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

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3 Project Overview A TMDL is a calculation of the maximum pollutant load that a water body can receive and still achieve water quality standards. TMDL mandates at both the federal and state levels require state DOTs and other transportation agencies to commit resources—such as BMP implementation, institutional controls, and funding—to maintain compliance with water quality regulations that address the federal Clean Water Act and state impaired waters programs. Through the construc- tion, operation, and maintenance of highway infrastructure, state DOTs may be contributing sources discharging to receiving waters. Therefore, departments are often assigned WLAs and named as designated management agencies or stakeholders in TMDL management plans. While highways are an important source of some pollutants, highways may only be a minor con- tributor of other pollutants; or the pollutant of concern (POC) may have its origin 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 (even if it is named as a stakeholder in a TMDL). Understanding highway runoff contributions and identifying the sources of pollutants and compliance strategies for developing cost-effective TMDL compliance plans are important steps for maximizing water quality benefit at the least cost. State DOTs are unlike typical MS4 permittees that are regulated for water quality. Typically, the MS4 permittees (e.g., cities) are fully encompassed within an individual watershed. There- fore, these agencies are named within one or two TMDLs for the impaired water body. How- ever, highways are linear in nature and cross many water bodies, so state DOTs may be named as stakeholders in numerous TMDLs throughout their states. With the growing number of impaired water bodies across the nation, state DOT practitioners need guidance to improve their understanding of the scientific background and regulatory basis of TMDLs and to assist them with implementation strategies, TMDL evaluations, and decision making for TMDL compli- ance. This guidance is designed to achieve these objectives and provide state DOTs with strategic approaches for TMDL compliance that are based on determining the sources and concentra- tions of pollutants in highway runoff and the relative contribution of highway runoff loads to receiving waters. The approach is founded on an analysis of runoff quality from highways and other land uses, as well as other pollutant sources—such as atmospheric deposition, soils, and groundwater—to facilitate the development of appropriate and equitable TMDL WLAs. This report provides pollutant-specific BMP control strategies that can be applied within the highway environment. For example, various pollutants within the highway right-of-way may be fully or partially controllable through the careful selection of road construction and winter sand- ing and deicing materials, maintenance of road shoulders and infrastructure, or installation of structural treatment BMPs. However, other pollutants, such as pathogens or nitrogen—depending on their source and transportability—may not be readily controllable by a state DOT, but they C H A P T E R 1 Introduction

4 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff still may need to be addressed as part of TMDL compliance. The challenges that state DOTs are faced with in regard to uncontrollable pollutants include addressing constituent loads—such as bacteria and nutrients—that do not originate from state DOT highways. For state DOTs to avoid receiving violations and fees (e.g., lawsuits) for noncompliance, it is important that they be aware of the targets and allocations that are assigned to them within a TMDL. Typically, the regulators are only focused on achieving water quality for the impaired water body and require compliance to be met as specified in the TMDL, regardless of the stakeholder limitations and constraints. This report equips the state DOT practitioner with strategies for negotiating the TMDL pro- cess, developing implementation plans to help allocate responsibilities and resources for the maximum overall benefit to receiving waters, and assessing the cost benefits for the designated management agencies. It also provides suggested watershed-based stormwater mitigation prac- tices and alternatives for state DOTs across the country in facing challenges with BMP design or site constraints. The fundamental goal of this project was to summarize and compare runoff quality from highways in comparison to other land uses to enable state DOT practitioners to distinguish appropriate WLAs and compliance measures. The following summarized components outline the process of how a state DOT practitioner can navigate TMDLs and develop a plan of compli- ance with TMDL requirements. Figure 1 shows how these components work together to guide the state DOT practitioner toward a successful and cost-effective compliance program. • State DOT TMDL Negotiation and Engagement (Chapter 2): This component guides the state DOT in how to negotiate TMDLs and engage in TMDL–WLA development with the regulator. It identifies approaches that the state DOT practitioner can take, beginning with the impaired water body listing and ending with TMDL development and implementation. • Significance of Stormwater Pollutants (Chapter 3): This component guides the state DOT in how to determine the significance of stormwater pollutants of concern for roadway TMDLs Figure 1. Components of the TMDL navigation and compliance process for state DOT practitioners.

