Chapter 13 Groundwater
Introduction Groundwater is all subsurface water that completely fills the pores and other open spaces in rocks, sediments, and soil. Groundwater is responsible for forming beautiful caverns. It is also an important source of freshwater.
Groundwater and the Hydrologic Cycle Groundwater is part of the hydrologic cycle and an important natural resource. Groundwater is recharged by infiltration from precipitation, streams, lakes, and other surface waters. Groundwater exits the subsurface through springs and human use. Table 12.1, p. 286
Groundwater and the Hydrologic Cycle As the world s population and industrial development expand, the demand for water, particularly groundwater, will increase. Most groundwater in the United States is used for irrigation of crops and public drinking water supplies.
Porosity and Permeability Groundwater is stored in open spaces in rocks called pores. Porosity is the percentage of a material s total volume that is pore space. Permeability is the capacity of rocks, soils or sediments to transmit fluids. Permeability is dependent on porosity, but also on the size of the pores and their interconnections. A porous rock with poorly connected pores would be impermeable. Fig. 13.1a, p. 315
Porosity and Permeability Types of Pores some rock types have more porosity than others Table 13.1, p. 315 Figure 13.1, p. 315
Notice that clays have abundant micropores. However, they tend to impermeable! Table 13.1, p. 315
Pore space a. A well-sorted sedimentary rock has high porosity, whereas b. a poorly sorted one has lower porosity. Openings resulting from solution Fractures c. In soluble rocks such as limestone, porosity can be increased by solution, whereas d. crystalline metamorphic and igneous rocks can be rendered porous by fracturing. Stepped Art Fig. 13-1, p. 315
Porosity and Permeability Aquifers are rocks, sediments or soils that are permeable or capable of transmitting groundwater. Sand and gravel are often good aquifers. Aquicludes are impermeable rocks, sediments and soils that are incapable of transmitting groundwater. Unfractured shales, clays and many metamorphic and intrusive igneous rocks are aquicludes.
The Water Table The water table is located between the zone of aeration and the zone of saturation. Zone of aeration - where water initially infiltrates, most of the pores are filled with air Zone of saturation a zone in which the pores are filled with water Fig. 13.2, p. 316
The Water Table Capillary Fringe - a thin saturated zone found in some fine-grained materials. It extends a few centimeters or meters above the water table. The water in the capillary fringe does not flow as groundwater, but hangs between the closely space particles. Fig. 13.2, p. 316
Fig. 13.2, p. 316
Groundwater Movement Water infiltrates downward under the influence of gravity through the zone of aeration to become groundwater in the zone of saturation. Fig. 13.3, p. 317
Groundwater Movement Some groundwater moves on the water table from high to lower elevations under the influence of gravity. Fig. 13.3, p. 317
Groundwater Movement Most groundwater moves from areas of high pressure to areas of low pressure, sometimes moving upward. Fig. 13.3, p. 317
Groundwater Movement Groundwater velocity varies greatly and depends on many factors. Generally, the average velocity of groundwater is only a few centimeters per day. Fig. 13.3, p. 317
Springs, Water Wells, and Artesian Systems Springs are found wherever the water table intersects the surface. Water by itself flows out of the springs. Fig. 13.4, p. 318
Springs, Water Wells, and Artesian Systems Perched water tables, which often have springs, are associated with localized perched aquifers within zones of aeration. Perched aquifers typically form on top of localized aquicludes. Fig. 13.4, p. 318
Springs, Water Wells, and Artesian Systems Water wells are humanmade openings dug or drilled into the zone of saturation. When the zone of saturation has been penetrated, water percolates into the well, filling it to the level of the water table. Fig. 13.5, p. 318
Springs, Water Wells, and Artesian Systems Modern wells use pumps, which create cones of depression on the water table surrounding the well. Fig. 13.5, p. 318
Springs, Water Wells, and Artesian Systems Cones of depression result when the rate of water removed from a well is greater than the rate flowing into the well from the aquifer. If the cone of depression reaches the bottom of the well, the well runs dry. Fig. 13.5, p. 318
Springs, Water Wells, and Artesian Systems Artesian systems - groundwater flows out of the well without pumping. In an artesian system, the aquifer is confined or "sandwiched" between two aquicludes. The confined groundwater builds up high hydrostatic pressure. Fig. 13.6, p. 319
Springs, Water Wells, and Artesian Systems Artesian systems For an artesian system to develop, three geologic conditions must be met: 1. The aquifer must be confined above and below by aquicludes, layers that are not permeable. Fig. 13.6, p. 319
Springs, Water Wells, and Artesian Systems 2. The aquifer is usually tilted and exposed at the surface so it can be recharged. 3. Precipitation must be sufficient to keep the aquifer filled. Fig. 13.6, p. 319
Springs, Water Wells, and Artesian Systems The dashed line, which originates at the elevation of the water table in the recharge area, defines the highest level to which well water can rise without pumping. Fig. 13.6, p. 319
Groundwater Erosion and Deposition Water from precipitation reacts with carbon dioxide (CO 2 ) in air and organic-rich soils to produce carbonic acid (H 2 CO 3 ). Carbonic acid readily dissolves calcite (CaCO 3 ) in limestones to produce caves. Salt deposits and other water-soluble rocks are also susceptible to dissolution and the formation of caves.
