CHEMISTRY OF SEDIMENTS IN A HIGH-GRADIENT MOUNTAIN STREAM IMPACTED BY MINING AS RELATED TO METAL TRANSPORT AND AQUATIC TOXICITY

An important source of metals to aquatic ecosystems is acidic, metal-rich effluents from mining sites. Natural processes and remedial activities both lead to the neutralization of acidity, which results in the removal of dissolved metals from the water column. This can produce sediment deposits and suspended particles with high metal concentrations. Metal-contaminated sediments and associated streamwater collected from a high-gradient stream reach (North Fork, Clear Creek) located in the Front Range of Colorado were studied. These sediments are dominated by very fine-grained (colloids and fine silts) iron oxyhydroxide precipitates that contain high levels of zinc, copper, and a number of other heavy metals. Sediments of this type are unusually susceptible to in situ formation, redissolution and aggregation/disaggregation processes. These fine-grained sediments are suspected to be the key to metal transport, fate and aquatic toxicity in this system. We examined seasonal and spatial variations in metals concentrations in dissolved, suspended sediment, and bed sediment phases. The importance of these types of sediments for metal toxicity through dietary exposure was examined.