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

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70 This chapter shows the state DOT how to evaluate its contribution in the watershed and the feasibility of potential BMPs. It provides considerations that state DOT practitioners can use to effectively implement compliance measures for a water body with a TMDL. The treatment practices identified in the pollutant-control strategies are critical to treat for the impairment within a watershed. However, prioritization of BMP is based on applicability, feasibility, per- formance, and cost. This chapter provides a stepwise framework of factors to help a state DOT select applicable BMPs based on site conditions. Upcoming chapters provide a more effective analysis of BMP performance and cost-effectiveness to further allow a state DOT to narrow the appropriate BMP for the impaired pollutant in a watershed. State DOT significance and specific applicability of a BMP vary among watersheds. There- fore, departments are provided with factors and feasibility constraints to consider prior to imple- menting structural BMPs and nonstructural controls. These factors not only consider the design guidance for a BMP but also the practicality of implementing and maintaining a BMP based on physical and financial constraints. In addition, prior to implementing a treatment control, mul- tiple issues must be considered, including any impacts on the underlying soil or groundwater. Therefore, it is important for a state DOT to know the impacts within specific watersheds to appropriately fund projects that reduce further alteration of an impaired water body or its tribu- taries. To optimize the budget allocations within a state DOT stormwater program, it is beneficial to rank water bodies based on their relative and immediate impacts on the environment. Stra- tegically prioritizing implementation of state DOT projects within sensitive (or higher priority) subwatersheds may provide a greater benefit to the overall condition of the water body than lower priority (less-sensitive) subwatersheds. This prioritization can be determined by using a metric scoring system for the most critical factors listed in Step 1 of the BMP prioritization methodology that follows and ranking them for each water body or TMDL. Site prioritization and feasibility are critical to provide the most appropriate water quality solution for the impaired water body. Best Management Practice Prioritization Methodology The framework provided in Figure 22 illustrates a stepwise procedure that state DOT practitioners can follow to prioritize project implementation options for a water body or TMDL. This framework builds from the compliance strategies identified in the previous chapter to prioritize the applicable locations for project implementation within a watershed. To further prioritize the BMP selection, analysis from the performance evaluations of BMPs and cost-effectiveness of BMPs identified in the upcoming chapters should be followed. The following steps are discussed in detail below: 1. Prioritize implementation locations: Possible implementation locations are compared based on prioritization factors. C H A P T E R 5 Evaluating Watersheds and Best Management Practices

Evaluating Watersheds and Best Management Practices 71 2. Assess feasibility of site conditions: Physical feasibility factors are used to determine viable locations and BMP types. 3. Prioritize BMP selection: Performance comparisons are conducted to compare between implementation alternatives. Step 1. Prioritize Implementation Locations An overall BMP selection framework for a state DOT should consist of multiple factors, including prioritizing impaired drainage areas. This first step in prioritizing BMP selection identifies factors that should be prioritized based on a quantified ranking system (based on low, medium, high, or a metric system rank) in order of effectiveness or suitability for the POC. The prioritization may vary among the various impaired watersheds due to regional and state agency priorities for certain water bodies. To prioritize water bodies addressed in a TMDL, the factors that should be considered by a state DOT include the following: • Impairment status: The overall impairment of the receiving water body based on required pollutant load reduction in comparison to the reference condition (or natural state of the water body). There may also be variations in impairment status for various tributaries to a major receiving water body. In addition, a receiving water body may have multiple pollutant impairments. • TMDL implementation deadlines: The final adoption of a TMDL and its relative deadline to achieve full compliance. • Regulatory agency water body priorities: Individual water body priorities identified by regulators within permits or due to factors such as human health impacts, fish kills, or other TMDL stakeholder concerns. • State DOT drainage area in watershed: The relative tributary drainage area of all highway areas (impervious and pervious or right-of-way) within each contributing impaired water- shed (TMDL or water body). • Proximity to receiving waters: State DOT highway areas (impervious and pervious) within direct contact of a receiving water body through natural conveyances. A buffer width (e.g., 1,000 ft) to the water body should be established for state DOT highway conveyances dis- charging directly into the impaired water body. This helps determine the percentage of department right-of-way area in direct contact with the receiving water body in comparison with the total department area in the watershed. A higher percentage of state DOT area directly connected to the receiving watershed should have higher priority within the list of TMDL watersheds in treatment control implementation due to the possibility of higher con- centrations and loads of POCs being discharged downstream from highways. Compliance Strategies (Chapter 4) 3. Prioritize BMP Selection BMP Performance Evaluation (Chapter 6) Cost-Effectiveness (Chapter 7) 2. Assess Feasibility of Site Conditions 1. Prioritize Implementation Locations Figure 22. Prioritization and feasibility framework process.

