SEASONAL-VARIATIONS IN THE GEOCHEMISTRY OF SHALLOW GROUNDWATER HOSTEDIN GRANITIC TILL

The groundwater chemistry in a small catchment in northern Sweden has been studied for a period of 15 months, including two snowmelt events. The groundwater was sampled from two wells in a slope facing a small stream. One well was placed at the top of the slope (28 m from the str eam), and the other was placed closer to the stream (13 m from the str eam). Both wells were completed to approximately the same elevation (1 .5 m below the stream bed). In addition to groundwater compositions, t he chemical composition of precipitation, soil water and stream water was investigated. The chemical composition of the groundwater in the w ell farther from the stream was fairly constant during the studied per iod. In the closer well the concentrations of Ca, Mg, Na, Sr, Si, and alkalinity decreased by approximately 10% during snowmelt in May. In c ontrast, the concentrations of Fe, Al, Ce and Cu increased. After the snowmelt, during June and July, the concentrations of Ca, Mg, Na, Sr, Si, and alkalinity in the closer well increased by a factor of 2-5 com pared with the concentrations prior to the snowmelt. By August, the co ncentrations had decreased to the same levels as before the snowmelt a nd remained constant until next snowmelt. To explain these variations the Ca/Sr ratio was used as a natural chemical tracer for different wa ter masses. It was found that the increased concentrations of Ca, Mg, Na, Sr, Si, and alkalinity during summer was caused by changes in the groundwater flow direction, which resulted in upwelling of deeper grou ndwater. At this time the area at the closer well acted as an unsatura ted discharge area. The concentrations of Fe, Al, Ce and Cu decreased rapidly after the snowmelt and then remained constant except for two p eaks in July and August. These variations were due to mixing with rapi dly percolating soil water from the E-horizon and melt water or rain w ater, which is supported by increased content of dissolved oxygen. (C) 1997 Elsevier Science B.V.