CHAPTER 2

Research Approach

Overview of Project Phasing

The overall project involved applied research, product development and testing, and guide development, implemented in a phased approach, as described below:

• Task 1a: The objective of Task 1a was to identify treatment mechanisms, focusing on media filtration or other materials, that can effectively address (via removal or transformation) highway runoff pollutants within the space constraints of the bridge environment. This was primarily a literature review and synthesis effort to recommend treatment processes.
• Task 1b: The objective of Task 1b was to develop BMP options that provide the treatment processes recommended in Task 1a, while also meeting design requirements related to structural integrity, hydraulic capacity, trash and debris management, operations and maintenance (O&M), and other factors. This involved applied research into the bridge structural and operational environments and a multi-objective design development process.
• Task 2: Interim Report 1 provided a summary of findings from Task 1 and recommended next steps for the project. It had two primary objectives:
  • Synthesize the findings of Task 1a and 1b to recommend prototype BMP options that proceeded to testing in Task 3.
  • Propose a Task 3 research strategy to support conceptual design development and fill data gaps necessary to support more general design guidelines.
• Task 3 involved new research, primarily conducted in a laboratory environment to help (1) support conceptual design development, (3) provide vetting or refinement of recommended treatment BMP options, and (3) fill key data gaps for conceptual design and/or guide development. Based on direction from the Project Panel, laboratory testing was supplemented with applied research into other topics.
• Task 4 included documentation and synthesis of results of Task 3 research and development of an outline of the Guide.
• Task 5 distilled the results of all preceding tasks into a Guide for use by DOT professionals to support decisions and conceptual design. Task 5 also included the development of case studies to supplement the Guide.

Task 1a Approach: Stormwater Treatment Processes and Options

Objective

The objective of Task 1a was to characterize target stormwater pollutants in highway bridge runoff, evaluate candidate treatment processes for target pollutants, and identify candidate filtration media that effectively removes and/or eliminates the toxicity of target pollutants (hereto referred to as pollutant removal effectiveness) from highway runoff. One of the primary goals of this literature review was to develop sufficient understanding of stormwater pollutants and removal and toxicity reduction processes to propose a filtration media for further testing in Task 3.

Methodology

Task 1a primarily involved a literature review. Based on previous research findings and the experience of the research team, we developed several starting assumptions:

• Copper and zinc are the target pollutants, as described in the NCHRP Project 25-61 Request for Proposal. Toxic organic compounds (such as 6PPD-quinone) are not considered target pollutants in this study, but these are likely addressed by many of the same processes that remove and/or reduce the toxicity of dissolved copper and zinc. Nutrients are also of interest in some areas, such as the Chesapeake Bay region.
• Due to presence of target pollutants in both dissolved and particulate-bound forms, both physical filtration and sorption are likely needed to effectively remove target pollutants from runoff.
• Because water quality targets vary by location, the pollutant removal effectiveness of filtration media should be compared to removal effectiveness standards, not specific water quality targets. We used the Washington State Department of Ecology Technology Assessment Protocol – Ecology (TAPE), enhanced (i.e., dissolved metals) removal standards as the pollutant removal effectiveness benchmarks for metals in this literature review (Washington Department of Ecology, 2018). We used Chesapeake Bay Stormwater Performance Standards (Chesapeake Stormwater Network, 2015) as a supplemental benchmark to assess expected levels of nutrient control provided by stormwater BMPs.
• Because of physical space constraints and weight limitations in the bridge deck environment, higher media flow rates would allow treatment devices to treat a greater portion of long-term runoff in a small footprint.
• Media selection will focus on candidate media blends with existing literature and track records, rather than on individual media components or new blends formed in this project from individual components.

Building on these initial assumptions, this review consisted of the following phases:

• Characterization of critical highway pollutants. We characterized the forms and abundance of zinc and copper in highway stormwater runoff that would be representative of bridge runoff.

• Evaluation of candidate treatment processes. Based on the stormwater characterization, we then evaluated treatment processes that potentially could be placed in the bridge environment that are effective in removing these pollutants.

