Box 1 Stages from Fracture Flow to Conduit Flow

As mentioned in Section 2.3 and displayed in Figure 5, when the water table drops during short-term drought or longer-term minor changes in climate, the vadose-zone karstification processes function at greater depth and when the water table rises, groundwater flows in solution openings formed under the earlier saturated and unsaturated conditions. Erosion along stream channels also lowers the water table over time by opening spring outlets at lower elevations. Additionally, as the water table rises and falls seasonally, this results in more mixing of water and increases dissolution at that current relative position of the water table. The Stages of development are described in more detail in this Box.

Stage 1

The evolution of the karst starts when the new limestone landscape is created from structural forces within the Earth’s crust while a network of fractures forms with bedding plane fractures and vertical joints that provide pathways for an active, gravity driven groundwater flow system to develop. This example shows an established water table and a hill slope spring above the valley-bottom stream. Initially, water that infiltrates through the vadose zone has minimal geochemical aggressivity because the dissolved carbon dioxide (CO₂) in the water is in equilibrium with the above ground atmospheric CO₂ partial pressure (pCO₂ ~ 10^-3.5 atmospheres). Additionally, groundwater flow is slow as the initial fractures are small and dissolution has not progressed.

Stage 2

Soil and vegetation form at the surface converting the barren landscape into an active ecological system and dissolution is increasing fracture openings and through-flow. Henceforth CO₂ is produced in the subsurface, both in the surficial organic soil zone and throughout the vadose zone as particulate and dissolved labile organic matter is dispersed throughout the vadose zone down to the water table. This partial pressure of CO₂ is substantially elevated in the fracture network. Therefore, the water that infiltrates through the vadose zone becomes chemically aggressive with carbonic acid (H₂CO₃) that dissolves the mineral calcite (CaCO₃) which makes up the limestone. In this stage, solution channels begin to form and karstification is initiated, however until channels become connected to drainage outlets, the rate of karstification is minimal and the water table remains near the land surface.

Stage 3

Solution channels at, or slightly below, the water table are connected to form a drainage outlet at a spring along the hill slope so that the spring feeds the stream. Less rainfall goes to the stream by surface runoff and more by subsurface channel flow and the water’s capability to enlarge channels increases. As channel connectivity increases, CO₂ emanating from the soil zone and the organic matter are transported deeper causing chemically aggressive water to spread farther and deeper. The water table declines nearly to the stream level because the bulk permeability in the channel network in the vadose zone is now so much larger than in the underlying fractured limestone.

Stage 4

The process of channel formation penetrates deeper and horizontal channels along bedding planes form with connections to the stream at which time the spring above the stream dries up. The volume of void space occupied by solution channels in the vadose zone continues to increase because of the generation and spread of CO₂ gas that immediately forms carbonic acid in the water wherever the CO₂ spreads. The water discharging at the spring gives off CO₂ into the atmosphere because the partial pressure of the CO₂ in the subsurface water is much higher than that of the atmosphere. This off-gassing causes CaCO₃ minerals to form at the spring. In this stage the water table drops to near stream level. If there were to be a relatively insoluble geologic stratum such as shale or mudstone at or near the elevation of the stream bed, then the karstification process would not penetrate deeper. However, in this illustrative example, there is only limestone and hence no stratigraphic constraint to the depth of penetration of this channel forming process over geologic time.

Stage 5

The channel formation process continues deeper and eventually a channel pathway forms that drains all the local water to outlets beyond the stream valley and hence the local water table drops below the stream level and the stream changes from a gaining stream to a losing stream. The stream may have flow in response to surface runoff from rainfall events, but this water quickly disappears downward through vertical channels to flow to a deeper valley some distance away. This elevation of this distant valley now governs the karst flow system.

Stage 6

The valley that now governs the depth of penetration of the karst flow system is at a lower elevation even farther away, hence outside of this field of view, such that this local karst flow system drains out the bottom of the figure to its primary drainage outlet in a distant valley at lower elevation. The process of karst deepening continues through geologic time so that some cave systems extend as deep as a kilometer or more. Nothing limits the depth of evolution of a cave system over geologic time except a geological stratum that is relatively insoluble such as shale, sandstone or granite or limits on the depths of valleys available to serve as the base level for the subsurface drainage system.

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