Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff (2019)

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.

111 State DOTs and other transportation agencies are increasingly being named Designated Management Agencies or stakeholders in TMDL implementation plans across the nation. As such, state DOTs are increasingly challenged to treat stormwater runoff from highways and meet TMDL WLAs within impaired watersheds. Due to varying site constraints and poorly defined highway pollutant source contributions, evaluating and selecting TMDL compliance approaches that are feasible for state DOTs to implement and contribute toward measurable improvements in receiving water quality and ecological function can be extremely difficult. Implementing stormwater BMPs in highly urbanized areas presents several challenges, includ- ing right-of-way availability, site constraints, geotechnical requirements, safety, and cost. These constraints limit the capabilities of a state DOT to implement control measures to treat the runoff from its right-of-way. In addition, while highways can be a significant source of some pollutants, they may only be a minor contributor of other pollutants. Or the POC may have its origins outside the highway right-of-way, unrelated to the operation or maintenance of the road. Consequently, the state DOT may have little to no control over the source of the pollutant. The portion of the total pollutant load attributable to the department may be small compared to the TLR needed, resulting in a nominal improvement in the receiving water and primary pollutant sources remaining unaddressed. Understanding highway runoff contributions and identify- ing the sources of pollutants are critical for developing cost-effective TMDL management plans that make the best use of public funds. Often a combination of strategies is needed to mitigate receiving water impacts and to meet overall TMDL goals. This project conducted research into both traditional and innovative approaches for achiev- ing TMDL compliance, many of which are well established and actively used by several state DOTs. The primary goals of this research were 1) to provide guidance on when and how to engage in the TMDL development process, 2) to identify and develop an approach of assessing state DOT impacts to the receiving water quality, and 3) to evaluate the compliance strategies that are most appropriate given various constraints and estimated state DOT contributions to pollutant loading. Compliance strategies have been developed for sediment, nutrients, metals, bacteria, and chloride. However, the compliance strategies presented could be adapted for other TMDL pollutants or stressors. Even before a TMDL is developed, state DOTs should consider being involved in the TMDL development process. At a minimum, the involvement should include reviewing and com- menting on the public review drafts of the TMDL to ensure the department’s perspectives and concerns are considered. More direct involvement may include sharing of data and providing tech- nical assistance or advice—where appropriate—in the development of WLAs. Such involvement may help ensure that all potential pollutant sources are considered and that the assigned WLAs are scientifically based and equitable. The major source of some pollutants may be naturally Conclusion

112 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff occurring in soils and groundwater, or some pollutants may be from atmospheric deposition. The TMDL must quantify these sources to appropriately assign WLAs. After a TMDL has been developed, state DOTs should assess the significance of the POCs for the impaired water body relative to the department load contributions. Various data sources and tools have been summarized to assist with this assessment. For example, runoff quality data from land use types, including highways with different AADT volumes, have been summarized. These data, along with estimates of runoff volumes, can be used to estimate relative pollutant loadings. Chapter 3 includes a land use load comparison protocol using SELDM, along with an example. If a TMDL pollutant must be addressed by a state DOT, a compliance strategy should be developed. The strategy should consider the controllability and treatability of the pollutant. Controllability refers to whether source controls may be effective at reducing load contributions, while treatability refers to whether pollutant loads may be reduced by implementing structural BMPs. In many cases, both source control and structural BMPs should be considered. However, in some cases, only one or the other is cost-effective. For example, chloride is typically best addressed through source control because it cannot be effectively removed with filtration or infiltration. In other cases, neither approach is cost-effective; the state DOT may want to consider an alternative compliance strategy, such as pollutant trading or restoration-based mitigation. Guidance has been developed to help state DOTs evaluate and compare the performance of various treatment BMPs based on water quality metrics and objectives. Existing references and tools have been summarized, and an example application of the NCHRP Report 792 BMP evalu- ation tools is provided. The process of prioritizing BMP implementation (structural or nonstructural), developing a statewide pollutant categorization, and considering sensitive watersheds provides a state DOT with the appropriate information to assess the best ways to use available resources for the greater benefit of each impaired water body. The report provides supporting references and case studies of innovative compliance measures that can be effectively implemented by state DOTs. While further research can reveal additional approaches to achieving TMDL compliance, the following guidelines are suggested for the state DOT practitioner: • Collection and characterization of monitoring data from state DOT highway runoff specific to an impaired watershed is the most effective way to determine the most accurate pollutant loads from the department. • Early involvement and participation of the state DOT in the TMDL development process reduces the possibility of inaccurate WLA assignments in watersheds where the department has negligible impact downstream. Additionally, if the portion of the total pollutant load attributable to the state DOT is relatively small compared to the TLR needed, the result is a nominal improvement in the receiving water, while primary pollutant sources remain unad- dressed. In addition, renegotiating a TMDL after adoption is challenging and may require additional state DOT capital costs and resources (e.g., funding of feasibility studies) to justify reopening the TMDL. • Consideration of alternatives—such as retrofits, cooperative implementation, stormwater banking, pollutant trading, off-site mitigation, off-site source control, and other holistic compliance strategies—commonly used by stormwater practitioners at state DOTs and other transportation agencies may save costs and benefit the department in controlling the loads discharged into downstream receiving water bodies. • Develop a TMDL implementation or compliance plan for state DOTs by pollutant type, since approaches for sediment-bound pollutants are different for chlorines, bacteria, nutrients, and others.

Conclusion 113 • TMDL compliance tracking and data management through a statewide online database with all state DOT facilities (e.g., inlets, outlets, BMPs, maintenance yards, and so on) can simplify the process of implementing appropriate treatment controls to address water quality issues. In addition, having some form of a statewide database can lead to benefits for departments during reporting on activities (e.g., annual reporting) to the environmental agencies. Asset management can be a key component in allocating state DOT resources for the necessary projects.