Rocks that are primarily composed of calcium carbonate constitute about 20 percent of the earth’s sedimentary rocks and they underlie about 40 percent of the land surface in the United States. These carbonate rocks consist of limestone and dolomite that contain groundwater in fractures and pore space which, in the year 2000, provided water at a rate of 5,840 million gallons per day from 16 principal carbonate-rock aquifers in the U.S. (Maupin and Barber, 2005). Despite their importance for water supply, many aspects of groundwater flow in carbonate aquifers are poorly understood. These aquifers, which are also referred to as karst aquifers, are particularly challenging for hydrogeologists to evaluate because they have a wide range of hydraulic characteristics related to fracturing and chemical dissolution. As a result of dissolution-enhanced fracturing, prediction of groundwater flow rates and directions in these aquifers requires an understanding of both the geometry of hydrostratigraphic units and their porosity and permeability characteristics. Where they are covered by thin soil or exposed at the land surface, there is little attenuation of surface pollution and the aquifers are especially prone to being contaminated.
Discussion Questions:
After going through the following text and graphics, answer the following questions about carbonate aquifers and their relation to water resources:
1. Briefly explain the mechanisms underlying the formation of fractures and karst (cave systems). Discuss why caves are often aligned with regional flow, and why bands of caves are observed at significantly different elevations.
2. Explain the processes that produce the following features. Discuss implications in permeability, variability, etc.:
- anticline, implications on fractured orientations
- syncline
- faults, permeable? impermeable?
- Karst formation
- joints, bedding planes, and fractures, cavities, caves
3. Fractured-rock aquifers are more easily contaminated than porous media aquifers but, once contaminated, fractured rock aquifers may be easier to decontaminate. Consider some geologic conditions when a fractured-rock aquifer may be just as difficult to decontaminate as a porous media aquifer.
4. Because many carbonate-rock aquifers conduct water through fractures, bedding plane partings, or solution enhanced fractures; discuss the limits of using groundwater flow models that use porous media flow assumptions to simulate groundwater flow. What are the limits to simply increasing hydraulic conductivity before porous media assumptions are violated? Consider some techniques that allow MAGNET4WATER models to be used in spite of violating porous media assumptions.
Figure 1 – Conceptual schematics of karst landscapes
Figure 2 – Karst aquifers in the United States. Source U.S. Geological Survey (https://water.usgs.gov/ogw/karst/kig2002/jbe_map.html) [accessed 3/15/20]
The primary porosity of carbonate rock ranges from just a few percent in older deeply buried rocks to almost 20 percent in younger near-surface rocks. Therefore, some carbonate-rock aquifers can still produce significant volumes of water without being fractured. Dolomites are chemically altered limestones that includes magnesium, which is chemically similar to calcium, into the chemical structure of calcium carbonate. Dolomites generally have higher porosities than limestones because of this chemical alteration. The most chemically pure limestones were precipitated in evaporite basins as seawater reached higher calcium carbonate concentrations. Fossils and mud can be incorporated into this rock structure but the resulting rock is usually so fine grained as to not detect grain size with the naked eye. Limestones can also form from detritus that is cemented by calcium carbonate. These detrital limestones originated in higher energy areas of warm ancient seas similar to some tropical parts of the world today. Often sea shells or other calcite-rich parts of sea creatures may be the detritus that is chemically glued together with calcium carbonate. With deep burial, these fragments form rock that generally has higher porosity and permeability than chemically derived limestone.
Figure 3 – Fossiliferous detrital limestone of the Ozark Plateaus region, Midwestern United States (Photograph from Norman Grannemann).
Because of solution weathering, carbonate-rock aquifers play an outsized role in understanding groundwater. Most springs emanate from carbonate-rock aquifers and their presence has intrigued humans for millennia. Springs have long been an important source of water and many people have an almost magical fascination with them. The springs developed because cracks and interconnected fissures in carbonate rocks can allow much more rapid water flow underground. When a fissure intersects the land surface at or below the water table, groundwater discharges as a spring. For aquifers composed of porous media only, this intersection would result in a seepage face, not a spring. In addition, almost all cave systems occur in carbonate rocks. Caves give humans an unusual opportunity to walk inside an aquifer and see directly how water moves underground. However, groundwater in a cave system is an atypical aquifer because caves move water underground more like a stream at the land surface.