Introduction 5 and how to identify impaired water bodies. It shows how to determine the circumstances where highway runoff is a significant source by identifying relative loadings and impacts downstream. It helps the state DOT identify POCs within the highway environment, impaired water bodies, and runoff concentrations for different land uses and other geographic variables (e.g., local soils, geology, and traffic volume). The predominant land use for the state DOT highway environment is primarily composed of impervious (roadway) and pervious areas. • Compliance Strategies Approach (Chapter 4): This component guides the state DOT in how to develop pollutant-specific compliance strategies. Compliance approaches—such as institutional controls, structural retrofits, or watershed stakeholder collaborative activi- ties (e.g., stream restoration)—may be available for a given pollutant. This chapter helps the state DOT determine if the TMDL WLAs and implementation plans are based on its expected source contribution and accurately reflect state DOT impacts and water quality mitigation capabilities. This chapter uses source analysis and runoff concentrations to iden- tify approaches feasible for mitigating TMDL pollutants on-site or off-site through various compliance measures. The chapter focuses on primary POCs for state DOTs, including sedi- ment, nutrients, metals, bacteria, and chloride. • Evaluating Watersheds and Best Management Practices (Chapter 5): This component guides the state DOT in how to evaluate its contribution in the watershed and the feasibility of potential BMPs. DOT practitioners can evaluate numerous factors to identify and priori- tize critical or sensitive water bodies requiring immediate mitigation. Factors considered may consist of the state DOT’s contributing drainage area, the level of water body impairment, direct water body impact, and more. Prioritization methodology based on state DOT signifi- cance and contribution within a watershed—among other factors—can be an important step toward identifying effective implementation measures in areas that present a greater water quality threat to the receiving water body. The chapter elaborates on BMP types that are most effective and practical for a site having limited watershed benefits with regard to aesthetics, groundwater recharge, and habitat value. Typical scenarios encountered by state DOTs when determining the feasibility of implementing effective BMPs for specific pollutants prior to the selection of a treatment BMP are presented. • Best Management Practice Pollutant Removal Performance (Chapter 6): This component guides the state DOT in how to determine the removal performance of proposed BMPs. It provides a framework for assessing pollutant removal performance after selecting an appropri- ate location and feasible BMP for a project. The performance is determined by the pollutant removal (i.e., volume and concentration reductions) for specific BMPs. • Best Management Practice Cost and Effectiveness Analysis (Chapter 7): This component guides the state DOT in how to construct a cost and effectiveness analysis for BMPs. It is guidance for state DOTs on unit costs for BMP types and relative effectiveness specific to pollutants. A BMP evaluation tool developed by NCHRP is presented for determination of life-cycle costs specific to BMPs. • Effectiveness of Innovative Solutions (Chapter 8): This component guides the state DOT in how to evaluate the efficiency and effectiveness of innovative compliance solutions. The department can identify various innovative compliance approaches for meeting TMDL goals through stormwater banking and crediting, pollutant trading, off-site mitigation, or other watershed approaches where state DOTs can directly fund non-state DOT mitigation that can possibly have more benefit than traditional on-site treatment approaches. Alternative strate- gies also consist of state-funded programs (i.e., grants) developed where the TMDL regulators fund projects to construct an off-site practice—or regional BMP—to manage the equivalent stormwater runoff that cannot be managed on site. Several successful innovative compliance strategies implemented by state DOTs have been depicted to encourage such strategies. In addition, this component guides the state DOT in how to compare on-site and off-site BMPs to watershed approaches for compliance. It identifies the benefits of on-site and off-site water

6 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff quality treatment for state DOTs. Several factors—including location, cost, and the overarch- ing goal of compliance—are considered in identification of the most feasible treatment solu- tion for an impaired water body. A key resource for this purpose is NCHRP Research Report 840 (Weinstein et al. 2017), which identifies a useful toolbox that lays out in-kind (i.e., structural BMPs) and out-of-kind (i.e., restoration-based) treatment approaches. Currently, application of a watershed approach is an important emerging tool for state DOTs to address TMDLs. This report provides solutions for state DOTs based on the research and evaluations com- pleted with regard to TMDL compliance for the highway environment. It summarizes various ongoing efforts by state DOTs recognized by the research team as effective measures of compli- ance, in addition to providing region-based innovative solutions. Alternatives include retrofits, cooperative implementation, and other compliance strategies that may benefit the state DOT in controlling its load discharge into downstream receiving water bodies.