Introduction

Treatability assessments are used to identify limitations to the rate or endpoint of bioremediation for a specific soil-contaminant combination. For treatability studies, the degradation pathways for the contaminant are generally known (see Chapter 4, Section 4.2.1), but the limitations in a particular soil or at a specific site are less well understood. The tremendous utility of treatability studies is in evaluating practical treatment regimes prior to full-scale implementation. The goal is to demonstrate practicability, optimize treatment design, and provide information for project planning. Sometimes this is an essential proving step for clients or regulators because choice of treatment depends primarily on urgency of remediation and cost. The cost-time relationship for different treatment types is illustrated in Chapter 1, Figure 1.1. The ability to predict the rate of bioremediation progress for a treatment scheme is particularly important in cold regions where costs are higher and treatment times are longer than in temperate regions.

In an effort to understand and improve the bioremediation process in cold regions, researchers have used treatability experiments to:

• identify the presence or absence of microbial activity for a particular contaminant or group of contaminants;

• determine optimum requirements, such as temperature, nutrients, oxygen, and water, for bacteria and fungi to metabolize contaminants in the soil regime;

• examine the effects that natural cycles, such as freezing-thawing and wetting-drying, have on microbial activity and degradation rate;

• estimate achievable endpoints;

• predict and compare treatment times and costs.

Treatability studies can involve in vitro microcosms with individual bacterial species or consortia from the soil incubated in liquid or slurry media, mesocosm studies with soils and natural microfauna, or field trials.

Introduction

Oil and fuel spills are among the most extensive and environmentally damaging pollution problems in cold regions and are recognized as potential threats to human and ecosystem health. It is generally thought that spills are more damaging in cold regions, and that ecosystem recovery is slower than in warmer climates (AMAP 1998; Det Norske Veritas 2003). Slow natural attenuation rates mean that petroleum concentrations remain high for many years, and site managers are therefore often forced to select among a range of more active remediation options, each of which involves a trade-off between cost and treatment time (Figure 11). The acceptable treatment timeline is usually dictated by financial circumstance, perceived risks, regulatory pressure, or transfer of land ownership.

In situations where remediation and site closure are not urgent, natural attenuation is often considered an option. However, for many cold region sites, contaminants rapidly migrate off-site (Gore et al. 1999; Snape et al. 2006a). In seasonally frozen ground, especially in wetlands, a pulse of contamination is often released with each summer thaw (AMAP 1998; Snape et al. 2002). In these circumstances natural attenuation is likely not a satisfactory option. Simply excavating contaminants and removing them for off-site treatment may not be viable either, because the costs are often prohibitive and the environmental consequences of bulk extraction can equal or exceed the damage caused by the initial spill (Filler et al. 2006; Riser-Roberts 1998).

Bioremediation of Petroleum Hydrocarbons in Cold Regions is written by a multi-disciplinary group of scientists and engineering professionals working in polar regions. The monograph is designed as a state-of-the-art guidance book to assist industry, environmental practitioners, and regulators with environmental cleanup in cold climates. The book can also be used for environmental science and remediation engineering seniors and graduate students who are preparing for a career in professional environmental practice or applied scientific research. The intent of this book is to articulate conditions unique to our cold regions, and present practical and cost-effective remediation methods for removing petroleum contamination from tundra, taiga, alpine, and polar terrain.

Oil and its refined products represent a significant proportion of the pollution found in the Arctic and Antarctic. This pollution is encountered at former military and industrial sites, scientific research stations, rural communities, and remote airstrips, while recent spills and releases tend to be associated with resource development and transportation mishaps. Bioremediation is recognized as potentially the most cost-effective technology for removing petroleum contaminants from ecosystems in cold regions.

Permafrost, suprapermafrost water, tundra, cold-tolerant microorganisms, short summers and long, dark winters, cold air and ground temperatures, and annual freezing and thawing of the active layer are but a few environmental characteristics of cold regions. Their prevalence limits practical remediation methods and has led to the development of innovative and pragmatic bioremediation schemes for use at contaminated sites in cold climates.

