A field guide to oil spill response on freshwater shorelines: chapter 4

Oil released into the environment changes due to a variety of biological, physical and chemical processes collectively referred to as “weathering”. These processes alter the behaviour and control the fate of oil in the environment and can affect the selection of appropriate strategies and treatment methods during a response. The following sections describe:

• the transport and weathering processes when oil is released in a freshwater environment or is stranded on shorelines
• natural attenuation of oil on shorelines
• the range of oil types
• ice and snow and the effects of winter conditions on oil behaviour and weathering
• key factors that differ between spills in freshwater and marine environments

4.1 Transport and weathering processes

Several physical and chemical processes are set in motion when oil is released into the environment that result in changes to the character and behaviour of the oil. Understanding how weathering can change oil characteristics and oil behaviour is key for the selection of response strategies and treatment. The weathering processes are the same for marine or freshwater environments but the effect on oil behaviour can be somewhat different due to the hydrodynamic and geographic conditions encountered in freshwater environments. The rate and extent of oil weathering are highly dependent on meteorological and hydrological conditions at the time of the incident and on the oil type.

Some of the weathering processes help the response as they remove oil from the environment whereas other processes can make the oil more persistent or more difficult to recover. The individual processes described in the following sections act together to weather oil, but the relative importance of each process varies in time (from hours to years) and space. Most of the weathering processes are pathways that move oil within and between environments and locations, as only photo-oxidation or biodegradation break down the hydrocarbons into other compounds.

Figure 4.1: Main weathering processes affecting the fate and behaviour of oil on water

4.1.1 Spreading

Spreading begins immediately when oil is released onto a water surface. The thickness of the oil layer decreases rapidly for all except highly viscous oils and can be reduced to a few millimetres or less within minutes to hours. The rate at which spreading occurs depends on the quantity of oil spilled and the initial viscosity. Low viscosity oil spreads more than highly viscous oils. As it spreads, a slick fragments into smaller patches or long bands under the influence of winds, currents, and waves. In rivers, fragmentation can be significantly increased by the presence of rapids, eddies or falls. Oil slicks are rarely uniform, and thickness can vary greatly from one location to another. Oil appearance provides key information to estimate oil thickness and potential volume (Table A1.6, ECCC 2018). Spreading can greatly affect response operations as thin oil layers are difficult to recover while fragmentation and the larger surface area occupied by the oil affects oil recovery rates (Figure 4.2). In small bodies of freshwater (small lakes and ponds) a release may occupy the entire surface area.

Figure 4.2: Fragmented oil on lake water surface, Lake Wabamun, AB (2005)

4.1.2 Drifting

Most oils initially float on the water surface and are transported under the action of surface water currents and winds. The trajectory of oil on the water surface of a large water body (lakes) can be estimated from knowing the friction parameters; movement will be influenced by water current and wind speed. In the case of streams or rivers, oil is typically transported downstream by currents with the wind pushing the oil towards one shore or the other (Figure 4.3). The configuration of the water body may have a significant effect on oil movement along shorelines as oil can accumulate in curves, bays and other areas. Drifting poses a significant challenge for oil spill response as oil is always moving on the water surface making timely and efficient equipment deployment difficult. In addition, the drift rate of oil at the surface may be affected by vegetation, as the oil tends to move more slowly through the vegetation than the water.

Figure 4.3: Oil drifting and spreading on river water surface, Chaudière River, QC (2013)