Groundwater Erosion and Deposition Sinkholes and Karst Topography Sinkholes are depressions in the ground formed by the dissolution of the underlying soluble rocks or the collapse of a cave roof. Fig. 13.8, p. 321
Groundwater Erosion and Deposition Karst topography largely develops by groundwater erosion in many areas underlain by soluble rocks, especially limestones. Fig. 13.7, p. 320
Groundwater Erosion and Deposition Features of karst topography include: Sinkholes, along with springs, solution valleys, disappearing streams, and caves Fig. 13.9, p. 322
Fig. 13.9, p. 322
Groundwater Erosion and Deposition Caves and Cave Deposits The dissolution of limestones by groundwater produces many depositional and erosional features, including caves. Caverns are large caves or a system of interconnected caves. Fig. 13.11 a, p. 324
Groundwater Erosion and Deposition The precipitation of calcite within caves creates a variety of interesting depositional features. Fig. 13.12, p. 325
Groundwater Erosion and Deposition Common cave deposits include: Stalactites Stalagmites Columns Drip Curtains Travertine Stalactites hang from the ceiling, stalagmites are on the "ground" Fig. 13.11 c, p. 324
Fig. 13.11 c, p. 324
Modifications of the Groundwater System and Its Effects Groundwater is a valuable natural resource that is being exploited rapidly. Overpumping and other modifications to the groundwater system can cause serious problems such as: Lowering of the water table Saltwater incursion Subsidence Contamination Fig. 13.13, p. 325
Saltwater Incursion A problem in coastal areas Fig. 13.14, p. 326
Water table Ocean Fresh groundwater Salty groundwater Cone of depression Water table Cone of ascension Recharge well Water table Pumping well Fresh groundwater Ocean Salty groundwater Pumping well Ocean Fresh groundwater Cone of depression Salty groundwater Stepped Art Fig. 13-14, p. 326
Subsidence Fig. 13.15, p. 327 Fig. 13.16, p. 327 Fig. 13.17, p. 327
Modifications of the Groundwater System and Its Effects Groundwater Contamination by humans from landfills, septic systems, toxic waste sites, and industrial effluents is becoming a serious problem. Fig. 13.18 b, p. 330
Modifications of the Groundwater System and Its Effects Groundwater Quality Excluding pollution from humans, groundwater quality is mostly a function of the: Kinds of materials that make up an aquifer Residence time of water in an aquifer Solubility of rocks and minerals in the aquifer These factors account for the amount of dissolved materials in groundwater and are responsible for such undesirable effects as hard water and iron staining.
Modifications of the Groundwater System and Its Effects Groundwater Quality If the total calcium and magnesium content of a water exceeds 120 mg/l, then the water is "hard." Hard water precipitates calcite scales in pipes, makes soap difficult to lather and may cause water heaters to overheat, produce hydrogen gas and explode. Water softeners remove calcium and magnesium from water and release sodium. Sodium may be a problem for those with high blood pressure.
Hydrothermal Activity Hydrothermal refers to naturally occurring hot water. Most hydrothermal water results from groundwater infiltrating deep within the Earth and being heated by magmas or simply through the geothermal gradient. Fumaroles, hot springs, and geysers are all hydrothermal features. Fig. 13.20, p. 333
Hydrothermal Activity Fumeroles are steam discharges in volcanically active areas. Hot springs are springs where the water temperature is higher than 37 C (human body temperature). Fig. 13.19a, p. 332
Hydrothermal Activity Hot springs Travertine or calcareous tufa - Calcite that precipitates from supersaturated hot spring water Yellowstone National Park Fig. 13.23a, p. 334
Hydrothermal Activity Hot springs Geyserite - Silica that precipitates from supersaturated hot spring water Yellowstone National Park Fig. 13.23b, p. 334
Hydrothermal Activity Geysers Hot springs which periodically eject hot water and steam with tremendous force Fig. 13.22, p. 333
Hydrothermal Activity Fig. 13.19b, p. 332 Fig. 13.21, p. 333
Geothermal Energy Geothermal energy is energy produced from Earth s internal heat. Comes from the steam and hot water trapped within Earth s crust It is a relatively clean form of energy that is used as a source of heat and to generate electricity. Fig. 13.24, p.335
Geothermal Energy 1-2% of the world s energy needs could be met with geothermal energy. Geothermal energy is currently used in Iceland, the US, Mexico, Italy, New Zealand, Japan, the Philippines, and Indonesia. Derived from mostly convergent zones and hot spots Fig. 13.24, p.335
End of Chapter 13