Naphthenic acids, zinc salts

Endpoint summary

Description of key information

Based on a water accommodated fraction loading rate (WAF), naphthenic acids, zinc salts have a 96-hour LC50 for fish of >100 mg/L and a 48-hour EL50 for Daphnia magna of 35 mg/L. The 72h-NOELR for algae growth rate inhibition was 1.0 mg/L, when based on biological relevance, and the 72h-EL50 was 4.0 mg/L.

Additional information

The acute toxicity of naphthenic acids, zinc salts to fish and Daphnia magna were determined in GLP-compliant tests (Charles River 2018) following OECD guideline 203 and 202, respectively. The inhibition of naphthenic acids, zinc salts to algae growth rate and yield was determined in a GLP-compliant test (Charles River 2018) following OECD guideline 201. The results are considered to be reliable for use.

 

Naphthenic acids, zinc salts is a complex UVCB with a large number of constituents, with varying solubility. As such, the ecotoxicity studies were conducted using Water Accommodated Fractions (WAFs) and the results have been reported based on loading rates. The reporting of loading rates for UVCB substances is based on the OECD 23 guidance:

 

Only a fraction of the total mass of the UVCB or other test material added to the test solution may be present in the WAF. The term “loading rate” in place of “nominal concentration” has therefore been advocated for expressing exposures of UVCBs that neither wholly dissolve nor completely form a stable dispersion or emulsion over the required test range (Girling et al., 1992). The loading rate is the mass to volume ratio of the UVCB to test medium used in the preparation of a WAF. Results can be calculated from the loading rates of the entire substance or specific fractions of the substance and are identified as either a median lethal loading (LL50) or median effective loading (EL50) value. Similarly, the No Observable Effect Loading Rate or NOELR can be calculated. The statistical methods used to determine LL50, EL50 and NOELR values are the same as those used to determine LC50, EC50 and NOEC values. (OECD. 2017. OECD Environmental Health and Safety Publications Series on Testing and Assessment No. 23: Draft guidance document on aqueous-phase aquatic toxicity testing of difficult test chemicals. Revised August 2017.)

 

Chemical analysis (zinc and TOC) has been conducted for all three studies to confirm the stability of the test item in solution. The measured zinc concentrations ranged between 93 and 160% of initial at the end of the test (93-102% for fish, 94-98% for Daphnia and 96-160% for algae). The measured TOC concentrations ranged between 68-158% of initial at the end of the test (72-82% for fish, 68-92% for Daphnia and 100-158% for algae), though it should be noted that results based on concentrations below 1.0 mg TOC/L are only indicative. The analytical results indicate that the concentrations of the substance were stable across the study periods, which supports the reporting of the results for the UVCB substance based on loading rates.