Chapter 16: A Hidden Reserve: Groundwater
Study Plan
Geothermal areas offer unique groundwater-caused surface features including hot springs, mud pots, and geysers. Shown here are the geysers at Yellowstone National Park.
Credit: Stephen Marshak
Guide to Reading
Chapter 16 continues the study of the compound that makes Earth unique in our solar system: liquid water. As explained in the previous two chapters, the greatest amount of Earth's water is salty and is found in oceans (Chapter 15), and the obvious freshwater supply flows on the surface in streams and lakes (Chapter 14). A much larger quantity of water, hidden below the surface and rarely seen, is the subject of this chapter.
Your author begins by reminding you that water moves between various reservoirs on, above, and under Earth's surface as part of a phenomenon known as the hydrologic cycle. Most groundwater begins as moisture that falls on land and infiltrates into the ground. Some remains close to the surface as soil moisture; more percolates downward through narrow, crooked channels in the bedrock. The reason water can exist and travel through "solid rock" involves a discussion of pore spaces, porosity (primary and secondary), and permeability. A rock layer is classified as an aquifer (confined or unconfined) or as an aquitard on the basis of its porosity and permeability (that is, its ability to hold and conduct water).
Water is a vital natural resource. To analyze a groundwater supply successfully and efficiently, hydrologeologists study geochemistry and subsurface structure in the potential resource area. They try to discern where the various layers lie. An ordinary well must be drilled down into the aquifer below the water table if it is to become a dependable, long-term water producer and not just a seasonal well or a dry well. They identify special features like perched water tables and artesian wells (flowing and nonflowing) and special regions like recharge and discharge areas, so these can be used advantageously and not create problems. They consider the head of the water table, the hydraulic gradient and hydraulic conductivity of a region, and the effects of local drawdown and cones of depression when deciding how much groundwater an area will yield. They use the formula called Darcy's law to calculate the discharge figure. Sometimes hydrologeologists don't have to search for groundwater; it reaches the surface on its own. You read about several types of geologic settings that yield springs and about the special setting that can produce an artesian spring.
Geothermal areas offer unique groundwater-caused surface features including hot springs, mud pots, and geysers that can serve as tourist or recreational attractions and sometimes as energy sources.
There are problems associated with groundwater usage, like water depletion. Are we carelessly exhausting a nonrenewable resource and ruining land areas in the process? Your author uses several examples in discussing this complex issue. In many locations it is true that:
- Mining of groundwater is occurring (withdrawal faster than natural recharge), so the water table is being lowered and the flow direction of groundwater is being reversed.
- Saline intrusion is causing wells to yield useless salty water.
- Irreversible pore collapse and land subsidence are occurring.
Groundwater contamination is another problem, one that's especially serious because it's so hard to clean up once it occurs. Society is growing increasingly concerned about putting any wastes in the ground that might pose a threat to local groundwater. You read about specific issues such as putting nuclear wastes into welded tuff at Yucca Mountain, Nevada, and about general matters like categories of contaminants, injection wells, contaminant plumes, and bioremediation.
The chapter concludes on a lighter note, with a discussion of unusual karst landscapes (with sinkholes, natural bridges, disappearing streams, and tower karst), and the fascinating world spelunkers explore: caves. Since groundwater is naturally slightly acidic, it dissolves rock (chiefly limestone) to produce cave networks of passages and chambers that sometimes, due to the precipitation of limestone out of solution, are decorated with dripstone speleothems (soda straws, stalactites, stalagmites, and limestone columns) and flowstone, and populated by unusual life forms that have adapted to life away from light.
In review, Earth's subsurface region is, like its surface region, a world of water, but in a subtle way. Few streams disappear into the ground and flow there, but a great quantity of water moves slowly underground through connecting microscopic pores, transporting materials in solution, indirectly sculpting the land, and providing fresh water for human needs when we're wise enough to figure out where and how to tap into the supply.
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