Abstract

Groundwater in shallow aquifers is commonly used for community water supply in cold climates. Shallow groundwaters are inherently susceptible to contamination from land-use and surface water intrusion threatening drinking water usage. We used a large-scale snapshot data set of stable water isotopes from shallow glaciofluvial aquifers used for drinking water supply in Northern Finland to assess surface water intrusion risks and recharge conditions. This data set was supplemented by long-term stable water isotope precipitation data, Geographic Information System proximity analysis and multivariate statistics. The isotope analysis suggest that a warm season contributes about 60% to the total annual precipitation in the region. This is reflected in the aquifers isotopic composition as it represents an approximately equal mixture of warm and cold season precipitation. Groundwater isotope data normalized to precipitation inputs by line-conditioned excess (lc-excess) was used to flag the water supply wells impacted by surface water intrusions. The proximity analysis showed some of the wells may be affected by intrusions from gravel pit ponds, lakes and peatland drainage. On the larger scale, the wells in coastal areas were more likely to have evaporated water (intrusion) compared to inland regions of Northern Finland due to lower water availability and the presence of man-made structures. This application of stable water isotopes with lc-excess is a useful approach not only for recharge studies but also within water management for supply well surface water contamination risk assessment.