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90 7.1 Introduction During the course of this work, a number of items related to the removal of dissolved heavy metals in stormwater were identified that deserve additional research. These include: ⢠Field testing of GFO ⢠Characterization of highway runoff ⢠Modification of PFC 7.2 Field Testing of GFO Conceptual BMPs The highest priority for future work should be to verify the results of this research through a field verification program. There are a number of reasons why the laboratory results may not be directly transferable to the highway environment. First of all, the synthetic stormwater used in the laboratory experi- ments did not include any solids. This was necessary in the lab because sediment would have clogged the small diameter columns used in the experiments, likely before the adsorp- tion capacity of the media was exhausted. The solids present in actual runoff will need to be reduced in a pretreatment step to avoid rapid clogging and blinding of the adsorptive media. Nevertheless, some solids will remain, so the adsorptive media will still be subject to some degree of solids loading, which will have a significant impact on both performance and main- tenance requirements. These items can only be investigated through a prototype field testing program. The laboratory work also demonstrated that the amount of organic matter and alkalinity have a very strong impact on the ability to remove dissolved copper from runoff. A field evalu- ation is critical for determining the precise levels at which adsorption becomes ineffective for dissolved copper removal. Although this work was focused on the removal of dis- solved heavy metals, field testing may identify other water quality benefits as well. GFO is frequently used in aquarium filters to reduce phosphate levels in the water to prevent algal growth and the original material used in these tests was actu- ally acquired from a business that sells aquarium supplies. It is very likely that this reduction in phosphorus will also occur when stormwater is treated. Phosphorus is generally the con- trolling nutrient in fresh water systems for eutrophication, and the tools available for reducing dissolved phosphorus concentrations are limited. Consequently, quantifying nutri- ent reduction in the field setting is also a priority. 7.3 Additional Characterization of Highway Runoff Many highway runoff characterization projects involve only a limited suite of constituents, typically those thought of as constituents of concern. These include solids, nutrients, and selected heavy metals. What this research has demon- strated is that many of the secondary constituents, normally not regarded as pollutants, have a very strong impact on the effectiveness of adsorption for removing dissolved metals. These other constituents include species such as carbonate (a component of alkalinity), NOM, and others. In order to determine whether GFO can be effective and used widely, a better characterization of stormwater from selected locations across the country should be encouraged. This characteriza- tion must include pH, TOC, and a full suite of inorganic ions, such as typically used by the U.S. Geological Survey when sampling natural systems. The results of this monitoring pro- gram can then be input into chemical speciation and adsorp- tion computer models to predict the effectiveness of GFO for dissolved metals removal for various regions of the country. 7.4 Incorporation of GFO in the PFC A PFC, which is also known as open graded friction course (OGFC), is a porous asphalt overlay increasingly used by DOTs as a wearing course that improves safety during wet C H A P T E R 7 Recommendations for Future Work
91 7.5 Incorporation of GFO in Filtration Systems GFO could also be incorporated into BMPs that include a filtration component as a unit process. One example is the MFD developed by WSDOT. The MFD mix is a mixture of crushed rock, dolomite, gypsum, and perlite. The crushed rock provides the support matrix of the medium; the dolo- mite and gypsum add alkalinity and ion exchange capacity to promote the precipitation and exchange of heavy metals; and the perlite improves moisture retention to promote the formation of biomass within the MFD mix. GFO could be incorporated uniformly throughout the mix or installed just above the underdrain, with the media filter providing a level of pretreatment. There are also a variety of proprietary, below-grade systems that include a filtration component. Many times these filters include a special mix of zeolites, perlite, activated carbon, and other constituents. GFO could be included as one of the com- ponents to improve dissolved metals removal. Whether the MFD or modification of a proprietary device is of interest, field testing would be needed to verify the pollutant removal prior to widespread implementation. weather. Recent research in the United States and Europe has demonstrated that pollutant concentrations in runoff are substantially lower than conventional asphalt or concrete surfaces (e.g., Eck et al. 2012). The primary pollutant reduc- tion process appears to be removal of solids and associated pollutants through sedimentation and filtration. PFC appears to be one of the better options for stormwater treatment for highways in that it provides treatment within the pavement itself, rather than requiring additional ROW, construction of structural treatment controls, and their continued mainte- nance. One drawback is the lack of effectiveness for dissolved constituents. A back of the envelope calculation suggests that there is suf- ficient porosity in PFC that GFO could be incorporated into the pavement after it is installed, which potentially increases the removal of both dissolved heavy metals and dissolved phosphorus. The initial evaluation could be carried out at the laboratory scale to determine the effectiveness for pollutant removal as well as the impacts on permeability and porosity of the PFC. Should the results be promising, a follow-up field study could then be conducted to determine the best method for placement on an actual highway and verification of the pollutant removal performance.
