2.3 Water Table Decline and Fluctuation Forms Interconnected Conduits

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. Additionally, as the water table rises and falls seasonally, this results in more mixing of water. Figure 5 illustrates the stages of karst aquifer evolution from a fractured flow system to a conduit flow system as a result of water table decline and rise. In the first stage (Figure 5a) the limestone has normal small fractures before chemical weathering, followed by a second stage in which chemically aggressive water dissolves calcite along the fracture walls. In this second stage (Figure 5b), some fractures have been enlarged but are insufficient to form a network of enhanced hydraulic conductivity and therefore the rock does not yet have conduit flow. In the third stage (Figure 5c), further dissolution has connected the enlarged fractures so that conduit flow begins. In the fourth stage (Figure 5d) enlargement and interconnection has increased the hydraulic conductivity of the interconnected paths so much that flow can be driven by a lower gradient, thus the water table declines and vadose-zone karstification processes occurs at deeper levels. This lowering of the water table may be increased if it is accompanied by down cutting of streams in the area. In the fifth stage (Figure 5e) a wetter period occurs, raising the water table and resulting in active conduit flow through what was the deep part of the vadose zone when the water table was lower.

Illustrations showing the evolution of a karst hydrologic system over geologic time

Figure 5  Illustrations (a-f) show the evolution of a karst hydrologic system over geologic time in six stages from a barren limestone landscape (no soil) with a fracture network with groundwater flow in connected fractures connecting to a nearby hillslope and ending with a fully developed karst hydrologic system that drains the local precipitation through the bottom to discharge at faraway places not within the topographic watershed:
a) Stage 1: Barren limestone with small fractures that contribute a small flow from the spring to the stream;
b) Stage 2: Soil develops causing generation of CO2 and organic matter that generates more CO2, both creating larger fractures and dissolution channels;
c) Stage 3: Connected dissolution channels form to cause lowering of the water table and spring flow from conduits formed along fractures increases, soil development continues;
d) Stage 4: Channel network deepens in the vadose zone as CO2 and organic matter are driven downward maintaining chemically aggressive water at deeper levels, and the stream is fed by karst drainage;
e) Stage 5: Channel network deepens below the stream bottom so that the stream feeds karst system; and,
f) Stage 6: Channel network drops below the local flow system with karst flow draining to a distant exit.
The enlargement of karst channels is driven by chemically aggressive subsurface water with its aggressivity continually renewed by production and dispersion of CO2 in the subsurface through the synergistic combination of hydrologic flow, downward erosion along surface stream channels, and hydrogeochemical processes. If limestone extends far and deep, then given sufficient geologic time, the karst system will extend far and deep. Modified from Wood and Cherry (2021).

The sixth stage (Figure 5f) is a mature karst with no overland flow, the water table below the image area, unknown distant discharge areas and dry caves. These stages can occur within a single geomorphic period. If crustal uplift or a major climate change occurs, then a different era of karstification can begin as is typical in the evolution of most karst systems. More detailed descriptions of the stages of karst aquifer evolution from a fractured flow system to a conduit flow system are provided in Box 1 along with a repeat of Figure 5 for the readers convenience.

Underlying horizontal beds of mudstones, sandstones, dolostones, or any rock that are both less soluble than limestone and less transmissive of water will restrict the downward movement of water resulting in conduits forming in the limestone directly above and along the dip of the less soluble formation. If the water encounters vertical fractures through these less soluble rocks and is still undersaturated with calcite, another deeper more soluble limestone layer will begin to dissolve. This occurs in both the unsaturated zone and saturated zone as changes in the rock facies focus water movement through the more permeable rock. These units of karst aquifers are called preferential flow units or zones (Williams and Kuniansky, 2016; Cunningham et al., 2006; Rose, 1972). If present, these facies changes result in a complex three-dimensional karst network as compared to a system with no facies changes, as in the stages of karst development shown in Figure 5.

Inland areas, such as in southwest China along the Li River (as shown in cover collage), dissolution of limestone often occurs over time at the level of the alluvial valley. In the karst areas of southwest China, Laos, and Vietnam, inland caves are common at the level of the current rivers and along alluvial plains and sunken valleys between karst towers (haystacks) or mountain ridges. Often these caverns have rivers running through them under the large haystacks or mountains. Foot trails follow and boats move between the villages in these sunken alluvial valleys (Khang, 1985). Additionally, there are many dry caverns far above the current river valleys. Worthington (2005) discusses how base level lowering results in the upper dry caves and the dissolution processes creating conduits and cave formation.


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.