Age and chemical evolution of groundwaters in Skagafjordur, Iceland

Groundwaters in the basaltic terrain of Skagafjordur, Iceland, have tempera tures ranging from ambient to about 90 degrees C. They are probably >10,000 years old in some instances. These waters have pH as high as 10 due to dis solution of mafic basaltic minerals and glass. Their major solute concentra tions, except for Cl and SO4, are controlled by a close approach to equilib rium with secondary minerals at temperatures as low as 10 degrees C. The cl ose approach to equilibrium at these low temperatures is a consequence of t he high reactivity of the mafic basaltic minerals, which act like strong ba ses, the low acidity content (CO2) of the parent surface water, the relativ ely high age of the water and the low content of soluble salts in the basal tic 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 (dis solve a larger quantity of the primary rock minerals) before the acid is co nsumed and the pH raised sufficiently to attain saturation with many second ary minerals. (C) 2000 Elsevier Science B.V. All rights reserved.