Our recommendation will be based on the site-specific conditions, remedial goals and our extensive field experience. Typically, basic site information (e.g., concentrations of the contaminants, soil type, depth of contamination, etc.) will narrow down the reagents that can be utilized and the appropriate application method. Once the reagent and application method have been determined, our qualified staff will begin to develop a site-specific design and associated cost that is based on many different factors.
Geo-Cleanse uses permanent injection wells for the following reasons:
- Permanent injection wells provide a dense network of process monitoring locations. Process monitoring will allow for real-time treatment program modifications and ensure that appropriate reagent concentrations and conditions are established. The process monitoring data collected are critical for daily optimization of the injection program. Injection through temporary injection points does not allow for process monitoring and real-time program modifications, which are essential when applying geochemical-sensitive reagents.
- Enables targeted oxidant injection to only those locations where remediation is necessary. This also ensures that the remediation is complete by monitoring and collecting field data to document that the program goals have been met prior to post remedial sampling and analysis.
- Installation is relatively inexpensive. Injection wells are typically installed using direct push and are constructed of CPVC.
- Geo-Cleanse can establish a larger radius of influence using permanent injection wells in conjunction with our patented equipment. Temporary injection locations, due to the nature of that type of delivery, can generally only distribute reagents a few feet from each location.
NOD assumptions do not strictly apply to catalyzed hydrogen peroxide. The reason is that the reaction is catalyzed, and the catalyst exerts a far greater affect on degradation rate than NOD. The precise pH, iron concentration, and presence of stabilizers control the degradation rate. This is very different than for permanganate, which is uncatalyzed and degradation is predominantly controlled by NOD; and is different than for activated persulfate, which although is a catalyzed system is a very different approach that is less sensitive.
Surfacing of reagents should not occur. Typically surfacing is due to injection at shallow depths (e.g., less than 4 ft below grade), injection of high reagent volumes or off-gases not being mitigated by a venting system. Geo-Cleanse will decline a site if contamination is less than 4 ft below grade or if appropriate, will recommend a soil mixing approach. Injection of high reagent volumes is typically not necessary; our injection volumes are between 5 and 15% of the total pore volume. Geo-Cleanse installs vent wells when catalyzed hydrogen peroxide is applied. The vent wells ensure that off-gases do not build within the subsurface and nearby structures. The vent wells provide a release point for the off-gasses (CO2 and O2) generated during CHP and will also serve as additional monitoring points to confirm reagent distribution and evaluate the geochemical conditions.
Permits range depending upon the state, country and remediation approach. We will assist our client with submitting the necessary paperwork and obtaining the required permits.
Each in-situ chemical remediation treatment program is different, but typically reagent costs, field crew and equipment will be the majority of our overall costs.
The Geo-Cleanse® Process is conducted under mildly acidic groundwater pH conditions (pH 4-6). This pH range is optimal for several reasons:
- Iron solubility is maintained and the soluble Fe+2 valence state is stable, which is important for reaction initiation. The mildly acidic pH allows us to rely primarily on naturally-occurring iron in the formation, rather than adding large quantities of expensive chelated iron. Under circumneutral pH conditions, Fe+2 is rapidly oxidized to Fe+3; Fe+3 can catalyze hydroxyl radical production but it is much less efficient than with Fe+2, which increases oxidant demands. It is also important to note that chelated iron is most commonly in the Fe+3 valence state.
- Dissolved bicarbonate species in groundwater are reduced or eliminated at pH less than about 6. This is important because dissolved bicarbonate is a relatively efficient hydroxyl radical scavenger present at concentrations much greater than the target contaminants, and thus under certain conditions hydroxyl radicals are much more likely to react with bicarbonate than the contaminants.
Our process is optimized for mildly acidic pH conditions for the reasons outlined above. In most cases, if the alkalinity is not exceptionally high, the mildly acidic conditions we seek to establish can be achieved and maintained in the field. However, this should be tested in the laboratory, and we acknowledge that the appropriate pH range might not be achievable.
We do not need to use chelators to maintain iron in solution; the mildly acidic conditions that we establish keeps iron in the soluble Fe+2 state. Chelators react very readily with hydroxyl radicals, and are typically used at concentrations far higher than the contaminants being targeted, thus hydroxyl radicals are normally hundreds or thousands of times more likely to react with the chelators than with the contaminants. This dramatically increases peroxide requirements, in addition to the lower efficiency caused by the Fe+3 valence state of chelated iron and the bicarbonate interference, which occurs at pH greater than about 6. Typical iron chelates (e.g., Fe-EDTA) cost on the order of $4-5 per pound; in the absence of mild acidity to improve iron solubility, the chelate is destroyed by reaction of the hydroxyl radical then the iron drops out of solution, thereby requiring constant addition of chelated iron throughout the treatment.
The amount of sulfate present following an ASP injection will be site-specific; however, 80 lbs of sulfate will be introduced to the subsurface for every 100 lbs of persulfate. This may be a concern if anaerobic bioremediation or attenuation is desired as a polishing step to reach final cleanup goals. Residual sulfate following ASP injection is typically 1,000 – 1,500 mg/L, and a sulfate concentration less than about 50 – 75 mg/L is typically utilized as the target for effective anaerobic bioremediation of chlorinated solvents. This condition will also create a high demand for additional electron donor.
Even though ASP and CHP both accept one mole of electrons and produce one mole of radicals, they differ significantly in molecular formula weights. Hydrogen peroxide’s and sodium persulfate’s formula weights are 34 g/mol and 238 g/mol, respectively. When calculating the oxidation of one mole of a contaminant, the difference in reagent volume is evident. For example, oxidation of one mole of TCE requires 0.45 lbs of hydrogen peroxide or 3.15 lbs of sodium persulfate. Along with reagent demand, the cost of sodium persulfate is approximately three times more expensive than hydrogen peroxide.
Temperature increase is site-specific and depends on a number of factors. We can limit temperature increase by modifying our injection approach, such as using a lower peroxide concentration or a more stable catalyst solution. In most cases, we typically utilize the exothermic reaction to desorb contamination into solution. Once in the dissolved phase, the contamination is more likely to react with the hydroxyl radical.
We may also utilize the exothermic reaction at sites that have viscous NAPL present (e.g., MGP or creosote sites). We can reduce the viscosity of the NAPL and passively (i.e., NAPL mounding up injection/monitoring wells) or actively (i.e., vacuum trucks or pumps) collect the NAPL during or following the injection event.
Our treatment programs include daily process monitoring, which consists of the collection and analysis of groundwater and off-gas samples collected within and adjacent to the treatment area. Please visit the Monitoring & Reporting page for more information regarding our process monitoring and how it is used to make real-time field changes to optimize our treatment programs.
- Due to our slightly acidic catalyzed hydrogen peroxide approach, dissolved metal concentrations typically exhibit a temporary increase during the injection program. Prior to field injection, soil buffering capacity tests are conducted on site soil to determine how much acid is required to lower the pH to a 4. The soil buffering results, along with other tests and factors, are used to develop our site-specific catalyst solution. Once in the field, pH is continually monitored to ensure the pH is between approximately 4 and 6, which prevents long-term pH shifts. Typically, pH returns to post-injection conditions within a few weeks. This process is also implemented during alkaline activated sodium persulfate injection programs.
- The oxidation reduction potential (ORP) is higher after a CHP treatment, and can also result from a temporary increase in turbidity. As the ORP returns to ambient conditions, the metals will return to their ambient concentrations.