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

Abstract

This paper demonstrates that both cation exchange, a commonly invoked mechanism, and silicate hydrolysis, which is less commonly considered, can produce Na-HCO3-type water in sedimentary rocks. Evolution of Na-HCO3 groundwater beneath the Oak Ridge Reservation, Tennessee, USA, was studied by comparing observed end-member groundwater composition from multiport samplers to compositions generated by reaction-path geochemical models. Observed groundwater compositions could be reproduced by either the silicate-hydrolysis model or the cation-exchange model. Secondary minerals precipitated in the silicate-hydrolysis model are similar to those present along fractures in the shale and carbonate host rocks, and observed molar Sr2+/Ca2+ ratios more closely resemble evolution from shale weathering. Both mechanisms should be considered to understand the evolution of Na-HCO3 groundwater.