In situ Chemical Oxidation: An Innovative Groundwater Remediation Technology

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Courtesy of Courtesy of AECOM

A promising new technology, in situ Chemical Oxidation (ISCO), has the potential to supplant the more widely used pump-and-treat groundwater remediation technology by destroying groundwater contamination in situ (in place). ISCO is the process of oxidizing contaminants in the groundwater and soil by injecting water containing high concentrations of an oxidant to the site of the contamination. Unlike the pump-and-treat process, there is no requirement for aboveground water treatment or disposal. ENSR scientists have found that in situ remediation can reduce groundwater clean-up times dramatically, and substantially reduce clean-up costs over time.

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Contaminated groundwater underlying a property is one of the most common environmental liabilities. It usually results from the release of contamination to the ground or below the ground surface where it is carried downwards and eventually (depending on the depth) into the groundwater. The contamination can have several effects: it can make an on-site water well unusable; volatile contaminants in the groundwater can diffuse upwards into buildings; or the chemicals can migrate off-site and contaminate the groundwater or surface water of neighboring properties.

The classic approach to groundwater remediation – “pump-and-treat” – has been to pump contaminated groundwater above ground, treat it to remove the chemicals, and then release it back to the environment (to a surface water body, the groundwater, or to a sewer). While this method can remediate a site, the process may take too long (decades are not uncommon; in some cases, centuries are required) to reduce the contaminant concentrations enough to achieve “clean”.

One alternative, ISCO, entails using an oxidant to chemically break down the compound of concern (COC) in the subsurface. An oxidant is a chemical that wants to “steal” electrons from another molecule. All organic compounds can be oxidized. Ideally, the final products of the oxidation of organic compounds are carbon dioxide and water, though incomplete oxidation can also occur, yielding smaller organic compounds. If the COC is readily oxidized by common and safe oxidants, and its oxidation products are benign, then ISCO is a potential remediation solution.

Limitations of Pump and Treat

Pump-and-treat is a quasi in situ process. Soil does not have to be excavated for treatment, however, contaminated water is extracted and treated above ground. The primary drawback to pump-and-treat remediation is that a long period of remediation often is required, as a result of the site’s hydrogeology and the chemical properties of the contaminants. The following physical factors of soil and groundwater affect the efficiency of pump-and-treat, and frequently make ISCO the preferred treatment method.

  • Sorption.

Soil contains billions of soil particles to which most contaminants of concern sorb or “stick”. In addition to the contamination measured in the groundwater, there is typically much more contamination sorbed to the soil. The ratio of contaminant sorbed to dissolved contaminant varies depending on the soil type and the contaminant.;

  • Retardation.

If all the groundwater in contact with the soil where the contaminant is also sorbed is pumped out, only the dissolved amount is removed. When new, “clean” groundwater flows in to replace the groundwater that was extracted, it flows into the soil that is still contaminated with sorbed contaminants and some of the sorbed contamination dissolves into this groundwater. Slowly, each time water is removed, some of the contamination is removed. Eventually, all of the contamination in the soil is released and removed with the extracted groundwater. Compared to the removal rate of groundwater from the ground, the removal rate of contamination is “retarded”.

  • Soil permeability.

Groundwater moves slowly through soils with low permeability. Consequently, the rate at which the contaminated groundwater can be extracted is very low. When combined with the phenomenon of retardation, the rate at which the total amount of contamination (dissolved and sorbed) is removed via pump-and-treat remediation systems can be very slow.

  • Soil heterogeneities.

Most subsurface soils, especially in the northern US, are comprised of layers of different soil types, each being some mixture of gravel (largest grain size), sand, silt (smaller grain sizes) and clay. The properties of the layers, which vary from inches to feet thick, often differ considerably. Typically, contamination sorbs more to the siltier, less permeable layers. Groundwater flowing through the soil tends to travel better through the most permeable (larger grain size) soil layers. When pump-and-treat systems are operated in a heterogeneous soil, most of the groundwater flows through the more permeable sandy layers, leaving behind the contamination in the low permeability layers.

  • Diffusion limitations.

As groundwater in the more permeable, sandier layers be-comes clean, contamination diffuses from higher concentrations in the low-permeability soil layers toward the lower concentrations in the flowing groundwater. The rate of diffusion is directly proportional to the difference between the two concentrations. In many cases, the clean-up levels in the groundwater are very low. Therefore, as pumping and treating progresses, the difference in concentrations between the hard-to-reach pores and the more permeable layers becomes smaller, and the rate of diffusion decreases, slowing the overall rate of cleanup.

