TRACING SURFACE WATER–GROUNDWATER INTERACTIONS AT LAKE PYHÄJÄRVI AND ITS INFLOWING RIVERS, SW FINLAND

In Finland, most of the aquifers are shallow, unconfined sand and gravel deposits, i.e. eskers, ice-marginal end moraine complexes or minor beach terraces on top of the Precambrian bedrock. The management of water resources has traditionally focused on either groundwater or surface water bodies. However, it is apparent that nearly all surface water features interact with groundwater.

The mesotrophic Lake Pyhäjärvi has a surface area of approx. 155 km², a perimeter length of 88 km and a mean depth of 5.5 m. Its drainage basin area is 616 km². There are two major rivers discharging into the lake and one outflowing river.

In the lake the discharge sites were identified via measuring temperature, EC and pH in the lake water, and measuring lake bed sediment temperatures. The fluxes of the order of 10^-3 and 10^-4 cm s^-1 were measured using seepage meters at the study locations. Aerial infrared photography was used to identify the groundwater discharge locations in the rivers.

General water chemistry and stable isotopic compositions are used to indicate the groundwater discharge from aquifers into the rivers and Lake Pyhäjärvi and to evaluate the proportion of groundwater in the rivers and the lake inshore.

Main ions, SiO₂, pH, EC and alkalinity were analysed from 64 groundwater, inshore water and surface water samples. Groundwater discharging into the lake is Ca-HCO₃-type water. The EC, Alkalinity, Cl, NO₃, SO₄, Na, Ca, Mg, and SiO₂ of groundwater is significantly higher than in lake water, while the pH of groundwater is more than one unit lower. In isotope ratios, there is a major difference between the groundwater (δ¹⁸O about -12‰ and δ²H about -86 ‰ (VSMOW)), and the lake water (δ¹⁸O about-7.5‰ and δ²H about -60‰ (VSMOW)).

According to cluster analysis (variables EC, Alkalinity, main ions, SiO₂, δ¹⁸O and δ²H), three main clusters were obtained: river water, groundwater/inshore water and lake water/inshore water. Mixing-ratios of groundwater and lake water were calculated using AquaChem; the proportion of groundwater being in order of some percents to some tens of percents in inshore. In the lake, the SiO₂ content and stable isotope values of waters are the most suitable variables to indicate the groundwater discharge and to calculate inshore water mixing ratios whereas in the river water, the SiO₂ content is the most useful one.