The effect of dissolution of volcanic glass on the water chemistry in a tuffaceous aquifer, Rainier Mesa, Nevada

Geochemistry of ground water associated with the Tertiary tuffs within Rainier Mesa, southern Nevada, was investigated to determine the relative importance of glass dissolution in controlling water chemistry. Water samples were obtained both from interstitial pores in core sections and from free-flowing fractures. Cation compositions showed that calcium and magnesium decreased as a function of depth in the mesa, as sodium increased. The maximum effect occurs within alteration zo^es containing clinoptilolite and montmorillonite, suggesting these minerals effectively remove bivalent cations from the system. Comparisons are made between compositions of ground waters found within Rainier Mesa that apparently have not reacted with secondary minerals and compositions of waters produced by experimental dissolution of vitric and crystalline tuffs which comprise the principal aquifers in the area. The two tuff phases have the same bulk chemistry but produce aqueous solutions of different chemistry. Rapid parabolic dissolution of sodium and silica from, and the retention of, potassium within the vitric phase verify previous predictions concerning water compositions associated with vitric volcanic rocks. Parabolic dissolution of the crystalline phase results in solutions high in calcium and magnesium and low in silica. Extrapolation of the parabolic dissolution mechanism for the vitric tuff to long times successfully reproduces, at comparable pH, cation ratios existing in Rainier Mesa ground water. Comparison of masstransfer rates of the vitric and crystalline tuffs indicates that the apparent higher glass-surface to aqueous-volume ratio associated with the vitric rocks may account for dominance of the glass reaction. INTRODUCTION Volcanic glasses are metastable supercooled liquids that ex'st due to an extreme reduction in nucleation, crystallization, and diffusion rates imposed by rapid increase in viscosity during initial rapid cooling. Rarity of natural glasses older than Cretaceous age tends to con*Art F. White and Larry V. Benson are currently associated with the University of California, Lawrence Berkeley Laboratory. Qi Q2 GEOCHEMISTRY OF WATER firm their metastable nature. Because of this relatively rapid reactivity, numerous researchers have suggested that volcanic glasses may contribute significantly to chemical composition of waters associated with volcanic rocks. The purpose of this study is to investigate the geochemical nature of ground waters contained in vitric and nonvitric tuffaceous aquifers of Rainier Mesa, Nev. , and to ascertain the importance of glass dissolution and alteration on the water composition. SETTING OF RAINIER MESA Rainier Mesa, an eroded volcanic plateau, is in southern Nevada, approximately 160 km northwest of Las Vegas; it is within the northcentral part of the Nevada Test Site operated by the U.S. Department of Energy. Rainier Mesa generally trends north-south and rises more than 750-1,000 m above neighboring basins; altitude of the mesa ranges between 2,250 and 2,340 m. The mesa is characterized by low precipitation, low relative humidity, and large daily variations in temperature. Average annual precipitation on Rainier Mesa is estimated to be 0.2 m per year.