THE COMPARATIVE HYDROCHEMISTRY OF 2 GRANITIC ISLAND AQUIFERS - THE ISLES OF SCILLY, UK AND THE HVALER ISLANDS, NORWAY

A comparative study is presented of granitic groundwaters from the Hva ler 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 groundw ater of both groups bears a strong marine signature, although the Hval er 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 m edian (and maximum) values of 2510 Bq/l (8520 Bq/l), 15 mu g/l (170 mu g/l) and 3.3 mg/l (4.4 mg/l), respectively, for Hvaler, compared with 140 Bq/l (200 Bq/l), 1.5 mu g/l (4 mu g/l) and 0.1 mg/l (0.27 mg/l) f or Scilly. Commonly employed drinking water limits for these parameter s are 500 Bq/l (Norwegian action level), 20 mu g/l (Canadian limit) an d 1.5 mg/l. The differences in groundwater contents of these elements between Hvaler and Scilly may be ascribed to: (i) differing trace elem ent compositions of the granites and fracture mineralisations; (ii) ra dically differing recent weathering histories; and (iii) hydrodynamic factors. (C) 1998 Elsevier Science B.V.