4.1.3 Evaporation

Evaporation is the transfer of the light volatile components of oil into the atmosphere as vapours and is the same process that occurs in a marine or terrestrial setting. The evaporation rate is controlled by oil composition, ambient temperature, winds, turbulence (waves, rapids) and the surface area of the slick. All oil types undergo evaporation to one degree or another. Generally, lighter oils (or products such as gasoline or diesel), warmer temperatures, greater sunlight exposure, higher winds and turbulence promote evaporation. With high intensity sunlight, the temperature of the oil can greatly exceed ambient temperature and will be the predominant factor affecting evaporation. Spreading and evaporation are closely related; as oil spreads the surface area of the oil increases so that a greater volume is exposed to evaporative processes. Evaporation can have a positive effect on the response as a significant volume of oil may be lost to the atmosphere, decreasing the oil volume that remains on the water or on shorelines. However, the remaining oil may have a higher viscosity and density due to the loss of the light fractions. Although airborne dispersion rates typically are high, vapours may be a health and safety risk for responders and the public as these may be toxic and flammable; for example, following a release into a narrow, steep-sided stream valley. Air monitoring should be implemented at the beginning of a spill to evaluate these hazards. Typically, evaporation takes place in the initial hours/ days of a release unless there is a continuous source of new oil.

4.1.4 Dispersion

Natural dispersion occurs when an oil slick is broken down by breaking waves and turbulence at the water surface to form oil droplets of various sizes in the upper layer of the water column. Oil droplets smaller than 70 μm may be held in suspension and remain in the water column, whereas larger droplets may re-surface forming slicks. Oil droplets that remain in the water column may eventually be diluted and biodegraded (depending on local microbial communities) where water depth is adequate for under-water dilution to take place. Oil dilution may be limited if the water depth is not sufficient relative to the volume of oil to enable the mixing process to take place, potentially resulting in adverse effects on the freshwater environment or water quality. Oil type and water energy levels are the two main factors influencing this process; low viscosity oils typically are dispersed more rapidly than higher viscosity oils. Natural dispersion can be beneficial for response operations as the water surface oil volume may decrease significantly. However, in a freshwater environment limited water depth may be an important factor influencing the extent of this process.

4.1.5 Emulsification

Emulsification occurs when either water droplets are incorporated into the oil to form a water-in-oil emulsion or oil becomes entrained within water droplets to form an oil-in-water emulsion. These processes can increase the volume of oil up to five times due to the addition of water in the oil matrix. This process is likely to occur when oil with a nickel/ vanadium concentration greater than 15 ppm or an asphaltene content above 0.5% is exposed to energy in the form of waves or turbulence (e.g. rapids, falls). Emulsification significantly increases the oil viscosity and density. Stable water-in-oil emulsions can be highly persistent whereas non-stable emulsions may separate into oil and water in calm conditions, when heated by sunlight, or when stranded on the shoreline. Emulsification has an important effect on response techniques as viscous oils are generally more difficult to recover and the increased volume generates greater quantities of liquid waste.

4.1.6 Sedimentation

Sedimentation of spilled oil occurs when oil droplets interact with and attach to suspended organic material or large sediment particulates (>1 mm in size) present in the water column. This change in oil character can increase the density and cause the oil to either sink or to remain suspended under water (i.e. submerged). This process is more common in freshwater environments as water density is lower than in sea water. Sediment may be mixed with oil that flows overland to a freshwater lake or river or has stranded on a shoreline or river bank (or bar) and is agitated by waves or currents. There are three distinctly different Oil Particle Aggregate (OPA) mechanisms and types of aggregates, which are important for the understanding of sedimentation processes (refer to text box).

The aggregate interactions depend on the:

• state of oil (dissolved, emulsified, floating)
• size and type of particles [colloidal (a mixture of microscopic dispersed insoluble particles suspended in another substance), granular, organic, inorganic]
• oil-particle interaction mechanisms
• settling/suspension characteristics

Section 7.3 provides a discussion on submerged and sunken oil in the freshwater environment.

4.1.7 Dissolution

A small fraction of the lightest components of the oil may dissolve in the water column. Components of interest are aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylene (BTEX) as they can be harmful to biota and water quality. However, as these compounds are also highly volatile, they tend to evaporate in a much larger proportion than they dissolve. As such, dissolution is considered a minor oil weathering process. The rate at which dissolution occurs is affected by oil type, spreading, droplet formation, weather conditions and water turbulence. In a freshwater environment, dissolution can be a significant concern as municipal drinking water intakes may be potentially exposed to dissolved hydrocarbons.

