Leaching from Roadways Stabilized with Additive Coal Combustion Products (CCPs): Data Assessment and Synthesis
Brown, Brigitte Leora
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Approximately 37% of the electrical power used in the United States is generated by coal-fired power plants. Air pollution control systems installed on coal-fired power plants collect solid byproducts of coal combustion, which are commonly referred to as coal combustion products (CCPs). Common CCPs include fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) residuals. Disposing CCPs in landfills or similar waste containment facilities is costly and land intensive, and many CCPs have useful engineering properties. Consequently, CCPs are often used beneficially in other products or applications, most notably as construction materials. Beneficial use of CCPs has many positive benefits in the context of sustainability including an annual reduction in greenhouse gas emissions by 11 million tons, fossil fuel consumption by 17 TJ, and water consumption by 121 GL, amounting to more than $11 billion (US) in total economic benefits. Field water quality data collected from roadways where fly ash or bottom ash was used as embankment fill or as a stabilizing agent in the base or subgrade was assessed for any potential risk of ground water and surface water trace element contamination. Trace element concentrations (e.g. As, Cd, etc.) were obtained for seven roadways in Minnesota, Wisconsin, Indiana, and Georgia, spanning the applications of fly ash base and subgrade stabilization, fly ash fill, and bottom ash fill. Direct assessments for field site water quality were conducted to provide a conservative evaluation of the risk of contaminating surface water and ground water with trace elements leached from roadway substructure applications using fly or bottom ash. Field site water quality data was directly compared to federal and state drinking and surface water quality limits. Elements evaluated were categorized as imposing ?no risk? when trace element concentration profiles were below water quality limits. When elemental concentrations from a roadway sections constructed with fly ash or bottom ash were not statistically different from concentrations eluted from control sections (constructed without fly ash or bottom ash) the element was categorized as imposing ?no additional risk? relative to that imposed by a roadway constructed using conventional materials. Water quality at the Indiana and Georgia sites was characterized as no risk or no additional risk imposed by leached trace elements from fly ash fill in embankments as compared to embankments constructed with conventional fill materials. Only up to 6 of 17 elements required further evaluation at the Minnesota and Wisconsin field sites. Elements requiring further evaluation were assessed indirectly to conservatively assess predicted field concentrations at a point of compliance (i.e., edge of right-of-way) 20 m away from the centerline of the roadway (CPOC@20). Predicted trace elemental concentration below water quality limits were categorized as imposing ?no predicted risk.? Field concentration data and Insert Appropriate Auto Text License Entry. If license is copyright, please delete other field parameters were inputs for WiscLEACH hydrologic and transport modeling software. Conservative assumptions built in and applied to WiscLEACH included taking the maximum observed field concentration as the input concentration and assuming no reactions (chemical or biological) or sorption takes place. The assumptions are conservative because they create a scenario that results in higher values of CPOC@20 than are likely to occur. Assumptions are reasonable because simulation with less conservative inputs resulted in CPOC@20 lower by one order of magnitude, which is significantly low on the scale of observed CPOC (1 to 0.1 ?g/L). Water quality at the Minnesota and Wisconsin sites requiring further investigation were characterized as no predicted risk, resulting in the characterization of Minnesota and Wisconsin sites as no risk, no additional risk, and no predicted risk imposed by trace elements leached from fly ash stabilization or bottom ash fill roadway substructures, as compared to roadways constructed with conventional construction materials. The reduction factor for each element at each site was calculated by dividing the initial concentration by the predicted concentration at the point of compliance to provide site specific context for quick estimation of potential concentrations to be realized at a point of compliance given the element concentration at the base of the byproducts layer. The reduction factor can also be used to estimate the allowable initial concentration given a maximum allowable concentration at a point of compliance. (e.g. water quality limits). The minimum RF of all roadways was 46, meaning the initial elemental concentrations expected at the base of the byproducts layer at levels equal to or below 46 times the water quality limit are predicted to impose no additional risk where the groundwater table was at least 1 m below the ground surface. The reduction factor can be applied to similar roadways, however roadways with different field conditions than evaluated in this study (especially those with a thicker stabilized layer or groundwater that is closer to the base of the pavement structure) should be evaluated to ensure eluted trace element concentrations meet the water quality standards at a point of compliance. Overall, the use of fly ash stabilization, fly ash fill, and bottom ash fill in roadway applications described in this study do not impact ground water and surface water quality making these beneficial reuse applications suitable with respect to trace element leaching based on obtained water quality data. Water quality data used in this study spans seven locations in four states, three substructure applications (fly ash stabilization, fly ash fill, and bottom ash fill) and 19 monitoring points and many years of monitoring (some more than a decade). Thus, it represents a substantial database to make inferences. Conclusions and RF can be applied to similar roadways, however roadways with different field conditions than evaluated in this study (especially those with a thicker stabilized layer or groundwater that is closer to the base of the pavement structure) should be evaluated using the analytical procedure provided herein to ensure eluted trace element concentrations meet the water quality standards at a point of compliance. These applications are indicated to be low risk and should not be prohibited by future regulations regarding the beneficial reuse of CCPs.
Coal combustion products, CCP, fly ash, bottom ash, additive roadway applications, roadway substructures, subgrade, subbase, base course, embankment fill, trace element leaching