The residence time distributions in the reference aquifers are established using available data, and augmented using new dating and conservative tracer techniques (CFCs, 3He/3H, 85Kr, 39Ar).
The age of water can be determined by measuring relative amounts of parent and daughter isotopes. Different methods, based on isotope and tracer techniques, are used in BaSeLiNe to date waters of different ages (Figure 1).
Historical data on groundwater quality and evolution for selected sites in the reference aquifers are being evaluated to interpret trends in quality over the decades since the aquifers began to be exploited.
These trends in water quality are important for understanding the changes in natural baseline and anthropogenic influences.