Tracing the origin of natural waters in a glacial till slope during snowmelt

A study of the origin of water sampled at a set of sampling points in the discharging area of a sloping forested glacial till, was performed during the two snowmelts of 1986 and 1987. Analyses of a limited number of chemical components and the 18O content were performed of the sampled water from the snowmelt of 1986. On the basis of the findings from these analyses, a tracer experiment was performed during the snowmelt of 1987, when potassium iodide (KI) was sprayed on top of the snow cover at two surfaces in the discharging area of the slope. The results from both snowmelt studies indicate that macropore channels of old root remains can conduct water rapidly through an unsaturated and frozen matrix without displacing the soil-bound water. Alkalinity and pH values of the sampled water from a macropore were similar to the alkalinity and pH values of sampled snow cores. Even the deeper ground water from a spring site was affected by the acid surge during the peak snowmelt of 1986, giving proof that macrostructures present in the till were promoting preferential water movement through the soil matrix. The surge was also detected with the 18O technique but was not as pronounced as in the values of the chemical components. The iodide tracer experiment during the snowmelt of 1987 also indicated the above-mentioned findings despite the large amount of tracer that was retained in the unsaturated soil matrix. The otherwise conservative tracer subsequently gave no distinguished breakthrough in the discharging water at the V-notch, where the runoff from the slope was measured. It was therefore not possible to determine the tracer turnover time during this snowmelt. However, one very important conclusion from this experiment is that iodide should be injected below the humus and the leaching horizons of podsols in order to get good saturated conditions and thereby prevent retention in the matrix.