Introduction to Karst Groundwater
An aquifer is any body of bedrock, or other earth material, from which a well or spring produces useable quantities of groundwater. All potable groundwater originates as precipitation. The rain or snowmelt percolates into the ground, and flows through the pores and fractures in the bedrock, to ultimately discharge from the aquifer and once again become surface water.
Many important aquifers are composed of granular materials such as loose sand and gravel or weakly cemented bedrock (see figure above). Groundwater flow in these aquifers is through the pores or spaces between the grains of sand or gravel, or through narrow fractures in solid bedrock, and is usually very slow. How quickly the water flows is partly dependent on how big the pores are, how interconnected the pores or fractures are, and how much energy (head or water pressure) is available to move the water through the aquifer. The small pore openings act as a filter, physically or chemically removing most bacteria, viruses, and polluting chemicals within a few tens to hundreds of feet. In shallow granular aquifers the water table mimics the land surface, and the slope of the land can often predict the direction of groundwater flow. Springs occur along creeks and rivers where the water table meets the land surface. Artesian springs occur where impermeable rocks, such as shale, underlie and cover (confine) most of the aquifer. The impermeable rock restricts the direction of groundwater flow and can direct flow in a confined aquifer to discharge many miles from where it started out. Recharge and discharge occurs where the confining unit is missing.
Technical Definition of Karst Aquifer*:
A body of soluble rock that conducts water principally via enhanced (conduit or tertiary) porosity formed by the dissolution of the rock. The aquifers are commonly structured as a branching network of tributary conduits, which connect together to drain a groundwater basin and discharge to a perennial spring.
Dr. Steve Worthington, independent geologist, personal communication, 2002
* Must produce useful quantities of water to a spring or well to qualify as an aquifer.
Karst Aquifers Are Different from Granular Aquifers
A karst aquifer is roughly analogous to a roofed-over creek. In comparison, granular or fractured bedrock aquifers have no equivalent "underground river" or channel. The drainage patterns of karst conduits resemble the branching pattern formed by streams flowing above ground across insoluble rocks. We are prevented from seeing the branching pattern of karst conduits because the streams are below ground. The disrupted topography of a karst terrain also prevents us from easily seeing on the surface the now-abandoned channels, relics that once carried water before the limestone dissolved. In karst aquifers, the conduits and caves drain the pore space between the limestone grains (intergranular or primary porosity) and the fractures (secondary porosity) formed by joints, bedding planes, and faults. The unconsolidated cover over the bedrock, narrow fractures in the bedrock, small conduits, and larger cave passages collectively form a karst aquifer. The openings forming the karst aquifer may be partly or completely water-filled. The elevation where all pores are filled with water in an aquifer is the water table. The water tables in karst areas can be highly irregular in elevation, however, because water-carrying conduits can develop at various elevations. Water may also be encountered in perched aquifers -- aquifers that are elevated above the lower, regional water table by less soluble, impermeable beds.
A karst spring receives drainage from all the sinkholes and sinking streams within its groundwater basin, equivalent to a watershed on the surface. The conduits carrying water from each point where water sinks join together underground to form successively larger passages with ever-increasing flow, which eventually discharges at a spring. An important way that karst aquifers differ from other aquifers is that a groundwater basin boundary may have little relationship to surface watershed boundaries. A stream flowing on the surface simultaneously shifts the watershed boundary as it erodes headward. When a cave stream erodes headward, however, it can extend beneath a ridge, leaving the surface unmodified, to capture flow from the adjacent watershed. Therefore, the actual watershed area of a karst spring may be much larger, or smaller, than is apparent from topographic maps. Sinkholes and valleys shown on maps may seem to be in one watershed yet drain to a far-away spring.
In many karst aquifers a large percentage of the water stored underground is perched, or suspended, above the main part of the aquifer in the "epikarst." The epikarst ("upon the karst") is the interval between the mostly unaltered bedrock and the topsoil. The water in the epikarst is stored in enlarged joints and bedding planes, spaces around pieces of float (rocks that have been detached from the bedrock), porosity within residual chert rubble, and the smaller conduits in the bedrock. Sinkholes are a reflection of the development of the epikarst and are sites of active transport of insoluble sediment and dissolved rock into the subsurface.
Karst springs occur where the groundwater flow discharges from a conduit or cave. Karst springs or "cave springs" can have large openings and discharge very large volumes of water. The sinkholes and sinking streams that drain to a large karst spring can be many miles away from the spring. Frequently, groundwater flow rises to the surface from a completely water-filled conduit. The depth of the clear water in the spring pool gives the water a deep blue color so they are termed "blue holes."
Spring distributaries are branched conduits or caves that discharge groundwater to multiple springs, commonly distributed along the bank of a short reach of the receiving stream. They are quite common in karst systems and were first described formally and scientifically from springs discovered along the Green River near Mammoth Cave. As the water flowing in the conduits nears a permanent surface-flowing stream into which it discharges, the water seeks the lowest available exit and is constantly creating new spring openings downstream. During higher flows the intermittently abandoned openings, or "cave springs," also discharge water. Along low-gradient streams, several openings may develop almost simultaneously, resulting in many springs draining a single groundwater basin.