John Wiley & Sons, Ltd.

Relative toxicity of bifenthrin to Hyalella azteca in 10‐day vs. 28‐day exposures

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Many watersheds in the Central Valley region of California are listed as impaired due to pyrethroid‐associated sediment toxicity. The Central Valley Regional Water Quality Control Board is developing numeric sediment quality criteria for pyrethroids, beginning with bifenthrin. Criteria are being developed using existing data, along with data from 10d and 28d toxicity tests with Hyalella azteca conducted as part of the current study. A single range‐finder and two definitive tests were conducted for each test duration. Median lethal concentrations (LC50s), as well as LC20s and inhibition concentrations (IC20s) were calculated based on measured whole sediment bifenthrin concentrations and interstitial water concentrations. Sediment LC50s were also corrected for organic carbon content. Average LC50s were not significantly different in 10d versus 28d tests with H. azteca: 9.1 and 9.6 ng/g bifenthrin for 10d and 28d tests, respectively. Average LC20 values were also similar with concentrations at 7.1 and 7.0 for 10d and 28d tests, respectively. Bifenthrin inhibition concentrations (IC20s) based on amphipod growth were variable, particularly in the 28d tests, where a clear dose response relationship was observed in only one of the definitive experiments. Average amphipod growth IC20s were 3.9 and 9.0 ng/g for 10d and 28d tests, respectively. Amphipod growth calculated as biomass resulted in IC20s of 4.1 and 6.3 ng/g for the 10d and 28d tests, respectively. Lack of a clear growth effect in the longer term test may be related to the lack of food adjustment to account for amphipod mortality in whole sediment exposures. The average carbon‐corrected LC50s were 1.03 and 1.09 μg/g OC for the 10d and 28d tests, respectively. Interstitial water LC50s were determined as the measured dissolved concentration of bifenthrin relative to interstitial water dissolved organic carbon. The average LC50s for dissolved interstitial water bifenthrin were 4.23 and 4.28 ng/L for the 10d and 28d tests, respectively. In addition, a set of 10d and 28d tests were conducted at 15°C to assess the relative toxicity of bifenthrin at a lower temperature than the standard 23°C test temperature. These results showed that bifenthrin was more toxic at the lower temperature, with LC50s of 5.1 and 3.4 ng/g bifenthrin in 10d and 28d tests, respectively. Amphipod growth at 15°C after a 28d exposure resulted in the lowest effect concentration of all experiments conducted (IC20 = 0.61 ng/g). This paper discusses how bifenthrin dose‐response data from 10d and 28d exposures inform development of sediment quality criteria for this pesticide for California Central Valley watersheds. This article is protected by copyright. All rights reserved

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