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Sediment toxicity

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The most sensitive endpoint is a 10-day NOEC (growth) for Hyalella azteca of 0.012 g Ag/kg dry weight (Call et al., 2006). This value was used to derive the sediment PNEC.  An assessment factor of 10 was applicable, and the value was normalised to 5 % organic content to give the derived PNEC of 
438.13 mg/kg dw.

Key value for chemical safety assessment

Additional information

In freshwater sediments spiked with silver nitrate, a high proportion of the dissolved silver fraction was not readily bioavailable to cause lethality to dipteran insect larvae (Call et al., 1999). Porewater concentrations of dissolved silver that killed 50% of the larvae were up to 275 times greater than the 10-day water only LC50 value of 57 µg/L, indicating that most of the dissolved fraction was not readily bioavailable to cause death. The 10-day LC50 values for Chironomus tentans, based upon nominal additions of Ag to sediments, were 2.75 and 1.17 g Ag/kg dry sediment. The lowest NOEC for growth was 0.2 g Ag/kg dry sediment.

Hyalella azteca held on sediments containing as much as 753 mg Ag/kg dw, as silver sulphide, for 10 days had normal growth and survival (Hirsch, 1998). No significant effect was observed on the growth of oligochaetes (Lumbriculus variegatus) exposed to laboratory-spiked sediments containing 444 mg Ag kg dw, as silver sulphide for 28 days (Hirsch, 1998).

Hyalella azteca exposed to Ag as AgNO3 over a 10-day period in two lake sediments resulted in 10-day LC50s of 0.084 g Ag/kg and 2.98 g Ag/kg dry sediment for sediment with a lower quantity of total Ag-binding ligands and a greater quantity of total Ag-binding ligands respectively (Call et al., 2006). The NOEC for growth in the former was 0.012 g Ag/kg dry sediment. These results demonstrate the differing Ag binding capacities of sediments. The sediment with the lower quantity of Ag-binding ligands was sandy (95.3%) with a low level of organic carbon (0.29% TOC). In contrast, the other sediment contained a greater proportion of small-grained particles (59% silt; 11.2% clay) and a higher lever of organic carbon (2.5% TOC).

As discussed above, sediment physicochemistry can affect the toxicity of silver in sediments. Berry et al. (1999) exposed the marine amphipod Ampelisca abdita for 10 days to two marine sediments with different amounts of acid-volatile sulphide spiked with silver (Sediment 1: 2.7 – 4415 mg silver/kg dw, Sediment 2: 2.7 – 72,770 mg silver/kg dw). The EC10 values for these two sediments were 4415 and 6626 mg/kg, respectively. In general, sediment treatments with an excess of acid-volatile sulphide (AVS) relative to simultaneously extracted metal were not toxic to marine amphipods.