Why strontium baselines should be based on pristine areas only: The disturbing effect of agricultural lime revisited

Based on twelve boreholes to groundwater on a meltwater plain in Jutland, Denmark, we document that the ⁸⁷Sr/⁸⁶Sr ratio of the groundwater is significantly higher in the pristine areas of heath and forests than in the adjacent farmland areas. The average ratio is 0.70798 in the farmland areas versus 0.71490 in the pristine areas. The sand of the vadose zone exhibits even larger differences with average values of 0.70959 in the farmland and 0.72205 in the pristine land. The strontium concentrations show similar changes, although the highest values here are found in the farmland areas. As the study area consisted entirely of pristine heathland until around 1900, the ⁸⁷Sr/⁸⁶Sr values, at that time must have been equally high everywhere. Today, only the pristine areas exhibit high values, whereas the values from the farmland are low. Strontium derived from agricultural lime is the only possible source for these low values. They cannot be caused by strontium from carbonate clasts buried deeper in the meltwater deposits, as it has previously been suggested. Such a hypothesis would require that the clasts were concentrated in the central farmland area and absent in the pristine areas, a highly improbable explanation considering the homogeneous geology of the plain. The new results confirm that strontium released from agricultural lime is highly mobile and not retained in the topsoil of the farm fields as earlier suggested. Ground- and surface waters in contact with farmland should consequently be avoided in the creation of strontium baselines.