Regenesis - Resources - faq

1. How long will it take for ORC or HRC to clean up my site?
We estimate that, in general, ORC and HRC increase biodegradation rates by 10x to 20x. However, the duration of an accelerated natural attenuation cleanup is very difficult to predict with accuracy due to uncertainty about the total mass (especially sorbed and/or residual phase) and site-specific biodegradation rates that can be achieved. Accelerated natural attenuation of contaminants via ORC or HRC is a less costly, less intrusive, and more organic remedial alternative that may take longer compared to an expensive, highly-engineered, physical or chemical treatment method with a well defined schedule. In contrast, ORC or HRC application can decrease the time to site closure when used to replace a poorly designed pump-and-treat system or other physical or chemical treatment method.

2. Will the contaminants rebound?
This is a function of two separate issues. There may be a spike in contaminant concentrations shortly (typically 1-2 months) after ORC or HRC injection. This is a result of physical disruption from ORC/HRC injection and accelerated desorption by biological surfactants associated with general growth of aquifer biomass. This rebound should decrease over time. Upon exhaustion of the ORC or HRC, a different type of rebound may occur. An increase in contaminant concentrations can occur if contaminants re-enter the previously remediated aquifer volume by advection or diffusion from outside the treatment area. For example, transport of contaminants from a source zone or free phase can increase concentrations in the treated area. Rebound can also occur by desorption of the contaminant from the soil grains into the remediated groundwater. Rebound can be mitigated by taking upgradient contaminant concentrations and sorbed phase mass in to account in the original design.

3. ORC vs. HRC for dichloroethene/dichloroethylene (DCE), vinyl chloride (VC), dichloroethane (DCA), or chloroethane (CA)?
Lower order chlorinated hydrocarbons are amenable to anaerobic reductive dechlorination processes; however, the rates of biodegradation for less chlorinated contaminants (e.g. DCE and vinyl chloride) can be slower under anaerobic conditions than under aerobic or semi-aerobic conditions. PCE is never amenable to in situ oxidative (aerobic) biodegradation processes, and TCE may be aerobically co-metabolized under certain conditions, but requires the addition of primary substrates like toluene or methane. In areas where PCE and TCE levels are relatively low, an aerobic enhancement strategy using ORC that targets DCE, VC, DCA, and/or CA may be more appropriate and result in faster biodegradation rates. The decision between using an HRC- or ORC-based approach should be based on:

Knowledge of the existing aquifer redox state and geochemical parameters (redox potential (ORP), dissolved oxygen, nitrate, iron, sulfate, methane, and total organic carbon concentrations)
Ease of shifting redox conditions to improve the rate of biodegradation. Is it easier to make the aquifer aerobic or anaerobic?