72 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff • Multibenefit projects: Efforts by stakeholders that improve water quality and provide groundwater recharge or habitat enhancement through implementation of infiltrating or vegetated BMPs. Implementation of larger regional BMPs to address hydromodification and water quality. • Traffic-related emission loads (average daily traffic classes): The average daily traffic rate or volume traveling in both directions of the highway, determining the variability in pol- lutant concentrations and loads. • Load-reduction potential: BMP retrofits to achieve TMDL pollutant-load reduction goals are ideally located downstream of higher pollutant-loading land uses. Subcatchments can be analyzed with regard to pollutant load and volume reduction potential based on a hydrologic budget and land use pollutant-load analysis. The effective applicability of this may be limited to BMP areas that treat runoff from multiple land uses due to comingled flows or run-on from adjacent land uses. • Planned future projects (optional): Known or upcoming planned projects in the watershed by state DOTs should be a factor to consider when prioritizing receiving water bodies. An increase in impervious or pervious area may impact the overall state DOT drainage area in a watershed, which indirectly alters the prioritization ranking. These rating factors should be simultaneously considered by state DOTs to identify the higher- priority watersheds and subwatersheds. As an example, a scoring mechanism can be developed that allocates one point to low-priority factors, two points to medium-priority factors, and three points to high-priority factors. The scoring scheme can be a combination of quantitative score and identification of priority water body based on community input. The quantification of scoring and prioritizing watersheds based on appropriate factors should also be discussed with local or regional stakeholders prior to final adaptation. Additionally, it can be at the dis- cretion of a state DOT to weigh each factor differently and based on its priority or importance to the agency. Alternatively, identify criteria for prioritization based on regional applicability. Then, these factors should be quantitatively ranked to identify the high-, medium-, and low- priority circumstances for appropriate treatment implementation. After each watershed has been classified with a priority for each factor and corresponding points, the state DOT should rank these watersheds for appropriate stormwater project implementation. The watersheds that received the higher number ratings should be considered the high-priority or sensitive watersheds. Prioritizing potential BMP locations helps direct state DOT resources toward addressing water quality for the most impaired or sensitive water bodies first. The state DOT can also quan- tify its drainage contributions within the watershed to identify whether internal or external compliance strategies are practical. An example of prioritizing watersheds and TMDLs in recent years has been completed by Caltrans. The department effectively implemented a reach-ranking methodology by considering four factors: • Impairment status, • Department drainage area contributing to the reach, • Proximity to receiving waters, and • Community environmental health impacts (California Department of Transportation 2015). This methodology assisted Caltrans in prioritizing projects within higher-impaired water- sheds (or subwatersheds) to allocate capital funds toward projects with a multibenefit outcome (water quality and improvement of highway conditions). An example of the Caltrans reach prioritization analysis used for TMDLs is shown in Appendix C.

Evaluating Watersheds and Best Management Practices 73 Step 2. Assess Feasibility of Site Conditions This next step outlines a list of factors for state DOTs to analyze and consider when identify- ing a BMP location based on installation and maintenance feasibility. This approach is intended to give departments options to analyze factors specifically applicable to a site of interest. Similar to prioritizing watersheds in Step 1, identification of a prioritizing approach for each identi- fied factor that the state DOT can use should be established. The prioritizing can be based on a scoring mechanism that is established using a semiquantitative rating that can consist of dis- tance, depth, or area rating (or low, medium, and high) while also considering input on criteria from the community (or other watershed agencies). Site conditions and watershed constraints should be identified using factors provided below to determine applicability prior to selection of BMPs. Implementation of a BMP at a specific location in a watershed should be based on numer- ous factors, including desirable conditions suited for the specific treatment control of interest that may lead to effective water quality results over an extended period. The implementation and selection of feasible BMP locations is dependent on the following factors: • Maintenance and safety access: The accessibility to maintain a treatment control safely is critical for consistent effectiveness of a BMP over an extended period. Regular maintenance of structural BMPs (e.g., vegetation removal and outlet cleaning) is important for the func- tion and applicability of a BMP. Any ineffective maintenance of a treatment control may lead to further water quality impairments and issues for the downstream receiving water bodies. Ease of access to treatment controls is based on practice configuration and construc- tion considerations to promote safety. To reduce maintenance-related costs and enhance safety, it may also be beneficial to provide an access lane or vehicular pullout to prevent lane shutdown. • Space and geometry requirements: A highly urbanized project area may not meet the space requirements to implement a large-footprint BMP. Identify and prioritize applicable BMPs that are compatible with the space available. For cost-effectiveness, it can be beneficial to consider the use of available space within the right-of-way, which could then result in the selection of a lower-ranked treatment practice. However, site constraints may limit the treat- ment area and require implementation of additional facilities to meet the water body alloca- tions. Highways with steep slopes may prohibit the use of high-performing BMPs, as well as limit the use of vegetated conveyance systems (e.g., biofiltration swales). Environmentally sensitive areas or protected habitats, wetlands, or other protected resources may also limit BMP applicability at specific locations within the watersheds. Additional considerations and guidance on the appropriate BMP site selection can be found in NCHRP Report 802 (Strecker et al. 2015). • Climate adaptability: Regions with cold climates versus semi-arid climates may present challenges for designing flow-based and volume-based BMPs. Areas with excessive freezing temperatures require deicing controls that may impact groundwater and the efficiency of the implemented treatment control (e.g., clogging). Additionally, excessive deicing controls (i.e., salts) on highways may have a negative consequence on the downstream water quality. Salts can also corrode metal components of structural BMPs (e.g., valves) and possibly cause further damage to the receiving water from additional pollutants discharged from the cor- roded metal. Dry weather regions make certain treatment controls—such as biofiltration or wetlands—ineffective. BMPs require adequate vegetation to remove POCs. Regions with low rainfall storm events may lead to a treatment BMP being more effective in volume reduction and POC removal than areas with higher intensities, where peak storm intensities are unable to be captured.