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• Identification of filtration media characteristics to meet project goals. We utilized literature and professional experience to summarize the filtration media characteristics needed to meet project goals, such as pollutant removal effectiveness, hydraulics (e.g., treatment flow rate), durability, and replacement/disposal.
• Review of blended media studies. We reviewed and summarized the results of studies of blended filter media mixes that potentially meet the project requirements. This focused on studies that have investigated copper and zinc removal (and preferably nitrogen and phosphorus removal) with actual stormwater vs. synthetic stormwater.
• Evaluation of candidate media blends. Based on the desired characteristics and performance studies, we developed recommendations for a filtration media blend and component characteristics to be tested in Task 3 of this project. We also identified candidate proprietary media blends.

Appendix A to this report documents the detailed methodology and findings of this task.

Task 1b Approach: Overall Stormwater Treatment Design

Objective

Task 1b focused on developing and evaluating potential conceptual designs for treatment of stormwater runoff from bridge decks. For the purpose of this effort, we focused primarily on retrofit of existing bridges. Treatment of stormwater from new bridges will require many of the same considerations, but with greater ability to incorporate treatment systems into the structural and conveyance design and with greater ability to incorporate design features that support maintenance access. New bridge designs could also more easily route stormwater to one of the abutments to avoid treatment systems on the bridge.

Methodology

Our overall approach for Task 1b included the following steps:

• Characterize the bridge environment. We sought to determine the typical ranges of physical conditions and design and operational constraints that affect stormwater treatment design, construction, and O&M in the bridge environment. We conducted interviews with state DOTs, obtained case studies, reviewed typical design criteria, and conducted rapid on-line and in-person visual surveys to inform our understanding of the potential issues (constraints) and opportunities for installing treatment systems on a bridge.
• Develop and prioritize design requirements. We reviewed the range of design requirements and constraints and sought to prioritize their relative importance and impact on the design process. As part of this process, we also began to develop design parameters and alternatives to address each requirement or constraint.
• Conduct design case studies. We applied potential on-bridge stormwater treatment designs to real bridge designs obtained from DOTs. We used these case studies to further develop and evaluate design alternatives when applied to example bridges.

Appendix B to this report documents the detailed methodology and findings of this task.

Task 2 Approach: Candidate Media Blend, Prototype BMP, and Research Plan (Interim Report 1)

Building on the findings of Task 1a and 1b, we synthesized design recommendations for candidate filtration media and an initial prototype BMP design. From this review, we also identified the aspects of the design process that are (1) most important for successful stormwater treatment in the bridge environment, and (2) least supported by available scientific or practical experience so that these could be prioritized for potential new research. The Interim Report produced in Task 2 defined the overall research plan for Task 3. The results of this synthesis are summarized in Chapter 3 of this report (Findings and Applications).

Task 3 Approach: Laboratory Testing and Supplemental Research

Laboratory Research Priorities and Objectives for Task 3

Based on the results of task 2, the research team and Project Panel identified the following primary research priorities for laboratory analysis:

• Investigate clogging processes of media to inform estimates of O&M maintenance intervals. This is a key driver of BMP sizing.
• Confirm water quality treatment performance of the candidate media for dissolved metals at a higher flowrate than previous testing of similar media. Flowrate is a key driver in BMP sizing.
• Evaluate water quality performance for additional pollutants, including 6PPD-quinone and phosphorus.
• Evaluate media bed depth needed to meet treatment objectives. This is relevant to determine the necessary depth of media and associated weight of BMPs.

The following sections describe the main elements of Task 3 laboratory research.

Media Procurement

Interim Report I described a recommended filtration media for testing. This media was based on high-performance bioretention soil media that has been developed and tested over approximately 8 years in Washington State. Geosyntec sourced the recommended filtration media from Walrath Landscape Supply company in Tacoma, Washington. The mix was customized to meet the needs of this project with respect gradation and composition. Geosyntec maintained notes about material quality and availability. Geosyntec also conducted research with suppliers into the ability to procure media elsewhere in the country.

Quality Assurance Project Plan Development

We prepared a QAPP for both the permeability and water quality treatment studies and obtained Project Panel review of this QAPP. Following Panel approval, the QAPP was implemented.

