CHEMICAL WEATHERING AND MELT WATER EVOLUTION AT THE MATANUSKA GLACIER SOUTH-CENTRAL ALASKA

Streams emanating from the outlets of glaciers often represent mixing between dilute meltwater and groundwater that has reacted with mineral assemblages. To help quantify this relative contributions of these two components at the Matanuska glacier, South Central Alaska, streamwater chemistry and water fluxes were analyzed over the course of a one-month field season in the summer of 2004. Water samples were collected from subglacial stream discharge points, and pH, conductivity, and temperature measured at each sample site. Water and suspended sediment samples were also collected daily at high and low flow, and also hourly for two diurnal periods from a stream that collects and integrates most of these glacial discharge points. Two downstream profiles capture the evolution of the dissolved components of the Matanuska river in the proglacial floodplain. All water samples were filtered on site through 0.45µm filters and stored in polyethylene bottles until analysis on an ICP-MS for major cations, including: Na+,Mg+, Si, K+, and Sr2+. The Alkalinity of selected samples was determined by Gran titrations. Discharge gradually increased form 500 cf/s in May to nearly 5500 cf/s in late June. An abrupt increase occurred on June 19, most likely due to snowmelt on the valley walls. 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 in the morning and high flows at ~8 pm in late afternoon. Generally, as the amount of discharge increased so did the concentration of suspended sediments. Conductivity ranged from 40-120 µmho and varied inversely with discharge, with the highest conductivity and ion concentrations observed at low discharge. We believe this is the result of the daily flux of low ionic strength glacial melt water mixing with higher ionic strength groundwater before exiting the glacier. In the downstream direction the pH ranged from 6.5-7.8, increasing downstream, along with changes in conductivity from 51-370 µmho. This difference in chemistry is the likely result of inputs from several tributaries that contribute solutes from the vegetated, soil-mantled hillslopes to the Matanuska River.