From “Leaching Characteristics of Asphalt Road Waste“ – a study from the University of Florida
“The results of this research project indicated that reclaimed asphalt pavement of the nature examined in this study poses minimal risk to groundwater as a result of pollutant leaching under normal land disposal scenarios. The results of the leaching tests indicated that in most cases RAP would pose minimal environmental risk when used as fill in regard to the leaching of the pollutants.”
Excerpts from the Description of RAP
Asphalt is a natural component of nearly all types of petroleum found in nature. By refining crude petroleum, various components can be separated and recovered, including asphalt. Asphalt has physical and chemical properties that make it of great interest to many engineers. It is a strong cement, extremely durable, readily adhesive, highly waterproof, and considered to be chemically inert. Asphalt is highly resistant to the reaction of many acids, alkalis, and salts. Asphalt is a viscoelastic substance that imparts both controllable elasticity and viscous flow to mixtures of aggregates with which it is commonly combined. Asphalt for pavement construction is called asphalt cement.
Asphalt cement is often added to aggregate to make asphalt concrete for construction of asphalt pavements for highways and parking lots. When these asphalt pavements are removed from a road surface, the by-product is commonly called reclaimed asphalt pavement (RAP). RAP consists of ground up asphalt pavement. For this study, RAP is defined as all asphalt road waste, large chunks or milled material that has been size-reduced, making it reusable as part of a new asphalt mixture.
…Other uses for the unused RAP have included the following: roadbase material for stabilizing dirt roads, pothole/rut filler, and roadway shoulder material. A proposed alternative for the management of RAP is use as fill material, whether by disposal in asphalt-only monofills or in beneficial reuse applications as clean fill.
Excerpt from the End of the Summary
…In all of the batch tests, measurements of VOCs (volatile organic compound), PAHs (polycyclic aromatic hydrocarbons), and selected heavy metals (Ba, Ca, Cr, Cu, Pb, Ni, and Zn) all were below detection limit (BDL) and were below applicable regulatory groundwater guidance concentrations. This indicates that the RAP samples tested posed minimal risk (in regard to leaching of the pollutants tested) under current waste policy in Florida. No comparisons can be made regarding the effectiveness of each test to leach pollutants because no pollutants were measured.
Column (lysimeter) tests were then performed on the same six RAP samples. Column tests are not a prescribed test procedure by regulatory agencies. They were conducted in this case to investigate leachate production from RAP under more realistic environmental scenarios. Approximately 60 lbs of RAP material filled a three foot section of each column. The RAP in these columns were subjected to either a saturated or unsaturated condition. Samples were analyzed for the same parameters as in the batch tests, but leaching was continued for a total of 42 days. The columns did leach large concentrations of ions such as calcium, sodium, sulfate, fluoride, and carbonate as a result of mineral leaching from the aggregate used in the asphalt pavement manufacture. Some organic material was leached. However, all priority pollutant VOC and PAH analyses had results similar to the batch test — none were detected. Leachate concentrations for selected heavy metals (Ba, Ca, Cr, Cu, Ni, and Zn) were below detection limit (BDL) except lead (Pb). Only one unsaturated lysimeter resulted in a leachate with a lead concentration above the groundwater guidance concentration (15 ppb). Leachate from the unsaturated lysimeter containing the Jacksonville sample had a lead concentration of approximately 28 ppb eight days into the sampling period. The concentration of lead decreased over time and then leveled off below the regulatory guidance concentration. In the saturated columns, all columns had detectable concentrations of lead ranging from 5 ppb to 38 ppb. Three out of the six lysimeters (two sites) were above the regulatory guideline. The lysimeters decreased over time to lead concentrations below the guidance concentration (and the detection limit) except the Jacksonville sample. The Jacksonville sample concentration decreased slightly but was still above the guidance concentration at approximately 18 ppb.
As discussed previously, the batch tests were more dilute than the column test. This condition helps to explain why lead concentrations were observed in the column study but not in the batch test. In a real world situation, rainfall and other surface water runoff would ultimately dilute leachate produced from a stockpile before it reaches the groundwater table or a nearby receiving body of water. This phenomenon is commonly referred to as dilution attenuation. Dilution-attenuation models used to determine the concentration of groundwater underneath waste piles and landfills commonly apply a dilution factor of 20 (EPA, 1996). In batch tests, dilution is usually assumed to be accounted for because of the high liquid-to-solid ratio. In the column study there was no dilution-attenuation factor involved. If the concentrations produced from the column studies were placed into a groundwater dilution model, results would likely be well below any regulatory guideline. Under most reuse circumstances, even if lead was encountered at levels of the highest amount measured in this study, the concentrations in the environment would be below acceptable regulatory levels for drinking water. Possible conditions of risk would be from RAP used as fill below the groundwater table in areas with little dilution.
Lead was observed in the greatest concentrations in the oldest RAP samples. This indicated that the lead was not a result of the aggregate or asphalt cement, but rather a result of vehicle traffic and emissions. Lead has been used for many years in leaded gasoline and in crankcase oil. Since vehicle accidents and accidental spills contribute to this contamination, there is a possibility that this contamination was site specific. Previous studies regarding asphalt road waste also found trace amounts of lead in some circumstances (Kriech, 1991). Lead was encountered in greater concentrations in older samples, indicating that the source of lead was prolonged exposure to vehicle traffic and emissions.
Most of the previous studies reviewed in Chapter 2 regarding RAP leaching were consistent with the results found in this study. Organic compounds do not leach from typical RAP under the conditions tested. Heavy metals are sometimes encountered. The literature indicated the presence of chromium, lead, and barium. Only lead was detected in this study and was attributed to prolonged exposure to traffic and vehicle emissions. The literature often referred to chromium resulting from slag used as aggregate. It should be noted that the aggregate used in the asphalt samples collected for this study was assumed to be natural aggregate (e.g. limerock). If other materials — especially waste materials such as slag, spent sandblast grit and ash — are used as aggregate, the results gathered here may not be applicable. It should also be noted that fresh asphalt was not tested, nor were extremes in temperatures evaluated.
The results of this research project indicated that reclaimed asphalt pavement of the nature examined in this study poses minimal risk to groundwater as a result of pollutant leaching under normal land disposal scenarios. The results of the leaching tests indicated that in most cases RAP would pose minimal environmental risk when used as fill in regard to the leaching of the pollutants. This study did not, however, address that implication of direct exposure.