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Ecotoxicological information

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Additional information

1. Aquatic toxicity: freshwater, short-term

establishing the dataset

In accordance to the approach followed in the RAR, only acute data from standardised test protocols were considered in the analysis for setting the reference value for classification. This is possible because numerous data are available, and it ensures that the tests were performed under rather well defined and standard conditions.

Still, the quality and some aspects of relevancy should be checked in a critical way when using the extensive datasets from the open literature, available for zinc. It is e.g. important to know the conditions under which the organisms were tested and cultured, because these conditions may result in acclimatisation and deviating toxicity response. The information on these test conditions is often scarce in non-standardised test reports.

The short-term aquatic ecotoxicity data base for zinc was reviewed according to the following principles:

  • the data accepted for setting the acute aquatic reference value in the RA (ECB 2008, Annex 1.3.2a, table 1) were as such also accepted and used for the present analysis. Prescriptions from standard protocols were strictly followed, e.g. data from an acute Daphnia test exceeding 48 hrs were not used.
  • Data that were rejected for use in the RA (ECB 2008, Annex 1.3.2a, table 2) were also not used for the present analysis. In this respect, data from studies that were accepted for use in the chronic database, but rejected for use in the acute toxicity database were reconsidered; this resulted in the acceptance of a few additional data.
  • In accordance to the approach followed in the RA, acute data obtained in natural waters that contained e.g. significant amount of DOC, were not used. Exception to this rule were data obtained on the N.-American Great Lakes waters, which were used, in accordance to the RA.
  • Fish data mentioned in the RA under “EHC 1996” were not used, since they were from a review, not from original study reports. These data are not influencing the outcome of the analysis, since they are all at the higher concentration level.
  • More recent (obtained after 1996 to the present) short-term acute toxicity data on standard organisms were included in the database.

After checking and updating the data base, the data are grouped per species as follows:

-pH: low (6 -<7) - neutral/high (7 -8.5)

-hardness: low/medium (<100mg CaCO3/l) and medium/high (>100 mg CaCO3/l).

If 4 or more data points were available on a same species, the geomean was calculated and used for the analysis.

Acute data – results

The short-term acute aquatic toxicity database covers 10 species (1 algae, 4 invertebrates and 5 fish species). The full set of EC50 values are presented together with the pH and hardness of the test media in the CSR. A significant number of data are available at both low and neutral/high pH.

 

Discussion: reference values for short term aquatic ecotoxicity

Table above presents an overview of the information available for short-term aquatic toxicity for zinc. It can be seen that significant number of data are available at both low and neutral/high pH.

At low pH, 2 values are available for 2 daphnia species. The values are similar. They were obtained at lower hardness, where the highest sensitivity is expected, which is confirmed by the value >530 µg/l, obtained on Ceriodaphnia dubia at high hardness. Algae are as a rule not tested under standardised conditions at low pH, but from chronic algae data (72 hrs NOECs), it is known that the sensitivity of algae is much lower at lower pH. Simulation with the biotic ligand model gives an aquatic ecotoxicity value for algae at pH 6 which is about 5 times higher than the one observed at neutral/high pH. Fish toxicity at low pH is also not critical in this respect, so the values for the daphnids are representative for the sensitivity of organisms to zinc at low pH. The lowest value observed for Ceriodaphnia dubia is used for the classification at low pH.  

At neutral/high pH, the value obtained on the algae Selenastrum capricornutum is the lowest of the dataset. This value is taken forward as reference value for classification at this pH. This value is obtained at low hardness conditions, where sensitivity is highest. The same algae species is also the most sensitive in the chronic aquatic toxicity database (see below) so this sensitivity pattern is consistent. Among the daphnids, Ceriodaphnia dubia is also here the most sensitive, and the lowest value comes close to the one for the alga. From the paired data, it follows that the Daphnids are more sensitive at lower hardness than at the higher hardnesses. The fish are also at this pH less sensitive to zinc, although the lowest value observed on O. Mykiss also comes close to the reference value. All together, the lowest values among the species show also here a consistent pattern, supporting the lowest value identified.

