff influent quality and cost data for BMPs. The user also has the option to use the default data for these values, which will provide sufficient accuracy for the vast majority of department of transportation (DOT) projects.