Advantages of ISCO

A notable advantage of ISCO compared to pump-and-treat is that it is a true in situ process. There is no requirement for aboveground water treatment or disposal. In situ remediation has become recognized as generally less expensive than prolonged pumping and treating and has become the approach preferred by regulators.

ISCO is not as subject to the physical limitations of pump-and-treat. Since ISCO does not rely on flushing of the contamination, retardation does not influence the success of the ISCO process. In addition, ISCO takes advantage of greater diffusion rates. During ISCO remediation, an oxidant is injected at very high concentrations into the aquifer. Because there is no oxidant in the deep soil pores and low permeability soil layers, there is a strong driving force for the oxidant to diffuse into these locations where COCs are sequestered and oxidize the contamination. Dissolved oxidant concentrations that “flood” the high permeable soil layer may be 1,000 times the dissolved COC concentrations that reside in the low permeable layer. Therefore, the rate of diffusion of the oxidant “inward” can be up to 1,000 times the rate of diffusion of the COC “outward”. Since the oxidant gets “used up” performing the oxidation, this tendency to diffuse will continue, rather than decrease over time the way the diffusion of contamination outward does.

Common oxidants include hydrogen peroxide, Fenton’s reagent (hydroxyl radicals, produced by reacting hydrogen peroxide with iron catalysts), potassium permanganate, ozone, and molecular oxygen. These oxidants have varying strengths of attraction for electrons, and thus have varying degrees of effectiveness on common COCs.

Oxidation can also be accomplished biologically by bacteria living in the subsurface. This process is bioremediation, which is distinct from ISCO because it is biologically mediated. ISCO can be used in tandem with bioremediation by chemically oxidizing recalcitrant compounds and creating products that are readily biodegradable.

Inorganic contaminants, such as arsenic or ferrous iron, can also be treated by ISCO. Arsenic has many different oxidation forms, some of which are more toxic and more mobile than others. ISCO can remediate an arsenic plume by immobilizing the dissolved arsenic.

Arsenic Immobilization

Arsenic was detected in groundwater samples at greater than the 50 ppb (parts per billion = micrograms/liter) drinking water level at a Superfund site in New England. The groundwater was highly reducing (the opposite of oxidizing) due to biodegradation of the organic materials formerly discharged to the subsurface. ENSR concluded that under these highly reducing conditions, normally insoluble ferric hydroxide became reduced and dissolved, releasing associated arsenic into the groundwater.

Modeling indicated that water with a dissolved oxygen (DO) concentration of 25 mg/L, injected at a rate of 6 gpm for years, would reestablish aerobic, oxidative conditions in the aquifer. Then, the arsenic would revert to the immobile form.

Oxygen was selected as the oxidant and a 10-gpm in situ oxidant delivery system was installed to super-oxygenate municipal water, then inject it into the aquifer. After one year of operation, groundwater laboratory analytical results for arsenic and chemical oxygen demand (COD) and field measurements of REDOX (reduction-oxidation) potential and DO were evaluated for trends. The data indicated that the ODS system was changing the groundwater back to oxidizing conditions as intended. In most wells around the infiltration gallery there are upward trends in DO and REDOX and downward trends in COD and arsenic.

In Situ Fenton-Like Oxidiation

Practitioners recently have published a number of articles describing the success of using hydrogen peroxide and iron based catalytic agents (also known as Fenton’s Reagent) to remediate groundwater contaminated with different organic contaminants at various sites. Two case studies are summarized in the adjacent sidebars.


In situ Chemical Oxidation offers many advantages to the remediation industry, including:

  • Significantly reduced cleanup times compared with pump-and-treat remediation – ISCO treats both dissolved and sorbed contaminants concurrently
  • The ability to treat sites previously considered technically impractical to remediate
  • Substantially lower overall remediation costs than pump-and-treat remediation - the capital costs for ISCO may be higher for some sites, but no above ground treatment systems are necessary and faster treatment means that the operation, maintenance, and monitoring expenditures for years of pumping and treatment will be avoided
  • ISCO may be used as an alternative to in situ bioremediation due to its ability to oxidize compounds that are not readily biodegradable and ISCO can also be used in concert with in situ bioremediation by oxidation of the recalcitrant compounds and allowing biodegradation of the oxidized breakdown product

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