4.1.8 Biodegradation

All aqueous environments (freshwater and marine) contain populations of microorganisms that use oil as a source of carbon and energy leading to its ultimate attenuation into carbon dioxide and water. Microorganisms are very opportunistic and oil degrading organisms multiply rapidly once in contact with exposed oil surfaces. Biodegradation is the mechanism by which hydrocarbons are naturally removed from the environment.

Typically, biodegradation occurs following emulsification or dispersion into small oil droplets. Heavier oils with high wax or asphaltene contents biodegrade more slowly than crude oils or products which primarily have a higher content of light and medium hydrocarbon compounds. Different microorganisms degrade different oil compounds and multiple communities interact in this suite of processes. Biodegradation occurs mainly at the oil-water interface and any increase of the oil surface area, such as the formation of small droplets through dispersion or OPA mechanisms (refer to text box), increases the rate of biodegradation. Oil type (generally lighter oil is easier to biodegrade), the presence of oxygen and nutrients (phosphorus and/or nitrogen) and temperature control the rate of biodegradation.

4.1.9 Photo-oxidation

Photo-oxidation is a chemical reaction promoted by sunlight to form oxidized compounds. Light products degrade slowly, whereas heavier products can form a protective surface layer (crust-like) making the oil more persistent (i.e. resistant to other weathering processes). Photo-oxidation contributes to the breakdown of hydrocarbon molecules.

4.2 Natural attenuation of oil on shorelines

The natural attenuation or removal of oil stranded on shorelines may occur by one or a combination of six processes, including physical, photochemical and microbial degradation mechanisms.

The four physical mechanisms include:

1. Evaporation: physical volatilization of light hydrocarbon fractions into an atmospheric (air) environment.

• Primarily a function of the available exposed surface area, the oil composition, wind velocity, sunlight intensity and ambient temperature.
• A pathway to photo-oxidation in the atmosphere or to deposition on land or water surfaces where the hydrocarbon molecules are subject to biodegradation.

2. Buoyancy partitioning by a rising water level, with or without wave energy: physical dispersion into an aquatic environment.

• Primarily a function of location and exposure to changing water levels, the internal cohesion of the oil and the adhesive properties of the oil.
• A pathway to OcPA and biodegradation.

3. Physical action by waves: physical partitioning and dispersion into an aquatic environment.

• Primarily a function of location on a shoreline with respect to wave action, the level of wave energy, the internal cohesion of the oil and the adhesive properties of the oil.
• A pathway to OcPA and biodegradation.

4. Aggregation by fines (OcPA): emulsification and physical dispersion into an aquatic environment with or without wave energy.

• Primarily a function of the available exposed surface area and the properties of the available fine sediments.
• Concurrent with biodegradation and physical dispersion into an aquatic environment.

The fifth process, photochemical degradation by oxidation, occurs on exposed oil surfaces. The aromatic hydrocarbons are sensitive to photo-oxidation, whereas saturates are more resistant. Photo-oxidized products are found in resins and polar fractions, compounds that are more resistant to biodegradation.

The sixth process by which oil is removed from shorelines is the same microbial activity that attenuates oil in an aquatic environment and involves direct in situ biodegradation of stranded oil and natural attenuation by microbial activity:

• Typically requires aerobic conditions and water. Anaerobic degradation is a much slower process than the aerobic process and involves sulfates and carbon dioxide that reduce the hydrocarbons to sulphide and methane.
• Primarily a function of the available exposed surface area, the oil composition, and ambient temperature.
• Concurrent with partitioning and natural dispersion into an aquatic environment.

4.3 Oil types

This section provides an overview of main physical/chemical properties, behavioural characteristics, and potential adverse effects of various types of oil in freshwater. The oil spill response community uses simple classification systems to cluster different oil types. The United States Environmental Protection Agency (US EPA) and United States Coast Guard (USCG) use a Group 1 to 5 based on density. ECCC (2016) used a 5-tier descriptive scheme (volatile, light, medium, heavy, solid). The latter schema is defined for this guide:

• volatile (gasoline products)
• light (diesel and light crudes)
• medium (intermediate products, medium crudes, fresh diluted bitumen or ‘dilbit’)
• heavy (residual products, heavy fuel oils, Bunker C, and heavy crudes, weathered diluted bitumen)
• solid (do not pour; bitumen, weathered Bunker C, tar, and asphalt)

Table 4.1 compares the physical properties of each oil category to freshwater.

