12 Abbreviated Glossary of Karst and Groundwater Terminology

This glossary includes terms used in this book. A more complete lexicon of karst terminology is provided by the United States Environmental Protection Agency (USEPA, 2002).

Allogenic Recharge

Recharge contributed by surface runoff carried into the karst aquifer by sinking or disappearing streams, but which originates through precipitation falling on areas underlain by non-karstic bedrocks. Allogenic recharge contributions and the catchment areas they derive from must be included in the water budget for karst aquifers even though they are geographically and geologically outside the physical boundaries of the karst system.

Autogenic Recharge

Recharge originating from infiltration of precipitation that falls on the area directly underlain by the karst aquifer, or from underground diversion of surface runoff that accumulated within the geographic boundaries of the area underlain only by the karstified bedrock.

Capillary Fringe

This is the saturated zone above a water table in which water pressure is less than atmospheric. The water table is defined by the surface below the capillary fringe where the water is at atmospheric pressure. Below the water table, pressure is greater than atmospheric. Where, in the capillary fringe, molecular forces between water molecules and the rock surface hold water via surface tension onto the pore walls. The thickness of the capillary fringe varies and decreases in thickness as pore diameters increase, but is generally less than 1 m thick. Figure Glossary-1 shows the vadose (unsaturated) zone and the phreatic (saturated) zone of a water table aquifer with insets showing the capillary zone in creviced rock and gravel rock.

Figure Glossary–1 Illustration of the interface between the vadose and phreatic zones. Modified from Poeter and others (2020).

Carbonates

Sedimentary rocks composed of at least 50 percent carbonate minerals that are often cemented. Carbonate minerals include calcite or aragonite (different forms of calcium carbonate), dolomite, many phosphate and sulfate minerals. The predominant mineral in limestones is calcium carbonate (CaCO₃). The predominant mineral in dolostone formations is dolomite-calcium magnesium carbonate (CaMg(CO₃)₂). These tend to be layered sedimentary rocks that accumulate from chemical precipitation of carbonate minerals or the accumulation of shells, reef and atoll structures in coastal or marine environments. Carbonates can also form by calcite precipitation from inland waters forming rocks called tufa.

CO₂ Partial Pressure

Dalton’s law of partial pressures states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases if the independent gas alone occupied the entire volume of the original mixture at the same temperature. It is important to note that the gas pressure is a function of temperature. Additionally, in the earth’s atmosphere the total pressure decreases with altitude.

Dalton’s Law of Partial Pressure: PTotal = Pgas1 + Pgas2 + Pgas3…

The pressure exerted by an individual gas in a mixture of gasses is known as its partial pressure.

Diagenetic Processes

The processes involved in the physical and chemical changes in sediments first caused by water-rock interactions, microbial activity and compaction after their deposition, but before lithification.

Dissolved Solids (or TDS—Total Dissolved Solids)

It is a measure of the concentration of all organic and inorganic dissolved substances (like minerals, metals, and salts) present in a water. Generally reported in milligrams per liter (mg/L).

Eogenetic Karst Aquifer

Karst aquifers composed of younger, near-surface carbonates that generally have higher matrix porosity and permeability than deeply buried carbonates. They were never deeply buried. Eogenetic karst could also be described as an epigene karst aquifer.

Epigene Karst Aquifer

Shallower karst aquifers formed by aggressive recharge descending from the land surface, as a consequence of the infiltration of precipitation, subsurface diversion of stormwater runoff and surface stream flow via sinks, and the subsequent movement of groundwater via karst features. These processes can occur with eogenetic and telogenetic karst aquifers as telogenetic refers to a carbonate that was buried, its porosity reduced and then uplifted to expose the aquifer to surface processes that may increase dissolution along fractures and bedding planes. An eogenetic karst aquifer has never been compacted via burial.

Epikarst

The uppermost weathered zone of carbonate rocks that possesses substantially enhanced porosity and permeability relative to the deeper parts of the rock mass. Epikarst stores and intermittently distributes infiltrated recharge water to the underlying karst aquifer’s unsaturated zone. It is an important water storage system that functions as a perched, leaky aquifer. Some studies suggest that water storage in the epikarst can be more significant than storage in the saturated zone of the karst aquifer.

Estavelle

An open ground orifice that supports flow both into and out of the groundwater system. It can be a sink hole that is not along a stream or can occur beneath a stream channel.

Evaporites

Layered crystalline sedimentary rocks that form from brines generated in areas where the amount of water lost by evaporation exceeds the total amount of water from rainfall and influx via rivers and streams. The mineral composition includes carbonates (especially calcite, dolomite, magnesite, and aragonite), sulfates (anhydrite and gypsum), and chlorides (particularly halite, sylvite, and carnallite), as well as various borates, silicates, nitrates, and sulfocarbonates. All evaporitic rocks are water soluble to varying degree.

