Some problems in the interpretation of isotope measurements in United Kingdom aquifers

Corrections are discussed which allow for the dilution of biogenic carbon in the determination of groundwater age by 14C measurements. A reappraisal is made of the procedure used in earlier work on groundwaters from the Lincolnshire Limestone, which allowed for diluting during an incongruent stage of dissolution. A new correction formula is derived for groundwaters which have been affected by both congruent and incongruent dissolution. The significance of such corrections in relation to other limitations imposed by the mixing of groundwaters of different ages is also discussed. Some of the practical problems encountered are illustrated from studies of groundwaters from the Carboniferous Limestone and Lower Greensand. In the Lower Greensand the problem is further complicated by uncertainty about the delta13C value for the rock carbonate as both fresh-water and marine carbonate cements appear to be present. Corroboration of groundwater ages have been sought from D/H and 18O/16O relationships. Equations are derived from the relationship between D/H and 18O/16O for rainfall based on data for a number of European stations. These are compared with that for groundwaters covering a wide age range. A relationship between 18O/16O in rainfall and surface air temperatures for northwest Europe is also derived and is then used in conjunction with known climatic conditions at the time of origin of the groundwaters to investigate a possible relationship between 18O/16O and groundwater age. (author)