Sediment toxicity in mid-continent great rivers (USA)

As part of the Environmental Monitoring and Assessment Program for Great River Ecosystems (EMAP-GRE), sediment samples were collected from 447 randomly selected littoral sites along the main channels of the Ohio, Missouri, and Upper Mississippi Rivers between 2004 and 2006. Toxicity of these sediment samples was measured using a 7-day Hyalella azteca survival and growth test. Sixty-five sites (14.5%) exhibited lethal toxicity, and 130 sites (29.1%) exhibited decreased growth. In the EMAP-GRE probabilistic sampling design, each sampled site had a weight associated with it that determined the length (and proportion) of the river represented by that sample point in the population. Weighted whole-river estimates indicated that of the 4721 river km sampled, sediment from 15.9 ± 3.0% of the river (752 ± 50 km) were lethally toxic, 27.4 ± 3.5% (1289 ± 57 km) were toxic by way of growth inhibition, and 40.0 ± 3.7% (1887 ± 68 km) exhibited either lethal or growth toxicity. Selected toxic samples were analyzed for 21 pesticides, 20 polychlorinated biphenyl congeners, and 6 polybrominated diphenyl ether congeners. For all of the samples tested, the concentration levels of these analytes were mostly lower than known toxicity thresholds, and neither unionized ammonia concentration nor osmotic stress (as measured by conductivity) could account for the toxicity found in sediments. The spatial pattern of sediment toxicity cannot be readily explained by urbanization or agricultural land use at the subcatchment scale. We speculate that the distribution of toxic sediment is more likely due to a combination of localized sources, including polluted tributaries, and the redistribution of contaminated sediments from upriver. The sediment toxicity results from this study will be used, in combination with other sediment, biologic, and habitat metrics and indicators collected in the EMAP-GRE study, to help interpret and assess the condition of the Ohio, Upper Mississippi, and Missouri Rivers.