Which of the Following Results From Groundwater Deposition

Groundwater Weathering and Deposition

Last Updated on Mon, 04 Jul 2022
| Geology

Chemical weathering of limestone by water causes the characteristic topography of karst areas.

Real-World Reading Link You might have seen an old gravestone, statue, or sculpture that has been weathered by acidic water. Similar processes form limestone caves underground.

Carbonic Acid

Acids are aqueous solutions that contain hydrogen ions. Most groundwater is slightly acidic due to carbonic acid. Carbonic acid forms when carbon dioxide gas dissolves in water and combines with water molecules. This happens when precipitation falls through the atmosphere and interacts with carbon dioxide gas or when groundwater infiltrates the products of decaying organic matter in soil. As a result of these processes, groundwater is usually slightly acidic and attacks carbonate rocks, especially limestone. Limestone mostly consists of calcite, also called calcium carbonate, which reacts with any kind of acid. The results of this reaction over time are shown in Figure 10.7. This process occurs above ground and below ground.

Figure 10.7 Carbonic acid has dissolved large portions of this limestone. This resulting formation is the Stone Forest in China.

Figure 10.7 Carbonic acid has dissolved large portions of this limestone. This resulting formation is the Stone Forest in China.

Dissolution by Groundwater

The process by which carbonic acid forms and dissolves calcite, can be described by three simple chemical reactions.

In the first reaction, carbon dioxide (CO2) and water (H2O) combine to form carbonic acid (H2CO3), as represented by the following equation.

CO2 + H2O * H2CO3

In the second reaction, carbonic acid splits into hydrogen ions (H+) and bicarbonate ions (HCO3). This process is represented by the following equation.

H2CO3 * H+ + HCO3-

In the third reaction, the hydrogen ions (H+) react with calcite (CaCO3) and form calcium ions (Ca2+) and bicarbonate ions (HCO3).

CaCO3 + H+ * Ca2+ + HCO3-

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Michele Burgess/Index Stock

The resulting calcium ions (Ca2+) and bicarbonate ions are then carried away in the groundwater. Eventually, they precipitate, which means they crystallize out of the solution, somewhere else. Precipitation occurs when the groundwater evaporates or when the carbon dioxide gas leaves the water. The processes of dissolving, called dissolution, and precipitation of calcite both play a major role in the formation of limestone caves, such as those shown in Figure 10.8.

Caves A natural underground opening with a connection to Earth’s : surface is called a cave or a cavern. Some caves form three-dimensional mazes of passages, shafts, and chambers that stretch for many kilometers. Some caves are dry, while some contain underground streams or lakes. Others are totally flooded and can be explored only by cave divers. Mammoth Cave in Kentucky, shown in Figure 10.8, ; is composed of a series of connected underground passages.

Most caves are formed when groundwater dissolves limestone. The development of most caves begins in the zone of saturation just below the water table. As groundwater infiltrates the cracks and joints of limestone formations, it gradually dissolves the adjacent rock and enlarges these passages to form an interconnected network of openings. As the water table is lowered, the cave system becomes filled with air. New caves then form beneath the lowered water table. If the water table continues to drop, the thick limestone formations eventually become honeycombed with caves. This is a common

– : occurrence in limestone regions that have been uplifted by tectonic

Figure 10.8 Groundwater dissolution forces.

and precipitation result in a variety of features in caves. Reading Check Explain how most caves form.

Identify which chemical reactions might be at work.

260 Chapter 10 • Groundwater

(l)Fritz Polking/Visuals Unlimited, (r)Adam Jones/Visuals Unlimited

260 Chapter 10 • Groundwater

(l)Fritz Polking/Visuals Unlimited, (r)Adam Jones/Visuals Unlimited

Figure 10.9 Karst topography is characterized by a landscape of sinkholes formed by dissolution of limestone. Identify what controls the rate of dissolution of bedrock in karst topography.

Karst topography Figure 10.9 shows some of the surface features produced by the dissolution of limestone bedrock. One of the main features is a sinkhole—a depression in the ground caused by the collapse of a cave or by the direct dissolution of limestone by acidic water. Another type of feature, called a disappearing stream, forms when a surface stream drains into a cave system and continues flowing underground, leaving a dry valley above. Disappearing streams sometimes reemerge on Earth’s surface as karst springs.

Limestone regions that have sinkholes and disappearing streams are said to have karst topography. The word karst comes from the name of a region in Croatia where these features are especially well developed. Prominent karst regions in the United States are located in Kentucky, Indiana, Florida, and Missouri. The Mammoth Cave region in Kentucky has karst topography that contains tens of thousands of sinkholes.

In karst areas, sinkholes proliferate, grow, and eventually join to form wide valleys. Most of the original surface has been dissolved, with the exception of scattered mesas and small buttes. The rate of the dissolution process varies greatly among locations, depending on factors such as humidity and soil composition. In humid areas, where there is more precipitation, more water infiltrates areas of porous soil, and dissolves the limestone in the subsurface.

Groundwater Deposits

Calcium ions eventually precipitate from groundwater and form new calcite minerals. These minerals create spectacular natural features.

Dripstones The most remarkable features produced by ground-water are the rock formations called dripstone that decorate many caves above the water table, as shown in Figure 10.10. These formations are built over time as water drips through caves. Each drop of water hanging on the ceiling of a cave loses some carbon dioxide and precipitates some calcite. A form of dripstone, called a stalactite, hangs from the cave’s ceiling like icicles and forms gradually. As the water drips to the floor of the cave, it may also slowly build mound-shaped dripstone called stalagmites.

Figure 10.10 Stalactites are dripstones produced by a buildup of minerals precipitated from groundwater.

Figure 10.10 Stalactites are dripstones produced by a buildup of minerals precipitated from groundwater.

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(t)Lloyd Homer/GNS Science, (b)Albert J. Copley/Visuals Unlimited

Section 2 • Groundwater Weathering and Deposition 261

(t)Lloyd Homer/GNS Science, (b)Albert J. Copley/Visuals Unlimited

Figure 10.11 Hard water contains high concentrations of minerals., which leave precipitates in household water pipes such as this one. Identify one of the likely precipitates in this pipe.

Figure 10.11 Hard water contains high concentrations of minerals., which leave precipitates in household water pipes such as this one. Identify one of the likely precipitates in this pipe.

Sometimes stalactites and stalagmites grow together to form dripstone columns. Increasingly, researchers are finding abundant and varied microorganisms associated with dripstone formations in caves. It is possible that these organisms play an important role in the deposition of at least some of the materials found in caves.

Hard water You are probably aware that tap water contains various dissolved solids. While some of these materials are added by water treatment facilities, others come from the dissolution of minerals in soils and subsurface rock and sediment. Water that contains high concentrations of calcium, magnesium, or iron is called hard water. Hard water is common in areas where the subsurface rock is limestone. Because limestone is made of mostly calcite, the groundwater in these areas contain significant amounts of dissolved calcite. Hard water used in households can sometimes cause problems. Just as calcite precipitates in caves, it can also precipitate in water pipes, as shown in Figure 10.11, and on the heating elements of appliances. Over time, deposits of calcite can clog water pipes and render some electrical appliances useless.

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