Several oil spill models have been developed during the last decades.  Most of these are physical models which can be separated into oil weathering models, trajectory models (predicts the route of an oil spill), or stochastic models (describing an impact area of an oil spill).  Examples of well-known models are the Oil Spill Contingency and Response (OSCAR) and the OILMAP models (www.sintef.no/Materialer-og-kjemi/Marin-miljoteknologi/Miljomodellering/Modellverktoy/OSCAR-Oil-Spill-Contingency-And-Response/; http://www.asascience.com/software/oilmap/index.shtml). Models have also been presented for predictions of oil behavior in ice-infested water (Drozdowski et al. 2011). Most of these models are physical models, but the OSCAR model also incorporates biodegradation of 25 pseudo oil compound groups, in addition to descriptions of the physical environment, physical-chemical fate processes and ecotoxicity (Aamo et al. 1997; Reed et al. 2000). In the OSCAR model, which is an industry standard in Norway, biodegradation is one of the fate processes together with physico-chemical processes like advection, spreading, evaporation, dispersion, dissolution, particle adsorption/dissolution, volatilization from water column, and seabed contamination. An example of vertical oil concentrations and mass balance after a simulated 60-day blowout is shown in Figure 5-4. However, biodegradation as part of the mass balance may be overestimated in the model, since degradation is determined on the bases of biotransformation, not complete biodegradation, and only compounds determined by gas chromatography-mass spectrometry (GC-MS) analyses are included.  

Figure 5-4. Simulation of a deep water blowout by the OSCAR model. [The left figure shows the oil concentration vertically in the water masses during a simulated 60-d blowout from 1600 m depth with a light paraffinic oil. The right figure shows the mass balance between different fate processes during the blowout period (from Brakstad et al. 2011).]

In the oil spill models biodegradation must be predicted at different temperatures. Oil biodegradation data in Arctic environments with cold seawater are limited, since most published studies have been performed at higher temperatures than relevant for these environments. In order to transform degradation data between different temperatures, plots have been used to transpose results of bacterial metabolism and growth at different temperatures. Temperature-related bacterial growth rates may be estimated by using modified Arrhenius plots (Arrhenius, 1889).  Ideally, Arrhenius plots should show temperature-related linearity.  However, in a study with psychrotrophic toluene-degrading strains of Pseudomonas putida grown on toluene or benzoate, growth rates had to be fitted using two linear segments at a temperature range of 4-30°C: one segment above and one below 17-20°C (Chablain et al. 1997).  When using Arrhenius plots, the temperature range should therefore not be too broad.  For water-soluble compounds, temperature-dependent biodegradation has been suggested to follow a Q₁₀-value, which is a relationship describing the degradation rate increases when temperatures are raised by 10°C increments.

Equation 1

Where R is the general gas constant (8,314·10^-3 kJ/mol·K), E_a is the activation energy (kJ/mol), T₁ is the reference temperature in Kelvin and T₂ is the actual temperature in Kelvin.  According to this approach, the degradation rates should double for every 10°C increase, resulting in an ideal Q₁₀ of 2.0.  The Q₁₀–values for the biodegradation of oil hydrocarbons in seawater were determined with a heavy fuel oil (Bunker C), and with winter or summer water samples from the North Sea.  When incubation temperatures of 4-18°C were used, Q₁₀–values of 2.4 and 2.1 were determined for waters in winter and summer, respectively, where biodegradation was measured as biological oxygen demand (Minas and Gunkel 1995).  Calculations of Q₁₀-values from a variety of studies have shown that the rule-of-thumb value (Q₁₀ = 2.0) is a fairly good approximation in a temperature range of 5–27°C (Andrea Bagi, personal communication).  However, for the narrow range and freezing temperatures of the Arctic the expectation that calculated Q₁₀ would remain close to 2 may not be valid.  For instance, a calculation of Q₁₀ in immobilized oil films (Statfjord B oil) based on data for 5 and 0°C (Brakstad and Bonaunet 2006) showed a value of 16.2 (Andrea Bagi, personal communication).  This may be caused by changes in the physico-chemical characteristics of this oil at these temperatures.  Thus, changes in oil characteristics at low seawater temperatures may affect the biodegradation models, and therefore predictions of oil degradation rates in Arctic seawater will require closer examination.