The effects of frozen soils on groundwater recharge and discharge in granitic rock terrane of the Canadian Shield

Hydrologic measurements on groundwater flow systems of the Canadian Shield during the winter period provide insight into groundwater dynamics and can support conclusions based on measurements of the more "active" spring-to-fa il, open-water period. To illustrate this, we present the results of detail ed and continuous year-round measurements of parameters such as groundwater levels, air/soil temperatures, soil heat flux and soil moisture content wh ich were made in upland recharge and wetland discharge areas of a local gro undwater flow system in granitic terrane of the Canadian Shield. Recharge to the groundwater flow system in the rocks of the study area occu rs as rain or snowmelt waters infiltrate exposed vertical and/or sub-horizo ntal soil-infilled fractures in outcrops of the upland area. During winter, soils in the fractures of the recharge area do not normally freeze below 0 .5 m depth and shallow (5-20 cm depth) soil temperatures are most often onl y 1-2 degrees C below freezing. During the spring melt period the temperatu re of these frozen soils remains near 0 degrees C for several weeks as the pore ice absorbs thermal energy necessary for the phase transition from ice to water. However, despite these soils being completely or partially froze n, infiltration and recharge to the groundwater flow system in the rock occ urs as shown by large and rapid rises in watertable and piezometric levels. In the groundwater discharge area of this flow system, near-surface soil te mperatures (5 cm depth) reached minimums of about -12 degrees C during the 1996 winter and freezing soils extended downward to more than 75 cm depth. During the spring melt period, as meltwaters add heat to the substrate, the se shallow soil temperatures also rise to near 0 degrees C and remain froze n for several weeks as latent heat of fusion of ice becomes a factor. Howev er, during the spring melt period, while ice covers the surface and soils a re still frozen in the discharge area, groundwater level rises are recorded in both the overburden and bedrock of the discharge area. This is attribut ed to hydraulic pressure being rapidly transmitted to the discharge area th rough the low storativity groundwater flow system in the rock from the larg e waterlevel increases occurring in the rock of the adjacent upland recharg e area.