Remediation Remediation is so difficult and expensive that of 400,000

hazardous waste sites identified by the General Account- ing Office, only 1300 of the most dangerous sites had been placed on a national priority list for cleanup by 1993 (Fig. 15–16). At present, approximately 217 of these projects have been completed, at an average cost of $27 million per site. Future cleanup rates depend in part on government enforcement and budget allocations.

Several processes are currently used to clean up a contaminated aquifer and the source of its contamination. Contaminated ground water can be contained by building an underground barrier to isolate it from other parts of the aquifer. If the contaminant does not decompose by natural processes, it may pollute the aquifer if the barrier fails. Therefore, additional treatment eventually must be used to destroy or remove it.

In some cases, hydrogeologists drill wells around and into the contaminant plume and pump the polluted ground water to the surface. The contaminated water is then collected in tanks, where it is treated to destroy the pollutant. Containment and pumping are often used si- multaneously.

270 CHAPTER 15 G RO U N D WAT E R

Figure 15–15 An oil refinery in New Jersey. New Jersey suffers from some of the worst ground-water pollution in North America as a result of heavy industry.

Ground Water and Nuclear Waste Disposal 271

of these radioactive waste products are useless and must

be disposed of without exposing people to the radioactivity. In the United States, military processing plants, 111 commercial nuclear reactors, and numerous labora- tories and hospitals generate approximately 3000 tons of radioactive wastes every year.

Chemical reactions cannot destroy radioactive waste because radioactivity is a nuclear process, and atomic nuclei are unaffected by chemical reactions. Therefore, the only feasible method for disposing of radioactive wastes is to store them in a place safe from geologic haz- ards and human intervention and to allow them to decay naturally. The U.S. Department of Energy defines a permanent repository as one that will isolate radioactive

Figure 15–16

A hazardous waste dump site photographed wastes for 10,000 years. 1 For a repository to keep ra- in 1989 shows violations of environmental protection laws.

dioactive waste safely isolated for such a long time, it

(Jeff Amberg/Gamma Liaison)

must meet at least three geologic criteria:

1. It must be safe from disruption by earthquakes and volcanic eruptions.

Bioremediation uses microorganisms to decompose

2. It must be safe from landslides, soil creep, and other

a contaminant. Specialized microorganisms can be fine-

forms of mass wasting.

tuned by genetic engineers to destroy a particular con-

3. It must be free from floods and seeping ground wa- taminant without damaging the ecosystem. Once a

ter that might corrode containers and carry wastes specialized microorganism is developed, it is relatively

into aquifers.

inexpensive to breed it in large quantities. The microorganisms are then pumped into the contaminant plume,

where they attack the pollutant. When the contaminant is E The Yucca Mountain destroyed, the microorganisms run out of food and die,

Repository

STUDY

leaving a clean aquifer. Bioremediation can be among the cheapest of all cleanup procedures.

In December 1987, the U.S. Congress chose a site near Chemical remediation is similar to bioremediation.

Yucca Mountain, Nevada, about 175 kilometers from If a chemical compound reacts with a pollutant to pro-

Las Vegas, as the national burial ground for all spent re- duce harmless products, the compound can be injected

actor fuel unless sound environmental objections were into an aquifer to destroy contaminants. Common

found. Since that time, numerous studies of the geology, reagents used in chemical remediation include oxygen

hydrology, and other aspects of the area have been con- and dilute acids and bases. Oxygen may react with a pol-

ducted.

lutant directly or provide an environment favorable for The Yucca Mountain site is located in the Basin and microorganisms, which then degrade the pollutant. Thus,

Range province, a region noted for faulting and volcan- contamination can sometimes be reduced simply by

ism related to ongoing tectonic extension of the Earth’s pumping air into the ground. Acids or bases neutralize

crust. Bedrock at the Yucca Mountain site is welded tuff, certain contaminants or precipitate dissolved pollutants.

a hard volcanic rock. The tuffs erupted from several large In some extreme cases, reclamation teams dig up the

volcanoes that were active from 16 to 6 million years entire contaminated portion of an aquifer. The contami-

ago. Later volcanism created the Lathrop Wells cinder nated soil is treated by incineration or with chemical

cone 24 kilometers from the proposed repository. The processes to destroy the pollutant. The treated soil is then

last eruption near Lathrop Wells occurred 15,000 to used to fill the hole.

25,000 years ago. In addition, geologists have mapped

32 faults that have moved during the past 2 million years adjacent to the Yucca Mountain site. The site itself is lo-

䊳 15.5 GROUND WATER AND NUCLEAR WASTE DISPOSAL

1 This number is derived from human and political considerations

In a nuclear reactor, radioactive uranium nuclei split into

more than scientific ones. The National Academy of Sciences issued a report stating that the radioactive wastes will remain harmful for

smaller nuclei, many of which are also radioactive. Most

one million years.

