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70 Chapter 7 Discussion Stormwater can contain a range of pollutants (e.g., nutrients, metals, and synthetic organics) that are adsorbed or incorporated into inorganic and organic solids conveyed by stormwater. Ponds effectively settle and remove these particulate pollutants. Increased pollutant removal can be engineered with the use of filtration to capture smaller particles that are slow to settle. The remaining constituents after settling and filtration are often operationally defined as dissolved. Removal of this fraction requires chemical binding or adsorption and formation of new particles that can be retained in the treatment media. This current study was designed to evaluate the capacity of two existing, full scale treatment systems that use ferric oxide in a sand medium, to chemically bind and retain dissolved metals in highway stormwater runoff. This study clearly demonstrated that ferric oxide can be effectively used in a full scale application receiving dissolved metals loads from natural highway stormwater runoff. It also appears that the use of ferric oxide in a sand medium can improve particulate metals removal. Ferric oxide in sand has the capacity to reduce dissolved metals to very low concentrations that approach or are below one part per billion. These findings are consistent with other studies (Genc-Fuhrman et al., 2007, Erickson et al., 2012) that have used a ferric oxide media in a laboratory to enhance pollutant removal or with studies of ferric oxide metal binding in the natural environment (Zanzo et al., 2017). Interpretation of the results of this exercise suggests that ferric oxide-sand filters may provide incremental benefits of increased metals removal as a return on the incremental investment in the ferric oxide filtration media, indicated by higher removal efficiencies and possibly lower whole life cycle unit removal costs. The importance of dissolved metals for aquatic toxicity is well known but also the removal of dissolved metals has value as this fraction may remain dissolved and concentrate in terminal or highly evaporative waterbodies that are often prevalent in the Western United States. Removal of the particulate metal is important also as metals can desorb and become dissolved once deposited into the aquatic environment. The ferric oxide-sand filter will likely be a more permanent sink for particulate metals compared to sand filters without media, or other BMPs such as ponds and wetlands. The potentially transitory nature of particulate metals was observed at the Woodlynn Avenue treatment cell where certain dissolved metals (e.g., copper) were released for a number of storm events even as particulate metals were removed. The Highway 36/61 site appeared to be a more permanent sink for metals and this was likely a function of the larger size of this treatment cell relative to the watershed and the lack of extended ponding above the filter bed. It should also be noted that data were provided in this report as concentrations rather mass removals. Most of the metals that were removed had consistent treated concentrations that did not vary notably between events. Hence, mass removed was simply a function of storm event rainfall depth. Mass removal estimates at sites with ferric oxide-sand filters can be estimated using the treated concentration data in this study and local hydrology. Data were collected and methodologies provided in this report to help the practitioner design a treatment cell that will be effective but also fit within the confines of the highway right of way. In many situations, there is not enough or no space to construct a ferric oxide-sand filter. The sand in these treatment cells
71 provides a medium that restricts flow and consequently allows time for the ferric oxide metals complex to form but it also helps to retain ferric oxide and newly formed iron-metal complexes. Attempting to build a smaller cell with ferric oxide alone may be challenging as iron can polymerize depending upon the iron media chosen and it is unknown if the contact time will be sufficient. The laboratory study with OGFC and ferric oxide shows some promise as metals were effectively removed by OGFC with ferric oxide imbedded in the pavement pore space. This approach may have application to bridge decks where there is limited opportunity for treatment, however, there is no literature available regarding the use of ferric oxide in asphalt pavements and thus the effect of ferric oxide on the mechanical performance in terms of rutting, fatigue, and thermal cracking of open-graded mixtures is unknown. This study did not evaluate the longevity of the ferric oxide media and the potential for metals release once the ferric oxide binding sites are filled. It may be warranted to match the longevity of the OGFC pavement and the longevity of the ferric oxide media to minimize the potential for metals release. It should also be recognized that this study was limited to two study locations, a single filtration medium (e.g., sand), climatic conditions that may not be representative of the prevailing climate in many states, and metals, pH, and other constituent concentrations that may be higher or lower than concentrations at other sites. These factors may affect performance, longevity, design and sizing, as well as construction and maintenance costs.
