Ex Situ Treatment of Groundwater

Most methods for ex situ treatment of hydrocarbon-contaminated groundwater at cold-climate sites are conventional methods of pump and treat that are commonly used by engineering firms at warmer sub-Arctic sites. For general information about these conventional pump and treat methods, the reader is referred to overviews provided by Nyer (1992), Eastern Research Group, Inc. (1996), and Cohen et al. (1997). Documentation of applications of pump and treat in cold regions has typically been in the form of unpublished, proprietary reports for clients.

With some applications of pump and treat, a combination of physical, chemical and/or biological processes may be employed. For example, Mitchell and

Table 19.4 In situ groundwater remediation approaches with limitations to consider for application in cold regions (see Table 19.3)

Technique

Description

Limitations

In situ groundwater treatment

Physical/chemical processes

Air sparging

Steam sparging/ flushing

Chemical oxidation

Hydrofracturing enhancement

Multi-phase extraction

Treatment walls/ permeable reactive barriers Vertical contaminant barriers

Biological processes

Intrinsic bioremediation

The injection of air below the water table in order to induce volatilization of contaminants into the unsaturated zone, which can be removed by soil vapor extractiona

Steam is forced into aquifer through injection wells to vaporize volatile and semivolatile contaminants, which are vacuum extracted from the unsaturated zone for treatmentb Brings chemical oxidants (e.g., permanganate, H2O2) into contact with subsurface contaminants to remediate the contaminationc

Injection of pressurized water through wells to crack low permeability and over-consolidated sediments; cracks are filled with porous media that serve as substrates for bioremediation or to improve pumping efficiencyd Simultaneous extraction of vapor phase, dissolved phase and separate liquid phase contaminants from vadose zone, capillary fringe, and saturated zonee Barriers allow the passage of water while causing the degradation or removal of contaminantsd Construction of vertical barriers such as slurry walls, grout curtains or sheet pile walls in subsurface to contain plumes of contaminated groundwaterf

Unmanipulated, unstimulated, non-enhanced biological remediation of an environment; i.e., natural attenuationg

Requires on-site power and ongoing maintenance; limited to warm season (with respect to subsurface temperature) for Arctic applications if contamination occurs in seasonally frozen active layer Requires on-site power and site crew for ongoing O&M; limited to warm season application

Limited by reactive capacity of added oxidant; may be compromised by unintended oxidation of non-target substances (e.g., sulfide minerals; natural organic carbon)

Site has to be accessible by heavy equipment — generally applied as a short-term (one-event) technique in warm season; requires follow-up with another method

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

Limited by sorptive capacity of wall/barrier

Barrier may be overtopped by groundwater flow if the annual average recharge rate exceeds the rate of evapotranspiration

May be too slow for effective site remediation.

(continued)

Table 19.4 (continued)

Technique

Description

Limitations

Biosparging

Phytoremediation

Bioslurping

Biofiltering

The injection of air or specific gases below the water table to enhance bacterial activity for remediationb

The use of natural plants to remove contaminants through bioaccumulation or through enhancing biodegradationa

Combines vacuum removal of petroleum hydrocarbon free product with in situ bioventing. Designed for removal of free-floating LNAPL on the water table as well as residual product in the vadose zonea Refers to treatment of groundwater via passage through a biologically active area in the subsur-faceg

Requires on-site power and ongoing maintenance; limited to warm season (with respect to subsurface temperature) for Arctic applications if contamination occurs in seasonally frozen active layer May be too slow for effective site remediation; limited to warm season; limited to applications for shallow water-saturated zones that are readily accessible to plant roots; limited to regions where plants can grow effectively; some jurisdictions may restrict use of non-native plant species Requires on-site power and site crew for ongoing O&M; limited to warm season application

Custom engineering design and installation may be expensive; requires ongoing subsurface monitoring; likely not practical for some settings (e.g., plumes in fractured bedrock)

O&M, operation and maintenance a Riser-Roberts (1998) b USEPA (2004b) c USEPA (2004a) d Van Deuren et al. (2002) e US Army Corps of Engineers (1999) f USEPA (1998) g Hazen (1997)

Friedrich (2001) reported the use of a bioreactor, in combination with oil/water separation, air sparging, filtration and sorption by activated carbon. The site was Komakuk Beach in Yukon Territory, Canada, along the Arctic Ocean coast, where the mean annual air temperature is -11.4°C. Pump and treat was applied over two summers. Vacuum pumps were employed to extract fuel-contaminated ground-water along with free phase hydrocarbons via a multiphase extraction system. Following oil/water separation, the groundwater was treated in a series of two bioreactors to promote biodegradation of the hydrocarbons by indigenous bacteria. The first in the series, a fixed-film bioreactor contained polypropylene balls as a growth medium, where groundwater was circulated, amended with urea and monopotassium phosphate as nutrients, and sparged with air. Next in series was a suspended growth bioreactor, where the groundwater was again aerated and circulated. After flow through a sedimentation tank, final treatment included bag filtration and adsorption via organically modified clay and activated carbon. Monitoring of the effluent from the first bioreactor indicated removal of 53-97% of benzene, toluene, ethylbenzene, and xylenes (BTEX) and 44-89% of the total petroleum hydrocarbons (TPH).

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