Equilibrium partitioning as the basis for an integrated laboratory and field assessment of the impacts of DDT, DDE and DDD in sediments

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Many of the most biologically productive portions of streams are backwater areas which support large populations of benthic macroinvertebrates. The sediments in these locations and their associated macroinvertebrate communities are frequently subjected to chemical inputs and physical perturbations. Historically, assessment of the effects of contaminants in sediments have emphasized chemical analyses and either laboratory toxicity tests or in-stream monitoring of benthic macroinvertebrate community structure. However, combining the chemical and biological approaches provides a more powerful assessment technique. Such an integrated approach, combining laboratory water-only and sediment toxicity tests with Hyalella azteca and Chironomus tentans, field surveys of benthic macroinvertebrate community structure and evaluation of chemical data using equilibrium partitioning theory was used to assess the effects of DDT, DDE and DDD (collectively termed DDTR) in the sediments of the Huntsville Spring Branch-- Indian Creek (HSB--IC) stream system in the southeastern USA. Benthic macroinvertebrate populations in the HSB--IC system still appear to be adversely affected by DDTR residues within the sediments even though DDT discharges to the stream were stopped over 20 years ago and a major remediation project was completed in the late 1980s. This conclusion is based on a weight of evidence approach which incorporates (1) the observed sediment toxicity to C. tentans and H. azteca in laboratory tests, (2) the identification of DDTR as the likely cause of effects observed during laboratory toxicity tests, (3) the absence of appropriate sensitive species from groups such as the Ephemeroptera, Plecoptera, Trichoptera and Amphipoda, (4) the presence of reduced numbers of both total individuals and species of chironomids and oligochaetes relative to nearby streams not contaminated by DDTR and (5) the observed distribution of benthic macroinvertebrates in relation to organic carbon-normalized concentrations of DDTR and equilibrium partitioning-based predicted sediment toxic units of DDTR