Confronting the vicinity of the surface water and sea shore in a shallow glaciogenic aquifer in southern Finland

Abstract

Abstract. The groundwater in a shallow, unconfined, lowlying coastal aquifer in Santala, southern Finland, was chemically characterised by integrating multivariate statistical approaches, principal component analysis (PCA) and hierarchical cluster analysis (HCA), based on the stable isotopes δ 2 H and δ 18 O, hydrogeochemistry and field monitoring data. PCA and HCA yielded similar results and classified groundwater samples into six distinct groups that revealed the factors controlling temporal and spatial variations in the groundwater geochemistry, such as the geology, anthropogenic sources from human activities, climate and surface water. High temporal variation in groundwater chemistry directly corresponded to precipitation. With an increase in precipitation, KMnO 4 consumption, EC, alkalinity and Ca concentrations also increased in most wells, while Fe, Al, Mn and SO 4 were occasionally increased during spring after the snowmelt under specific geological conditions. The continued increase in NO 3 and metal concentrations in groundwater indicates the potential contamination risk to the aquifer. Stable isotopes of δ 18 O and δ 2 H indicate groundwater recharge directly from meteoric water, with an insignificant contribution from lake water, and no seawater intrusion into the aquifer. Groundwater geochemistry suggests that local seawater intrusion is temporarily able to take place in the sulfate reduction zone along the freshwater and seawater mixed zone in the low-lying coastal area, but the contribution of seawater was found to be very low. The influence of lake water could be observed from higher levels of KMnO 4 consumption in wells near the lake. The integration of PCA and HCA with conventional classification of groundwater types, as well as with the hydrogeochemical data, provided useful tools to identify the vulnerable groundwater areas representing the impacts of both natural and human activities on water quality and the understanding of complex groundwater flow system for the aquifer vulnerability assessment and groundwater management in the future.

Comparison of the AVI, modified SINTACS and GALDIT vulnerability methods under future climate-change scenarios for a shallow low-lying coastal aquifer in southern Finland

“…1). These areas consist of permeable aquifer media (gravel and sand), and the groundwater level is close to soil surface and rapidly responds to precipitation during the snowmelt period and heavy rainfall events (Luoma et al 2015).…”

“…NO 3 , Cl, for the validation of the assessment methods. Instead, the integration of multivariate statistical approaches with hydrogeochemical data (Luoma et al 2015) provided useful tools to classify the vulnerable groundwater areas representing the impacts of both natural and human activities on water quality. The vulnerability index maps based on the AVI and SINTACS were consistent with the geochemical data.…”

“…The Z thicknesses were approximately 9-21 times thicker than the thickness of the aquifer, which indicates the thickness of the freshwater zone covering all of the aquifer section, which agrees well with the geochemical and monitoring results indicating a small contribution of seawater to the aquifer. However, these three wells are located in the GALDIT high vulnerability index map areas; nevertheless, the geochemistry of groundwater from Obs10, which is located near the coastline and the water intake well, suggests sulphate reduction in the mixed zone between fresh and seawater, indicating that local seawater intrusion may temporarily take place in this area due to the intrusion of seawater either from sea-level rise or the influence of groundwater pumping (Luoma et al 2015).…”

“…In this study, the predicted levels of vulnerability were compared with the results of a hydrogeochemical study by Luoma et al (2015), which included the analysis of water samples from 15 sites and monitoring data from 15 observation wells (depth and temperature for inland observation wells, and depth, temperature and electrical conductivity (EC) for wells located along the coastline) during the years 2009-2010 and 2012. The contribution of seawater to the aquifer was calculated based on the chloride (Cl) concentrations of groundwater, seawater and freshwater using the following equation (Appelo and Postma 2005):…”

“…The integration of multivariate statistical approaches (principal component analysis (PCA) and hierarchical cluster analysis (HCA)) with conventional classification of groundwater types), as well as with the hydrogeochemical data from Luoma et al (2015), identified the vulnerable groundwater areas based on the impacts of both natural and human activities on water quality. The natural impacts on groundwater vulnerability were observed in most parts of the aquifer area, which are still uncontaminated.…”