Geochemical and isotopic evolution of high-pH groundwater in a carbonate-bearing glacigenic aquifer, SW Finland

The glacigenic Virttaankangas aquifer in SW Finland is characterized by groundwaters with high pH values of up to 9.5. High pH values exceeding 9 are uncommon for shallow groundwaters from unconsolidated glacigenic formations on crystalline bedrock. To evaluate the geochemical evolution of groundwater, water samples were collected from 17 sites. The samples were analysed for the isotopic composition of oxygen, hydrogen, dissolved inorganic carbon (DIC) and strontium, the activities of radiocarbon and tritium, and for the basic chemical composition. Groundwaters in the Virttaankangas aquifer system show a considerable range in water pH from 6.4 to 9.5. The isotopic and chemical characteristics of the groundwater in the aquifer units were observed to vary depending on the aquifer mineralogy, groundwater residence time and the openness of the system to CO₂ exchange. The pH values were noted to increase in parallel with the contents of Ca and DIC and δ¹³C_DIC. In higher pH values, the groundwaters had attained saturation with respect to calcite. These chemical changes were interpreted to result from dissolution of calcite, present in trace amounts in the Virttaankangas sediments. Specifically, the highest pH values were attributed to calcite dissolution under closed system conditions at a low partial pressure of CO₂.