Using Hydrochemistry and Stable Isotopes to Indicate Groundwater-Surface Water Interaction at Lake Pyhäjärvi, SW Finland

General water chemistry, PCE-concentrations and 18O isotopic compositions are used to verify the groundwater discharge from esker aquifers into Lake Pyhäjärvi (average depth 5.5 m, surface area 154 km2) and to evaluate the proposition of the groundwater in inshore water. Discharge sites were identified via measuring temperature, electrical conductivity (EC), and pH in the lake water; and measuring lake bed sediment temperatures. Mini-piezometer data suggests that the groundwater flux is from the aquifer to the lake at seven study locations. The flux at the study locations was measured using seepage meters to be on the order of 10-3 and 10-4 cm/s. The ratios of the isotopes of oxygen present in groundwater and lake water (18 samples altogether) are used to distinguish groundwater discharge into the lake. There is a significant difference between groundwater (δ18O about -12‰ SMOW) and surface water (δ18O about -8‰ SMOW) and therefore the isotope data is used both to confirm the discharge and in mixing calculations, too. Main anions and cations, pH, EC, alkalinity, dissolved oxygen, TOC, total phosphorus and total nitrate were analysed from 12 samples: seven groundwater samples, four inshore water samples and one surface water sample. Groundwater samples were taken using seepage meters and mini-piezometers. The EC of groundwater is about twice the magnitude of that of lake water, while the pH of groundwater is more than one unit lower. In addition, groundwater differs from surface and inshore water due to lower TOC values and higher nitrate and chloride concentrations. Otherwise groundwater discharging into the lake has major ion concentrations typical of the shallow unconfined aquifer in the area (Ca-HCO3-type water). Cluster analysis was made on 8 variables (EC, dissolved oxygen, K, Na, Ca, Cl, HCO3, and NO3) of the samples and two main clusters were obtained: groundwater and surface water/inshore water. This indicates the usefulness of general water chemistry in verifying the original source of the water. Mixing-ratios of groundwater and surface water in inshore water were calculated using AquaChem. Proportion of groundwater in inshore water samples range from 6 to 34% near the shoreline. PCE and its degradation products (remnants of a contaminant spill at a dry-cleaning facility near the esker adjacent to the lake) are used as a tracer in the aquifer, which intercepts the northeastern shore of the lake. PCE concentrations in four samples from discharging groundwater (3 from mini-piezometers,1 from a well) confirm the computer modeling estimation (Artimo 2002) that PCE concentrations in polluted groundwater discharging into the lake would be between 10 and 30 μg/L over most of this region in 2010. The maximum analyzed PCE concentration is 95 μg/L. References: Artimo, A. 2002. Application of flow and transport models to the polluted Honkala aquifer, Säkylä, Finland. Boreal Environment Research 7: 161-172. Korkka-Niemi, K., Rautio. A, Wiebe, A. 2009. Methods for investigating groundwater surface water interaction at Lake Pyhäjärvi, SW Finland. In: 6th National Geological Colloquium 4.-6.3.2009, program and abstracts. Publications of the Department of Geology. Series A 3.University of Helsinki, 28 p.