Evolution of groundwater in a carbonate-bearing glacigenic aquifer, SW Finland: hydrochemical and isotopic evidence

Full text: Unusually high pH values, up to 9.5, are encountered in the glacigenic Virttaankangas aquifer, southwest Finland. This is in strong contrast to the pH values normally recorded in Finnish shallow groundwaters, averaging 6.4. Chemical and isotopic (18O, D, 13C, T, 14C) compositional parameters were determined in order to evaluate the origin of the unusual chemical composition of the groundwater at Virttaankangas. In addition, the mineralogical composition of the aquifer material was examined. The Virttaankangas groundwater formation has been studied extensively, because it has been selected as the site of an artificial recharge plant, intended to start operation in 2007. The project is the largest artificial groundwater scheme in northern Europe. The high pH values have been suggested to be related to the presence of carbonate minerals in glaciogenic sediments. However, only erratic carbonate boulders have been observed, and no outcrops containing carbonate rock have been discovered at the site. The Virttaankangas aquifer is located in a NW-SE esker chain composed of Quartenary deposits. In the studied area of 54 km2 the total thickness of sediments above the bedrock varies from 30 to 90 m. The formation of the stratigraphical succession occurred in three stages. The core of the esker with coarse sand and gravel was deposited in the first stage. The second stage consists of an interlobate glaciofluvial ridge and fine-grained glaciolacustrine deposits. Locally the glaciolacustrine deposits form a confining interlayer holding a perched water table. The uppermost, third stage, is composed of sorted littoral sands formed during the regression of the former Baltic Sea. Accordingly, three distinct hydrogeological units can be identified: 1) an unconfined, highly conductive aquifer in the core of the esker, 2) a semi-confined aquifer, with fairly low hydraulic conductivity underlying the confining interlayer, and 3) an unconfined perched aquifer in the littoral deposit overlying the confining sediments. The different hydrogeological units have distinct hydrochemical and isotope geochemical characteristics. The shortest residence time of groundwater is encountered in the perched aquifer of Unit 3, where seasonal δ18O and δD variations are evident, and the tritium (ca. 10 TU) and radiocarbon (ca. 100 pmC) contents reflect those of recent precipitation. The perched water is highly unsaturated with respect to calcite (log SIcalcite << 0). Chemically this water type is a typical shallow groundwater with a low pH value of < 7, low electric conductivity (EC) and low δ13C of dissolved inorganic carbon (DIC) of < -20 per mille (unpublished data). Oldest groundwaters are present in Unit 2, where some bomb tritium (ca. 21 TU) is still observed. None or minor seasonal variations are present in δ18O and δD of Unit 1 and Unit 2. Groundwater in Unit 2 is saturated for calcite (log SIcalcite ∼ 0) and pH values are high, 9.2 - 9.5, exceeding values observed elsewhere in the aquifer. There is a small increase in the concentration of DIC from 0.2 mmol/L in Unit 3 to 0.4 mmol/L in Unit 2, suggesting dissolution of calcite. Existence of calcite as a source of DIC is confirmed by higher δ13C (DIC) values in Unit 2, ranging from -17 per mille to -12 per mille. Concomitantly, the 14C content of DIC has been lowered due to dissolution of old 14C-free carbonate minerals. The δ13C value of about -4.66 per mille was measured from calcite in glacigenic sediments. In the unconfined aquifer (Unit 1), groundwater is slightly unsaturated for calcite (log SIcalcite ≤ 0), however, the DIC content is approximately 0.5 mmol/L, which suggests slightly different evolution of DIC compared to Unit 2. Also pH is somewhat lower, 8.5 - 9.2. Both tritium (12-16 TU) and 14C (80-90 pmC) concentrations are variable depending on the proportion of recent precipitation and the groundwater flow from Unit 2. Mineralogical studies of the sediments in the Virttaankangas for mation confirmed that very small quantities (0.5 % at most) of clastic, fine-grained (ca. 0.1 mm in diameter) calcite is present in the glacigenic units 1 and 2. However, the littoral sands of Unit 3 are nearly devoid of calcite. Calcite dissolution appears to be sufficient to explain the hydrochemical and isotope geochemical characteristics of the Virttaankangas aquifer. The reaction between groundwater and calcite in the Virttaankangas groundwater system was modelled using the PHREEQC program. A modelled evolution of DIC and pH in groundwater as calcite is dissolved to the point of saturation is in harmony with the chemistry of groundwaters collected from units 2 and 3 in the northeast side of the esker. The model represents closed system dissolution under conditions of a relatively low PCO2 of 10-2.65. In a closed system, pH increases significantly as calcite dissolves. It seems that dissolution of calcite under closed or partially closed system and relatively low CO2 conditions explains the exceptionally high pH values in the Virttaankangas aquifer. Also the δ13C values of DIC in groundwater are consistent with the origin of carbon from soil calcite and soil CO2. (author)