Extracts (petroleum), heavy naphthenic distillate solvent

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 petroleum products 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 are expressed either as (a) the dilution, or % WSF, or (b) the concentration of dissolved hydrocarbons expressed in mg/L (CONCAWE, 1992b). 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 (1992b). 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 products 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 short-term rainbow trout (Oncorhynchus mykiss) limit test (OECD 203; KS=1), 10 fish/replicate were exposed to the Water Accommodated Fraction (WAF) of a heavy paraffinic distillate solvent extract (PSG 1860; CAS # 64742-04 -7) at nominal concentrations of 0 and 1000 mg/L. The NOEL was ≥ 1000 mg/L (BP Oil Europe, 1994a).

Long-term toxicity to fish:

No chronic toxicity studies have been carried out on fish with DAEs. Composition information, derived using two dimensional gas chromatography, has been used in conjunction with the PETROTOX model to calculate this endpoint.

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 value is 20.01 mg/L based on mortality (Redman et al., 2010b).

Short-term toxicity to aquatic invertebrates:

The data on the acute toxicity of DAEs to Daphnia magna confirms that there is variability in toxicity of test samples within this category, probably as a result of variable composition of DAE. In a biomimetic extraction (BE) screening study conducted using WAFs at 100 mg/L loading rate with five DAEs (including CAS# 64742- 05-8 and 64742-04-7) only one sample (a light paraffinic distillate solvent, CAS# 64742-05-8) proved to have a positive BE result, all others were non-detect (ExxonMobil, 2010e). 

In a subsequent supporting 48-hour short term Daphnia magna toxicity test (OECD 202; KS=1), daphnids were exposed to WAFs of two DAE samples at a nominal concentration of 100 mg/L. The samples included the one with the positive BE result, a light paraffinic distillate solvent (CAS# 64742-05-8) and one of the samples with a non-detect BE result, a heavy paraffinic distillate solvent (CAS# 64742 -04 -7). The 48-hour EL50 values were estimated to be <100 mg/L and >100 mg/L, respectively (ExxonMobil, 2012b).

In a subsequent key static 48 -hour short-term Daphnia magna toxicity test (OECD 202; KS=1), daphnids were exposed to WAFs of the sample with the positive BE result, a light paraffinic distillate solvent (CAS# 64742 -05 -8) at nominal concentrations of 0.1, 1.0, 10, 100, and 1000 mg/L. The 48 -hour EL50 was calculated to be 35.9 mg/L and the NOEL was 0.1 mg/L based on mobility (ExxonMobil, 2012a).

In another supporting static 48 hour short-term daphnia (Daphnia magna) toxicity test (OECD 202; KS=1), 40 animals/loading were exposed to the WAF of distillate aromatic extract (PSG 1860; CAS# 64742-04-7) at a nominal concentrations of 1000 mg/L. The EL50 was > 1000 mg/L. The NOEL was determined by immobilisation, and was ≥ 1000 mg/L (BP Oil Europe 1994b).

Long-term toxicity to aquatic invertebrates:

In a key semi-static 21-day long-term Daphnia magna test (OECD 211; KS=1) using the WAF of Distillate Aromatic Extract (CAS 64742-05-8), ten replicate test chambers (each containing one daphnid) were prepared for each treatment group and control. Two actual loadings were tested, namely 0.1 and 1.1 mg/l. The 21 day NOELR for reproduction was 0.1 mg/l, the LOELR was 1.1 mg/l (ExxonMobil Biomedical Sciences (2012).

In a supporting semi-static 21 -day long-term Daphnia magna reproductive test (OECD 211; KS= 1), 40 animals/loading were exposed to the water accommodated fraction of distillate aromatic extract at nominal concentrations of 0, 10, and 1000 mg/L. Concentrations of the test substance were not determined during the study. The 21 day EL50 was >1000 mg/L and the NOEL was 1000 mg/L based on immobilisation and reproduction, respectively (BP Oil Europe, 1995).

Toxicity to aquatic algae and cyanobacteria:

The data on the acute toxicity of DAEs to Pseudokirchneriella subcapitata confirms that there is variability in toxicity of test samples within this category, probably as a result of variable composition of the DAEs. In a biomimetic extraction screening study conducted using WAFs at 100 mg/L loading rate with five DAEs (including CAS# 64742- 05-8 and 64742-04-7) only one sample (a light paraffinic distillate solvent, CAS# 64742-05-8) proved to have a positive BE result, all others were non-detect (ExxonMobil, 2010e).

In a subsequent supporting 72 -hour alga (Pseudokirchneriella subcapitata formerly Selenastrum capricornutum) toxicity test (OECD 201; KS =1) the freshwater alga was exposed to WAFs of two DAE samples at a nominal concentration of 100 mg/L. The samples included the one with the positive BE result, a light paraffinic distillate solvent (CAS# 64742-05-8) and one of the samples with a non-detect BE result, a heavy paraffinic distillate solvent (CAS# 64742 -04 -7). The 72-hour EL50 values were estimated to be <100 mg/L and >100 mg/L, respectively (ExxonMobil, 2012d).

In a subsequent key 72 -hour alga (Pseudokirchneriella subcapitata formerly Selenastrum capricornutum) toxicity test (OECD 201; KS =1), the freshwater alga was exposed to WAFs of the sample with the positive BE result, a light paraffinic distillate solvent (CAS# 64742 -05 -8) at nominal concentrations of 0.1, 1, 10, 100, and 1000 mg/L. The 72 -hour EL50 was calculated to be 18.8 mg/L and the NOEL was 0.1 mg/L based on the slope of the growth rate (ExxonMobil, 2012c).

Toxicity to microorganisms:

Composition information, derived using two dimensional gas chromatography, has been used in conjunction with the PETROTOX model to calculate this endpoint.

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 hydrocarbon substance and membrane-water partition coefficients (Kmw) that describe the partitioning of the hydrocarbons between the water and organism. Results of computer modelling to estimate aquatic toxicity show no acute toxicity to freshwater microorganisms at or below its maximum attainable water solubility (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.