Geomorphic Period

Geomorphology is the study of the origin and evolution of topography and bathymetry. Sometimes described as the study of landforms and landscapes. A period predominated by erosion over decades or centuries could be described as a geomorphic period.

Hydraulic Conductivity

In groundwater, a constant of proportionality, symbolized by the letter K. It is a coefficient in Darcy’s law as shown here:

Q = −KA(Δh/ΔL)

where:

Q	=	rate of water flow (L3T-1)
K	=	hydraulic conductivity (LT-1)
A	=	column cross-sectional area (L2)
Δh/Δl	=	head gradient across the column of length L (dimensionless)

This is illustrated by Figure 33 and discussed in Section 4.1 of this book.

Hypogene Karst Aquifer

Deeper karst aquifers formed in the subsurface by aggressive recharge moving upward from ground water under artesian conditions. Hypogene karst is generally located in, or near, regions of tectonic, volcanic, or high-temperature geothermal activity past or present.

Hyporheic Zone

The layer of streambed where exchange or cycling occurs between groundwater and surface water and considered important in stream nutrient cycling, in moderating stream temperature regimes, and in creating unique habitats within streams (Figure Glossary-2).

Figure Glossary–2 – Hyporheic zone, from Winter and others (1998).

Hysteresis

Hysteresis occurs when a system’s output is based on its history. For example, concentration of a chemical constituent increases more rapidly during the increase of discharge from a storm (rising limb) and decreases more slowly during the recession period (falling limb) at a spring or river. There are many definitions of hysteresis as the phenomenon is noted in many natural and engineered systems and the definition is often context dependent. In hydrology soil moisture tension, river discharge, and suspended sediment concentration exhibit hysteresis. Hysteresis plots have different forms that depend on their context. In the example above, a hysteresis plot of the chemical-concentration time series will form a loop.

Figure Glossary-2 shows an example of hysteresis with turbidity and streamflow data. These data are not from a karst area, but this is illustrative by using a) a natural-logarithm, transformed streamflow and turbidity time series; then b) a plot of an autocorrelation function indicating time lags between the streamflow and turbidity; and finally, c) the hysteresis loop when the log streamflow (y-axis) and log turbidity (x-axis) are plotted together. The data comes from two sites (DIFF and SFLIL), but the sites are not important—rather it is important to understand that the DIFF site has a clockwise hysteresis loop from beginning to end of the storm and the SFLIL site has two storm events plotted together on one graph and two counterclockwise loops. These two sites behave quite differently with respect to the relationship between time lag between rising storm peak and turbidity peak.

Figure Glossary 3 – Example of hysteresis with turbidity and streamflow data from two different stream sites labeled DIFF and SFLIL: a) natural logarithm, transformed streamflow and turbidity time series; b) autocorrelation function indicating time lags between the streamflow and turbidity; and, c) hysteresis loop with log turbidity (y-axis) and log streamflow (x-axis). For the DIFF site, turbidity peaks before streamflow peaks (top a); autocorrelation (top b) indicates a positive time lag; and hysteresis (top c) loops in a clockwise manner. Thus, for the DIFF site, in-channel material and streambank erosion are the dominant sources of suspended solids. Two storms are shown for the SFLIL site (bottom a) and turbidity peaks after the streamflow peak for both storms indicating the same negative time lag for both storms (bottom b); the hysteresis loops are counterclockwise (bottom c). Thus, for the SFLIL site, upland material and erosion of upper streambanks are important components of the suspended solids. From Porter and others (2020).

Karst Window

A large surface opening that allows the aquifer to be directly exposed to land surface. Generally formed by the collapse of the ground surface into a conduit forming a sink hole which is a window to underground flow in the karst aquifer.

Mesogenetic Karst Aquifer

Aquifers generally composed of deeply buried older carbonate rocks that experienced karstification before burial with matrix porosity and permeability reduced during burial.

Newtonian Fluid

Newtonian fluids are named after Isaac Newton, who first used the differential equation describing the relation between the shear strain rate and shear stress for such fluids. This differential equation is the simplest mathematical model of fluids that accounts for viscosity. While no fluid fits the definition perfectly, many common liquids and gases, such as water and air, can be assumed to be Newtonian for practical calculations under ordinary conditions. A Newtonian fluid has a constant viscosity tensor that does not depend on the stress state and velocity of the flow. If the fluid is also isotropic (that is, its mechanical properties are the same along any direction), the viscosity tensor reduces to two coefficients, describing the fluid’s resistance to continuous shear deformation and continuous compression (or expansion), respectively.

