Natural freshwater sediments from two Oregon sites were spiked with copper in the laboratory and two static toxicity tests were conducted with series of copper concentrations ranging from 59 to 10,600 mg/kg of dry sediment. Water (800 ml) was added to 1‐liter test beakers over the sediment (200 ml) and repeatedly removed and replaced to lower the aqueous copper concentrations. When copper concentrations stabilized, i.e., attained equilibrium between water and sediment, the midge Chironomus tentans, the cladoceran Daphnia magna and the amphipods Hyalella azteca and Gammarus lacustris were introduced as test animals in solid‐phase sediment toxicity bioassays. Based on dry‐weight sediment copper concentrations, LC50 values (48‐h for Daphnia, 10‐d for other organisms) ranged from 681 to 2,296 mg/kg.

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Sediment toxicity tests were conducted with the freshwater amphipod Hyalella azteca using sediments ranging in total organic carbon (TOC) concentration from 3 to 11% and spiked with either DDT or endrin. While the toxicity in the DDT-spiked sediment decreased with increasing TOC content, the TOC content of the endrin-spiked sediment had little apparent effect on toxicity. The 10-d sediment dry-weight LC50 for DDT was 11 .0 pg/g at 3.0% TOC and 49.7 pg/g at 10.5% TOC. The sediment dry weight LC50s for endrin at 3.0 and 1 I .2% TOC were 4.4 and 6.0 pg/L, respectively. These results are important in evaluating the carbon normalization theory, which assumes that the availability of nonpolar organic compounds is reduced in the presence of higher concentrations of TOC. The differences in adsorption and toxicity between DDT and endrin are presumably related to the smaller carbon (&.) and octanol/water (I&) partition coefficients of endrin.

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