An Overview and Preliminary Data of a Five-year Water Quality Study of the Yukon River

The U.S. Geological Survey is conducting a five-year water quality study of a 3000 km reach of the Yukon River from its headwaters in the Yukon Territory, Canada to Pilot Station, Alaska near its mouth and just above tidal influence. The water quality of the Yukon River, the fourth largest river system in North America, is poorly documented and may be changing in response to warmer temperatures. Permafrost regions in the Yukon River drainage basin are melting. As permafrost melts, the frozen soil is transformed into biogeochemically active zones. Runoff moving through and across these active zones is hypothesized to increase the flux of solutes to Yukon tributaries and the main stem, ultimately changing the Yukon River water chemistry. The objectives of the five-year study are: 1) Establish a baseline describing the general water quality of the Yukon River and its major tributaries as a reference to measure future changes. This objective will be achieved by sampling the Yukon River and two of its major tributaries at five fixed sampling sites every two weeks from March through September for a total of five years; 2) Identify processes that affect or control the water quality of the Yukon River and its tributaries. This objective will be achieved by intensive sampling campaigns at high flow in June and low flow in late August during 2002-2004. The first reach and its tributaries from Eagle, AK to the Dalton Highway were sampled during the summer of 2002. The second reach, from the highway to Pilot Station will be sampled during the summer of 2003. The third reach from Whitehorse, Canada to Eagle, AK will be sampled during the summer of 2004. Along with discharge, a suite of geochemical parameters, including suspended sediment, quantitative mineralogy, major ions, trace elements and nutrients are being measured with emphasis on dissolved and particulate carbon, gas flux and generation of methyl mercury. Discharge and water quality sampling using equal discharge increments provide instantaneous fluxes. In addition, the occurrence and fate of constituents of environmental concern, such as mercury and other toxic metals, are being addressed. Identification of the sources and sinks of these constituents, in addition to the processes controlling their fate and reactivity, will provide an important frame of reference to assess water quality changes in the basin that may result from permafrost melting and a warmer climate. Preliminary data from fixed station sampling and the 2002 summer sampling campaign indicate that water quality in the Yukon River drainage basin is sensitive to the hydrologic state of the river. Seasonally and spatially, dissolved organic carbon (DOC) ranges from 1 to 54 mg/L, carbon dioxide ranged from 33 to 735 umols/L, and major ions such as sulfate and calcium range from 4 to 74 mg/L.