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Abstract

Weathering of black shales leads to elevated metal concentrations in both surface water and stream sediments. In spite of the recent focus on black shales, few data exist on the ecological impacts of this process particularly on aquatic organisms. The key objective of this study was to determine the impact of trace metal concentrations in sediments upon aquatic organisms. To achieve the above objective, stream sediment samples were collected from streams draining black shale and limestone (used as a reference stream) lithologies located in central Arkansas between June 2003 and January 2004. Trace metal concentrations were measured by the dynamic reaction cell inductively coupled plasma mass spectrometry (ICPMS; Perkin Elmer DRC II) following EPA 6020 methodology. Sediment samples were tested for toxicity using standard EPA protocols. The trace metal concentrations in sediments and acute toxicity test findings using midge larvae, Chironomus tentans with endpoints measured as growth and survival is presented. Our results showed that there are significant differences in survival of the midge larvae among the study sites and also among the different sampling occasions. Percent survival of the midge larvae in the sediments derived from black shales was lower than that observed in the limestone-derived stream sediments. Significant differences in growth of the midge larvae were also observed among the sites with the control and reference stream sediments having higher growth than the black shale stream sediments. Though our measured metal concentrations in the black shale-derived sediments were below the Effects Range-Low, there is a great potential of metal accumulation in the fine sediment fraction particularly during baseflow regimes. At the time, metals can be concentrated in the fine sediment fraction due to the low discharge and less dilution. The study thus far has shown that the black shale metal-enriched stream sediments have both lethal and sublethal effects on aquatic organisms and higher organisms through food chain transfer.

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