Hydrochemical coupling of a glacial borehole–aquifer system

Abstract

Measurements of the electrical conductivity of subglacial water provide a useful complement to measurements of pressure and turbidity. In the summer season, fluctuations of conductivity can be attributed to changes in water transport, water provenance and subglacial residence time. These explanations are unlikely to apply during the winter season because surface melt sources are not active and the subglacial water system is predominantly unconnected. Thus, fluctuations in water conductivity during the winter months seem paradoxical. To introduce a quantitative basis for comprehending such phenomena, we develop an interpretative model of the hydrochemical interaction between a water-filled borehole and a subglacial aquifer. The electrical conductivity of water near the borehole–aquifer contact is affected not only by diffusion but also by advective transport of solute between the two reservoirs in response to pressure forcing of the system. Using records of ice strain, water pressure and electrical conductivity from unconnected boreholes in Trapridge Glacier, we demonstrate that changes in borehole geometry caused by ice-strain events provide a plausible mechanism for at least some of the observed fluctuations of electrical conductivity. Conductivity records provide information regarding advective coupling of the borehole–aquifer system that is not available from pressure records alone.

Channelized, distributed, and disconnected: subglacial drainage under a valley glacier in the Yukon

“…However, the limited electrical conductivity and turbidity measurements also indicate that relatively little water might actually flow in the distributed system (Oldenborger et al, 2002). Unlike in the data in Hubbard et al (1995), there are no diurnal variations in electrical conductivity.…”

“…The borehole originally disconnected from the main group on 11 July, but reconnects on several occasions during periods of high water pressure in the active drainage system, disconnecting when water pressure subsequently drops. Note that for the first two reconnections, S2 on 21 July and S4 on 24 July, the switching events are clearly associated with large drops in conductivity as seen in panel (g), suggesting an inflow of meltwater that has spent less time in contact with the bed (Oldenborger et al, 2002).…”

“…By this, we mean the formation of individual Röthlisberger-type ("R") channels, incised into the base of the ice by dissipation-driven melting (Röthlisberger, 1972). Formation of channels should cause the mean water pressure to drop, as the focusing of water discharge causes larger channel wall melt rates that have to be offset by faster creep closure, driven by larger effective pressures (Nye, 1976;Spring and Hutter, 1982). It can also account for the increased sensitivity of the pressure response to the inferred melt input, and the reduction of dye tracer transit time observed in other glaciers (Nienow, 1993).…”

“…Alternatively, storage capacity could be relatively localized, so that water does not need to flow everywhere. Oldenborger et al (2002) show how water pressure variations with no corresponding change in conductivity can be observed over an impermeable bed. However, the proposed mechanism requires the boreholes to be disconnected, and would not operate on hydraulically connected boreholes as in this case.…”

“…By implication, when water drains through channels, an increase in water supply should increase effective pressure around the channels, and slow the glacier down (Nye, 1976;Spring and Hutter, 1982;Schoof, 2010).…”