The environmental site assessment and remediation industry is relatively new and dynamic, and like any new field, innovations are rapidly altering processes and techniques utilized to achieve site cleanup and closure. Recently, new methods and innovative technologies, designed to treat contaminants in-situ via injection of slurry mixtures, are gaining recognition in the environmental remediation field. The system uses discrete chemical and/or biologically based injections, as opposed to a permanent infrastructure. Remediation is accomplished with a direct-push drill rig, injectate, mixing tanks and pumps. Cleanup can be accomplished in a matter of weeks or months, as they do not require the same semi-permanent piping infrastructures, equipment compounds, onsite technicians, utility connections and permits.

One of the most critical factors in successfully installing an injectate is overcoming delivery challenges, i.e. getting the treatment reagent to the subsurface impacted area such that adequate contact is made (between injectate and solute/contaminant) to achieve targeted concentration reductions. The science behind these technologies is sound, but successful full-scale implementation beyond the laboratory necessitates a proper three-dimensional distribution of injectate in the subsurface. In order to achieve a successful design, a thorough site conceptual model is required relative to other remediation technologies. Identification of geologic heterogeneities and a quantitative understanding of the contamination profile are crucial. A haphazard approach of injecting a variety of injectates without developing the site conceptual model ultimately will lead to either project failure by under dosing a site, or cost overruns by overdosing a site, hence wasting resources.

Environmental remediation injection technologies fundamentally promote either abiotic chemical reactions, biologically mediated reactions, or a combination of the two. Following are descriptions of several chemical and biological injectates and technologies.

Chemical injectates

Direct push exploration and injection systems can be faster than traditional drilling methods. Also, they can be used in restricted areas such as shown above where the overhead wires would be a problem.

Chemical oxidation: Chemical oxidation is defined as an abiotic reaction where electrons are transferred from electron donors to electron acceptors, and in the process the constituents of concern (COCs) undergo transformation to less harmful end products. Often, a variety of reactions will occur, sometimes with the intermediary products having equal or worse toxicity than the original COC. Generally, end products are CO2 gas and water. Various oxidants can be used to perform the reactions, with most utilizing the formation of a species called a free radical. Free radicals, or simply radicals, are unstable species that are highly reactive, which give this technology the ability to degrade a variety of organic chemicals quickly. Typical oxidants are permanganate, persulfate, hydrogen peroxide/Fenton's reagent and ozone. Many proprietary oxidants are also available.

Reductive dechlorination: As with biotic reductive dechlorination, abiotic reductive dechlorination happens when a chlorine atom is removed from an organic compound. Abiotic reductive dechlorination is often accomplished by the addition of zero-valent (elemental) iron (ZVI). ZVI is typically applied in permeable reactive barriers with funnel and gate systems to control the flow of the groundwater. This technology degrades chlorinated organics through an oxidation/reduction reaction where the ZVI is oxidized to ferrous or ferric iron, as the chlorinated compound is reduced, which removes the chlorine atoms from the COCs.

Carbon slurry bioremediation/abiotic reductive dechlorination: This technology utilizes activated carbon that is impregnated with elemental iron in an injectable slurry that first adsorbs contaminants in-situ, and then promotes abiotic reductive dechlorination for chlorinated organics. It is unique in that it immediately reduces observed COC concentrations, as the activated carbon immobilizes the contamination, while typical abiotic dechlorination is allowed to proceed within the activated carbon complex. During this process, groundwater quality is protected through the cleanup cycle by the activated carbon's adsorptive capabilities.

Biological injectates

Direct push rigs can penetrate many surfaces including concrete, asphalt or even frozen ground.

Reductive dechlorination: Biotic reductive dechlorination takes place when a chlorine atom is biologically removed from an organic compound. This process is utilized to degrade chlorinated organics. Biotic reductive dechlorination/dehalogenation may be accomplished by introducing a substrate (electron donor mixture), such as lactate, molasses, acetate or other non-toxic hydrocarbon. Biotic reductive dechlorination is most often mediated with hydrogen acting as the electron donor. There are many carbon-based substrates that can be naturally degraded and fermented in the subsurface that result in the generation of hydrogen. The substrates most commonly added to facilitate biotic reductive dechlorination include soluble substrates (molasses, lactate, acetate), high-viscosity fluid substrates (vegetable oils), low-viscosity fluid substrates (vegetable oil emulsions), and proprietary injection products.