Permeability and Clogging Study

Permeability and clogging tests were performed by the University of Texas to provide guidelines with respect to maintenance frequency. The experimental approach involved running a series of falling head, rigid wall hydraulic conductivity tests with sequential additions of stormwater solids. Changes in permeability with addition of the solids was used to determine the relationship between solids loading and permeability (K). Permeameter cells were set up for the selected media blend and the effective hydraulic conductivity was determined. After the measurement, a defined mass of stormwater solids was loaded in the permeameter with the media and the permeability was re-measured. This procedure was repeated until the filtration rate fell below 25 inches/hr, which allowed us to determine the relationship between solids loading and hydraulic conductivity. The test was run in triplicate for the selected media.

Appendix C contains the permeability and clogging study laboratory report, which includes greater detail on the study methodology and results.

Water Quality Treatment Column Study

The water quality testing study employed a series of column experiments to evaluate the water quality treatment performance of the recommended non-proprietary media blend described in Interim Report I and in the Laboratory Permeability and Clogging Study (Laboratory Report 1). Since the main goal of the research project was to develop BMP with a footprint small enough to install on a bridge, which requires a relatively high treatment rate, this study evaluated pollutant removal performance in a system with a loading rate of 50 in/hr and compared removals at this higher rate to removal at more typical loadings of 14 in/hr. A media similar to the selected blend has been tested extensively at approximately 14 in/hr. Therefore, the comparison in this study between performance at 14 in/hr and 50 in/hr was intended to determine how increased loading affects treatment performance compared to a baseline that has been more thoroughly tested in field conditions.

Performance of the media blend for removal of dissolved copper and dissolved zinc was of primary interest with dissolved phosphorus of secondary interest. The metrics for removal of these contaminants were based on benchmarks set by the Washington State Department of Ecology.

Performance for zinc and copper was be compared to Washington State Technology Assessment Protocol Ecology (TAPE) benchmarks (Washington State Department of Ecology, 2018):

• 30 percent removal of dissolved copper for the influent concentration range of 0.005 mg/L to 0.02 mg/L, and
• 60 percent removal of dissolved zinc for the influent concentration range of 0.02 mg/L to 0.30 mg/L

In addition, the research team identified 6PPD-quinone as an emerging constituent of concern for inclusion as another secondary objective, because it is associated with tire wear debris and has been demonstrated to be toxic to a variety of fish species.

In contrast to the permeability and clogging study, the water quality study focused on removal of dissolved solutes. While both dissolved and total metal and phosphorus concentrations were analyzed from effluent samples, the influent water was settled to avoid clogging of the columns during the testing period, therefore the total concentrations reported herein are not intended to representative of highway runoff.

Appendix D contains water quality treatment study laboratory report, which includes greater detail on the study methodology and results.

Supplemental Research

Methodology and Parameters for Structural Evaluation

The research team reviewed state and national structural guidelines. From this review, we summarized common methodologies and the variables needed to conduct structural analysis on various types of bridge designs. The research team also summarized AASHTO Manual for Assessing Safety Hardware (MASH) criteria as it pertains to structural assessment of edge barriers. From this research, we distilled general calculation guidelines for how to assess the suitability of BMP placement on an edge barrier. This was included in Chapter 5 of the Guide.

Research O&M Equipment

The research team researched typical bridge maintenance protocols and equipment via published literature and professional contacts. The research team used the findings of this research to: (1) describe the O&M protocols and equipment that would be needed for O&M of on-bridge BMPs.

Feasibility Analysis and Conceptual Design Case Studies

The objective of this research was to provide worked examples of the feasibility analysis and conceptual design process recommended by this research. This was initiated in Task 3 by identifying and obtaining information on case study bridges. It was completed in Task 5. See the approach description associated with Task 5.

The results of supplemental research were incorporated into the Guide.

Task 4 Approach: Synthesis of Laboratory Findings and Development of Sizing Guidelines (Interim Report 2)

Interim Report 2 summarized the findings of the Task 3 laboratory analysis. From these findings, the research report distilled three key elements to support the design of on-bridge BMPs:

• Establishment of an upper limit of design media filtration rates to meet water quality treatment objectives
• Determination of necessary media bed depth to meet water quality treatment objectives
• Development of a sizing approach that accounts for water quality treatment performance and estimated maintenance intervals, incorporating local rainfall patterns.

