Age and chemical evolution of groundwaters in Skagafjordur, Iceland

Groundwaters in the basaltic terrain of Skagafjordur, Iceland, have temperatures ranging from ambient to about 90°C. They are probably >10,000 years old in some instances. These waters have pH as high as 10 due to dissolution of mafic basaltic minerals and glass. Their major solute concentrations, except for Cl and SO₄, are controlled by a close approach to equilibrium with secondary minerals at temperatures as low as 10°C. The close approach to equilibrium at these low temperatures is a consequence of the high reactivity of the mafic basaltic minerals, which act like strong bases, the low acidity content (CO₂) of the parent surface water, the relatively high age of the water and the low content of soluble salts in the basaltic rock. Waters that receive a larger supply of acids, e.g. from decaying organic matter in soils or sediments, need to react more with the rock (dissolve a larger quantity of the primary rock minerals) before the acid is consumed and the pH raised sufficiently to attain saturation with many secondary minerals. (C) 2000 Elsevier Science B.V. All rights reserved.