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PETROTOX
PETROTOX is a spreadsheet based model designed to determine the toxicity of petroleum products to aquatic organisms. The model was designed by the Conservation of Clean Air and Water in Europe (Concawe), a European environmental research agency dedicated to understanding petroleum based products and their environmental implications. Petroleum products are composed of a unique mixture of hydrocarbons that each have specific physicochemical properties when released. Using lab collected data and a library of information, the PETROTOX model predicts the toxicity of the petroleum in the environment through a combination of partitioning methods.

History and Intent of the Model
Concawe was established in 1963 by oil companies looking to research the environmental impacts of the petroleum industry. Concawe research covers a wide breadth of topics including fuel emissions, air quality, soil contamination, waste, and water quality. Three principals of the work overseen by Concawe are: sound science, transparency, and cost-effectiveness. PETROTOX was designed in 2007 by Concawe to model the toxicity of petroleum mixtures to aquatic organisms. The efficiency of the model allows users to quickly estimate toxicity of petroleum products if released into the environment.

Concepts Used in the Model
Petroleum products are complex mixtures of various sized hydrocarbons that each behave differently in the environment. Exact compositions of many petroleum products remain unknown, making hazard and risk assessment of these products difficult. PETROTOX was developed as a tool to deal with these complications. The model relies on the assumption that the overall behavior of a petroleum product can be predicted by its substance composition, which is characterized by its hydrocarbon blocks (HBs).

Hydrocarbon Block Method (HBM)
Petroleum is a mixture of various sizes of hydrocarbons. Depending on the composition of the mixture, different types of petroleum will behave differently when released, such as accumulation in the gas, liquid, or solid phases of the environment or degradation. However, hydrocarbons of similar molecular size behave in similar manners. In the Hydrocarbon Block Method (HBM), hydrocarbons of similar sizes are grouped together in an effort to create “blocks” or groupings that simplify the complex mixture when considering how toxic they are in an environment.

In PETROTOX, a structure library of over 1500 hydrocarbons is integrated into the model. The hydrocarbons are grouped based on physicochemical properties identified from the structure and the HBM. The percent of each of the hydrocarbon blocks in the petroleum of concern then translates to a prediction of the petroleum properties.

Water Accommodated Fractions (WAF)
The hydrocarbons that makeup petroleum are either soluble or insoluble in water. When considering the potential exposure in the aquatic environment, the hydrocarbons that are soluble in water are of more concern for acute toxicity. When conducting toxicity tests with petroleum substances, a known amount of petroleum is added and then allowed to dissolve until it reaches equilibrium with the water. The process that allows partitioning of the soluble hydrocarbons into the water is labeled as the Water Accommodated Fraction method (WAF), and the WAF is used for further toxicity testing. In a three-phase model (air, water, and nonaqueous-phase liquid), the concentrations of hydrocarbons in each phase can be estimated.

Target Lipid Model (TLM)
The Target Lipid Model (TLM) is based on the assumption that at a specific narcotic chemical concentration in the lipids of an organism, most likely a fish, mortality will occur. The TLM model has been modified in the PETROTOX model to account for the fact that it is not based on long term, or chronic, organism exposures. In the case of hydrocarbons mixtures, the model assumes the hydrocarbons behave additively in mixtures since they have a similar mechanism that causes toxicity.

Input Variables
In order for the PETROTOX model to predict toxicity, various measurements need to be defined by the user. The required variables include: PETROTOX takes the measures of the petroleum composition and organizes it based on the structural class of the compounds via the hydrocarbon block method (HBM). The information regarding the dimension of the chamber, the amount of petroleum added, and the organic carbon concentrations are used in the WAF portion of the model to determine what phase the HB are going to partition into when in the chamber. The PETROTOX model also allows users to account for the modifications due to higher concentrations of suspended particulate matter from algal cells present in the water sample. The Kow is used in the TLM model with the partitioning information to predict toxicity.
 * Petroleum substance composition (Hydrocarbon Blocks)
 * Exposure chamber water and headspace dimensions (L)
 * Amount of petroleum added to the exposure chamber (g/L)
 * Particulate organic carbon concentrations (mg C/L)
 * Octanol-water partitioning coefficient (Kow)
 * Desired model endpoint

Validation of Model
Various PETROTOX estimates have been validated thought laboratory testing to ensure that predicted values are comparable to observed values. The three-phase-partitioning model (WAF) has been validated for both gasoline and crude oil in experimental observations. PETROTOX consistently over predicts acute and chronic toxicity for fish. This may be due to the increased volatilization in lab studies due to the addition of bubbling oxygen through the test chambers for the fish or that the acute toxicity time frame may be insufficient to allow for the target lipid equilibrium to be reached. Model predictions for heavy and less water-soluble petroleum substances, such as heavy fuel oil, are also inconsistent with experimental observations. The PETROTOX model assumes that there are no abiotic or biotic losses of the petroleum in the closed vessel during preparation, however this may be an oversimplification of the real environment.

General Use and Potential Applications
PETROTOX is primarily used to predict the toxicity of petroleum hydrocarbons that have contaminated an area based on biological endpoints of interest. Accidental release or improper handling of petroleum products pose a significant threat to aquatic organisms and due to the variation in physiochemical properties of hydrocarbons, there are few reliable methods for predicting the toxicity of a spill. PETROTOX utilizes previously gathered information within Concawe databases to model the effects of any hydrocarbon toxicant based on its composition.

After running the model to convergence, valuable information is calculated including solubility and toxicity of each hydrocarbon. This model allows researchers to derive dose-response data and LC50 predictions, both essential to understanding the petroleum toxicity. PETROTOX predicted values are comparable to measured values in the field and are complementary to other methods used in environmental risk assessment to characterize complex hydrocarbon structures.

PETROTOX can be used to calculate the toxicity of a petroleum or its environmental risk limits. PETROTOX has been used to study the developmental effects of fish embryos (particularly Oncorhyncus mykiss) exposed to heavy fuel oils using the WAF for both chronic and acute toxicity. In addition to looking at petroleum's effects on specific species, PETROTOX can also be used as a tool for the characterization of biodegradability and aquatic toxicity of atmospheric distillate hydrocarbons.