In conclusion, the reference values for the Zn++ion that are used for the aquatic toxicity hazard assessment of Zn++are:

  • for low pH: 0.413 mg Zn/l (based on single lowest value for Ceriodaphnia dubia)
  • for the neutral/high pH: 0.136 mg Zn/l (based on single lowest value for Selenastrum capricornutum (=Pseudokircherniella subcapitata)

2. Aquatic chronic toxicity: freshwater

Chronic data - establishing the dataset

In this analysis, like in the RAR, the results of the chronic aquatic toxicity studies are expressed as either the actual (measured) concentration or as the nominal (added) concentration (Cn). The actual concentrations include the background concentration (Cb) of zinc. Because of the “added risk approach”, the results based on actual concentrations have been corrected for background, if possible. This correction for background is based on the assumption that only the added concentration of zinc is relevant for toxicity. In case both actual and nominal concentrations were reported, the results are expressed in the RAR (and in this CSR) as nominal concentrations, provided the actual concentrations were within 20% of the nominal concentrations.

Many of the reported aquatic toxicity data (either actual or nominal) represent total-zinc concentrations, i.e. the dissolved plus particulate fraction. However, the results are regarded as being dissolved-zinc concentrations, because under the conditions that were used in the laboratory tests, it is assumed that the greater part of zinc present in the test waters was in the dissolved fraction. This is especially true for the long-term studies, e.g. by using flow-through systems, in which particulate matter (suspended inorganic material and/or organic matter) was removed from the artificial test waters or natural waters. The fact that in ecotoxicity testing the nominal added concentration of zinc is very close to the actually measured zinc concentration, is also demonstrated by the many data reported in the papers of the chronic aquatic ecotoxicity database. Also in static and flow-through acute toxicity studies with several saltwater species, dissolved zinc was greater than 93% of the total zinc. Therefore, the PNECaddvalues derived from the aquatic toxicity studies are considered to be relevant for dissolved zinc.

The chronic aquatic toxicity dataset for zinc was checked according to the general criteria for data quality:

-study design preferably conform to OECD guidelines or equivalent

-Toxicological endpoints, which may affect the species at the population level, are taken into account. In general, these endpoints are survival, growth and reproduction.

- whether or not NOEC values are considered chronic is not determined exclusively by exposure time, but also by the generation time of the test species, e.g. for unicellular algae and other microorganisms (bacteria; protozoa), an exposure time of four days or considerably less already covers one or more generations, especially in water, thus for these kinds of species, chronic NOEC values may be derived from relatively short experiments. For PNEC derivation a full life-cycle test, in which all relevant toxicological endpoints are studied, is normally preferred to a test covering not a full life cycle and/or not all relevant endpoints. However, NOEC values derived from tests with a relatively short exposure time may be used together with NOEC values derived from tests with a longer exposure time if the data indicate that a sensitive life stage was tested in the former tests.  

-If for one species several chronic NOEC values (from different tests) based on the same toxicological endpoint are available, these values are averaged by calculating the geometric mean, resulting in the “species mean” NOEC.

-If for one species several chronic NOEC values based on different toxicological endpoints are available, the lowest value is selected. The lowest value is determined on the basis of the geometric mean if more than one value for the same endpoint is available.

-In some cases, NOEC values for different life stages of a specific organism are available. If from these data it appeared that a distinct life stage was more sensitive, the result for the most sensitive life stage is selected. The life stage of the organisms is indicated in the tables as the life stage at start of the test (e.g. fish: yearlings) or as the life stage(s) during the test (e.g. eggsàlarvae, which is a test including the egg and larval stage).

-Only the results of tests in which the organisms were exposed to zinc alone are used, thus excluding tests with metal mixtures.

-Like in the RAR, unbounded NOEC values (i.e. no effect was found at the highest concentration tested) are not used.

