Distillates (petroleum), chemically neutralized light

Endpoint summary

Additional information

Discussion on methodology

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 semi-static 96 -hour acute rainbow trout (Oncorhynchus mykiss) test (OECD 203; KS = 1), 7 animals/dose were exposed to solvent, naphtha (petroleum), heavy aromatic kerosine at nominal concentrations of 0, 0.2, 0.7, 2.0, 5.0, 17.0, and 50.0 mg/L. Some fish were observed swimming abnormally and immobilisation was also observed. The LL50 was 2 to 5 mg/L. The NOEL is 2.0 mg/L (Shell, 1994).

 

In four reliable supporting semi-static 96-hour acute rainbow trout (Oncorhynchus mykiss) studies, the LL50 ranged from 2 to 100 mg/L; with the NOELs ranging from 6.8 to 10 mg/L (Exxon 1995a,b,c, Shell 1995).

 

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 0.098 mg/L based on mortality (Redman et al., 2010b).

Short term toxicity to aquatic invertebrates:

In a static 48 -hour acute daphnid (Daphnia magna)test (OECD 202; KS = 1), 20 animals/dose were exposed to kerosine petroleum, hydrodesulfurised at nominal concentrations of 0, 0.1, 0.3, 1.4, 6.8, and 34 mg/L. The EL50 was 1.4 mg/L with a 95% confidence interval of 1.0 to 2.0 mg/L. The No Observed Effect Loading (NOEL) rate was 0.3 mg/L determined by immobilisation (Exxon, 1995d).

The toxicity for four kerosine products was tested using WAF methodology. In these reliable supporting studies the 48 -hour EL50 (loading rate resulting in 50% immobilization of Daphnia) varied between 1.9 and 89 mg/L. The NOELs for these tests varied between 0.3 and 40 mg/L (Shell 1995, Shell 1994, Exxon 1995e, Exxon 1995f).

Long term toxicity to aquatic invertebrates:

In a 21-day semi-static chronic reproductive toxicity test (OECD 211; KS = 1) onDaphnia magna, hydrodesulfurised kerosine was evaluated using water accommodated fraction methodology.The actual loading rates were 0 (control), 0.08, 0.19, 0.48, 1.2, and 3.0 mg/L.Under the conditions of this test, the 21-day chronic reproductive NOEL for kerosine is 0.48 mg/L. The LOEL is 1.2 mg/L. The EL50 based on reproduction is 0.89 mg/L (ExxonMobil, 2010).

Toxicity to aquatic algae:

 

In a key 72-hour toxicity test (OECD 201; KS = 1), cultures of the freshwater plant Raphidocelis subcapitata were exposed to solvent naphtha (petroleum), heavy aromatic kerosine at nominal concentrations of 0, 0.1, 0.4, 1.0, 3.0, and 10.0 mg/L under static conditions. The 72-hour NOEL and EL50 values based on most average specific growth rates were 1.0 mg/L and 1 to 3 mg/L, respectively. The % growth inhibition, based on average specific growth rate, in the treated algal cultures as compared to the control ranged from 7.1 to 270%. Signs of phytoxicity include a reduction in growth rate (Shell, 1994).

In two reliable supporting algae toxicity studies (Selenastrum capricornutum) for two different kerosines products (Exxon, 1995h, 1995i) tested using the WAF methodology and following OECD Guideline 201, the 96 -hour EL50 (loading rate resulting in 50% decrease in growth of algae) varied between 5.0 to 6.2 mg/L (based on both area under the growth curve and average specific growth rates). The NOEL for these tests varied between 0.4 and 6.2 mg/L. In another reliable supporting algae toxicity study (Raphidocelis subcapitata) for hydrodesulfurised kerosine (Shell, 1995) was tested using WAF methodology, the 72 -hour EL50 was 10 to 30 mg/L WAF (based on both area under the growth curve and average specific growth rates). The NOEL was 10 mg/L. In another supporting algae toxicity study testing sweetened kerosine (Exxon, 1995g), on Selenastrum capricornutum, the 72-hour EL50 was 3.7 mg/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 72-hour LL50 value for Tetrahymena pyriformis, one of the most sensitive microorganism species, is 677.9mg/L (Redman et al., 2010b).

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