EQUILIBRIUM PARTITIONING AS THE BASIS FOR AN INTEGRATED LABORATORY AND FIELD ASSESSMENT OF THE IMPACTS OF DDT, DDE AND DDD IN SEDIMENTS

Many of the most biologically productive portions of streams are backw ater areas which support large populations of benthic macroinvertebrat es. The sediments in these locations and their associated macroinverte brate communities are frequently subjected to chemical inputs and phys ical perturbations. Historically, assessment of the effects of contami nants in sediments have emphasized chemical analyses and either labora tory toxicity tests or in-stream monitoring of benthic macroinvertebra te community structure. However, combining the chemical and biological approaches provides a more powerful assessment technique. Such an int egrated approach, combining laboratory water-only and sediment toxicit y tests with Hyalella azteca and Chironomus tentans, field surveys of benthic macroinvertebrate community structure and evaluation of chemic al data using equilibrium partitioning theory was used to assess the e ffects of DDT, DDE and DDD (collectively termed DDTR) in the sediments of the Huntsville Spring Branch-Indian Creek (HSB-IC) stream system i n the southeastern USA. Benthic macroinvertebrate populations in the H SB-IC system still appear to be adversely affected by DDTR residues wi thin the sediments even though DDT discharges to the stream were stopp ed over 20 years ago and a major remediation project was completed in the late 1980s. This conclusion is based on a weight of evidence appro ach which incorporates (1) the observed sediment toxicity to C. tentan s 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 pre sence of reduced numbers of both total individuals and species of chir onomids and oligochaetes relative to nearby streams not contaminated b y DDTR and (5) the observed distribution of benthic macroinvertebrates in relation to organic carbon-normalized concentrations of DDTR and e quilibrium partitioning-based predicted sediment toxic units of DDTR.