Recent chemical changes in Gardermoen Aquifer : effect of cation exchange on the composition of groundwater

Abstract

Recent analysis of water samples from Gardermoen Aquifer, the largest unconfined groundwater reservoir in Norway, revealed increasing trend of calcium and magnesium concentrations. The international Oslo Airport, situated above the aquifer, discharges several volume of runoff contaminated by deicing chemicals into the subsurface. Propylene glycol and Sodium formate are the principal components of deicing chemicals widely used in the airport during winter season. Sodium formate is a soluble compound and readily dissociates in to its ions upon reaching the groundwater. Soil samples were taken from borehole near the eastern runway in the airport ground. These samples were analysed with respect to mineralogical composition, grain size distribution, carbon content, exchangeable ions and cation exchange capacity. Data from chemical analysis of snow collections and previous water samples are used to identify the major processes that govern the increasing trend of calcium and magnesium. Cation exchange capacity of the soils was determined by saturating the sample with a NH4NO3 and SrCl2 solutions independently. The results obtained range between 1.5- 3.5 meq/l, with calcium being the dominant ion on the exchange sites. Geochemical modelling also strengthens the evidence of cation exchange processes being the dominant factors involved in the recent changes in groundwater chemistry.