SUBGLACIAL MIXING OF MELTWATER AND GROUNDWATER AT THE MATANUSKA GLACIER, ALASKA

The chemistry of streams emanating from glaciers is determined by mixing between meltwater and groundwater. To quantify this relationship at the Matanuska glacier, Alaska, streamwater chemistry and water fluxes were analyzed over a one-month field season in the summer of 2004. Water samples were collected from subglacial stream discharge points where pH, conductivity, and temperature were also measured. Water and suspended sediment samples were collected daily, at high and low flow, from a stream that collects and integrates most of these glacial discharge points. To characterize daily fluctuations in the system, hourly water samples were collected over two diurnal cycles. Water samples were filtered on site and stored in polyethylene bottles until analysis using ICP-MS for major cations, including: Na+,Mg+, Si, K+, and Sr2+. Conductivity-temperature-depth gauges were installed at two different stream discharge points to continuously record temperature and conductivity. Discharge gradually increased from 500 cfs in May to nearly 5500 cfs in late June. An abrupt, snowmelt driven, increase occurred on June 19. By July 1 discharge became steady as glacial melt becomes the predominant source. Diurnal fluctuations in discharge were punctuated by low flows at (8 am) and high flows at (8 pm). Generally, as the amount of discharge increased so did the concentration of suspended sediments. Conductivity ranged from 40-120 µm and varied inversely with discharge. Highest conductivity and dissolved ion concentrations were observed at low discharge and the lowest conductivity and ion concentrations were observed just after high discharge. This is most likely the result of the daily flux of low conductivity, low dissolved ion concentration glacial melt water mixing with higher conductivity, higher dissolved ion concentration groundwater before exiting the glacier.