-If the NOEC was <100 µg/l, the separation factor between the NOEC and LOEC should not exceed a factor of 3.2.

-If the EC10 was used as NOEC equivalent, the EC10 should not be more than 3.2-times lower than the lowest concentration used in the test.

-Like in the RAR, only the results of tests with soluble zinc salts are used, thus excluding tests with “insoluble” zinc salts (ZnO, ZnCO3), unless dissolved zinc is measured.  

 

Referring to the EU RA on zinc (ECB 2008), all the data that were accepted for deriving the freshwater PNEC in the RA (ECB 2008, Annex 3.3.2.A. part I) were as such also accepted for the present analysis. On the other hand, the data that were considered not useful for the purpose of PNEC derivation in the RA (ECB 2008, Annex 3.3.2.A. part II), were also not used for the present analysis.

 

The relevancy of the long-term aquatic ecotoxicity data base for PNEC derivation was further checked in accordance to the same principles as those applied in the RA (ECB 2008). Relevancy was checked:

1) related to the zinc background: results obtained in unpolluted test media (water, sediment or soil) are used, thus excluding tests that were performed in media containing high to very high background Zn concentrations, i.e. in case the control media contained zinc concentrations that are clearly above Zn concentrations normally encountered in relatively unpolluted environmental compartments. A pragmatic cut-off level of 30µg Zn/l, in accordance to decisions taken in the RA (ECB 2008) was set for this.

In accordance to the RA zinc (ECB 2008), data obtained in tests where the zinc background concentration was much lower than the natural background for EU waters, were also not used for PNEC derivation. In accordance to the RA (ECB 2008), a level of 1µg/l Zn was set as a cut-off for this. 

 

2) related to test medium conditions: Zinc ecotoxicity to aquatic organisms is a function of the physicochemical characteristics of the water. Parameters such as hardness, pH, dissolved organic carbon (DOC) are well-known drivers for zinc ecotoxicity. For this reason, it was considered important in the EU RA to select ecotoxicity data that were obtained under test conditions similar to the conditions observed in EU waters. Based on information related to the parameters mentioned above in EU waters, the following boundaries for EU relevancy for pH, hardness have been used in the RA (ECB 2008) and also in the present analysis for data selection, also considering OECD test guidelines:

pH:                             minimum value: 6

                                   maximum value: 9

Hardness:                   minimum value: 24 mg/l (as CaCO3)

                                   maximum value: 250 mg/l (as CaCO3)          

As indicated above, background zinc concentration was also considered in the RA to be a factor influencing the toxicity response of organisms to zinc; to avoid influence of acclimatisation towards very low or very high zinc concentrations (not occurring in the EU waters), a minimum value for soluble zinc was also set in the RAR for data selection: “around 1 µg/l” (ECB 2008).

Data obtained under conditions failing these relevancy criteria were not used for PNEC derivation in the present analysis. For a detailed description of the relevancy criteria and their application in the RA, see the RAR (ECB 2008).

It is realised that the selected ranges of the three criteria will not cover all European aquatic systems, e.g. specific aquatic systems in the Scandinavian countries. In particularly, hardness is much lower in the Scandinavian countries, although also other abiotic parameters differ from the ‘average’ situation in European freshwaters. Therefore, a “soft water PNECadd, aquatic” has been derived in the RA process, in addition to the generic PNECadd, aquatic.The present analysis however relates to the development of a generic PNEC for EU waters.

DOC: Tests have been considered relevant for the present analysis if DOC concentrations in the test media are between 0 mg/l and 13 mg/l. In most test solutions, DOC is not present.

 

The extensive dataset on chronic aquatic toxicity in the RA (ECB 2008) was also updated with new information. This information was screened for the same criteria as those described above.

For details on data selection see the CSR zinc.

 

Results

The 23 distinct chronic species ecotoxicity values that were used for the SSD in the present analysis are summarised in the CSR. The “species mean” NOEC values used for PNEC derivation (freshwater PNECadd, aquatic), range from 19 to 530 µg/l.

