A single injection has been used to effectively contain and treat petroleum hydrocarbon and chlorinated solvent plumes using a specially formulated activated carbon (AC) based technology. This result has been documented at hundreds of sites. It’s a welcome result over some in-situ chemical oxidation methods (which can still be very applicable!) where multiple injections are needed to attain the design contaminant mass removal, and to overcome contaminant back diffusion (a.k.a. “rebound”) from soils.
The key to successful remediation using activated carbon, specifically Trap and Treat® applications, is in both the short-term containment of the contaminants, and the subsequent long term treatment of the contaminants once they are adsorbed onto the AC.
AC, long trusted for purification of water, air, and waste streams, is now confidently and effectively deployed for cleaning up contaminated soil and groundwater in-situ.
When treating a waste stream above ground, contaminated media flows into the AC matrix and a “clean” effluent emerges. But in the subsurface, the remedial design is not based on groundwater contaminant concentrations alone.
Delivery of AC into the water table should immediately decrease dissolved phase contaminant mass. This disrupts the established soil/water mass equilibrium, and in turn drives desorption of contaminants from the soil matrix into the aqueous phase. The key to long-term remediation is to account for both groundwater and soil contaminant mass.
Application of AC can very effectively keep dissolved concentrations low, while continuously treating the contamination (in the carbon matrix).
Five Subsurface Treatment Design Considerations
Dosing is a key consideration. And dosing is more complex than it might first appear. Contaminant adsorption rates by AC are dependent upon:
- Type of Contaminant: Specifically contaminant molecule size and complexity. Generally, larger and more complex molecules (i.e. petroleum hydrocarbons) will adsorb to the activated carbon at a much higher mass of contaminant per kg of AC than simpler molecules (i.e. vinyl chloride).
- Contaminant Concentrations: The activated carbon will adsorb more contaminant mass the higher the contaminant concentration. For example, look at the statistics for benzene in the following table:
Carbon Adsorption Capacity
10 40 1 1 0.1 0.03 0.01 0.0007
- It may be possible to trap LNAPL levels of PHCs, due to the higher carbon adsorption capacities at higher concentrations.
- Total Contaminant Mass: Consideration of the total contaminant loading is critical. As contaminants are adsorbed out of groundwater, expect an equilibrium shift. New contaminant molecules will desorb from the soil, re-contaminating the groundwater. Sufficient carbon loading is required to account for this phenomenon.
- Soil Type: Contaminant mass back-diffusion rates and targeted AC injection volumes will be dependent on soil types. Dosing of AC must anticipate the response due to lithology at each depth and area of the site. The great news is that AC is known to address the long term contaminant back diffusion from the aquifer matrix.
- Injection Strategy: Delivery should ensure adequate dispersal and distribution to provide lateral and vertical coverage across the impacted zone. However, dosing should not necessarily be uniform. Rather, dosing should match the distribution of contaminants in both the horizontal and vertical planes. Higher pockets of contamination in tight soil pockets should be met with higher doses of AC, delivered directly to those intervals. A strong conceptual site model will inform the remedial design.
Understanding and accounting for these dosing factors is truly the edge that leads to a highly successful plume remediation.
Want to learn more about how to treat a groundwater plume using a single injection event? Get in touch with Vertex.