Self‐Organized Permeability in Carbonate Aquifers

Abstract

Advances over the past 40 years have resulted in a clear understanding of how dissolution processes in carbonate rocks enhance aquifer permeability. Laboratory experiments on dissolution rates of calcite and dolomite have established that there is a precipitous drop in dissolution rates as chemical equilibrium is approached. These results have been incorporated into numerical models, simulating the effects of dissolution over time and showing that it occurs along the entire length of pathways through carbonate aquifers. The pathways become enlarged and integrated over time, forming self-organized networks of channels that typically have apertures in the millimeter to centimeter range. The networks discharge at point-located springs. Recharge type is an important factor in determining channel size and distribution, resulting in a range of aquifer types, and this is well demonstrated by examples from England. Most carbonate aquifers have a large number of small channels, but in some cases large channels (i.e., enterable caves) can also develop. Rapid velocities found in ground water tracer tests, the high incidence of large-magnitude springs, and frequent microbial contamination of wells all support the model of selforganized channel development. A large majority of carbonate aquifers have such channel networks, where ground water velocities often exceed 100 m/d.

Isotopes as Tracers of Water Origin in and Near a Regional Carbonate Aquifer: The Southern Sacramento Mountains, New Mexico

“…A continuum of aquifers exists in carbonate rock [27]. At one extreme, carbonate dissolution leads to wide solution cavities that self-organize into dendritic drainage networks discharging through large springs; water flow rates are commonly 10 2 to 10 4 m/day over distances of 10 3 to 10 4 m. At the other extreme, solution cavities are narrow and of limited interconnection, generating an aquifer with lower flow rates and discharge through many small springs.…”

The Karst Paradigm: Changes, Trends and Perspectives

“…Porosity comprised by integrated void-conduit systems is commonly low (within 0.05−3%), comprising only a small portion of total porosity of the rock media, but it provides for high hydraulic conductivity of aquifers (up to 1 m/s and higher) and transmit almost all (up to 99.9%) flow (Worthington et al 2000). Additionally, the high degree of flow concentration is illustrated by a high proportion of large springs in regions underlain by soluble rocks, as compared to non-karstic regions, and the high efficiency of karstic flow systems is illustrated by very high flow velocities, commonly within 10 3 -10 4 m/day (Worthington & Ford 2009), i.e. 5−7 orders of magnitude higher than typical velocities of groundwater movement in non-karstic flow systems in the zone of intense circulation.…”

Spatially resolved information on karst conduit flow from in-cave dye tracing

“…At a distance of 700 m upgradient of the spring, the two cave streams merge just ahead of SP-3 and form a single, large phreatic cave passage towards the main outlet. It has been demonstrated by numerical modeling that solutionally enlarged pathways will form such dendritic drainage structures in karst aquifers, as increasing flow and dissolution are self-enhancing processes (Gabrovsek et al, 2004;Worthington and Ford, 2009;Dreybrodt et al, 2010). Although numerous flow paths exist in the epikarst and vadose zone due to discrete and diffuse infiltration, only a few preferential flow paths prevail with increasing flow distance (Fig.…”

“…Karst aquifers are characterized by strong heterogeneity and anisotropy related to the diverse distribution of solutionally enlarged conduits in the carbonate rock (Worthington and Ford, 2009). Groundwater flow and contaminant transport in karst aquifers are difficult to predict because of the unknown configuration and geometry of the conduit network.…”