Groundwater Treatment

Various methods have been used, tested or proposed for the remediation of petroleum-contaminated groundwater in cold regions. These methods can be broadly divided into ex situ and in situ remediation approaches (Tables 19.3 and 19.4). The ex situ remediation methods that have been applied in cold regions are essentially variations of pump and treat, where the treatment component may include physical processes (e.g., oil-water separation, air-stripping), chemical processes (e.g., sorption to granular activated carbon), or biological processes (e.g., the use of bioreactors). In situ techniques for remediation of hydrocarbon-contaminated groundwater can also use physical techniques (e.g., construction of a barrier, air sparging), chemical treatment (e.g., permanganate addition), biological processes (e.g., bio-stimulation with nutrients), or a combination thereof. For example, air sparging can promote the physical removal of hydrocarbons from groundwater via volatilization, but it also can stimulate bioremediation by introducing oxygen to the water (i.e., biosparging). The use of natural attenuation to remediate a plume of dissolved hydrocarbons in groundwater relies on a combination of physical (e.g., dispersion), chemical (e.g., sorption to particle surfaces) and biological (e.g., degradation by microorganisms) processes to control the extent and impact of the plume.

One of the main advantages of conventional ex situ methods is that they have generally been demonstrated to be applicable under cold climate conditions. Some of the main disadvantages of the ex situ treatment methods are that:

• They tend to be relatively expensive

• They require a source of power to maintain pumping, which may be challenging at remote sites

• They require an on-site worker for ongoing operation, monitoring and maintenance activities, and

• In cold regions, they are seasonal because the extraction of groundwater for ex situ treatment is limited by freezing conditions that persist through much of the year.

There has been growing interest in the use of in situ treatment methods for treatment of hydrocarbon-contaminated groundwater. These methods offer potential cost-savings, largely because they may require little or no on-site power generation, they typically require limited operation and maintenance activities, and they can potentially be applied year-round. The main disadvantages of these in situ techniques are that:

• Some of these methods are in the developmental stage as emerging technologies, and

• Their applicability for cold regions is often not yet well established.

It is useful to consider some general statistics and trends in groundwater remediation, for all contaminant types and all climate regions, as reported by the United States Environmental Protection Agency (2007) for more than a thousand National Priority Sites:

• From 1982 through 2005, more than 90% of groundwater treatments used pump and treat methods.

• In situ groundwater treatment applications have been increasing, from none in 1982 through 1986 to a high of 31% in 2005.

• Applications of pump and treat alone decreased from about 80% before 1992 to around 20% after 2000.

• The use of monitored natural attenuation has been increasing, comprising almost half of all selections made in 2005. Monitored natural attenuation is the "reliance on natural attenuation processes... to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods.These in situ processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants" (United States Environmental Protection Agency 1999).

• The most common in situ technologies include air sparging, bioremediation, chemical treatment, permeable reactive barriers, and multi-phase extraction.

• Applications of in situ bioremediation and chemical treatment have increased significantly in recent years.

• In situ groundwater remediation applications generally have shorter operating periods than pump and treat remedies.

Though the above trends are not specific to cold-climate sites, they suggest that interest will continue and grow in testing and advancing in situ techniques for remediation of hydrocarbon-contaminated groundwater in cold regions.

Table 19.3 Ex situ groundwater remediation approaches with limitations to consider for application in cold regions

Technique

Description

Limitations

Ex situ groundwater treatment

Physical/chemical processes

Air stripping

Carbon adsorption

Phase filtration/ separation

Volatile organics are partitioned from extracted ground water by increasing the surface area of water exposed to air. Aeration methods include packed towers, diffused aeration, tray aeration, and spray aerationa Removal of hydrophobic organic contaminants from the aqueous phase to carbon (e.g., granular activated forms) by physical and chemical forcesb Use of filter membranes and/or conventional oil-water separator to remove non-aqueous phase emulsions of hydrocarbons from water

Biological processes

Constructed wetlands

Bioreactors

Use of natural geochemical and biological processes inherent in an artificial wetland ecosystem to accumulate and remove contaminants from influent watersa

A contained vessel in which biological treatment takes placec

Requires on-site power and site crew for ongoing O&M; limited to warm season application

Requires on-site power and site crew for ongoing O&M; limited to warm season application

Requires on-site power and site crew for ongoing O&M; limited to warm season application

Cost of construction of artificial wetlands may be high; restricted to warm season operation; requires assessment and monitoring of impact to aquatic ecosystem Requires on-site power and site crew for ongoing O&M; limited to warm season application

O&M, operation and maintenance aVan Deuren et al. (2002) »Riser-Roberts (1998) cHazen (1997)

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