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In undertaking environmental risk assessments the best use of all available ecotoxicity data should be made. However, the assessment of ecotoxicity data for many petroleumproducts is complicated since several different test methods and procedures have been used. As petroleum products contain a mixture of substances with a range of solubilities a critical aspect with respect to interpreting the validity of ecotoxicity tests is how the test media is prepared. Although not always explicitly stated most of the data generated in the period up to the early 1990s originated from experiments in which a "water soluble fraction" (WSF) was tested. WSFs are prepared by mixing the petroleum product with the aqueous test medium (e. g. 25 to 50 mL product with 1 L of medium). After mixing the test solutions are then allowed to stand, the aqueous phase is separated and dilutions of this medium are used in testing the species under study. The results areexpressed either as (a) the dilution, or % WSF, or (b) the concentration of dissolved hydrocarbons expressed in mg/L (CONCAWE, 1992). A disadvantage with these WSF studies is that it is not possible to convert the quoted result to the amount of product that must be added to a given volume of aqueous medium to produce the effect.

The problems of preparing test media for oil products were recognised in the early 1990s. As a consequence the recommended method, which enables ecotoxicity assessments of petroleum products to be interpreted, was to determine the amount of test substance that must be equilibrated with the test medium to produce a specified level of effect. This is the so-called "loading rate" or water accommodated fraction (WAF) methodology as developed by Girling et al. (1992) and reported in CONCAWE (1992). Even with these laboratory based studies there are doubts about their value in the context of risk assessment owing to the fact that once a petroleum product is released to the environment its constituent substances will partition to the various compartments (water, sediment, soil and air) in accordance with their physico-chemical properties. The assumption being that in the receiving environment the substances will be degraded and transformed in accordance with their individual susceptibilities to physical, chemical and biological degradation processes and will exhibit effects in accordance with their individual toxic potencies.

Discussion on mechanisms of toxicity and PNEC derivation
In an attempt to better understand the potential for adverse effects of a product, the effects of a product's constituent substances (hydrocarbon blocks) can be integrated in such a way that an overall assessment of their combined effects can be made. For the assessment of toxic effects it is important that the method of integration meets the assertion that effects can only be integrated for substances that share the same mode of toxic action. All components of petroleum products exhibit non-polar narcosis effects on organisms.

Under ideal circumstances a PNEC for a hydrocarbon block would be derived from ecotoxicological test data for one or more components that are representative of that block. The TGD sets out how this can be done either by applying an Assessment Factor to the lowest acute ecotoxicological effect or chronic no effect concentration or by applying statistical extrapolation methods to a number of data points. For petroleum products this was not a practical option since the majority of its mass is comprised of chemical components that cannot be accurately described by a chemical structure (and which may not have a unique CAS number) and for which there is an absence of ecotoxicological data. Under such circumstances the only practical option is to estimate a PNEC using a relationship between physico-chemical descriptors of a component or a hydrocarbon block and concentrations resulting in ecotoxicological effects or absence of an effect. This is the hypothesis encompassed by the Target Lipid Model (TLM) described by McCarthy et al. (1991).

The theory underpinning the TLM is that the concentration of a substance in a lipid that is responsible for the onset of a non-polar narcosis effect is the same when expressed on a molar basis for a range of taxonomic groups e. g. fish, invertebrates and algae. Consequently the toxic potency of a substance depends upon its capacity to achieve the threshold concentration within an organism. There are a number of variables that determine this capacity, key of which are the solubility of the substance in water and lipid and its molecular size. In an application of the theory, DiToro et al. (2000) have published a non-polar narcosis-based QSAR for predicting the aqueous concentration of a hydrocarbon substance that induces a specified level of biological effect. The QSAR relates biological effect to the log Kow of the substance. Log Kow is a function of the solubility of a substance in water and lipid (octanol) but is limited by molecular size because large molecules cannot pass through biological membranes.

In the absence of measured ecotoxicity data for a substance the TLM and associated QSARs provide a theoretical basis for predicting the ecotoxicity of a substance. By extension of the theory it should also be possible to evaluate the toxicity of a mixture of substances provided that they have the same mode of toxic action. McGrath et al. (2004) have validated the theory by characterising the aquatic toxicity of six gasoline blending streams and have showed that predicted and measured toxicity were in good agreement.

Having established procedures that enable the toxicity of a mixture of hydrocarbons to be predicted, McGrath et al. (2004) have also utilised statistical theory developed by a number of workers to define an acute species sensitivity distribution for narcotic chemicals. A relationship has been established enabling the concentration of a hydrocarbon substance to be determined that will affect a specified proportion of the species present in a community. By setting the proportion to a notional low level (e. g. 5%), a hazard concentration (HCx where x is the proportion that might be affected i. e. 5%) is obtained. The HCx has similarities with a hazard concentration derived by applying statistical extrapolation procedures described in the TGD to a set of test substance data. It can also be considered analogous to, and used for risk assessment in the same way as, a PNEC derived by applying an Assessment Factor (AF) specified in the TGD to a lowest acute EC50 or LC50 value in a data set.

Short term toxicity to fish:

In a key, semi-static 96 hour rainbow trout (Oncorhynchus mykiss) acute toxicity test (OECD 203, KS=1), seven fish/replicate were exposed to the Water Accommodated Fraction (WAF) of distillates (petroleum), light catalytic cracked (MRD-10-576) at a nominal concentration of 0.30 mg/L. The LL50 was >0.30 mg/L based on mortality (ExxonMobil 2011b).

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater fish 96-hr LL50 value is 0.156mg/L.

Long term toxicity to fish:

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater fish NOEL (No Observed Effect Level) value is 0.029 mg/l based on mortality.

