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Toxicity to microorganisms

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Effects of boric acid on micro-organisms was estimated using the standardized OECD 209 method, Hanstvelt and Schoonmade (2001) reported 24% inhibition at 175 mg-B/L, and an EC10 can be calculated from their data as 58 mg-B/L. A NOEC was reported by the study authors of 17.5 mg-B/L, but was not determined via any statistical means. In actuality, there was 4% inhibition at this exposure, so it is an EC4, thus more stringent than intended by the OECD 209 protocol. An EC10 value was calculated by linear regression from the data which showed a good fit (R2= 0.863) with a value of 58.0 mg-B/L. This study complies with GLP practice, is rated as highly reliable in quality, and is consistent with earlier tests of activated sludge, however they were not published.

Gerike et al.(1976), using an earlier version of the OECD activated sludge method reported LOEC and NOEC of 120 and 20 mg-B/L, respectively. Guhl (1992, 2000) reported activated sludge LOEC and NOEC of 50 and 20 mg-B/L, respectively.

Other reports include tests of the microbe,Pseudomonas putida, with NOEC values ranging from 7.6 to 1040 mg-B/L However, these study reports are of varying reliability and it appears that the high value (1040) is incorrect. The geometric mean value of the Henkel studies is 50.8 mg-B/L (EC10 values of 340 and 7.6 mg-B/L).

Guhl (1992, 2000) reported on ciliate growth inhibition to Entosiphon sulcatum, Opercularia bimarginata, and Paramecium caudatum, with NOEC values of 10, 15, and 20 mg-B/L, respectively. Bringmann and Kuehn (1980b) reported a toxicity threshold (EC5) to the ciliateUronema pardacziof 23 mg-B/L.

The data on sewage treatment plants appears consistent, specifically that no effects are seen at boron exposures of 8 to 20 mg-B/L. The TGD notes that in some cases the PNECSTPvaries because different application factors are applied per Table 4-6 and in such cases expert judgment should be used to determine which effect value should be used. In the present case, the application factor applied to activated sludge drives the PNECSTPfrom that calculation significantly below the PNECSTP calculated from ciliate inhibition tests. Pseudomonas inhibiton. This disproportionate result suggests that the appropriate PNECSTPwould emphasize the data rather than choice of application factor. Thus use of the most sensitive public data point (from the data set:Opercularia bimarginata NOEC – 3d = 10 mgB/l) will be used in order to estimate PNECSTP.