Figure 4 – Scientists study a sinkhole entrance to a cave system in Florida (Photograph from Norman Grannemann). There are many sinkhole features worldwide. For example, the Yucatan Peninsula contains about 6,000 cenotes, the term used for water-filled sinkholes in Mayan culture.
Caves that form in carbonate rock units are among the most well-known geological features. The myth that groundwater occurs as underground rivers probably originates from streams flowing in caves. Over thousands or millions of years, the physical and chemical effects of water moving through limestone can cause small openings to enlarge. As the smaller openings coalesce, extremely large caverns can develop.
The process begins with limestone exposed at or near land surface. Groundwater filtering down from the land surface acquires acids from organic processes in the soil profile and, as this acidified water moves through natural openings in the geologic framework, the openings in carbonate rocks are enlarged. The natural openings may be from primary porosity during deposition of the limestone or they may be from joints, fractures, and faults from crustal tectonism. As the groundwater seeks out these natural openings, the acids attack and dissolve the exposed carbonate rock. Residues remaining after this reaction are transported down gradient with the migrating water and the openings in the limestone are slightly enlarged. As the openings increase in size, they begin to coalesce and form conduits capable of sustaining higher flow rates. Conduits are generally horizontal in the early stages of development but become more complex as the conduit system develops. As horizontal conduits develop at different elevations, gravity-driven water will utilize steeply-inclined joints or fractures to connect the conduits into a series of caverns. Collapse of the roofs of caverns into the voids below is a common mechanism for the interconnection of this maze. When this roof collapse involves the land surface, circular water-filled depressions dot the land surface. This characteristic feature is commonly called a “sinkhole” and the resulting landform is called “karst topography”.
Figure 5 – Preferential horizontal water flow along bedding planes in limestone rock (Photograph from Norman Grannemann).
Regardless of the size of these caverns, karst processes form hydrologic pipelines capable of carrying huge volumes of water in underground rivers. Unlike porous media flow in typical sedimentary rocks, which transmit water at a few feet per year, flow rates in limestone caverns can be measured in miles per hour. In well-developed karst terrain (for example, central Florida), entire rivers can disappear into or emerge from the subsurface. In addition to the water transmissivity associated with karst, sinkholes connect aquifers directly to the land surface and provide avenues for manmade contamination.
Although caves, sinkholes, springs, and underground rivers are fascinating to observe, most carbonate rock has not undergone the degree of weathering that produces karst features. Rather, a continuum of fracturing and dissolution produces a broad range of conduit and pore-space openings. This range of fracturing results in a corresponding range of hydraulic properties. For example, an unfractured, chemically precipitated limestone may have hydraulic properties more similar to a confining unit than an aquifer. A limestone with low porosity and widely spaced fractures may be a marginal aquifer, whereas, a limestone with many solutionally enhanced fractures may be an excellent aquifer especially if many interconnected conduits have formed. Karst systems that include sinkholes and caves are at the extreme end of a continuum of solution enhanced fractures. White (1993) estimates that the critical aperature size for conduit flow in carbonate rock aquifers is about 1 cm. Some conduits may have limited flow because they are plugged by debris that results from solution weathering and some may not be connected to other conduits to form a flow system.
One of the most well-known carbonate-rock aquifers in the world is a Silurian-aged reef system that is known for the erosion resistant limestone that creates Niagara Falls in Canada and the United States. Bedding plane and vertical fractures make this rock unit an excellent aquifer over the outcrop and subcrop area that extends from Wisconsin to Canada.
Figure 6 – Limestone rocks of the Niagara Escarpment at Niagara Falls, Canada and the United States (Photo from Norman Grannemann).
References:
Maupin, M.A., 2005, Estimated withdrawals from principal aquifers in the United States, U.S. Geological Survey Circular 1279, 46 p.
White, W.B., 1993, Analysis of karst aquifers, in Regional ground-water quality, W.M. Alley, editor: Van Norstrand Reinhold, New York, NY, p. 471-489.