Persistence of quartz disequilibrium in groundwater flows

This paper shows how constraints can be placed on the prediction of quartz equilibration distance in subsurface water flows. This is achieved here by coupling kinetic equations for the simple (SiO_2(quartz)-H₂O_(water)) system to various groundwater flow parameters (compaction flow, aquifer, convection flow, and hydrothermal system). This approach determines: (1) the order of magnitude of the downstream distance necessary for equilibrium to be attained, when a flowing solution is initially out of equilibrium; and (2) the minimum flow velocity required to sustain a sufficient quartz undersaturation or oversaturation over a given distance. Geological conditions under which specific water-rock interactions take place can be constrained with those calculations. Water flow in most subsurface environments, as idealized here, tends to equilibrate with the quartz-rich hosting medium on a very short distance (≤ 1 m). Major exceptions are: (1) the topmost part of a compacting pile, (2) shallow and fast-flowing aquifers, and (3) most hydrothermal systems.