74 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff • Groundwater depth: Shallow depth of groundwater can prohibit the implementation of infiltrating BMPs due to contamination or impacts to the water table. Shallow groundwater depths can also pose a risk to structural stability. • Groundwater contamination: The relative permeability of soil is critical when determining BMPs to use in locations that may have the potential to contaminate groundwater due to low organic content. The risk to groundwater may be greater than that of treating the runoff in locations with known soil or groundwater contamination. • Soils and geotechnical considerations: Assessment of geotechnical issues—such as slope instability, impacts on foundations or retaining walls, or overall settlement changes—is criti- cal to determine the risk associated with implementing an infiltrating BMP. Implementing infiltrating BMPs can further saturate the soils and lead to slope failures or additional geo- technical concerns. • Groundwater recharge: A site may benefit from infiltrating BMPs due to their capability of recharging groundwater. Decompaction of soil may increase infiltration at the project site and recharge the groundwater. However, decompaction of soil is not suggested if shallow ground- water is present. Identification of the most beneficial location for implementing an infiltrating BMP (due to its direct connection between the surface and the underlying aquifer) may be the most effective and efficient way to recharge groundwater. Available Tools and Resources to Assess Best Management Practice Feasibility–Applicability This section describes the applicable tools and resources that are available for state DOTs to prioritize factors and determine suitability of BMP implementation based on site conditions. Critical factors are not limited to the ones identified above. However, state DOT-specific factors can be analyzed for region- or site-specific applicability of BMPs. System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN). EPA’s SUSTAIN tool has been developed to assist with developing appropriate implementation plans and practices for water quality objectives (EPA 2018B). SUSTAIN was developed to provide a comprehensive watershed modeling and analysis tool for identifying optimal stormwater management solutions. The BMP siting tool module is part of the overall SUSTAIN interface and helps determine suitable locations for typically used structural BMPs in urban watersheds. This tool can be used by the state DOT to identify suitable sites for implementing control measures in high-risk receiving watersheds locally or at a watershed scale. The SUSTAIN tool emphasizes the variability in BMP selection due to these suitability factors: • Drainage area • Drainage slope • Imperviousness • Hydrologic soil group • Land uses • Groundwater table depth • Road buffer • Stream buffer • Building buffer The BMP siting tool includes a criteria matrix comparing BMP types to suitability factors. The tool identifies suitable locations and populates them with the default values listed in Table 38. The user can change or update the criteria based on preference or region information to further narrow down BMP locations and types based on site-specific constraints. Default criteria for BMP suitable locations used in SUSTAIN are shown in Table 38. The BMP siting tool interface

