PRELIMINARY ASSESMENT OF THE SOURCES, DISTRIBUTION AND TRANSPORT OF POTENTIALLY TOXIC METALS IN THE ANCHORAGE WATERSHED, ALASKA

The Anchorage Watershed lies within the Municipality of Anchorage, Alaska and includes the four major stream drainages of Ship Creek, Chester Creek, Campbell Creek, and Rabbit Creek. All of the streams have their headwaters in the mountains of the Chugach Range and flow through urbanized areas of the Municipality of Anchorage before emptying into the Cook Inlet of the Gulf of Alaska. Recently, elevated concentrations (above 10 ppm) of As have been discovered in several drinking water wells in the City of Anchorage. In addition, other trace elements such as Pb, Cu, Zn, Cd, and Ni as well as Fe occur in these water sources. This has been the motivation to examine the sources, transport, and fate of potentially toxic metal(loid)s for the entire Anchorage Watershed. This study is ongoing and the preliminary results of the surface water investigation are presented here. In the summer of 2003 water and suspended sediments were collected once a month during May, June, July, and August from 24 sites and streambed sediments were collected once during June. All samples were analyzed for elemental concentrations by ICP-MS. The results of this study show that most trace metals occur in low concentrations (a few ppb) in the dissolved phase of the streams with the exception of Zn and Cu which occur at concentrations up to 10 ppb. Instead, the primary means of metal transport is by suspended sediments in all of the streams. For example Zn occurs in the suspended load at concentrations ranging from 9,800 – 17,000 ppm, As from 1,200 – 1,600 ppm, and Pb from 120-300 ppm. In addition, the less than 63 micron fraction of bed sediments contain elevated concentrations of many trace metals including Zn 1,000 – 2,400 ppm, Cu 200 – 600 ppm, and Cr 300 – 500 ppm. Elevated concentrations of many of these same elements have also been found in streambed macro-invertebrates. Trace metal isotopes of Pb and Ni also indicate that there are different sources (ie. geogenic and anthropogenic) of metals contributing to the total load of the streams. Although this data is preliminary, it indicates that there are elevated concentrations of potentially toxic metal(loid)s in many different environmental media in the Anchorage Watershed and is motivation for further study.