Table 4.1: Typical physical properties of oil types and freshwater (from Emergencies Science Division of Environmental Science and Technology Centre, Oil Properties Database; Fingas 2001; ITOPF 2011)

Category of oil type	Density (g/ml)	Viscosity (cSt)	Distillation characteristics % boiling: below 200 °C above 370 °C	Pour point (°C)
Volatile (e.g. gasoline)	0.75	1	100 0	–
Light (e.g. diesel)	0.85	1 to 5	30 100	-35 to -1
Medium (e.g. typical crude, including dilbit)	0.85 to 0.90	10 to 50	15 to 40 45 to 85	-40 to 30
Heavy (e.g. fuel oil)	0.95 to 0.98	1,500 to 15,000	2 to 5 30 to 40	-10 to 10
Solid (e.g. bitumen)	>1	>50,000	–	–
Water	1	1	100 0	0

4.3.1 Volatile

Oils in this category are non-persistent as they are highly volatile and evaporate rapidly. They have high concentrations of toxic compounds, some of which are soluble in water, that may result in localized effects in the water column and on shoreline resources. Since they are not persistent and pose potential safety issues for responders, there generally is less requirement for response activities to control, contain or recover the released oil.

4.3.2 Light

Light oils are characterized as being relatively volatile and persistent. They contain some concentrations of toxic compounds, which may be soluble in water. They may result in longer-term oiling of shoreline resources and have the potential for acute subaqueous effects due to dissolution, mixing and sorption onto suspended sediments. Light oils behave the same in all aqueous environments.

4.3.3 Medium

Medium oils are generally more persistent than lighter oil types. They typically have less than 1% soluble fraction and potentially may cause significant and long-lasting oiling of shorelines and effects to waterfowl and aquatic furbearers (e.g. beaver, muskrat) as they have a high adherence potential.

Unweathered diluted bitumens are included in this category. However, in comparison to other commonly transported oils, many of the chemical and physical properties of diluted bitumen, especially those relevant to environmental effects, differ substantially. Primarily, the differences are the high density, viscosity, and adhesion properties of the bitumen component of the diluted bitumen that become the dominant properties as the oil weathers.

4.3.4 Heavy

Heavy oils have few light fractions and there are few compounds that readily evaporate or dissolve. These viscous products spread more slowly than products with a lower viscosity and frequently break down into discrete patches and tar balls when dispersed rather than forming slicks. They weather slowly and may sink in freshwater, which makes them difficult to detect and recover. Oil persistence on shorelines may be long (months to years). Shoreline treatment is typically required.

4.3.5 Solid

Solid oils likely sink as the density exceeds that of freshwater. In waters with little current, product movement and transport may be minimal.

4.3.6 Introduction to unconventional oils and biofuels

Conventional oil is typically referred to as crude oil (i.e. liquid petroleum), flowing naturally or capable of being pumped without further processing or dilution; this field guide deals primarily with crude oils and petroleum products derived from crude oils. Unconventional (or non-conventional) oil is derived from other sources, such as, light shale oil (e.g. Bakken) and oil sands bitumen, transported as diluted bitumen (dilbit); note that this field guide considers dilbit in the medium-heavy oil type categories, depending on its degree of weathering. Biofuels (e.g. ethanol-blended gasoline, biodiesel, and vegetable oil) are often promoted by some energy industries and governments as an alternative to conventional petroleum fuels. This section briefly summarizes the behaviours, fates, and potential response techniques for light shale oil, ethanol-blended gasoline, biodiesel, and vegetable oil, highlighting those differences from conventional oils.

