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Sediment toxicity

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Description of key information

Whole sediment long term toxicity tests have been performed with the annelid L.variegatus, the fatmucket clam L.siliquoidea, and with the midge Chironomus riparius (Lockwood et al, 2011b,c ; Hooftman, 2000 ; Gerke, 2011b). Lockwood et al (2001b,c) reported nominal NOEC-values of 80.6 mg NB/kg dry wt for L.siliquoidea, and an unbounded NOEC of <100.8 mg B/kg dry wt for L. variegatus (25% effect). Hooftman (2002) reported a nominal NOEC value of 180 mg/kg dry wt. The test that was conducted by Gerke (2011b) yielded a measured unbounded NOEC of 37.8 mg/kg dry wt (as mean total measured concentration). 

No studies with marine species/mare sediments are included in the assessment report.

Key value for chemical safety assessment

Additional information

The high water solubility of boric acid and corresponding low sorption to sediment means that a sediment-only exposure is not possible. Standard protocols involve spiking the test substance to the sediment at the initiation of the study. The overlying water is not spiked. However, boric acid will readily dissolve from the sediments during the course of a sediment study, so the test organisms will actually experience both water-borne and sediment-borne exposures.

The Hooftman study (2000) had a static test design using 6 test concentrations (18-320 mg B/kg dry wt.) spiked in a formulated sediment. Only nominal values were reported by Hooftman. In the test design of Gerke (2011b), a formulated sediment was spiked with 6 different B concentrations between 6.5 and 200 mg B/kg (as nominal concentrations). After spiking, dilution water (i.e. overlying water) was added to the prepared formulated sediment where after midge larvae (C. riparius) were added after 2 days of equilibration for 28 days toxicity testing.

Lockwood et al (2011b,c) used a semi-static test design with 20% renewal, ensuring that B-levels in the sediment were maintained by spiking the overlying water.

 

The long term toxicity results on survival/emergence for C. riparius could be completely explained by the boron concentrations measured in the water column. Assuming that quivalent fate processes take place in the other sediment testing set-ups, a similar conclusion could be drawn for L.siliquoidea. Therefore, the additional contribution of boron adsorbed to sediment to the observed toxicity is considered to be negligible. This observation comes as no surprise since the estimated Kd values for boron are very low (mean value of 1.94 and 3.0 L/kg for respectively freshwater and marine sediment; 3.5 L/kg for suspended solids), indicating that boron has no tendency to adsorb to sediments.

The weight of evidence provided by the lack of partitioning (Kd estimates) and the results of the water only/whole sediment toxicity tests indicate that it is unlikely that boron will exert toxic effects via the sediment compartment and that the derivation of a PNEC sediment is not warranted for boron and could waived based on exposure considerations.

Additional information on these studies can be found in the Background Document “Environmental effects assessment of boron”, which is attached in the technical dossier in IUCLID Section 13.

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