The 87Sr/86Sr ratios of lake waters from eastern Central Sweden, a region consisting mainly of 1.7-2.0 × 109 years old granitoid rocks are reported and discussed. The ratios range from 0.722-0.739, and are similar to lake and stream waters in geologically similar areas.

The annual variation of the isotope ratio in two small lakes varied in the range of 0.7323-0.7339 and 0.7313-0.7325 respectively, except at ice clearing in spring when the ratio of surface water was lowered by a contribution from melting snow. Small lateral differences also exist in Lake Mälaren, probably because its complex outline hinders complete mixing of its water.

The Sr isotopic ratio variations with respect to bedrock geology suggest that a more important influence on the ratio is the mineralogic composition of the Quaternary deposits (mainly till).

The great difference between the ratios of rain water (0.709-0.710) and those of rocks in old Precambrian platforms with granitic rocks (0.720 and greater) results in waters (ground, lake and river) with ratios around 0.720-0.740. This means that the 87Sr/86Sr ratio is an excellent tool in studies of hydrologic processes, such as mixing of waters and exchange reactions between water and mineral matter.