Light shale oil

Light shale oil is a type of light crude oil recovered from shale oil reservoirs, typically by hydraulic fracturing (“fracking”) techniques. Shale oils occasionally have high hydrogen sulphide (H2S) concentrations, thereby posing significant air quality risks early in the response. Bakken shale oil was the flammable product in the Lac-Mégantic, QC incident (2013; Section 9.1.6). Overall, Bakken oils tend to have increased levels of natural gas liquids relative to conventional crude oils due to the isolation of pockets of petroleum within the shale formation that are only recently “fracked”. The higher vapour pressure and lower boiling point are the reasons for the increased flammability risk of Bakken shale oil in comparison to other light crude oils; otherwise, the spill behaviours of light shale oil are not so very different from other types of light crude oil.

Ethanol-blended gasoline

Ethanol is an alcohol that has a very low viscosity and density in comparison to freshwater; however, it is very soluble in both gasoline and water. When spilled into the aquatic environment, ethanol will mix with the water and may enhance solubility of the gasoline. The introduction of ethanol into fuel changes the oil’s physical and chemical properties and therefore may alter the effectiveness of current spill response techniques.

One of the primary concerns associated with gasoline spills is ground- water contamination in areas where the water is extracted for human use. The ability of ethanol to increase the amount of petroleum hydrocarbons in the water is an important concern.

As ethanol is both volatile and highly soluble in water, spill response decisions typically are more concerned with managing effects and mitigating damages than with containment and recovery. Current best practices for response should follow typical procedures for gasoline fuels. However, the partitioning into surface water and co-solvent behaviour of ethanol-blended gasolines is not fully understood; toxicities of these mixtures to various organisms and the biodegradation of the gasoline portion of the mixtures require clarification.

Biodiesel

Biodiesel fuels can have varying chemical composition depending on the source material (e.g. canola oil, waste fry oils from restaurants, rendered animal or fish fats, etc.) and this is likely to influence their fate and behaviour in the aquatic environment.

Biodiesel is highly soluble in petroleum-based diesel and may be blended in a similar manner to ethanol with gasoline. Biodiesels have densities in the range of conventional diesels and float on freshwater; however, the viscosity of biodiesel tends to be higher, especially at lower temperatures. Some biodiesels can even become solid at temperatures approaching the freezing point of water. Biodiesels are not highly soluble in water but disperse readily. Biodiesel does not have a high vapour pressure and would not be expected to evaporate to a significant extent.

Petroleum diesels have been found to be 5-10 times more acutely toxic to aquatic organisms than pure biodiesels. Biodiesels degrade rapidly and may create a high oxygen demand in the receiving aquatic environment, which results in low oxygen conditions for aquatic organisms.

Current best practices for response should follow typical procedures for light oil; as biodiesels degrade rapidly, bioremediation may be a practical solution for treatment of biodiesel-affected shoreline substrates.

Vegetable oil

Vegetable oils are not soluble in water, do not evaporate, do not form water-in-oil emulsions, and do not disperse in water.

These oils may have similar environmental effects to petroleum oil spills, such as: coating of fur, feathers and gills; creating high biological oxygen demand; and harmful alteration, disruption, or destruction of shoreline habitats. Constituents or metabolic products of vegetable oils (e.g. free fatty acids) may be toxic to biota.

Current best practices for response should follow typical procedures for light oils.

4.4 Ice and snow and effects of winter conditions on oil behaviour and weathering

4.4.1 Freshwater ice formation

The process by which freshwater ice forms is very different from that of sea ice because, unlike most substances, freshwater becomes less dense as it nears the freezing point. Very cold, low-density freshwater stays at the surface of lakes and rivers, quickly forming an ice layer on the top. In contrast to freshwater, the salt in ocean water causes the density of the water to increase as it nears the freezing point, and very cold ocean water tends to sink. As a result, freshwater ice forms more quickly than sea ice because the saltwater must sink away from the cold surface before it cools enough to freeze. A fewer number of below-freezing temperature days are required to initiate ice growth in freshwater environments as compared to marine or brackish water (Sections 3.1.3 and 3.2.4).