Short term toxicity to aquatic invertebrates:

In a key static 48 hourDaphnia magnaacute toxicity test (OECD 202, KS=1), five daphnids/ replicate were exposed to the Water Accommodated Fraction (WAF) of light cracked gas oil (MRD-10-595) at nominal concentrations of 0.05, 0.15, 0.45, 1.35 and 4.05 mg/L. The 48 hour EL50 was 0.32 mg/L based on mobility (ExxonMobil 2012a).

 

In a supporting static 48 hourDaphnia magnaacute toxicity test (OECD 202, KS=1), five daphnids/ replicate were exposed to the Water Accommodated Fraction (WAF) of light cracked gas oil (MRD-10-578) at nominal concentrations of 0.05, 0.15, 0.45, 1.35 and 4.05 mg/L. The 48 hour EL50 was 0.7 mg/L based on mobility (ExxonMobil 2012b).

 

In a supporting static 48 hourDaphnia magnaacute toxicity test (OECD 202, KS=1), five daphnids/ replicate were exposed to the Water Accommodated Fraction (WAF) of light catalytic cracked gas oil (MRD-10-576) at nominal concentrations of 0.102, 0.256, 0.640, 1.60 and 4.00 mg/L. The 48 hour EL50 was 0.51 mg/L based on mobility (ExxonMobil 2011c).

 

In a supporting static 48 hourDaphnia magnaacute toxicity test (OECD 202, KS=1), five daphnids/ replicate were exposed to the Water Accommodated Fraction (WAF) of light thermal cracked gas oil (MRD-10-604) at nominal concentrations of 0.32, 1.6, 8.0, 40 and 200 mg/L. The 48 hour EL50 was 5.8 mg/L based on mobility (ExxonMobil 2012c).

 

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater invertebrate 48-hr EL50 value is 0.319mg/L.

 

Long term toxicity to aquatic invertebrates:

In a key, semi-static 21 day Daphnia magna chronic toxicity test (OECD 211, KS=1), one daphnid/replicate was exposed to the Water Accommodated Fraction (WAF) of light catalytic cracked gas oil (MRD-10-576) at nominal concentrations of 0.05, 0.10, 0.18, 0.34 and 0.65 mg/L. The 21 day NOELR was 0.05 mg/L based on growth and reproduction and 0.18 mg/L based on mortality (ExxonMobil 2012d).

 

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater invertebrate NOEL (No Observed Effect Level) value is 0.053mg/l based on immobility and numbers of live young produced per adult by Day 21.

 

Toxicity to aquatic algae and cyanobacteria:

In a key, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of light cracked gas oils (MRD-10-595) at nominal concentrations of 0.05, 0.23, 1.01, 4.56 and 20.5 mg/L. The 72 hour EL50 was 0.51 mg/L and NOELR was 0.05 mg/L based on growth rate (ExxonMobil 2012e).

 

In a supporting, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of light cracked gas oils (MRD-10-578) at nominal concentrations of 0.05, 0.23, 1.01, 4.56 and 20.5 mg/L. The 72 hour EL50 was 0.93 mg/L and NOELR was 0.05 mg/L based on growth rate (ExxonMobil 2011d).

 

In a supporting, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of light catalytic cracked gas oils (MRD-10-576) at nominal concentrations of 0.10, 0.32, 1.02, 3.28 and 10.5 mg/L. The 72 hour EL50 was 0.53 mg/L based on growth rate. The 72 hour NOELR was 0.1 mg/L (ExxonMobil 2011e).

 

In a supporting, static 72 hour Pseudokirchneriella subcapitata growth inhibition test (OECD 201, KS=1), 10000 cells/mL were exposed to the Water Accommodated Fraction (WAF) of thermally cracked gas oils (MRD-10-604) at nominal concentrations of 0.32, 1.6, 8.0, 40 and 200 mg/L. The 72 hour EL50 was 8.82 mg/L and the NOELR was 0.32 mg/L based on growth rate (ExxonMobil 2012f).

 

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated algal 72-hr EL50 value is 0.202mg/L.

 

Toxicity to microorganisms:

The aquatic toxicity was estimated using the PETROTOX computer model, which combines a partitioning model (used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading) with the Target Lipid Model (used to calculate acute and chronic toxicity of non-polar narcotic chemicals). PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a petroleum substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated 40-hr EL50 value forTetrahymena pyriformis, one of the most sensitive microorganism species, is 1.954mg/L and the estimated NOEL is 0.241 mg/L.

Some information for this category has been generated using the models PETROTOX and/or SPARC. The QMRFs for PETROTOX and SPARC are attached in IUCLID Section 13, with the associated QPRF.

The toxicity of cracked gas oils to algae, daphnia and fish are generally very similar based on the results of experimental studies. Cracked gas oils have a 96 hour LL50 to rainbow trout (Oncorhynchus mykiss) of >0.30 mg/L. They have a 48 hour EL50 to Daphnia magna of 0.32 mg/L and a 21 day NOELR to Daphnia magna of 0.05 mg/L based on growth and reproduction and 0.18 mg/L based on mortality. Cracked gas oils have a 72 hour EL50 toPseudokirchneriella subcapitataof 0.51 mg/L with a 72 hour NOELR of 0.05 mg/L based on growth rate. Based on these values, cracked gas oils meet the criteria under EU CLP Regulation (EC No. 1272/2008) for classification as Aquatic Acute Toxicity 1 (H400: Very toxic to aquatic life) and Aquatic Chronic Toxicity 1 (H410: Very toxic to aquatic life with long lasting effects).