Biostimulation: Biostimulation does not involve adding organisms, but rather simply implies adding nutrients or electron acceptors to the aquifer to facilitate bioremediation. Most bioremediation technologies can be classified as biostimulation. Additives are usually introduced to the subsurface through injection or by gravity feed into an existing well network. Electron acceptors are often the typical rate-limiting reactant in the bioremediation process. Biostimulation can be classified as either aerobic or anaerobic.

Enhanced aerobic bioremediation: Aerobic bioremediation is a biotic process in which indigenous or supplemental microorganisms (bioaugmentation) residing in soil and groundwater use oxygen as an electron acceptor to biodegrade COCs. Degradation is accomplished when microbes transfer electrons from the COCs (electron donor) to oxygen (electron acceptor) via biological processes, and subsequently the COCs undergo transformation to less harmful end products (generally water and CO2 gas). In order for the microorganisms to proliferate, a sufficient amount of oxygen must be available. In practice, oxygen is typically the principal growth-limiting factor for COC-degrading microbes. Oxygen can be added using in-situ mechanical systems (e.g. air sparging, soil vapor extraction, biosparging, bioventing); by injecting industrial grade oxygen; through hydrogen peroxide infiltration; by injecting a propriety compound; or by injecting ozone. Enhanced aerobic bioremediation may treat most organic chemicals, but is generally a much less aggressive technology than chemical oxidation.

Enhanced anaerobic bioremediation: Anaerobic bioremediation is a biotic process in which microorganisms may utilize non-oxygen electron acceptors to biodegrade COCs. Enhanced anaerobic bioremediation may also be defined as the addition of an electron donor species to promote the reductive dechlorination process. Non-oxygen electron acceptors include sulfate, nitrate, ferric iron, manganese IV, or CO2. A sufficient amount of these electron acceptors and/or substrates are required to sustain the biologically mediated degradation of COCs. Anaerobic bioremediation may be a slower process than aerobic, but it can efficiently degrade some compounds that are not reducing aerobically, such as many chlorinated and non-chlorinated organics.

Bioaugmentation: Bioaugmentation involves introducing microbes (non-native organisms) known to have a propensity to biodegrade specific COCs. This technique may be utilized in the event that the existing population of microorganisms is not adequately promoting the desired biologically mediated reactions. This inoculation is typically conducted via injection or periodic slug addition by gravity feed into an existing well network.

Carbon slurry bioremediation: This technology uses activated carbon filled with tailor-made facultative microorganisms, electron acceptors, and nutrients in an injectable slurry that first adsorbs contaminants in-situ, and then promotes anaerobic biodegradation via oxidative mechanisms of petroleum organics (primarily through sulfate reduction). It immediately lowers observed COC concentrations in the impacted matrix (soil and/or groundwater), as the activated carbon immobilizes the contamination, where a more traditional biodegradation process takes place within the activated carbon complex. During this process, groundwater quality is protected through the cleanup cycle by the activated carbon's adsorptive capabilities.

Conclusion

Portable mixing systems allow the injectate to be prepared onsite and placed by pump or gravity as needed.

These injection-based remediation technologies can require additional upfront costs when compared to long-term cleanups, but overall cost savings can be realized by avoiding years of monitoring, maintenance and reporting costs. These new technologies can be applied at almost any impacted site that would typically be approached using the standard in-situ or ex-situ remediation technologies. In addition, injection technologies rival excavation/source removal (with transportation/disposal costs soaring) when site closure is required in the shortest timeframe possible.

Sites involved with real estate property transactions, third-party liability concerns, or associated with sensitive receptors are especially good candidates for injection-based remediation because an expedited closure can be achieved. Injection technologies also rival traditional remediation systems as contaminants are often destroyed in-situ, where many traditional remediation systems often simply transfer contaminants from one phase to another, which perpetuates liability from a contaminant exposure perspective.

These new injection technologies are challenging scientists and engineers to develop more thorough site conceptual models, and obtain a more complete understanding of chemical and microbiological processes to treat contaminants. Successful projects will most likely be achieved by understanding where contaminants are found within the subsurface, how concentrated they are, and what technologies or combination of technologies may be applied to remediate them. PE