Abstract
Abstract. The groundwater in a shallow, unconfined, low-lying 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 δ2H and δ18O, 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, KMnO4 consumption, EC, alkalinity and Ca concentrations also increased in most wells, while Fe, Al, Mn and SO4 were occasionally increased during spring after the snowmelt under specific geological conditions. The continued increase in NO3 and metal concentrations in groundwater indicates the potential contamination risk to the aquifer. Stable isotopes of δ18O and δ2H 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 KMnO4 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.
Highlights
Shallow permeable aquifers located next to the sea shore may face greater risks and challenges in water consumption and management compared with inland water intakes
Aquifers along the coastline are especially vulnerable to seawater intrusion, either due to sea level rise or storm surges (e.g. Oude Essink, 1999, 2001; Barlow, 2003; Pulido-Leboeuf, 2004; Oude Essink et al, 2010; Rasmussen et al, 2013; Luoma et al, 2013; Luoma and Okkonen, 2014), which will presumably be accelerated by the changing climate (IPCC, 2000, 2007; Nicholls et al, 2007)
This study clearly demonstrated that the geochemistry of groundwater in the shallow, unconfined, low-lying coastal aquifer in Santala has spatial and temporal variability depending on the geological, anthropogenic, seawater and climate factors
Summary
Shallow permeable aquifers located next to the sea shore may face greater risks and challenges in water consumption and management compared with inland water intakes. Besides these, increasing water demand for the population and industries as well as changing land use practises as a result of human activities, e.g. exceeding the water intake, gravel excavation pits, car parking, and groundwater contamination risk areas, can expose shallow aquifers to contamination. S. Luoma et al.: Surface water and sea shore in a shallow glaciogenic aquifer in southern Finland (Scibek and Allen, 2006; Scibek et al, 2007; Jyrkama and Sykes, 2007; Okkonen and Kløve, 2011; Okkonen, 2011; Luoma and Okkonen, 2014). Climate change would potentially affect the groundwater, and the surface water. For sustainable groundwater resource management and land use planning, it is important to understand the hydrogeological processes and the interactions between groundwater and surface water, and factors affecting groundwater quality
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