Englacial and Subglacial Water Flow Elucidated by Active and Passive Borehole Experiments: Bench Glacier, Alaska

Slug and dye dispersion experiments are commonly used tests which provide information regarding flow parameters in streams and aquifers. These same tests can be performed in a glacial setting to aid in quantifying subglacial and englacial flow paths and characteristics and their evolution through time. Eleven slug tests and 7 dye dispersion experiments were conducted in 2 borehole grids on Bench Glacier, Alaska in June, 2006. One borehole grid contained 4 holes arranged in a square, and spaced 20 m apart. The other grid consisted of 3 boreholes spaced 8 m apart, and arranged in a triangle. Slug tests were conducted in various boreholes over a 10 day period to test the flow characteristics as well as the temporal and spatial changes occurring within the englacial and subglacial hydrological systems. Slug tests were performed by injecting a 75 liter slug of water in a designated borehole and monitoring the pre and post-slug water levels with pressure transducers in the injection hole as well as up to five adjacent boreholes. Englacial and subglacial dye tracing was performed in boreholes by injecting Rhodamine WT dye at a specific depth and then monitoring dye concentrations with two logging borehole fluorometers. Six of the dye injection experiments were accompanied by slug tests in the same borehole to enhance the understanding of the flow mechanics and water routing within the borehole and at the glacier's bed. Results from the combination slug/dye experiments include: (1) induced oscillations in water level (underdamped response curve) accompanied by oscillations of varying frequency in dye concentration at the borehole-bed interface; (2) slow draining of the perturbed water level (overdamped response curve) with no dye movement at the bed or directly below a documented englacial fracture. No inter-borehole connectivity was documented in any of the slug tests conducted. These experiments are used to document and quantify englacial and subglacial passageways and improve conceptual models regarding oscillatory slug test responses in boreholes.