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.

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.

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?

The additional demand factors in the Summary of Estimated ORC Requirements table on the ORC spreadsheet are safety factors that account for uncertainty about potential sinks for oxygen. These factors are used to increase the ORC dose by the specified multiplier and provide contingency to account for the many uncertainties inherent in subsurface investigations and in-situ remediation projects. Potential sources of unexpected ORC demand include higher than expected contaminant mass (in the form of residual phase NAPL present as hot spots), natural organic matter, or reduced inorganic species, such as ferrous iron, manganese, or sulfides.

Additional oxygen demand factors are given for four categories:

1. Individual species like BTEX components, MTBE, reduced inorganics, etc.
2. Total Petroleum Hydrocarbons (TPH)
3. Biological Oxygen Demand (BOD)
4. Chemical Oxygen Demand (COD)

The user chooses the category on which to base the ORC design cost using the radio buttons in the table. The demand factors for each category are based on the degree to which each category provides a reasonable estimate of oxygen demand. For example, individual species like BTEX, MTBE, reduced inorganics, etc. (Category 1) are the most specific, least conservative measurement of oxygen demand, and COD (Category 4) is the most general, most conservative measurement of oxygen demand. The default additional demand factors for the each category are given in the following table. Although the default values summarized here have led to the design of many successful ORC applications, they can be modified based on the user's knowledge of the site characteristics and contaminant measurements.

Category	Additional Demand Factor	Comments
Individual species like BTEX components, MTBE, reduced inorganics, etc.	5	Measurements of individual species or sets of species will often underestimate the oxygen demand to a significant degree, so this category is given a high additional demand factor.
Total Petroleum Hydrocarbons (TPH)	2	TPH is given an additional demand factor of 2, because TPH is a more thorough measurement that will account for the background hydrocarbon contamination not considered in the individual species measurements. However, TPH does not include chemical reactions that consume oxygen (such as the oxidation of ferrous iron), so a safety factor must be used.
Biological Oxygen Demand (BOD)	2	BOD accounts for oxygen demand that is available to microorganisms and does not account for chemical reactions that consume oxygen. Thus, BOD is assigned a demand factor of 2.
Chemical Oxygen Demand (COD)	1	COD accounts for chemical and biological oxygen demand and is assigned a demand factor of 1 because it typically over represents the oxygen demand placed on ORC.

The inclusion of a safety factor/additional demand factor is a typical engineering design technique used to provide a conservative estimate for the amount of HRC necessary for subsurface systems that have a high degree of variability and are minimally sampled for chemical/hydrological parameters. The safety factor/additional demand factor provides contingency to account for the many uncertainties inherent in subsurface investigations and in situ remediation projects. Potential sources of unexpected or contingent HRC demand include higher than expected contaminant mass (in the form of residual phase NAPL present of hot spots) and uncertainty about the quantity of HRC required for the reduction of manganese, iron, or sulfate.

The microbial demand factor accounts for non-targeted microbial demand for lactic acid and H₂ for processes that are not directly involved with either reduction of the chemical species included in the software (iron, sulfate, etc.) or in reductive dechlorination of VOCs. It includes microbial inefficiencies associated with lactate and/or H₂ metabolism and lactate use to grow biomass.

Yes, if the following conditions are met: 1) the wells are constructed properly; 2) the aquifer is capable of accepting the planned volumes of low % solids ORC slurry (range of 10-20%); 3) if the wells are flushed with sufficient clear water chaser (we suggest a minimum of 1-2 well volumes or until the water present is clear); and 4) post-injection clearing of the well screen using a steel wire brush. NOTE: it is critical that all ORC be removed and flushed from the well screen post-ORC injection. Failure to follow these guidelines may result in a reduction in well pack permeability and reduced percentage of well screen openings. For more information on the use of ORC in monitoring wells, please refer to Technical Bulletin 2.3.4: Use in Existing Monitoring Wells.

Yes, if a well-defined vadose zone contaminant mass is present and there is sufficient soil moisture to activate the release of oxygen from ORC. Additionally, there must be enough moisture to support biological activity. Typically, bacteria are located in thin films of water and contaminant that coat soil, and these thin films must have enough area to provide good contact between bacteria, oxygen, and contaminant.

ORC application in the vadose zone consists of a series of closely spaced injection points to which a dilute slurry of ORC and water is delivered. The dilute ORC/water mixture is used to mound and spread the ORC and provide moisture to the vadose zone.

To evaluate if ORC is exhausted, contaminant concentrations and dissolved oxygen (DO) should be measured. Use of dissolve oxygen (DO) alone as an indication of ORC exhaustion may mislead you. The measure of DO is a function of the DO in excess of what is consumed by the aquifer (i.e. the aquifer's capacity to process DO). When DO consumption in the aquifer is high due to biological activity DO measurements will be low. Thus, contaminant levels should also be used as an indicator of ORC longevity.

If ORC application was within the previous 6-9 months, TPH/BTEX remains low, and little or no DO is present, ORC is most likely releasing oxygen, and it is being consumed in contaminant biodegradation. However, if TPH/BTEX has increased to the original, baseline levels or has reach a plateau that lasts for several months, and it is beyond the 6-9 month release period, ORC has probably been exhausted and new contaminant influx or desorption has occurred. As with all contaminants a careful review of groundwater elevation changes should be factored into any of the above issues. A significant change in groundwater elevation typically affects the concentration of contaminants in groundwater.

Yes, but the successful application of nutrients to subsurface systems is a complex issue that usually requires significant knowledge of the site and possibly even microcosm experiments in the laboratory. When properly applied, nutrients may increase rates, but the cost and additional work may not be worth the benefits. Injection of nutrients without site knowledge and analysis can give negative results such as nutritional imbalances, microbial fouling of the aquifer, and deterioration of local water sources. Our extensive experience with the application of ORC shows that has probably never failed as a direct result of insufficient nutrients. Typically, oxygen is the limiting factor in contaminant biodegradation, not nutrients like nitrogen or phosphorus. Thus, a lack of nutrients does not prevent biodegradation when ORC is used. For more information on the use of nutrients, we suggest a review of Cookson's discussion on nutrient addition. This discussion can be found in Appendix B of Bioremediation Engineering: Design and Application by John T. Cookson. However, note that the material on nutrients in this reference was intended for biopiles or composting and might not be sophisticated enough for use in the subsurface.

Over the past 7 years Regenesis has reviewed, in detail, hundreds of ORC projects (out of 7500 total sites with ORC application ) and the typical longevity for ORC is 6-9 months. Shorter (3-6 months) or longer (9-12 months) time frames occur for under specific conditions, but the vast majority of sites fall into a range of 6-9 months. ORC will release oxygen faster at sites with high contaminant or organic loading and rapid groundwater velocity (> 0.5 ft/day) than at sites without these characteristics. For more information on the longevity of ORC, please refer to Technical Bulletin 1.1.1: Laboratory Studies.

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.

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.

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.

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.

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.

RegenOx can treat a wide range of contaminants from chlorinated solvents to petroleum hydrocarbons at various types of sites such as dry cleaners, MGPs, service stations and industrial sites. Some of the contaminants treated by RegenOx include:

• BTEX
• MTBE
• TBA
• PCE, TCE, DCE, VC
• #2 Heating Oil
• Naphthalene

RegenOx is not recommended for treatment of PCBs (polychlorinated biphenyls), PCP (pentachlorophenol ) and pesticides. The recalcitrant nature of these compounds has shown resistance to chemical oxidation technologies.