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Chapter 19: A Hidden Reserve: Groundwater
Guide to Reading
Chapter 19 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 found in oceans (Chapter 18), and the obvious freshwater supply flows on the surface in streams and lakes (Chapter 17). A much larger quantity of freshwater, 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), an aquiclude, or an aquitard on the basis of its porosity and permeability (that is, its ability to hold and conduct water).
Freshwater is a vital natural resource. To successfully and efficiently tap a groundwater supply, hydrologists study geochemistry and subsurface structure in the potential resource area. They are concerned with the ion content of the water (ion concentration, precipitates, and hard water). They try to discern where the unsaturated zone (or vadose zone), capillary fringe, water table, and saturated zone (or phreatic zone) lie. An ordinary well must be drilled down past the first three layers and into the fourth if it is to become a dependable, long term water producer and not just a seasonal well or a dry well. Hydrologists identify special features like perched water tables and artesian wells (flowing and nonflowing) and special regions like recharge and discharge areas, so they can be used advantageously and not create problems. They consider the hydraulic head, 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 hydrologists don't have to search for groundwater; it reaches the surface on its own. You will read about several types of geologic settings that yield springs and about the special setting needed to produce an artesian spring.
Geothermal areas offer unique groundwater-caused surface features including hot springs, mud pots, and geysers that serve sometimes 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.
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, become 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. There are few streams that disappear into the ground and flow there, but there is a great quantity of water moving 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. « Return to Chapter 19 Study Plan