Groundwater and the International Year of Caves and Karst: Explore, Understand, Protect.

Abstract

Groundwater is sometimes described as “the life blood” of our planet. Hydrologically, this analogy is most accurate for karst aquifers. They are triple-permeability systems. Flow occurs between the bedrock matrix (tissue), fractures (capillaries), and conduits and caves (veins and arteries). Groundwater storage, chemistry, and permeability are distinct in each part of the system. All parts of the system are important. The dissimilarity is in perception. Everyone knows to not cut a vein or artery, but until recent years, conduits and caves were largely ignored in many hydrogeologic studies. Awareness, and more importantly, understanding of karst systems is growing. The number of universities, governmental research and management programs, and consulting companies with karst expertise has expanded substantially over the past 20 to 30 years. Recognizing that 25% of the United States is karst, or related pseudokarst terrain, the U.S. Congress established the National Cave and Karst Research Institute in 1998 to focus on its study and management, and to educate the country about its characteristics, needs, challenges, and value. To accelerate this growth of awareness and knowledge, the International Union of Speleology established 2021 (extended through 2022 due to COVID) as the International Year of Caves and Karst. Its theme is “Explore, Understand, Protect,” because each step is necessary for the next to be effective. Karst aquifers are formed by bedrock dissolution. Caves, sinkholes, underground streams, and the world's highest yield wells and springs are typical features, although not all karst aquifers are overlain by karst landscapes. Caves, once dismissed as geologic curiosities, are now demonstrated as the primary permeability structures of karst aquifers. These groundwater systems include permeabilities spanning 11 orders of magnitude. Wells typically tap the sections of lesser permeability. Pump tests provide valuable data, but they are now known to reflect conditions around a well and do not usually characterize the karst aquifer. Convergent flow to features of greater permeability results in cave streams and the springs they feed. They carry water from all portions of their groundwater drainage basins. Monitoring cave streams and springs most accurately characterizes karst aquifers. Maps of caves, usually drawn by cave explorers, provide priceless insights into how karst aquifers are organized and have evolved. The classical “top-downward” or epigenic origin of karst aquifers is now joined by models of “bottom-upward” or hypogenic evolution, as well as flank margin evolution in coastal regions which has also been preserved and found far inland from modern seas. Caves and conduits (essentially caves hydrologically and geochemically, but too small for human entry) provide habitat for aquifer organisms. Some have been used to define the presence or absence of aquifer connections and divides invisible to some hydrogeologic methods. Microorganisms are now recognized as critical to permeability enhancement of some karst aquifers. The higher permeabilities of karst aquifers offer benefits, such as rapid recharge, but also problems. As triple permeability systems, karst aquifers are highly complex. They are also the most vulnerable to contamination and depletion. They are the only aquifers contaminated directly by solid waste reaching the water table. Pathogens, chemicals, metals, and other pollutants often travel through these aquifers at rates of hundreds of meters per day in difficult-to-predict directions. Porous media style plumes are rare in karst aquifers. Meanwhile, wells that tap high permeability flow paths are most prone to pumping at the highest rates possible rather than the highest rate sustainable. Most blood in the human body is stored in tissues and capillaries and the same is true of karst aquifers. At any point in time, caves and conduits store about 6% of a karst aquifer's water. However, over time, about 99% of the water flowing through a karst aquifer flows through caves and conduits. Similar conditions occur with surface watersheds, where no one would consider ignoring the stream channels when studying those systems. The growth in awareness and understanding that caves and conduits are features of primary importance within the triple permeability networks of karst aquifers, and that such aquifers serve as the sole or primary source of water for about 700 million people worldwide, is reason to celebrate the International Year of Caves and Karst. The scientific and practical benefits in karst groundwater use and protection are the greater reason I encourage you to continue to “Explore, Understand, Protect,” karst aquifers far into the future.