Regenesis - Resources - faq

1. Can HRC be used in the vadose zone?
We do not recommend using conventional HRC in the vadose zone. For certain sites with compact, well-defined contaminant mass, we may recommend (after a thorough technical evaluation) use of HRC primer. HRC primer is a low viscosity, free flowing version of HRC. A key difficulty with using conventional HRC (a thick, viscous, honey-like material that slowly releases lactate upon contact with water) in the vadose zone is obtaining sufficient HRC coverage to create anaerobic conditions in the typically aerobic and unsaturated vadose zone. In contrast, HRC primer rapidly releases lactate, is liquid enough to flush the contaminated area, and can quickly create an anaerobic treatment zone. For HRC primer treatment in the vadose zone, closely spaced injection points must be used and additional irrigation may be necessary. For these reasons, use of HRC primer in the vadose may be economically unfeasible. For vadose zone sites with large, highly contaminated plumes in shallow areas that are easily oxygenated, the amount of HRC primer necessary for treatment, and thus the cost, may be prohibitive.

2. How frequently do I need to reapply HRC?
HRC has a release profile of at least 1 year. It is has been documented to last up to 2.5 years on a few sites, but the vast majority fall into at least a 1 year release profile, with ongoing release occurring at many study sites. For more information on the longevity of HRC, please refer to page 5 of Technical Bulletin 2.8.1: Longevity of HRC in the Aquifer on the website.

3. What is the radius of influence of HRC?
The radius of influence of HRC is a function of lithology and groundwater velocity and varies from site-to-site. During installation activities the lithologic conditions will dictate how far HRC will be moved from the injection rod, higher permeability soils will allow for the material to be moved farther than less permeable ones. In addition, injecting larger quantities of material per vertical foot will aide in the physical movement of the HRC during installation.

HRC's radius of influence after injection is driven either by advection (groundwater velocity) or chemical diffusion. The faster groundwater is moving the larger the influence HRC and its derivatives will have. If groundwater is moving slow (less than ten feet per year) the main river will be chemical diffusion. Under slow moving groundwater conditions Regenesis believes that the HRC and its derivatives can move at a rate of approximately one-foot per month.

4. How does one detect the presence of HRC in an aquifer system?
The presence of HRC in an aquifer can be determined using both qualitative and quantitative methods. The parameters used to qualitatively evaluate an aquifer are dissolved oxygen (DO) and oxidation-reduction potential (ORP). When HRC is introduced into an aquifer it facilitates reducing conditions. Under reducing conditions the DO concentration should be at or near zero and the ORP also be zero or less.

The qualitative measurements used for detecting HRC in an aquifer are Total Organic Carbon (TOC) and metabolic acids which includes acetic, butyric, lactic, propionic, and pyruvic acid. The TOC test simply measures the levels of carbon present in a groundwater sample (aquifer). HRC is a polylactate ester which by definition is a high molecular weight carbon source. Therefore the injection of HRC into an aquifer will increase the overall mass and concentration of TOC.

HRC is used to accelerate in situ biodegradation rates of chlorinated hydrocarbons via anaerobic reductive dechlorination processes. Reductive dechlorination is one of the primary attenuation mechanisms by which chlorinated solvent groundwater plumes can be remediated. During reductive dechlorination the lactic acid (CH3CHOHCOOH), a primary component of HRC, present in the compound provides electrons (Hydrogen) to facilitate the process. During this process other acids such as acetic, butyric, propionic, and pyruvic can be created. Hence, testing for metabolic acids can show the presence of HRC and its derivatives in an aquifer.

5. Is it necessary to heat HRC before injection?
HRC is manufactured as a viscous gel with a viscosity of approximately 20,000 centipose and should always be heated prior to injection. The duration of heating is heavily dependent on the ambient conditions at the time of installation. Ideally, HRC should be heated to at least 105 °F before it is injected. Reaching this benchmark during when the ambient temperature is greater than 80 °F is fairly easy but under chilly conditions can be difficult and time consuming. Ultimately heating the HRC will make it much easier to inject into the subsurface.

6. How frequently does HRC need to be applied?
The frequency that HRC needs to be applied varies from site-to-site and is dependent on the site's application strategy and goals. There are two treatment approaches, grid- and barrier-based, used for HRC projects. A grid-based application pattern can be used to treat a specified area while a barrier-based approach can be used to stop plume migration or treat a lengthy (> 500 feet) plume. The grid-based approach is typically used to treat the zones of higher concentrations and is intended to be a single application. Barrier-based applications are often used to cut-off plume migration when a source is still present on the subject site. Therefore the frequency of application will be dictated by the presence of the source. If the source is not removed groundwater will continue to be impacted and in most cases the barriers will need to be recharged every 12-18 months.

7. How long does HRC last in the groundwater environment?
Since HRC is a time-release product, one of the central issues in site design and one of the most frequently asked questions is "How long does it last?" The short answer is that the basic HRC now sold, specifically defined as a formulation of Glycerol Polylactate (GPL), diluted with glycerol to a viscosity of 20,000 centipoise (cP), is estimated to stimulate reductive dechlorination within the aquifer for about 12 months. The longevity is a function of basic product chemistry and certain biological and geochemical features of the aquifer. HRC has been shown to have a direct effect on contaminants resulting from the lactic acid release and other secondary effects, related to the formation of other organic acids that "borrow hydrogen" and recycled biomass, that have a more prolonged effect on aquifer conditions (up to 18 months).