4.4.2 Shoreline ice types

The range of shoreline ice types includes:

• glacial cliff
• glaciers that reach a lake create an ice shoreline that can ‘calve’ as ice breaks off the glacier front
• the ice front of a slow moving or retreating glacier may melt without calving
• edges of seasonal land fast ice
• ice that is ‘fastened’ to the shoreline or the bottom and does not move with currents or winds (Figure 4.4: top panel)
• surface ice layers (an “ice foot”)
• layers of ice (or an “ice foot”) form seasonally although in high latitudes may persist through the open water season – the lake-ward or river channel edge of the ice foot is often a vertical or steep face
• frozen wave splash, spray, or swash can form a coat of ice on a shoreline or backshore surface (Figure 4.4: middle panel)
• freshwater flowing downslope from the backshore towards the shoreline can freeze
• shoreline sediments in which the interstitial water is frozen
• ice can form within a shoreline when water freezes in the interstitial spaces of sediment to effectively create an impermeable substrate
• permafrost exposed in a cliff at the shoreline
• erosion of the tundra can expose permafrost
• individual ice floes, ice pressure ridges, granular or slush ice
• ice floes (or sheets) of various sizes can be stranded on a shore these originate from the break-up of lake or river ice
• ridges form where solid ice grounds and buckles under the onshore pressure (Figure 4.4: bottom panel)
• many forms of ice can be driven against the shoreline by wind or current action, including granular or slush ice

Figure 4.4: Ice and snow on freshwater shorelines: ice ‘fastened’ to shoreline (top panel); frozen wave splash (middle panel); ice ridges visible during spring thaw (bottom panel)

On shorelines with seasonal ice, the ice forms on the surface of the sediment or bedrock in the form of frozen swash or spray or an ice foot. In these situations, both the surface layer of ice and the underlying geological substrate of the shoreline are considered when planning a response. Ice surfaces do not support significant plant or animal life.

4.4.3 Snow-covered shorelines

Snow can be present on any shoreline type with seasonal snow that is layered on top of the sediment or bedrock of the shoreline. The character of the snow surface can be highly variable, ranging from:

• fresh powder with a soft surface, or drifting snow
• a loose granular surface that results when powder or packed powder thaws then refreezes and re-crystallises, or from an accumulation of sleet
• a hard, dry, crusty surface
• wet slush

As snow accumulates in depth over time, it is common to find a vertical variation in density and porosity. Typically, this steady accumulation is interrupted by the effects of freeze-thaw cycles and wind. As the air temperature oscillates around the freezing point, layers of ice are generated as snow melts during warm daylight temperatures and freezes at night when temperatures drop below zero. If this freeze-thaw cycle is accompanied by precipitation, a range of features can form that may include alternate layers of snow and ice.

Wind action can strip the loose crystals on the surface to expose denser layers of snow below. Blown, powdery snow accumulates in hollows, depressions, or wind shadows. The snow layer itself is not considered to be a sensitive environment. When selecting oil removal tactics, the nature and sensitivity of the underlying sediment, vegetated or bedrock substrates must be considered.

4.4.4 Effects of winter conditions on oil behaviour and weathering

The transport and weathering processes summarized in Section 4.1 commence as soon as oil is spilled into the environment. However, their relative importance will vary depending mainly on the oil type and volume and environmental conditions. Winter conditions will have a significant effect on oil weathering processes. Generally, colder temperatures will increase oil viscosity, slow spreading on the water surface and reduce the evaporation rate. In cold weather, oil spills in ice-free water behave similarly to those in warmer conditions but with a reduced weathering rate. The effect on weathering processes in ice-covered water is more complex. In ice-covered water, the interaction of oil with ice affects the rate at which these processes are taking place. Generally, once oil is spilled in ice-covered waters, spreading is limited by the presence of ice, as ice acts as a natural barrier that keeps the oil more concentrated with a greater thickness. This reduction in spreading has far-reaching implications (mostly positive) in terms of extending response times and limiting the extent of the oiled area. In water with an ice coverage > 60%, ice provides natural containment significantly limiting oil movement and, in some cases, providing protection for sensitive resources on the shoreline (Figure 4.5). The presence of ice reduces natural dispersion and emulsification rates, as short waves are dampened by ice floes.

