The comparative hydrochemistry of two granitic island aquifers: the Isles of Scilly, UK and the Hvaler Islands, Norway

A comparative study is presented of granitic groundwaters from the Hvaler Islands, south-eastern Norway (11 samples) and the Scilly Islands, south-western England (10 samples). The islands display similar bulk lithologies (peraluminous S-type, U/Th-enriched granites) and land use, but differing glaciation and hence weathering histories. The groundwater of both groups bears a strong marine signature, although the Hvaler Islands display less marine influence and a greater degree of water-rock interaction. The most interesting hydrochemical dissimilarities concern the health related trace elements Rn, U and F. These display median (and maximum) values of 2510 Bq/l (8520 Bq/l), 15 μg/l (170 μg/l) and 3.3 mg/l (4.4 mg/l), respectively, for Hvaler, compared with 140 Bq/l (200 Bq/l), 1.5 μg/l (4 μg/l) and 0.1 mg/l (0.27 mg/l) for Scilly. Commonly employed drinking water limits for these parameters are 500 Bq/l (Norwegian action level), 20 μg/l (Canadian limit) and 1.5 mg/l. The differences in groundwater contents of these elements between Hvaler and Scilly may be ascribed to: (i) differing trace element compositions of the granites and fracture mineralisations; (ii) radically differing recent weathering histories; and (iii) hydrodynamic factors.