CHALLENGES IN QUANTIFYING EXCHANGE BETWEEN GROUNDWATER AND SURFACE WATER WHEN SURFACE WATER ICES OVER

Determination of rates of exchange between groundwater and surface water is particularly difficult in temperate regions when surface water ices over during winter. Exchange usually is focused in the near-shore margins of lakes, wetlands, rivers, and often in streams. However, it is at these shallow, near-shore margins where the ice freezing front can extend to the bed, preventing or diverting flow across the sediment-water interface. If flow is diverted to deeper water, farther from the near-shore margins, sediment permeability commonly is smaller than in the wave-washed littoral zone, reducing exchange between the lake and adjacent porous media. The extent of the ice effect depends largely on the direction and rate of flow across the sediment-water interface. Flow from groundwater to surface water can minimize or prevent ice formation at or within the bed by delivering heat along with the discharging groundwater. Frozen surface water and accumulated snow on top of the ice can further mitigate ice formation in the sediments by provide insulation during the coldest mid-winter periods. However, if flow is from surface water to groundwater, surface water directly beneath the ice commonly is at or near 0 degrees C and flow stops when surface ice extends into the bed sediment. Even where the bed remains unfrozen, flow rates are reduced during winter months because of the colder, more viscous water that reduces hydraulic conductivity by approximately 50 percent compared to surface-water temperatures during summer. Flow either decreases, or hydraulic gradients increase, during winter in response to viscosity driven reductions in hydraulic conductivity in a cold-water plume emanating from the lake. Monitoring rates of exchange based on measured hydraulic gradients during winter can be misleading if the effects of reduced hydraulic conductivity are not taken into account.