In Situ Biological Treatment of Groundwater

In situ bioremediation of petroleum plumes in groundwater may involve "active" techniques to enhance the biodegradation of hydrocarbons, as outlined in Table 19.4 and below. In contrast, intrinsic bioremediation (i.e., natural attenuation) is a

"passive" approach that takes advantage of the unassisted natural biological processes that attenuate contaminants in groundwater, such as microbial degradation of petroleum hydrocarbons (Weidemeier et al. 1999). Active in situ Bioremediation Techniques

Some applications of active in situ bioremediation at cold climate sites have been reported. Examples include:

• In situ biosparging with biostimulation (Soloway et al. 2001)

• In situ aeration with bacterial inoculation and addition of unspecified "biogenic" substances (Pawelczyk et al. 2003)

• Bioventing to treat both soil and groundwater (Barnette et al. 2005).

To date, most such reports have limited information regarding the final outcome (success or failure) and limitations of the remediation techniques employed. Also, some of the relevant publications (Carss et al. 1994; Shields et al. 1997; Pawelczyk et al. 2003) lack details regarding specific technologies or materials that were used. Consequently, there is a need to provide detailed case studies that conclusively demonstrate the applicability of in situ active bioremediation techniques for hydrocarbon-contaminated groundwater in cold regions.

Positive results were reported for some in situ bioremediation approaches, measured as disappearance or reduction of BTEX concentrations (Carss et al. 1994; Barnette et al. 2005), decline in TPH/oil concentrations (Carss et al. 1994; Pawelczyk et al. 2003), oxygen loss (Carss et al. 1994), and/or shrinkage of the plume (Shields et al. 1997). Some interpretations of field results (Curtis and Lammey 1998) or laboratory test results (Billowits et al. 1999; Cross et al. 2003) have suggested that in situ biostimulation with nutrients might enhance the bioremediation of the hydrocarbon-contaminated groundwater at cold climate sites. Following field investigations that included sulfate injection tests, Van Stempvoort et al. (2007a, b) suggested that it might be helpful to add sulfate as an electron acceptor to enhance in situ biodegradation of gas condensate plumes in groundwater in Western Canada, where groundwa-ter temperatures were reported to range from 5 to 9°C. Passive in situ Bioremediation

In a current review, Van Stempvoort and Biggar (2008) reported evidence that intrinsic bioremediation of petroleum hydrocarbons is a near-ubiquitous process in petroleum-contaminated groundwater in cold regions. This review noted that positive indicators for intrinsic bioremediation had been found at 16 sites in North America and 10 sites in Scandinavia and the adjacent Baltic region of Europe. Overall the annual air temperatures at these sites ranged from -12°C to 8°C. The contaminated subsurface media at these sites included sand or sand/gravel aquifers, fractured rock, gravel fill over peat, and silt/clay deposits. In these studies, the only reported complete lack of intrinsic biodegradation of hydrocarbons in groundwater was a study of a plume of a complex mixture of contaminants at Fairbanks, Alaska (Richmond et al. 2001). However, other studies at Fairbanks (Westervelt et al. 1997; Braddock et al. 2001) have indicated evidence for significant intrinsic bioremedia-tion of hydrocarbon plumes.

Perhaps the coldest site with published evidence for intrinsic biodegradation of a hydrocarbon plume in groundwater is one located near Barrow, Alaska (Braddock and McCarthy 1996). Here the air temperature averages -12°C annually, and rises above freezing for about 90 days each year. Soil and groundwater in sand and gravel deposits at this site had been contaminated by gasoline and jet fuel spills in the 1970s. Braddock and McCarthy (1996) reported that there were localized shallow groundwater flow systems in a thin unfrozen layer above permafrost, with groundwater temperatures ranging from 1.2 to 7.4°C. They found that 20 years after fuel was spilled, concentrations of BTEX remained elevated in the ground-water near the spill locations. Compared to groundwater outside of the plume, inside the plume the concentrations of oxygen and nitrate were lower and ferrous iron, sulfide and microbial populations were higher. These results suggested that aerobic and anaerobic microorganisms were associated with hydrocarbon degradation in the plume, utilizing oxygen, nitrate, sulfate and ferric iron as electron acceptors. Microcosm tests at 10°C indicated greater benzene mineralization potential in groundwater sampled from the plume than in groundwater sampled outside the plume. In other laboratory tests, hydrocarbon mineralization rates were stimulated by nutrient additions. Braddock and McCarthy (1996) reported that the strategy to manage the plume would incorporate intrinsic bioremediation, along with construction of a barrier to contain the plume by inducing permafrost mounding.

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