Carbonate mineral dissolution and the impacts of flood water exchange between conduit and matrix

Studies of flow and dissolution in karst aquifers have traditionally focused on conduits, and contributions from matrix sources have been generally ignored. This conceptual model works for limestone that has been deeply buried and had its matrix permeability obliterated by cementation (telogenetic) so that most aquifer recharge occurs by conduits. In limestone that has not been deeply buried (eogenetic), however, high matrix permeability is preserved, resulting in greater degrees of interaction between conduit and matrix porosity than occurs in telogenetic limestone. Interactions between conduit and matrix porosity, and how these interactions affect flow and dissolution within aquifers, is particularly important in karst aquifers subject to dynamic changes in head gradients between conduits and aquifers. These exchanges are particularly prevalent in transitional areas between confined and unconfined aquifer conditions. In such transitional areas, allogenic runoff from confining units can increase river stages on adjacent unconfined aquifers faster than infiltration of local rainfall can increase groundwater heads. As a result, normal hydraulic gradients between the aquifer and rivers are reversed, causing river water to flow into springs. Simultaneously, rapid increases in conduit hydraulic head will also reverse gradients between the conduit and the aquifer, driving exchange from the conduit to the matrix. Because allogenic runoff is highly undersaturated with respect to calcite after storm events, dissolution resulting from this exchange can be extensive. The magnitude of dissolution that occurs is a function of the calcite saturation state of the allogenic runoff, the surface area of the limestone in contact with the water, and the time that water remains in contact with the limestone. In eogenetic aquifers, the magnitude of dissolution that occurs during spring reversals should be higher than in telogenetic aquifers because of their elevated matrix permeabilities, which allows for both greater volumes of water exchange and greater mineral surface area. Quantitative comparisons of the dissolution in eogenetic and telogenetic karst aquifers have not hitherto been made. We present the results of modeling experiments that compare the magnitude amount of dissolution that would occur as a result of spring reversals in telogenetic and eogenetic settings. Water fluxes and residence times were calculated using MODFLOW and Conduit Flow Processes (CFP) and dissolution was estimated using PHREEQC geochemical modeling software. Maximum aquifer residence times of allogenic runoff were twice as long in telogenetic aquifers , but because most water was restricted to the conduit, dissolution was limited. Volumes of exchange and dissolution in eogenetic settings were orders of magnitude higher.