CONTRIBUTIONS OF BEDROCK WEATHERING TO THE CHEMICAL COMPOSITION OF STREAMS IN THE ANCHORAGE WATERSHED, ALASKA

The contribution of chemical weathering of bedrock to stream water chemistry for streams in the Anchorage Watershed was examined using natural samples and experimental weathering. The purpose of the project was to aid in determining the source(s) of elements in the Anchorage watershed. The four major streams in the Anchorage Watershed were sampled and the concentrations of major and trace elements both as dissolved and total concentrations were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) at UAA. Hand sample of rock were collected from the primary bedrock units within the McHugh Complex underlying the Anchorage Watershed. Whole-rock chemistry of the rocks was determined by X-Ray Fluorescence (XRF) at The Ohio State University. Two of the bedrock samples were weathered experimentally in the laboratory and the resultant solutions were analyzed for major and trace elements using ICP-MS. The chemistry of the experimental solutions was compared to the stream and bedrock chemistry. This will allow for comparisons of the chemistry of bedrock, streamwater, and experimental weathering solutions to be made and to determine if the stream water chemistry is primarily controlled by natural weathering processes. Preliminary results indicated that the stream water has higher concentration of Pb (0.83-1.8 ppb), Zn (0.84-27.6 ppb) and Cr (170-292 ppb) as compared to the solutions derived by the weathering experiment. In addition, ä18O and äD isotopic compositions of meteoric water, stream water and groundwater provide insight into the sources of water in the streams.