BIOGEOCHEMICAL PATHWAYS AND LAND-USE ASSOCIATIONS OF POTENTIALLY TOXIC METALS IN THE ANCHORAGE WATERSHED, ALASKA

Previous studies indicate that streams in the watershed of Anchorage, AK, contain elevated concentrations of several Potentially Toxic Metals (PTMs) in the water, suspended and streambed sediment and benthic macro invertebrates. Some elements of interest include As, Pb, Cd, Cr, and Cu. Currently, we are developing a geochemical model of the sources, distribution, and uptake of PTMs by aquatic organisms in the watershed and their relationship to land-use patterns. Sampling sites were chosen that represent the watercourses of each of the four major streams in Anchorage, with attention given to nonurbanized and urbanized reaches. Sampling sites upstream of all development provided background or geogenic levels of PTMs. Downstream sites provided information on potential input due to runoff from various land use types, including residential, commercial, industrial, and parkland. Total and dissolved water samples, the less than 63 micron fraction of streambed sediment, various species of benthic macroinvertebrates (Plecoptera, Ephemeroptera, Trichoptera, etc.), and slimy sculpin (Cottus cognatus) were collected at various sites throughout the summer of 2005. In addition, lake sediment core samples from Westchester Lagoon and Campbell Lake were collected and extruded in 1cm increments to assess PTM deposition coinciding with land use change over time. The streambed sediments and lake sediments were leached with 2 M HCl to recover environmentally available PTMs. Whole-organism acid digestion of the invertebrates and fish was performed using HNO3 and H2O2. All water, sediment, and biological samples were analyzed for major and trace element concentrations by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Lake sediment cores show trends of decreasing PTM concentrations with depth. For example, approximate Pb concentrations decreased from 860 ppm at 1cm depth to 90 ppm at 50 cm, Co decreased from approximately 190 ppm at 1cm to 90 ppm at 25 cm, V decreased from 800 ppm at 1cm to 400 ppm at 50 cm, and Zn decreased from approximately 3000 ppm at 1cm to 900 ppm at 60 cm likely indicating recent increases in anthropogenic inputs of these elements. ArcGIS was used to classify sampling locations according to land use from existing Municipality of Anchorage data.