Evaporation and biodegradation still take place in ice-covered waters, but the low temperatures usually associated with the presence of ice reduces the rate at which they occur.

Figure 4.5: Ice limits oil movement

Direct interaction of oil with ice will also affect oil behaviour and weathering. Figure 4.6 summarizes the various ways oil can interact with ice. When oil that naturally floats is spilled under the ice it can drift under the ice layer because of currents and movement of ice floes, potentially accumulating in naturally formed reservoirs due to ice roughness. These accumulations will vary in size, but significant quantities of oil could be trapped under the ice in this manner. Some of the oil could be dislodged by currents and continue to drift under the ice. Several studies have set the threshold for movement of oil under ice at 0.5 knots (0.25 m/s). Oil can become encapsulated within the ice structure in winter conditions when new ice is being formed. In some cases, this process can happen rapidly (between 18 and 72 hours) once oil is trapped under the ice surface. As soon as oil is encapsulated, the normal weathering processes cease keeping the oil fresh. The crystal structure of freshwater ice is very different than sea ice and the lack or paucity of brine channels typically affect the timing and process of oil migration during the thaw periods.

Figure 4.6: Oil and freshwater ice interaction processes

In springtime conditions with melting and warming of the ice, encapsulated and trapped oil can vertically migrate through leads in the ice and reach the surface of the ice forming pools of fresh oil (Figure 4.7). The rate of vertical migration will depend largely on the oil viscosity (i.e. less migration for higher viscosity oil).

Figure 4.7: Oil surfacing through a lead in a lake

The lower oil weathering rate generally observed in ice-covered waters could represent an advantage for response effectiveness in some spill scenarios. However, the presence of ice creates operational difficulties that can offset the advantages provided by the reduced weathering rate. Section 6.1.3 introduces response to oil in ice on shorelines and oil under ice.

If oil is spilled on ice, evaporative loss will be the main weathering process as spreading will be limited by surface ice roughness and by snow absorption. Because of this, oil accumulations on the ice surface are expected to be limited in size and fairly thick. Ice is essentially impermeable; however, oil may penetrate where surface cracks are present. The presence of an ice foot or a frozen layer of ice prevents oil from contacting the shoreline substrate. Oil washed onto the exposed surface of ice, in any of the various forms, is not likely to adhere except when the air temperature is below freezing. Oil on the shore or stranded on the shore-zone ice during a period of freezing temperatures can also become covered and encapsulated within the ice. During a thaw cycle or if the surface of the ice is melting and wet, oil is unlikely to adhere to the ice surface and remains on the water surface or in shore leads.

Oil may be splashed over the ice edge or stranded above the limit of normal wave or current action. The stranded oil can then be incorporated into the shore-fast ice if temperatures fall below freezing again. If oil becomes stranded on the substrate in between ice floes and on the floes themselves, its behaviour would be influenced by a combination of ice and that substrate material. Ice in shoreline sediments, either frozen interstitial or groundwater, can prevent the penetration of stranded oil.

The behaviour of oil on a snow-covered surface depends on the:

• type of oil
• type of snow (fresh, compacted, or containing ice layers)
• air temperature
• surface character (i.e. flat or sloping)

If a spill is on the surface of the snow, oil that is above its pour point migrates vertically and horizontally. Oil migrates horizontally from a spill at the base of the snow cover. Oil that is below its pour point could penetrate minimally and run off laterally across the snow’s surface. Oil usually penetrates rapidly into the snow column but may be hindered by layers of ice in the snow column that have formed as a result of the freeze-thaw process. As light oil can migrate laterally tens or hundreds of metres within snow, it may be difficult to detect. Oil detection canines (dogs) have been used to successfully locate subsurface oil in snow (Section 7.4).

Snow is an effective natural oil sorbent. The oil content may be very low (less than 1%) in the case of light oils or if the oil has spread over a wide area. The proportion of oil to snow depends on the type of oil and the character of the snow. Snow absorbs more medium crude oils than light products. For example, one cubic metre (m³) of snow can absorb up to 200 L of light oil and as much as 400 L of medium oil. Oil content is lowest for firm, compacted snow surfaces in below freezing temperatures and highest for fresh snow conditions.

