Multivariate statistical analysis of groundwater geochemistry to characterize flow in the Kurikka buried valley aquifer system, Western Finland

Abstract

The Kurikka buried valley aquifer system (Western Finland) contains significant groundwater resources in coarse-grained sediments alternating with till layers. Over the past 10 years, this multilayered aquifer system has been the object of growing interest to increase the water supply to the towns of Vaasa, Kurikka and nearby municipalities. In this context, it is important to understand the groundwater flow system to assess its sustainable exploitation rate and implement sustainable management of this resource. The goal of this study was to assess groundwater quality in the Kurikka aquifer system and interpret the geochemical data to better understand groundwater flow patterns. This goal was achieved through the geochemical characterization of groundwater and the use of multivariate statistical analysis to interpret results.  The study area (600 km2) encompasses 4 buried valleys connected to the main Kyrönjoki valley. Compilation of historical geochemical data (56 samples) from 2011-2021 was completed in June-August 2023 by a large groundwater sampling campaign (42 samples) from observation wells, bedrock boreholes, production wells and springs, covering all parts of the study area. Samples were analyzed for major ions, minor and trace elements and tritium analyses were performed on a subset of 25 samples. Multivariate statistical analysis (Hierarchical Clustering and Principal Components) was carried out based on 18 physicochemical parameters for 98 samples. Five water groups emerged from the hierarchical classification. The first three clusters (C1-C2-C3) represent water from sediments, cluster 4 corresponds to water from the bedrock in the upgradient areas and cluster 5 represents water from the bedrock deep beneath the buried valleys. The major recharge area is located to the west of the study area, in the topographic highs where less evolved, tritiated waters were found (C3). From the recharge area, groundwater flows to the north, east and south-east. A similar groundwater evolution from Ca-HCO3 to Na-HCO3 water types was observed in both sediments and bedrock in the recharge area (C4). This suggests there is either an evolution within the buried valleys themselves or the buried valleys act as discharge and mixing feature for the evolved bedrock waters. Groundwater from the northernmost buried valley and the northern part of the Kyrönjoki valley (C1) are geochemically distinct from the rest of the study area and contain tritiated waters, reflecting a different context of modern esker with a shallower system. Bedrock groundwater (C4-C5) are characterized by a lower pCO2 value and higher pH. While one bedrock borehole beneath the central Paloluoma buried valley showed a more evolved water type and was tritium-free, fresh groundwater was still found until 100 m depth, suggesting deep active flow in bedrock. This study will be complemented by an additional dataset of groundwater residence time tracers (3H, 14C) and isotope data (87Sr, 18O/2H) that will provide more information on groundwater origin and support the interpretation of the evolution of the water groups found in the Kurikka aquifer system.