272 CHAPTER 15 G RO U N D WAT E R

Yucca Wash

G4 Surface Facility

Canyon fault

Bare Mountain

Road fault

fault

Abandoned

Wash fault 0 1 2

Mile

Figure 15–17

A map of the proposed Yucca Mountain repository shows numerous faults where rock has fractured and moved during the past 2 million years. (Redrawn from Geotimes, January 1989)

cated within a structural block bounded by parallel faults ground water might disperse more rapidly than predicted. (Fig. 15–17). Critics of the Yucca Mountain site argue

Furthermore, critics point out that construction of the that recent earthquakes and volcanoes prove that the area

repository will involve blasting and drilling, and these is geologically active.

activities could fracture underlying rock, opening con- The environment is desert dry, and the water table

duits for flowing water.

lies 550 meters beneath the surface. The repository will To stop development of the Yucca Mountain site, the consist of a series of tunnels and caverns dug into the tuff

state of Nevada refused to issue air quality permits to op- 300 meters beneath the surface and 250 meters above the

erate drilling rigs at the repository. In December 1995, water table. Thus, it is designed to isolate the waste from

U.S. Energy Secretary Hazel O’Leary announced that ground water. However, geologists have suggested that

permanent storage for spent nuclear fuel cannot begin at an earthquake could drive deep ground water upward,

Yucca Mountain before the year 2015. Supporters of where it would become contaminated by the radioactive

the repository argue that we need nuclear power, and wastes. In addition, because radioactive decay produces

therefore as a society we must accept a certain level of heat, the wastes may be hot enough to convert the water

risk. Furthermore, the Yucca Repository is safer than the to steam. Steam trapped underground could build up

temporary storage sites now being used. At present, enough pressure to rupture containment vessels and cav-

29,000 tons of radioactive waste lie in unstable tempo- ern walls.

rary storage at nuclear power plants across the United Other scientists are concerned that slow seepage of

States.

water from the surface will percolate through the repository site to the water table sometime between 9000 and 80,000 years from now. The lower end of this estimate

䊳 15.6 CAVERNS AND KARST

is within the 10,000-year mandate for isolation. If the

TOPOGRAPHY

climate becomes appreciably wetter, which is possible over thousands of years, ground-water flow may accel-

Just as streams erode valleys and form flood plains, erate and the water table may rise. If rocks beneath the

ground water also creates landforms. Rainwater reacts site were fractured by an earthquake, then contaminated

with atmospheric carbon dioxide to produce a slightly

Caverns and Karst Topography 273

acidic solution that is capable of dissolving limestone. This reaction is reversible: The dissolved ions can precipitate to form calcite again. (Recall that limestone is composed of calcite.)

CAVERNS Most caverns, also called caves, form where slightly

acidic water seeps through a crack in limestone, dissolving the rock and enlarging the crack. Most caverns form at or below the water table. If the water table drops, the chambers are opened to air. Caverns can be huge. The largest chamber in Carlsbad Caverns in New Mexico is taller than the U.S. Capitol building and is broad enough to accommodate 14 football fields. While caves form when limestone dissolves, most caves also contain fea- tures formed by deposition of calcite. Collectively, all mineral deposits formed by water in caves are called speleothems . Some are long, pointed structures hanging from the ceilings; others rise from the floors.

When a solution of water, dissolved calcite, and carbon dioxide percolates through the ground, it is under pressure from water in the cracks above it. If a drop of this solution seeps into the ceiling of a cavern, the pressure decreases suddenly because the drop comes in con- tact with the air. The high humidity of the cave prevents the water from evaporating rapidly, but the lowered pressure allows some of the carbon dioxide to escape as a gas. When the carbon dioxide escapes, the drop becomes less acidic. This decrease in acidity causes some of the dissolved calcite to precipitate as the water drips from

Figure 15–18 Stalactites, stalagmites, and columns form as the ceiling. Over time, a beautiful and intricate stalactite

calcite precipitates in a limestone cavern. Luray Caverns, grows to hang icicle-like from the ceiling of the cave

Virginia. (Breck P. Kent)

(Fig. 15–18). Only a portion of the dissolved calcite precipitates as the drop seeps from the ceiling. When the drop falls to the floor, it spatters and releases more carbon dioxide. The acidity of the drop decreases further, and another

a dealer’s lot fell into the underground cavern. Within a minute amount of calcite precipitates. Thus, a stalagmite

few days, the sinkhole had grown to about 200 meters builds from the floor upward to complement the stalac-

wide and 50 meters deep and had devoured additional tite. Because stalagmites are formed by splashing water,

buildings and roads (Fig. 15–20). they tend to be broader than stalactites. As the two fea-

Although sinkhole formation is a natural event, the tures continue to grow, they may eventually join to form

problem can be intensified by human activities. The

a column. Winter Park sinkhole formed when the water table dropped, removing support for the ceiling of the cavern. The water table fell as a result of a severe drought aug-

SINKHOLES mented by excessive removal of ground water by

If the roof of a cavern collapses, a sinkhole forms on the

humans.

Earth’s surface. A sinkhole can also form as limestone dissolves from the surface downward (Fig. 15–19). A

Remediation Remediation is so difficult and expensive that of 400,000