Non-Point Source

A source of contaminants that comes from widely distributed or pervasive environmental elements, such as the diffuse source of bacteria and nutrients from pastures, or fertilizers and pesticides applied over agricultural fields or golf courses.

Phreatic Zone (Zone of Saturation or Saturated Zone)

Part of an aquifer below the water table in which pores and fractures are saturated with water. Immediately above the water table is the capillary fringe (a saturated zone of negative pressure owing to surface tension between the solid matrix and water molecules). Above the capillary fringe is the vadose zone. The phreatic zone size and depth may fluctuate with changes of season, and during wet and dry periods. See Figure Glossary-1.

Point Source

A source of contamination that comes from a fixed-point location, such as a sewage outfall to a stream or a leaking oil or gas well.

Post-Depositional Structural Deformation

Sedimentary rocks tend to be deposited in horizontal layers and can be deformed after deposition (post-depositional). Any changes to the layering of the rock are termed deformation. Deformation can be caused by desiccation causing cracks; or by dewatering or volcanic/tectonic events causing faulting and folding.

Sinkhole

An area of ground that has no natural external surface drainage—when it rains, the water stays inside the sinkhole and typically drains into the subsurface. Sinkholes can vary from a few meters across to hundreds of kilometers and are typically between 1 and 30 m deep. Some are shaped like shallow bowls or saucers whereas others have vertical walls; some hold water and form natural ponds. Some allow surface runoff to directly enter a karst conduit system. Sink holes can be a Karst Window or Swallet (Throat or Swallow Hole).

Swallet (Throat or Swallow Hole)

A place in a limestone stream channel where water disappears underground into a small sinkhole that may be called a snake hole. Swallets are generally smaller than most sinkholes or karst windows. In the latter, all streamflow diverts underground during times of minimal overland runoff.

Stream Discharge Hydrograph

A graph showing the rate of flow (volumetric discharge) versus time past a specific point in a river, channel, or conduit (Figure Glossary-4). The components contributing to a stream hydrograph are surface runoff, interflow, and groundwater flow. Surface runoff is the overland runoff that occurs when the rate of rainfall exceeds the infiltration capacity of the soil and surface depressions are full. Interflow is the lateral flow of water occurring in the unsaturated zone that returns to the surface or a streambed without reaching the saturated zone. Groundwater flow is the component of streamflow that moves into the streambed from the saturated zone of an aquifer.

Figure Glossary–4 – Example of a stream discharge hydrograph for a single storm with a) showing the flow components that sum together to from the total streamflow as shown in b). The inflection point shown on b) is the beginning of what is called the recession limb of the hydrograph when surface overland flow has ceased, interflow is beginning to recede, and the majority of flow entering the stream is from groundwater.

Telogenetic Karst Aquifers

Aquifers generally composed of long-buried carbonate rocks now uplifted to near the surface with matrix porosity and permeability reduced owing to deep burial, and/or diagenetic changes (for example, interstitial cementation—a process that can occur without deep burial) that are exposed to weathering and subaerial and subaqueous erosion, creating porosity from dissolution along fractures, joints, and bedding planes. These could also be described as epigene karst aquifers.

Thermodynamically Saturated

Term used in chemistry for chemical or gas dissolution in water. At a specific temperature and pressure, a gas, for example Oxygen, has a maximum concentration and that is the saturation concentration. For a mineral such as calcite, there is a maximum concentration that can be dissolved into a solution at a specific temperature or pressure.

Thermodynamically Undersaturated

More gas or chemical can be dissolved into the water at a given temperature and pressure.

Vadose Zone

Also termed the unsaturated zone, is between the land surface and the top of the phreatic zone, the position at which the groundwater is at atmospheric pressure. Hence, the vadose zone extends from the top of the ground surface to the water table. See Figure Glossary-1.

Water Salinity

Natural waters contain dissolved solids. The ranges of total dissolved solids (TDS) used for defining the salinity of water in the USA (Stanton et al., 2017) is as follows:

• Fresh: < 1,000 mg/L (milligrams per liter);
• Brackish: 1,000 to 10,000 mg/L; and
• Saline: > 10,000 mg/L of dissolved solids.

Seawater generally has a dissolved-solids concentration of 35,000 mg/L. The secondary maximum contaminant level, a nonmandatory standard that only applies to public water systems in the USA, advises a level of 500 mg/L for dissolved solids (USEPA, 1991), although numerous water supplies exceed this level. Water with dissolved solids levels exceeding 1,000 mg/L is generally considered undesirable for human consumption.