Oil causes snow to melt. Crude oils cause more melting but spread less than gasoline, which spreads faster in snow and over a larger area. Light oils, such as diesel, can move upslope in snow through capillary action as they spread. Fresh snow blowing over oil tends to stick to the oil and migrate down into it, which increases the amount of material to be recovered.

Evaporation is the single most important weathering process for oil trapped in snow. The limited available test data show that oil covered by snow continues to evaporate, although at a lower rate than oil directly exposed to air, and eventually to approximately the same degree as it would if spilled on the water during summer. The actual rate of evaporation is a complex function of several variables including snow diffusivity (related to the degree of packing), oil properties, air temperature, wind speed, and the thickness of the oiled layer.

4.5 Overview of differences between freshwater and marine environments

Fundamental aspects of shoreline treatment decision-making and response (including SCAT and shoreline treatment objectives, strategies, and tactics) apply in all environments. There are however important differences between freshwater and marine environments due to variations in water levels and water exposure/processes in tidal, lake and flowing water environments, which in turn affect oil stranding, oiling band width and behaviour, and natural removal potential and treatment tactics. Some of the basic differences between fresh and marine environments that may affect oil behaviour and oil spill response include water density, fetch, water levels and flow, biological environment, and water intakes. These differences are introduced in the following sections.

4.5.1 Water density

The average density of sea water at the surface is 1.025 kg/L, which is denser than freshwater and pure water (density 1.0 kg/L at 4 oC) due to the presence of dissolved salts. The freezing point of sea water decreases as salt concentration increases, and the likelihood of oil density exceeding that of water increases as the water density decreases. The result is that denser oils can sink more readily in freshwater.

4.5.2 Fetch

Fetch (i.e. the extent of the shorelines exposure to waves/energy) in freshwater environments is typically much smaller in comparison to marine environments, where waves are unimpeded by landforms/barriers for greater distances. Wave heights in freshwater are therefore not typically able to increase to the size of those achieved in the marine environment; however, waves heights of approximately 9 m have been recorded in Lake Superior. Wave height affects response operations with respect to safety, equipment suitability and efficacy, and affects oil behaviour.

4.5.3 Water levels and flow

The fluctuation of water levels in the marine environment is dominated by tides, i.e. predictable changes in water levels as a result of gravitational forces of the moon and the sun. Large bodies of freshwater like the Great Lakes are affected by tides, however this effect is small (on the order of cm) and is masked by seiches. Seiches occur when strong winds and rapid changes in atmospheric pressure push water from one end of the lake to the other resulting in an oscillating wave which can be a few metres high (Figure 3.5). In freshwater environments, water levels have long-term, annual, and short-term variations that are affected by factors such as precipitation, water storage over many years, and variations that occur with the changing seasons. Additionally, physical structures (e.g. dams, weirs) may be used to regulate water level and flow (Figure 3.11). Ice melt and spring freshet can cause extremes in water levels and flow, moving oil large distances down rivers and into backshore areas.

4.5.4 Biological environment

Similar shoreline types in different environments (i.e. marine, lake, river) typically have different productivity and sensitivity characteristics. Biota (i.e. plants and animals) vary from marine, to brackish to freshwater environments. The plant and animal species of any given environment must be considered during the decision-making process in a response, particularly if there are species-at-risk. It should be kept in mind that the majority of scientific and technical knowledge and experience with respect to biological effects and shoreline sensitivity to oil comes from marine oil spills.

4.5.5 Water intakes

Water intakes, used in both marine and freshwater environments, are at risk of contamination during an oil spill. Water intakes may be used for cooling water for power plants and process water for various industrial sites. Shutting down of water intakes during an oil spill may have a major effect on the facility. Municipal drinking water intakes commonly used in freshwater environments (canals, rivers, lakes, reservoirs) are a high priority as they are directly related to public safety.