Background

Human activities in the Arctic and Antarctic have resulted in releases of a suite of compounds that are harmful to human and environmental health including crude oil and petroleum products. In this chapter the terms crude oil and petroleum products refer to the actual liquids, and the term petroleum hydrocarbons refers to compounds such as benzene that make up the liquids. Most releases that occur in the Arctic and Antarctic are insignificant in volume; however, several larger terrestrial releases have taken place. Arguably, the largest release to have occurred in the Arctic took place north of the city of Usinsk, Russia (65°N), in the Kolva River Basin (Vilchek and Tishkov 1997; AMAP 1998). By one estimate 103,000-126,000 tonnes of crude oil (other experts estimate the release to be as high as 318,000 tonnes) was released over a 2-3 month period from multiple leaks in a pipeline system that continued to pump oil even though the pipeline was leaking (Vilchek and Tishkov 1997).

Recent relatively large releases of crude oil have occurred on the Trans Alaska pipeline. In 2001, a hole was shot in the pipeline near the village of Livengood, Alaska (65°N), resulting in the release of approximately 265,000 l (Spiess 2001). Corrosion of a crude-oil transit line at Prudhoe Bay, Alaska, caused between 761,000 l and 1,010,000 l of crude oil to be released to the tundra (JIC 2006). Relatively smaller releases associated with fuel storage and transportation for industrial activities, predominately mining, and for communities in the Arctic occur with more frequency then the larger more notable releases. Such releases are a result of vehicle accidents, such as a fuel tanker truck roll-over at a large zinc and lead mine in northwest Alaska (68°N) resulting in the release of approximately 10,000 l to tundra (ADEC 2004a), or mishaps related to fuel storage, such as an approximate 9,500 l release that occurred in the village of Point Hope, Alaska (68°N), due to overfilling of a storage tank (ADEC 2004b). Rike et al. (2003) described the presence of petroleum hydrocarbons in soil at a site near the village of Longyearbyen, on Spitzbergen Island in the Svalbard archipelago (78°N), most likely resulting from small releases of petroleum over time at a fire extinction training site.

Relatively smaller releases of petroleum products have occurred in the Antarctic as well. A majority of these releases are due to poor waste management practices at research stations. In the hope that burial in frozen ground would contain waste, most waste from research stations were disposed of in dumps with little to no engineered containment systems (Snape et al. 2002). In addition, minimal attention was given to petroleum spills, owing to the belief that the frozen environment would contain the compounds (Snape et al. 2002). Investigations illustrated that containment of contaminants in this manner is not feasible. Contaminants are mobile in this environment during thawing and thawed periods much in the same manner as in more temperate environments. In addition, cryoturbation and erosion uncovers buried contaminants, exposing them to transport processes both in ground water (suprapermafrost) and surface water (Snape et al. 2001).

To understand movement of petroleum and petroleum hydrocarbons through freezing and frozen soils in the Arctic and Antarctic, an understanding of the fundamental principles of immiscible fluid (in this case petroleum) movement through unfrozen soil is required. Several authors have presented thorough descriptions of the movement of immiscible fluid, commonly known as non-aqueous phase liquids (NAPL), through unsaturated soils (Mercer and Cohen 1990; Wilson et al. 1990; Poulsen and Kueper 1992). Petroleum is considered a light non-aqueous phase liquid (LNAPL), as the specific gravity of the fluid is less than unity. The remainder of the discussion will focus on petroleum.

Released at or near the ground-surface, petroleum will move downward through unsaturated soil toward the water table. Due to the immiscibility, the fluid migrates as a distinct liquid, separate from the air and water present in the unsaturated soil. Water and petroleum are held in the pore space of partially saturated soils by capillary forces. As petroleum migrates downward, air and possibly some water are displaced from the pore space. Once in soil pore space, individual petroleum compounds will dissolve into soil water according to the specific solubility of each compound and its mole fraction. Solubility of these compounds is low, since most petroleum hydrocarbons are non-polar. Sorption of petroleum hydrocarbons onto natural organic matter in the soil results from the non-polar nature of these compounds. The high volatility of relatively low molecular weight petroleum hydrocarbons dissolved in soil water results in partitioning of a fraction of these compounds into the gas phase. The mixture of gaseous petroleum hydrocarbons and air becomes soil gas in the pore space.

Infiltrating petroleum follows a path through unsaturated soil that is dictated by the properties of the soil encountered; primarily, permeability and pore structure. Results from field studies performed by Poulsen and Kueper (1992) illustrated how small variations in permeability result in extreme heterogeneous distribution of NAPL and some lateral migration, which is also a result of capillary forces.

Capillary forces immobilize a fraction of petroleum in the pore space as the main body of the liquid moves downward through porous medium. Results from a visualization study conducted by Wilson et al. (1990) showed that immobilized NAPL was mostly contained in pore throats and in thin films between soil water and soil gas. Soil water was also contained in pore throats that were bypassed by infiltrating NAPL, and soil gas filled the larger pore bodies.

Infiltrating petroleum that reaches the capillary fringe, sometimes referred to as the nearly saturated zone, will spread laterally as a result of the relatively high water saturations in this zone. For spill volumes that generate sufficient head to displace the water in the capillary fringe water, petroleum that migrates further downward to the water table may displace water from saturated pores and cause depression of the water table. As the water table rises and falls seasonally some petroleum is immobilized or entrapped in the capillary fringe and possibly below the water table during high water level conditions. This immobilized petroleum consists of small pockets (or ganglia) of liquid disconnected from the main body of organic liquid (Wilson et al. 1990). A dissolved phase plume results in the saturated zone below the water table, from petroleum contained above and below the water surface.

Was this article helpful?

0 0

Post a comment