The relevant findings are summarized in Chapter 3 of this report.

Task 5 Approach: Guide and Case Studies

As part of this task, we distilled the results of all preceding tasks into a Guide for use by DOT professionals to support decisions and conceptual design. Task 5 also included the development of case studies to supplement the Guide.

Major elements of the Guide’s development included:

• Development of a structured process for decision-making and design.
• Packaging and presentation of the results of subsequent research in a format targeted toward DOT practitioners working on individual bridges.
• Synthesis of feasibility screening criteria (fatal flaw analysis) based on the combinations of conditions identified in research that are infeasible or very challenging.
• Preparation of template specifications for the recommended high-performance media.
• Description of the recommended prototype BMP, including design elements, weight estimates, and O&M needs
• Research and summary of proprietary BMP options, including discussion of pros and cons and options for performance-based specifications to avoid sole source procurement.
• Recommendations for BMP placement and conveyance system design.
• Presentation of the sizing approach developed in Task 4, with results for additional precipitation stations added to improve geographic coverage.
• Guidelines for determining O&M needs and procedures.
• Guidelines for evaluation of alternatives and decision-making.
• Supplemental guidelines for whole lifecycle cost estimating.
• Supplemental guidelines for scoping structural analyses, including structural load ratings for bridge edge barriers.

In parallel with the development of the Guide, we also prepared five case studies of on-bridge BMP design concepts, three illustrating hypothetical conceptual design processes for real bridges, one showing an actual proposed design for a bridge, and one showing a completed on-bridge stormwater retrofit project.

For hypothetical conceptual design studies, the research team worked with the Project Panel to selected bridges that cover a range of common conditions and would be meaningful to study. Selection criteria included:

• Bridges had a design type that is not highly unique.
• Bridges were intentionally selected to include those where on-bridge stormwater treatment might be a necessary technical solution if there was a regulatory requirement to do so. For example, these bridges currently drain directly to the water body and do not have flat land areas below the bridge that would support a land-based BMP.
• Bridges were selected that did not have obvious fatal flaws based on initial characterization of existing conditions, such as those described in Chapter 2. This intentionally biases the analysis but supports more meaningful case studies.
• Bridges had a range of traffic volumes, lengths and water body sizes.

The three hypothetical case studies included:

• Case Study 1: Monitor Merrimac Bridge Memorial Bridge-Tunnel, Virginia. This is a long viaduct-type concrete bridge across the Hampton Roads Harbor that presents no opportunity route water to land.
• Case Study 2: Nisqually River Bridge, I-5, Washington State. This is an at-grade metal truss bridge over the Nisqually River, which is a relatively small river that supports salmonid runs.

• Case Study 3: Cannon Road Bridge, Georgia. This is a two-lane at-grade concrete bridge over a relatively small creek with relatively limited separation from flood event high water.

The two real project examples were selected based on examples with sufficient available data. There are very few examples of on-bridge stormwater treatment that the research team is aware of. These bridges include:

• Case Study 4: Molise Viaduct, Molise Italy. This is a long viaduct-type bridge over a water supply reservoir that is key to the region. It is proposed to be retrofit with spill containment and stormwater treatment in 2025 or 2026 as part of major rehabilitation work.
• Case Study 5: Broadway Bridge, Portland, Oregon. This bridge was retrofit with stormwater treatment BMPs as part of a larger rehabilitation project in a the early 2000s.

For each case study, we worked with the applicable organization to obtain information about the bridge and/or the BMP design.

For hypothetical case studies, we generally followed the structured approach described in the Guide to step thought the characterization of conditions, assessment of BMP placement opportunities, development of stormwater capture and conveyance approaches, BMP sizing options, and O&M approaches. Each included a rough estimate of capital and O&M cost. For each real bridge project, we attempted to obtain similar information, as available.

Each of these studies was intended to offer different insights and examples regarding the conceptual design process for on-bridge stormwater BMPs. These were not intended to be a cross-section of all bridges.

Case studies were documented in the appendix to the Guide.