Chapter 14: Groundwater Fig 14.5b
OBJECTIVES Recognize that groundwater is a vital source of accessible freshwater. Describe how groundwater forms below the water table. Explain the origin of aquifers, the controls of groundwater flow, and the origin of oases and hot springs. Describe how the movement of groundwater through soluble rock creates caverns and a distinctive surface topography.
OBJECTIVES Explain how groundwater is exploited as a resource and how the unique qualities of groundwater render it susceptible to contamination by human activities. Identify how plate tectonics governs the location and overall shape of some aquifers, as well as groundwater flow, by creating the mountains that influence topography and act as recharge areas for aquifers, as well as the faults that bring groundwater to the surface.
Groundwater Groundwater is a small fraction of Earth's total water, but it constitutes 22% of freshwater resources. Fig. 14.1
The Water Table The water table is the upper surface of the zone of saturation. Fig. 14.2
Porosity and Permeability Porosity is the amount of open space between sediment particles. Permeability is the amount of interconnectedness between sediment pores. Fig. 14.3
Groundwater Supply Groundwater supply depends on rates of discharge and recharge. Fig. 14.5a
Wells can be used to remove groundwater from an aquifer. If the water table drops, wells may run dry. Wells and Aquifers Wells drilled into an aquifer must penetrate below the water table. Note the lower level of the water table during the dry season. The water table feeds wells and springs, and changes in the level of the water table from the wet to the dry season affect the availability of water. The change shown here, for example, would cause the shallow well (on the right) to dry up during the dry season. Fig. 14.6
Unconfined Aquifers The upper surface of an unconfined aquifer is at the same level as the water table. Fig. 14.7
Confined Aquifers Confined aquifers can generate hydraulic head in artesian wells. Fig. 14.9
Heated Groundwater Heated groundwater can form geysers, hot springs, and fumaroles. Fig. 14.11
The dissolution of underground rock layers by groundwater can lead to the formation of sinkholes. Sinkholes come in two types: solution sinkholes and collapse sinkholes. Sinkholes Fig. 14.15
In areas with soluble bedrock, groundwater dissolution can lead to the formation of karst topography. Karst topography is characterized by sinkholes, monoliths, caverns, and disappearing streams. As seen here, groundwater migrating though the zone of aeration dissolves limestone rock, then moves along the water table to form interconnected subsurface channels as dissolved rock is transported to streams on the surface. The stream continues to erode and the water table drops, causing caves to form in the abandoned channels. Karst Topography Fig. 14.17
Risks of Overdrawing Wells As water is removed from a well, it produces a cone of depression. In coastal regions, this can lead to saltwater contamination of wells. Fig. 14.19
Risks of Overdrawing Wells Because salt water is denser than freshwater, it would normally sink below any freshwater in an aquifer. As freshwater is withdrawn and cones of depression form, the water pressure is reduced, which allows the saline water to migrate upward, causing a cone of ascension. Fig 14.21
Contamination of Groundwater Human actions can contaminate groundwater in a variety of ways. Fig. 14.23
Groundwater and Plate Tectonics Plate tectonics influences or has influenced Temperature of groundwater and existence of geothermal resources Topography that affects location of groundwater resources Topography that affects flow of groundwater, recharge of aquifers Ancient flow of warm, saline groundwater Fig. 14.12
Summary Groundwater Is the most widespread and accessible source of freshwater on Earth Is stored in aquifers, which are layers of porous and permeable sediment or rock Can be found in confined or unconfined aquifers Can form geysers, fumaroles, and hot springs when it is heated Can produce sinkholes, monoliths, disappearing streams, and caverns when it dissolves underground rock layers Is susceptible to contamination from saltwater, landfills, agricultural runoff, septic tanks, and other sources Is influenced by plate tectonics regarding location and flow