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2 Chapter 1 Introduction 1.1 Statement of Project Needs and Objectives There are several best management practices (BMPs) that are available and applicable to the treatment of highway runoff. These include vegetative swales, ponds, wetlands, sand filters, and infiltration approaches. These BMPs are good at removing particulate-bound pollutants by settling, filtering, and, in the case of wetlands, settling, uptake, and incorporation of pollutants into biological matter (e.g., natural organic matter). However, a longstanding goal of stormwater treatment is the removal of the stormwater pollutant fraction that cannot be readily settled or filtered. This fraction is often considered dissolved although it may also consist of colloidal particles. In addition to being capable of removing dissolved constituents, this BMP needs to be cost-effective and have a treatment-system footprint that is reasonably sized and can still treat the volume of water needed to meet a projects goals. For the case of metals and aquatic life protection, it is particularly important for a BMP to be capable of dissolved metals removal as it is the dissolved form that is considered more bioavailable than the particulate form and has the greater potential to cause aquatic toxicity (Niyogi and Wood, 2004). Furthermore, US EPA water quality criteria were often developed from laboratory studies using metal salts that were largely dissolved and the proper evaluations of BMP effectiveness with respect to meeting instream water quality standards for metals should consider the dissolved metal fraction. While there are several media that may be employed to remove dissolved metals from stormwater (Genc- Fuhrman, 2007), the media chosen for this current study is ferric oxide. Field scale testing of ferric oxide was recommended as an outcome of NCHRP Report 767: Measuring and Removing Dissolved Metals from Storm Water in Highly Urbanized Areas (Barrett et al., 2014), a laboratory study that considered several metals and media with testing focused on the capacity of ferric oxide to remove copper and zinc from synthetic and natural highway stormwater runoff. The study demonstrated the capacity of ferric oxide to remove copper and zinc from synthetic and natural highway runoff but it was also noted that the stormwater chemistry of highway runoff, including natural organic matter, carbonate, ionic strength, and pH can have a significant effect on the capacity of ferric oxide for metals removal. Hence, treatment performance expectations based upon equilibrium chemistry and laboratory studies needed to be confirmed as part of full scale study. Conceptual BMP designs making use of ferric oxide were proposed as part of NCHRP Report 767 as well as the use of ferric oxide within the pore space of open graded friction course (OGFC) pavement. Treatment cell type designs identified in NCHRP Report 767 included a vault-type system with ferric oxide imbedded filtration media and a scupper system for bridge decks. For this current study, full scale vault and swale-type filtration BMPs with ferric oxide were evaluated with consideration of their application in urban and rural environments. Ferric oxide in OGFC roads, which is more suited to conditions with limited land available for treatment (e.g., bridge decks), was investigated as a proof of concept in the laboratory. 1.1.1 Project Objectives The objective of this research is to evaluate the effectiveness and life cycle cost of using ferric oxide (iron) media to promote dissolved metals removal from highway stormwater runoff. The research was designed
3 to field test two existing and different full scale filtration-type BMP configurations that have incorporated ferric oxide media to remove dissolved constituents from natural stormwater runoff. This research builds upon the recommendation of NCHRP Report 767. Specifically, this research was design to evaluate the concentration and mass based removal efficiencies for dissolved metals in stormwater runoff from roadways for the following BMPs using ferric oxide: (a) open graded friction course (OGFC) pavement with ferric oxide incorporated into the pavement pore space, (b) vegetated-type full scale filtration BMPs established adjacent to roadways where the filtration media incorporates ferric oxide, and (c) vaults-type full scale filtration BMP with ferric oxide amended filtration media. Metals studied (i.e., copper, nickel, chromium, arsenic, zinc, and lead) are those that are commonly found in highway runoff and have the potential to affect downstream aquatic life. Iron was also monitored as an indicator of treatment system function and condition. The study using OGFC and ferric oxide was designed to be a âproof of conceptâ with respect to the potential use of ferric oxide in OGFC rather than an evaluation of the engineering or practical feasibility of this approach. BMPs studied were intended to be passive and provide treatment by filtration rather than infiltration. A further outcome of this study was the development of lifecycle cost information for ferric oxide sand filters and calculation of the annualized cost per unit of load reduction performance. The life cycle costing effort was intended to build on NCHRP Report 792: Long Term Performance and Life Cycle Costs of Stormwater Best Management Practices (Taylor et al., 2014). 1.1.2 Scope of the Report This report includes the result of two years of monitoring data collected for existing vault-type and a swale-type ferric oxide filter and a laboratory investigation of OGFC pavement with ferric oxide. The monitoring study was designed to (1) collect performance data for full scale ferric oxide filters for dissolved metals, (2) develop an understanding of those factors (both chemical, hydrologic and hydraulic) that affect dissolved metals removal performance, (3) develop design guidance for the implementation of full scale ferric oxide to optimize dissolved metals performance, and (4) evaluate the cost-benefit of ferric oxide filters for the treatment of metals in highway runoff. Results of the laboratory study with OGFC and ferric oxide are provided to only demonstrate the potential dissolved metals treatment benefit of incorporating ferric oxide in OGFC or similar porous pavement types. 1.1.3 Intended Users and Uses Intended users include state and local transportation agencies and staff, highway design engineers and scientists, as well as any governmental entity or organization managing roadway runoff and water quality. This study can be used as part of the implementation of total maximum daily load studies (TMDLs) where load reductions are estimated for different BMPs. More broadly, this study could be used to assess the expected load reduction and receiving water instream metals concentration for existing or proposed roadways. 1.1.4 Relationship with Other NCHRP Publications This research builds upon the recommendation of NCHRP Report 767. The life cycle costing tool provided as part of NCHRP Report 792 will be used for filtration BMPs which are similar to the ferric oxide sand filters evaluated in this study. Other relevant NCHRP publications include NCHRP Report 474: Assessing the Impacts of Bridge Deck Runoff Contaminants in Receiving Waters (Dupuis, 2002), NCHRP Synthesis 444:
4 Pollutant Load Reductions for Total Maximum Daily Loads for Highways (Abbasi and Koskelo, 2013), and NCHRP Report 640: Construction and Maintenance Practices for Permeable Friction Courses (Cooley, et al., 2009). This study also provides potential treatment approaches for bridge deck runoff that were not previously available when NCHRP Report 474 was published.