Asphalt millings, also known as Reclaimed Asphalt Pavement (RAP), are a widely used construction material created by grinding up old asphalt surfaces. RAP is popular for driveways, road bases, and pathways due to its cost-effectiveness and sustainability as a recycled product. Utilizing RAP conserves natural resources and reduces construction waste sent to landfills. However, because RAP is derived from decades of road use, questions about its environmental impact are valid. The material’s convenience must be balanced against the potential for chemicals to affect the surrounding ecosystem and water sources.
The Chemical Makeup of Reclaimed Asphalt Pavement
Reclaimed asphalt pavement is primarily composed of stone and sand aggregates bound together by an aged asphalt cement binder. The environmental concern arises from the presence of trace compounds trapped within this binder and the aggregate matrix. The two main classes of contaminants are Polycyclic Aromatic Hydrocarbons (PAHs) and various heavy metals.
PAHs are a group of chemicals that form naturally during the incomplete burning of organic substances, such as oil, coal, and garbage. These compounds are present in the original asphalt binder, which is derived from petroleum, and can also accumulate over time from vehicle exhaust and oil spills on the roadway. Certain PAHs, like benzo[a]pyrene, are classified as known or probable human carcinogens, making their potential release a significant health and environmental consideration.
Heavy metals, including lead, zinc, copper, and cadmium, are detected in RAP samples. Lead is often found in older millings, tracing back to leaded gasoline and older pavement sealants. Zinc and copper may be present from vehicle tire wear and other traffic-related sources. The concentration of these metals varies widely depending on the age and traffic history of the original pavement.
Risks of Contaminant Leaching and Runoff
The compounds within asphalt millings become an environmental risk when water moves through the material via two primary mechanisms: leaching and surface runoff. Leaching occurs when rainwater or groundwater percolates vertically through the porous, unbound millings, dissolving chemical contaminants into the soil. This process can potentially transport PAHs and heavy metals downward into the groundwater, which may serve as a source for drinking water.
Studies indicate that the overall leachability of these contaminants is low because the asphalt binder effectively encapsulates the chemicals. However, contaminants can exceed regulatory limits under specific conditions, with lead and arsenic occasionally observed at concerning concentrations in leachate. This risk increases when millings are placed in highly acidic environments (pH 4 or lower), as low pH increases the solubility and mobilization of certain metals.
Surface runoff presents a risk of horizontal contamination, especially from uncompacted surfaces. During heavy rain events, water washes fine, contaminant-laden particles and surface-level PAHs into storm drains, streams, and other surface water bodies. This migration poses a direct threat to aquatic life and contributes to sediment contamination in adjacent ecosystems. The loose nature of unsealed millings allows for greater water infiltration and particle movement, increasing the potential for both leaching and runoff.
Guidelines for Environmentally Responsible Use
Because the environmental risk is tied to the use of unbound millings and poor site conditions, specific precautions can minimize the potential for harm. Proper compaction is a primary mitigation step, as it significantly reduces void spaces within the millings and decreases permeability. A densely compacted surface limits the volume of water that can infiltrate and dissolve contaminants, thereby reducing the rate of leaching.
Location is a major factor; millings should never be placed where they can directly drain into sensitive areas like wetlands, streams, or potable water wells. Stormwater runoff controls, such as installing silt fences or diversion berms, are best management practices to prevent the transport of fine particles into surface waters. Millings should also not be used in proximity to vegetable gardens or areas where children may have frequent direct soil contact.
The most effective method to mitigate environmental risk is to seal the millings, typically using a final layer of new, hot-mix asphalt or a chemical sealant. Sealing prevents both water infiltration and the physical erosion of fine particles, effectively encapsulating the contaminants. The use of RAP in highly acidic environments, such as industrial sites or mine reclamation projects, should be avoided unless the material is fully contained and leachate is actively monitored.