Remediation technologies such as Soil Vapor Extraction (SVE) and Electrical Resistance Heating (ERH) are effective remedial approaches when properly designed and applied to suitable sites. But like a doctor’s prescription, they can treat the condition while causing unintended side effects. Under some conditions, remedial systems may have the unintended consequence of actually enhancing concentrations of non-target contaminants. Enhancement can arise when treatment unintentionally concentrates contaminates— the energy added via the remedy accumulates low level concentrations until they creep up into toxic and/or regulatory thresholds. In other cases, the remedy changes the subsurface chemistry and begins to mobilize previously immobile constituents. This is referred to as Technologically Enhanced Contamination.
Soil Vapor Extraction + ERH = PCBs?
One example is where this can occur is when ERH and SVE are paired together. ERH superheats the subsurface volatilizing contaminants, while the SVE extracts and treats the vapors. Similar to a doctor prescribing two medications, the two systems can have an additive effect. However, in this case the “side effects” of the systems can increase the concentrations of the non-target contaminants.
The increased subsurface temperature from ERH allows normally less mobile compounds—like polychlorinated biphenyls (PCBs)—to move more freely in the subsurface. Similar to heating oil in a hot pan—the PCBs can now migrate in the subsurface more easily. The action of the SVE system can then drive these mobile PCBs to the extraction points where they concentrate to high levels. After a period of operation, low level PCB concentrations in the subsurface and within the SVE vessels can be concentrated into levels dangerous to humans and/or trigger regulatory thresholds. It is important to always be aware of any potential side effects and safety concerns.
ERH and Metal Concentrations
Through ERH, the subsurface is superheated allows for the rapid breakdown or mobilization of target compounds—including chlorinated solvents. However, ERH can also indirectly result in a large uptick in metals concentrations in groundwater. Because the extreme heat changes the groundwater geochemistry.
The chemistry is fairly straightforward. When chlorinated compounds degrade, they lose chloride ions. This increases their concentration in groundwater where they increase corrosivity. Naturally occurring metals in the subsurface exposed to this corrosive groundwater are then readily dissolved. This can be as innocuous as a high salt (NaCl) or MgCl concentrations, but can also lead to increased iron, lead and arsenic concentrations in groundwater as conditions allow for them to be more readily dissolved.
How can this affect your Project? Remedial technologies are powerful tools that can clean up contaminated Sites. However, it is important to remember that changing the site conditions can have side effects. At Fleming-Lee Shue, Inc. we know one size doesn’t fit all, and to expect—the unexpected.