PNEC derivation

All adequate chronic data on fish, invertebrates, algae and plants were considered together in a species sensitivity distribution (SSD), and the PNECwas calculated by means of statistical extrapolation, using all available chronic NOEC values as input. The database is indeed sufficiently large and answers the basic requirements to use an SSD, since it covers the required 8 different taxonomic groups and > 10 test organisms.

Since the log-normal distribution significantly fits the data, this distribution was used for the SSD (like in the RAR). Other conditions to apply statistical extrapolation were also met (see CSR, discussion on the safety factor to be applied to the HC5). 

 

Because of the inclusion of 6 additional species, the species sensitivity distribution (SSD) that was calculated for the present analysis is slightly different from the one of the RAR (2008). For a detailed discussion on the uncertainty related to the SSD and the HC5, and the derivation of the PNEC, see the CSR.

As a result of the analysis, a PNEC freshwater of 20.6 µg Zn/l was derived.

3. Aquatic chronic toxicity: marine waters

For zinc, a specific effects assessment was made and a specific PNEC was derived for the marine environment, since there is a vast dataset available on marine ecotoxicity. This specific approach is also more reflective of the toxicity of zinc in the marine environment given the different speciation and bioavailability of zinc in salt – and freshwater, and differences in physiology of saltwater organisms. Given the vast amount of available toxicity data, statistical extrapolation was used to derive the marine PNEC. This marine effects assessment is following an added risk approach, as applied for the freshwater.

 

 

Sources of data

The ecotoxicological data were derived from original papers, published in peer-reviewed international journals. Literature and environmental databases, including AQUIRE (US EPA), MARITOX, ECETOC, and BIOSIS, as well as review articles covering zinc in marine waters were searched and reviewed for sources of relevant and reliable chronic toxicity data on zinc. Only original literature was used.

 

Data reliability and relevancy

Selection of ecotoxicity data for quality was done according to a systematic approach as presented by Klimisch et al. 1997. Standardized tests, as prescribed by organizations such as ASTM, OECD and US EPA, are used as a reference when test methodology, performance and data treatment/reporting are considered. A detailed description of data reliability and relevancy is provided in the CSR.

 

PNEC saltwater

Ecotoxicity database for zinc on species of the marine aquatic environment

The marine zinc database largely fulfils the species and taxonomic requirements for input chronic toxicity data as explained in the RIP R. 10 guidance (at least 10 species NOECs and 8 taxonomic groups). Indeed, 39 species mean NOECs based on 48 NOEC values, from 9 taxonomic groups covering three trophic levels were found to fulfil the relevancy and reliability requirements as explained by Klimisch et al. 1997. The marine zinc database includes 4 micro- and 5 macro-algae species, 4 annelid species, 6 crustacean species, 5 echinoderm species, 9 mollusc species, 1 nematod species, 1 cnidarian species and 1 fish species. The geometric mean values of the species NOECs are presented in the CSR.

 

Statistics on the species sensitivity distribution (SSD)

Given the multitude of relevant high quality toxicity data, statistical extrapolation was used for PNEC determination. As the approach taken is based on added risks, the results of the toxicity tests based on measured concentrations were corrected for background zinc concentration. Given the wealth of experimental data, no alternative method i.e. assessment factor approach was applied for the PNEC determination.  

Following the RIP R. 10 guidance, different distributions may be used for the SSD. Fitting of the chronic zinc toxicity data was assessed towards the log-normal frequency distribution (default distribution).  To be conform with the approach taken in the Zn RAR 2008, it is the lognormal distribution which was used to provide a basis for setting the PNEC saltwater, in spite of a better fit with the Weibull statistical distribution.   It is noted also that the PNEC is a PNECadd., i.e. the background concentration needs to be considered in the compliance assessment exercise. The 5thpercentile value of the SSD (the HC5), set at 50% confidence value, using the lognormal distribution function, results in a value of 6.09 µg zinc/L. For further details on the setting of the PNEC from the SSD, see the CSR zinc.