SUMMARY

Stormwater Retrofit Programs and Practices Through Third-Party Partnerships

Because many existing highways were constructed without stormwater control facilities, some state departments of transportation (DOTs) have been required to retrofit these highway facilities with improved treatment and flow control measures. Stormwater retrofits are permanent stormwater facilities constructed to reduce, minimize, and treat flows from existing transportation infrastructure. The motivations for these retrofits include National Pollutant Discharge Elimination System (NPDES) stormwater permits, state water quality and runoff regulations, prescribed actions resulting from Endangered Species Act consultation, and total maximum daily load (TMDL) findings. State DOTs typically hold one of two types of NPDES permits, either a municipal separate storm sewer system (MS4) or a transportation separate storm sewer system (TS4) permit. As the quality and quantity of stormwater runoff becomes subject to increased regulatory scrutiny, state DOTs are interested in exploring techniques that use watershed-based strategies to improve treatment effectiveness. To more economically accomplish these watershed-based strategies, some DOTs have experimented using partnerships with third parties. The approaches taken to develop these partnerships have been diverse.

This synthesis will document these diverse approaches taken by state DOTs regarding stormwater retrofit partnerships with third parties. This objective will be met by identifying on a nationwide basis the motivations and methods used to retrofit transportation infrastructure with permanent stormwater facilities and by documenting the use of third-party partnerships undertaken to accomplish this work. The variation of the types of these partnerships will be explored as well as the barriers to and benefits of these types of arrangements. The synthesis includes both a review of pertinent literature and a survey of state DOTs targeted at their water quality staff. Detailed case examples were then developed using interviews with selected DOTs identified by the survey.

The survey, included in Appendix A, was disseminated to all states, as well as Puerto Rico and the District of Columbia. Of the 52 state DOTs, 42 responded. The state of the practice is varied. Some agencies have full-blown programs with both flow control and stormwater quality treatment facilities widely deployed. Others have essentially none of these facilities. A significant fraction of the respondents has some experience either constructing or maintaining facilities using third-party partnerships. Of the 42 respondents, 18 responded that they have third-party partners to help maintain their facilities, and 20 have had partners help in constructing their facilities. Of the state DOTs with third-party partnerships, the total fraction of their facilities built or maintained through third-party partners is low. Only three states have more than 10% of their stormwater facilities constructed with partnership agreements, and only four states have more than 10% of their facilities maintained under such agreements.

Stormwater runoff can cause challenges both through its increased quantity and decreased quality. Measures to treat both problems are often closely linked but based upon the results of the survey, quantity control measures are more widely constructed than quality improvement measures. As part of the survey, DOTs were invited to submit their design and planning guidance documents. Twenty states submitted materials DOTs use for design, construction, and maintenance of stormwater infrastructure. These documents provided insight to the state of the practice in these states. Further, stormwater literature was reviewed for guidance in the areas of regional and watershed-based facilities and the structuring of partnerships. Most of this literature agrees that larger regional facilities for stormwater quantity control and smaller, more localized infrastructure for stormwater quality are complementary techniques with different benefits and challenges.

Survey questionnaire responses were reviewed to identify state DOTs that could provide expertise in both retrofit programs and partnership experience and that were willing to be interviewed as case examples. The Florida, North Carolina, California, and Rhode Island DOTs participated in these follow-up interviews. These case examples demonstrate both positive experiences and challenges that can occur with such partnerships. Two of the states, Rhode Island and California, rely upon TMDL-listed and other sensitive waters to drive the selection of partnership projects. Florida maintains robust partnerships with other MS4 agencies throughout the state. North Carolina has included as part of its TS4 permit the requirement that they construct 70 retrofit projects over the 5-year life of the permit.

The treatment of non-DOT water is one area that could effectively lead to partnerships. Non-DOT water, sometimes called offsite flow, is stormwater runoff that originates outside the DOT right-of-way and then flows through the DOT right-of-way. Based on the interviews, the treatment of non-DOT water is the genesis of many of the partnerships that do exist. These interviews suggest that besides offsite flow treatment, partnerships are also founded because of the advantages of pooled funding. There is also sometimes a need for various stakeholders to join together to address a TMDL or other regulatory finding in which they share responsibility.

In recent years, green infrastructure techniques have been refined to better mimic the natural infiltration of runoff, whereby runoff is naturally filtered as it passes through the soil. The EPA issued a technical memorandum in 2007, Using Green Infrastructure to Protect Water Quality in Stormwater, that outlines and advocates for these techniques. Green infrastructure techniques are decentralized and are typically constructed near the location of the development in an attempt to treat the runoff close to where it is generated. Large detention ponds and storm drainage conveyance projects, often thought of as regional facilities ripe for third-party partnerships, are the opposite of green infrastructure approaches. Design concepts, agreement formats, or regulatory approaches that would encourage partnerships to develop decentralized stormwater quality features are areas that could use more research.

One primary driver identified for partnerships was funding. DOTs typically control funding streams that can provide design funds for a partnership project at the conceptual stage. Few partners have access to funding sources at the early stage of project development. Additional research could help establish for DOTs and third parties a comprehensive guide on funding partnership projects at various phases of development and delivery. This guidance could provide criteria for DOT-funded conceptual design, as well as approaches that all partners could use to pursue grants and other funding sources once the design is complete and feasibility well known.

All of the state DOTs interviewed as case examples believe that watershed-focused permanent stormwater facilities offer both water quality performance and economic advantages

over onsite facilities. Further research on the relative water quality performance of watershed-focused permanent stormwater facilities versus that of onsite facilities could document these performance and economic advantages and lead to an increase in their use. Beyond the water quality performance of these facilities, the cost-effectiveness and the identification and quantification of economies of scale could also contribute to an expansion of partnerships and retrofit facilities.