Altogether, 1272 groundwater samples were collected from domestic wells drilled into fractured crystalline Precambrian bedrock in the Tampere Region, southern Finland. The Arsenic (As) concentration in the drilled bedrock wells varied from <0.05 to 2230 μg/L with median of 2.5 μg/L. 22.5% of total samples exceeds the safe drinking water limit of 10 μg/L and is much higher than the median for bedrock groundwater from all of Finland (0.16 μg/L, n = 263). High As groundwater was found in the felsic and mafic metavolcanic bedrock areas. Piper diagram showed that the majority of groundwater samples are Ca-HCO_3 type. Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA), showed that As is related to Mo, F~-, B,Br~-, Na,HCO_3~-, Ca, Na, U, Sr, pH, EC, and well depth. This may reflect their origin host rocks and groundwater evolution mechanism in both oxidizing and reducing conditions.
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