Groundwater Remediation: Restoring Nature's Balance

Groundwater remediation is the process of treating polluted groundwater by either removing the contamination or converting it into harmless products. This is important as groundwater is a source of drinking water, is used by farmers to irrigate crops, and is used by industries to produce everyday goods.

There are two types of groundwater remediation: in-situ (in place or on-site) and ex-situ (off-site). In-situ remediation is less expensive as it involves cleaning the water where it is, rather than removing and treating it elsewhere. Ex-situ remediation, on the other hand, may be beneficial when subsurface contaminant levels are high as it prevents further damage at the current location.

Groundwater remediation methods can be categorised into biological, chemical, and physical treatment technologies. Biological methods use microorganisms, organic matter, and plants to clean contaminated groundwater. Chemical methods can be achieved through chemical precipitation, oxidation, ion exchange, and carbon absorption. Physical methods, such as the traditional pump and treat approach, involve physically pumping out the contaminated groundwater and then purifying it using materials that absorb the contaminants.

In situ remediation: cleaning groundwater in its current location

In situ remediation is the process of cleaning groundwater where it is currently located, rather than removing and treating it elsewhere. This method is often chosen over ex situ remediation because it is less expensive and minimises the risk of further damage to the site.

There are several in situ remediation techniques that can be used to clean groundwater. One such technique is bioremediation, which is an engineered technology that modifies environmental conditions to encourage microorganisms to destroy or detoxify contaminants. Bioremediation can be applied above ground or below ground and has become one of the most widely used technologies for contaminated site treatment due to its relatively low cost, adaptability, and efficacy.

Another in situ remediation technique is the use of permeable reactive barriers (PRBs). PRBs can be biological or chemical in nature. Biological PRBs, or bio-PRBs, use microbial metabolic activity to degrade or detoxify pollutants, while chemical PRBs use chemical processes to achieve the same goal.

Other in situ remediation techniques include in situ sorption and biodegradation, in situ chemical reduction (ISCR), enhanced anaerobic biodegradation, in situ chemical oxidation (ISCO), and enhanced aerobic biodegradation.

The choice of in situ remediation technique depends on the specific conditions of the site, such as the type and extent of contamination, as well as the surrounding environment. Overall, in situ remediation is an important aspect of protecting the environment and ensuring the safety of drinking water.

Ex situ remediation: removing groundwater to be cleaned elsewhere

Ex-situ remediation involves removing contaminated groundwater from the site and treating it elsewhere. This method is more expensive and time-consuming than in-situ remediation, but it has the advantage of preventing any further damage at the current location. This approach is sometimes more beneficial when the levels of contaminants in the subsurface exceed those that can be treated via in-situ remediation.

The process of ex-situ remediation begins with the excavation of the contaminated groundwater. This can be challenging and disruptive, particularly if the site is located under infrastructure such as roads or buildings. Once the contaminated water has been extracted, it is then transported off-site for treatment.

There are several methods for treating contaminated groundwater, which can be categorised as physical, chemical, or biological treatments.

Physical Treatments

The "pump and treat" method is one of the most widely used groundwater remediation technologies. This process involves pumping groundwater to the surface and then using either biological or chemical treatments to remove the impurities. Other physical treatment technologies include air sparging and dual-phase vacuum extraction.

Chemical Treatments

Chemical treatments for groundwater remediation include ozone and oxygen gas injection, chemical precipitation, membrane separation, ion exchange, carbon absorption, aqueous chemical oxidation, and surfactant-enhanced recovery.

Biological Treatments

Biological treatments for groundwater remediation include bioaugmentation, bioventing, biosparging, bioslurping, and phytoremediation.

Biological treatment: using microorganisms, organic matter, and plants

Biological treatment methods use microorganisms, organic matter, and plants to remediate groundwater. These methods are advantageous because they can treat a wide range of pollutants and nutrients, resulting in energy and chemical generation with efficient organic removal. They also

Chemical treatment: using chemical precipitation, oxidation, ion exchange, and carbon absorption

Chemical treatment is a remediation process that involves the use of chemical processes to remove or neutralise pollutants from groundwater. This can be achieved through chemical precipitation, oxidation, ion exchange, and carbon absorption.

Chemical Precipitation

Chemical precipitation is a widely used method for removing heavy metals from groundwater. The process involves pumping water to the surface and using chemical reactions to convert soluble heavy metals into insoluble metals that can be settled and/or filtered out. The pH is adjusted, and chemicals are added to stimulate precipitation and promote the formation of larger particles through aggregation. Common precipitants include carbonates, sulfates, sulfides, lime, and hydroxides.

Oxidation

In situ chemical oxidation (ISCO) is a technique used to lower the concentrations of contaminants in groundwater. This is achieved by introducing strong chemical oxidisers, such as Fenton's reagent (hydrogen peroxide catalyzed with iron) and potassium permanganate, into the contaminated medium. These oxidisers destroy chemical contaminants and can be used to treat a variety of organic compounds, including chlorinated solvents and gasoline-related compounds.

Ion Exchange

Ion exchange is a process that removes dissolved metals and inorganic chemicals, such as perchlorate, from liquids. The liquid is passed over a resin bed where ions, such as cations and anions, are exchanged between the resins and the contaminated materials. The resins attract metallic ions, and after the resin is exhausted, some can be regenerated for reuse, while others are designed for single use.

Carbon Absorption

Activated carbon (AC) is an emerging technology for in situ remediation of groundwater. AC has a high adsorption capacity for contaminants and can be combined with secondary chemical or biological amendments to stimulate contaminant transformation. AC-based technologies have been applied at sites contaminated with petroleum and chlorinated solvents, where complex hydrogeology results in persistent plumes. The large surface area of AC can also promote microbial attachment and enhance biodegradation processes.

Physical treatment: using pump and treat methods to physically remove and purify groundwater

Pump and treat methods are a common physical treatment technology used to remediate contaminated groundwater. This process involves pumping groundwater from wells to an above-ground treatment system that removes the contaminants.

Pump and treat methods are considered safe and are the most common cleanup method for groundwater. They are used to clean up contaminated groundwater and keep it from moving to other areas, such as drinking water supplies, wildlife habitats, or recreational rivers and lakes. This technology is often used once the source of groundwater contamination, such as leaking drums and contaminated soil, has been treated or removed from the site.

The process typically involves installing one or more "extraction wells" to pump contaminated groundwater to the ground surface, either directly into a treatment system or into a holding tank until treatment can begin. The treatment system may consist of a single cleanup method, such as activated carbon or air stripping, to clean the water. The cleaned groundwater can then be discharged into sewer systems or re-injected into the subsurface.

It is important to monitor a pump and treat system to ensure the extraction wells and treatment units operate as designed. Additionally, the groundwater is sampled to ensure the contaminant plume is decreasing in concentration and is not spreading.

While pump and treat methods have been successfully operated at numerous sites for many years, data has shown that their performance decreases over time. As a result, significant amounts of residual contamination may remain unaffected by continued treatment. Due to these limitations, pump and treat methods are now primarily used for free product recovery and control of contaminant plume migration.

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