Effects of increased upward flux of dissolved salts caused by CO2 storage or other factors

Abstract

Injection of CO2 in deep saline aquifers is being considered to reduce greenhouse gas emissions to the atmosphere, and this process is expected to increase formation pressure with the potential consequence of increasing the upward flux of saline water. Saline and brackish groundwater occur naturally at shallow depths above many sedimentary basins, where an upward flux of solutes could degrade the quality of freshwater aquifers and affect aquatic ecosystems. Relatively large fluxes of salty water could be transported upward along preferential paths, like faults or improperly abandoned wells. Diffuse upward flow through the natural stratigraphy could also occur in response to basin pressurization. This process would be slower, but diffuse upward flow could affect larger areas than flow through preferential paths, and this motivated us to evaluate the associated effects. We analyzed idealized 2D and 3D geometries representing the essential details of a shallow, freshwater aquifer underlain by salty groundwater. First, we simulated the development of a freshwater aquifer by flushing out saline water as an initialization step, followed by simulations of a pulse-like increase in the upward flux from the basin. The results showed that increasing the upward flux from a basin increased the salt concentration and mass loading of salt to streams, and decreased the depth to the transition of fresh/salt groundwater. The magnitude of these effects varied widely, from a small, slow process that would be challenging to detect, to a large, rapid response that could be an environmental catastrophe. The magnitude of the increased flux, and the initial depth to the fresh/salt transition in groundwater controlled the concentration and mass loading of water discharging to streams and the change in the depth to the fresh/salt transition. We also identified impact categories for salt concentration, mass loading, and freshwater aquifer thickness, and used these categories to characterize the severity of the response. This showed that the impact depends on the magnitude of upward flux. Impact appeared minor when the upward flux was smaller than a few tenths of the magnitude of recharge, but it could be significant when the upward flux was greater than the recharge, according to the 2D analyses. Hydrostratigraphy will also play an important role by localizing the discharge of salty water, according to 3D analyses. The major contribution of this work is that it shows how a large increase in diffuse upward flux from a basin could cause significant problems, but a small increase in upward flux may occur without significantly impairing the shallow freshwater flow system. This heightens the importance of understanding interactions between shallow and deep hydrologic systems when characterizing CO2 storage projects.