Evaluating Watersheds and Best Management Practices 75 is available in ESRI ArcView 9 with various geographic information systems data layers of the suitability criteria. The output from the BMP siting tool narrows areas that meet the user-specified criteria for the project site or selected BMP. SUSTAIN has six additional modules that can be used to generate runoff and pollutant loads from land uses, route flows, evaluate cost-effective BMP placement, and strategically select applicable BMPs. SUSTAIN continues to be used by vari- ous agencies, practitioners, and other watershed groups in developing TMDL implementation plans and approaches for pollutant reduction or compliance and identifying green infrastruc- ture strategies to reduce volume or flow. NCHRP Report 802: Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual This research report identifies surface runoff volume-reduction practices to control water quality in the downstream water bodies within urbanized areas. It also identifies other benefits to implementing volume-reduction practices, such as reducing pollutant load, increasing ground- water recharge (infiltration or retention), and reducing channel erosion. This report provides guidance for implementing BMP practices within highways in urbanized areas with varying site conditions. The study focuses on identifying the criteria that dictate implementation of any volume-reduction approaches, including the effectiveness and desirability of treatment controls based on the physical setting or the project site design. The physical setting factors that are criti- cal to BMP selection include climate, soils, groundwater conditions, topography, and watershed characteristics. The project type factors that are important for BMP selection consist of project type, highway type, grading, drainage, maintenance access, and highway landscape and vegeta- tion. Additional information with regard to each of these factors can be accessed in NCHRP Report 802 (Strecker et al. 2015) for screening and selection of BMPs. BMP Site Suitability Criteria Drainage Area (acre) Drainage Slope (%) Imperviousness (%) Hydrological Soil Group Water Table Depth (ft) Road Buffer (ft) Stream Buffer (ft) Building Buffer (ft) Bioretention <2 <5 >0 A–D >2 <100 >100 – Cistern – – – – – – – <30 Constructed Wetland >25 <15 >0 A–D >4 – >100 – Dry Pond >10 <15 >0 A–D >4 – >100 – Grassed Swale <5 <4 >0 A–D >2 <100 – – Green Roof – – – – – – – – Infiltration Basin <10 <15 >0 A–B >4 – >100 – Infiltration Trench <5 <15 >0 A–B >4 – >100 – Porous Pavement <3 <1 >0 A–B >2 – – – Rain Barrel – – – – – – – <30 Sand Filter (nonsurface) <2 <10 >0 A–D >2 – >100 – Sand Filter (surface) <10 <10 >0 A–D >2 – >100 – Vegetated Filter Strip – <10 >0 A–D >2 <100 – – Wet Pond >25 <15 0 A–D >4 – >100 – Note: – = not applicable. (Source: EPA 2009A). Table 38. Default criteria for BMP-suitable locations used in SUSTAIN.

76 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff Step 3. Prioritize Best Management Practice Selection This last step identifies the most critical factors to consider in BMP selection and implemen- tation. Typically, for any BMP consideration it is valuable to prioritize the applicable BMPs based on the performance and effective cost of a BMP. In addition to prioritizing the selection of a BMP, it is critical to consider opportunities through collaborative efforts and multibenefit projects. These factors can also be considered by a stakeholder task force group—including the state DOT—as part of selecting the most applicable BMP for each project. The following aspects are important when considering BMP type selection. Evaluation methods for performance and cost are further discussed in Chapters 6 and 7, respectively. • Performance: The primary and most effective load reduction is achieved through volume loss or decrease in runoff volume discharged downstream. The pollutant removal algorithm (see Chapter 6) identifies runoff volume loss through infiltration, evapotranspiration, and reuse. Identification and prioritization of volume-reducing BMPs over other BMP types may provide greater benefits for the project area. • Cost: The evaluation of various alternatives based on new construction or retrofit of BMP implementation is important to obtain the most efficient solution for enhancing the water quality. Life-cycle costs for BMPs (e.g., construction, operation, and maintenance) should also be considered when prioritizing applicable BMPs. • Collaborative partnership: Consider the opportunity for project integration with other efforts, such as opportunity for public–private partnerships and public–public partnerships, watershed scale planning, or project inclusion with broader state DOT planning and capital improvement projects. A collaborative approach may be beneficial due to its practicality in a location where water quality directly benefits all watershed stakeholders. • Multibenefits: Quantify the economic, social, and environmental benefits, which are ancillary to the primary project goal. These factors may include habitat provisions, recreational or aesthetic value, traffic calming, and property value increase. The relative ability of the prac- tice to provide ecosystem services and recreation opportunities may influence community buy-in of the BMP. Implementing a regional BMP that provides social benefits (e.g., job creation), flood control, reduced greenhouse gas, and added recreational value (through stream restoration or park enhancements) should be considered. Aesthetic improvements to existing conditions can increase the economic value of a neighborhood, along with addressing hydromodification. The benefits of green infrastructure (e.g., LID or LID BMPs) extend beyond water quality to improved community livability, groundwater recharge, habi- tat, and air quality.