Environmental isotopes and hydrochemistry in groundwater of Gran Canaria

The 18O and D contents of Gran Canaria groundwater samples are linearly correlated with altitude for sampling sites where the tritium content is significant, indicating local recharge. In the northern part of the island the (delta18O, deltaD) correlation has a slope of 7.35+-0,67, while in the southern part the slope is 4.17+-0.28. In the south, therefore, the isotopic altitude effect is mainly governed by partial evaporation during the rainfall, and the isotopic gradients with altitude are greater. These altitude effects allow the determination of the mean elevation of the recharge area for the large number of water samples containing little or no tritium. The sites, which are recharged at much higher elevation, yield highly mineralized water. The distance travelled and the time spent by groundwater within the aquifer is roughly related to the difference between the recharge area elevation, calculated from the isotope composition, and the actual altitude of the sampling site. A few points with high mineralization appear to be locally recharged on the basis of their stable isotope composition. This means that the salt content also derives from local dissolution. In one case the salt content derives from seawater intrusion. (author)