Modeling alternative paths of chemical evolution of Na-HCO3-type groundwater near Oak Ridge, Tennessee, USA

This paper demonstrates that both cation exchange, a commonly invoked mecha nism, and silicate hydrolysis, which is less commonly considered, can produ ce Na-HCO3-type water in sedimentary rocks. Evolution of Na-HCO3 groundwate r beneath the Oak Ridge Reservation, Tennessee, USA, was studied by compari ng observed end-member groundwater composition from multiport samplers to c ompositions generated by reaction-path geochemical models. Observed groundw ater compositions could be reproduced by either the silicate-hydrolysis mod el or the cation-exchange model. Secondary minerals precipitated in the sil icate-hydrolysis model are similar to those present along fractures in the shale and carbonate host rocks, and observed molar Sr2+/Ca2+ ratios more cl osely resemble evolution from shale weathering. Both mechanisms should be c onsidered to understand the evolution of Na-HCO3 groundwater.