Introduction

It is well established that microbial activity is slower at low temperatures, and that there is a corresponding decrease in biodegradation rates (Paul and Clark 1996; Walworth et al. 1999; Scow 1982; Ferguson et al. 2003b; discussed in Chapter 4). As temperatures fall to near the freezing point of water, biomineralization of hydrocarbons practically ceases. Evaporation rates are also slower at low temperature, although diesel products and more volatile fuels continue to volatilize below 0 °C. For most cold regions, soil is typically unfrozen for only 6–8 weeks, affording a short in situ or passive ex situ treatment season. Even when the ground is thawed, temperatures are generally lower than optimal for hydrocarbon-degrading bacteria (Braddock et al. 2001; Rike et al. 2003).

At their simplest, thermally enhanced bioremediation schemes aim to increase microbial activity by increasing soil temperatures and extending the period when the ground is unfrozen. Modern integrated designs go much further – they typically incorporate some form of venting to promote volatilization, and deliver nutrients, oxygen, and water to hydrocarbon-degrading bacteria in attempts to optimize bioactivity. They are also designed to prevent off-site migration of contaminants and nutrient-enriched waters.

Relative to other remediation options, thermally enhanced bioremediation is a low-cost treatment option (see Chapter 1, Figure 1.1). It is typically much cheaper than bulk extraction and disposal or on-site combustion/desorption treatments, perhaps by a factor of five or more, but approximately two to four times more expensive than landfarming (Chapter 9).

Introduction

In this book, current scientific knowledge and practical experiences with bioremediation of petroleum-contaminated soils in cold regions are reviewed and compiled. We now more fully understand the inter-relationships between cold temperatures, soil and water properties, and biological processes. This aids decision making about practical remediation treatment for petroleum-contaminated sites in cold regions. Landfarming and enhanced bioremediation schemes have emerged as viable soil treatment methods that offer a number of advantages over other methods. Nevertheless, work still needs to be done to optimize these methods, and with regards to evaluating phytoremediation and rhizosphere enhancement potentials for cold soils.

Two emerging technologies have been identified that could offer significant cost savings; low-cost heating and controlled-release nutrient systems are described briefly here (see also Chapter 8). In addition, natural attenuation has received little rigorous evaluation for use in cold soils. The main limitation for natural attenuation in cold regions is the low rate of degradation, coupled with off-site migration that can be relatively rapid in soils or gravel pads that have a poor adsorption capacity. Permeable reactive barriers are one groundwater treatment technology that could buy time for slower in situ techniques such as natural attenuation to take place. An outline of emerging permeable-reactive barrier technology is presented here, although full-scale trials are not yet complete. It is possible that such in situ techniques, when coupled with aeration, sparging and biostimulation could offer methods for groundwater treatment in cold regions.

Introduction

Landfarming has been described as “a simple technique in which contaminated soil is excavated and spread over a prepared bed and periodically tilled until pollutants are degraded” (Vidali 2001) but, in practice, it can be either an ex situ or in situ technique. Landfarming generally uses a combination of volatilization and biodegradation to reduce hydrocarbon concentrations. For biodegradation to be effective, stimulating aerobic soil microorganisms is essential; this is commonly accomplished by adding nutrients and mixing the soil to increase aeration. Aerating the soil in this way also increases the loss of hydrocarbon contaminants to the atmosphere via volatilization. Volatilization of diesel and lighter hydrocarbons greatly assists the remediation process but it is less effective for heavier molecular weight hydrocarbons such as crude oil.

For in situ landfarming it is possible to treat only relatively shallow layers of soil where reasonable oxygenation can be maintained. In ex situ landfarming, excavated contaminated soil is spread as a thin layer in a treatment bed that is often lined with an impermeable layer to control leaching and runoff. Ex situ landfarming can be as simple as soil spread in a cleared area or it can be a major construction with contouring or drainage systems or both for removal of excess water. Plumbing can also be used for the application of water, either alone or in combination with nutrients or other amendments, to the landfarm surface.