How Does Geology Affect the Disposal of Low-Level Radioactive Waste?

RER-32

Audeen W. Fentiman
P. Andrew Karam
Ronald B. Meyers

Geologic features such as rock formations and the type of soil present must be considered when deciding where to place buildings and other structures. Rock formations and soil types can affect the way water flows on the surface and through the ground. Similarly, movement of contaminants in the water will be affected by geologic features. Information on geology is, therefore, important when selecting a site for a low-level radioactive waste disposal facility. In fact, under state and federal law, the site for such a facility must meet a number of geologic conditions. This fact sheet presents some basic concepts in geology and defines some of the geologic terms used in Ohio statutes governing the development of a low-level waste disposal site in Ohio. The subject areas covered are: types of rocks; earthquakes, faults and fractures; glaciers; and soils. Specific geologic conditions that the low-level waste disposal site must meet are described in another fact she? RER-34, "What Are the Siting Criteria for a Low-Level Radioactive Waste Disposal Facility in Ohio?"

Types of Rocks

The three major types of rocks are igneous, metamorphic, and sedimentary. Igneous rocks form when melted rock cools underground or flows as lava from volcanoes and cools. Igneous rocks are also called crystalline rocks. Metamorphic rocks form when solid rocks are re-heated or subjected to high pressure and change their form. Sedimentary rocks form when, over millions of years, tiny pieces of sand, clay, silt, or other materials are compressed and cemented together. All rocks to a depth of several thousand feet in Ohio are sedimentary rocks. Because sedimentary rocks are important in Ohio, they will be discussed in more detail in the following paragraphs.

Conglomerates, sandstone, shale, limestone and dolomite, are sedimentary rocks. The major differences among the types of sedimentary rocks are the size of the grains (or particles) and the types of minerals from which they were formed. Variations in grain size and mineral type account for most of the differences in the strength of the rocks and the ease with which water moves through them.

Sedimentary rocks are formed when particles are pressed together, usually over millions of years, in a process called lithification or consolidation. Conglomerates are made when gravel-sized particles are consolidated with some sand. Because this type of rock has relatively large grains, it can also have large holes (or pores) between the grains. This allows water to move through conglomerates relatively quickly. Sandstone is formed from grains of sand that can range from very coarse to very fine. However, the grains are sometimes packed so that water can move relatively easily through sandstone.

Shales are made from clay and mud that have lithified over time. Clays have extremely small mineral grains. As a result, the pores in shale are very small, making it difficult for water to pass through the rock.

Limestone and dolomite are formed when organisms that make shells from minerals dissolved in the water die, and their shells sink to the bottom of the ocean or lake. These rocks also form when dissolved minerals precipitate, forming small grains that settle to the bottom. The small grains and shells are lithified to form limestone if calcium is present or dolomite if calcium and magnesium are present.

Limestone and dolomite usually have the smallest grains and, unless fractured, do not transmit much water. However, these rocks are often fractured, and when water enters these fractures, the rocks dissolve rather easily. The process of dissolving limestone or dolomite is known as solution weathering. Mammoth Cave in Kentucky and the sinkhole plains around it are excellent examples of what can develop when limestone dissolves. These caves and sinkholes are known as karst features, and are illustrated in Figure 1. Water flows very rapidly through karst features. In some areas, these dissolved limestones and dolomites contain a major water supply.

Figure 1. Karst Features Typical of Mammoth Cave, Kentucky

Earthquakes, Faults, and Fractures

Earthquakes occur when rocks are under so much stress that they break. When this force is transmitted to the ground surface, it causes shaking and damage. Most major earthquakes occur near the edges of continents, for example, in California and Japan, but they can occur in Ohio, too. The amount of damage an earthquake causes depends on its strength, which can be measured on the Richter Scale. An earthquake with a strength (or magnitude) of 3 on the Richter Scale would barely be felt, while a very strong earthquake would have a magnitude of 7 or higher. The world-wide frequency and typical effects of earthquakes of various magnitudes are presented in Table 1. The best records available for Ohio show about 120 earthquakes over the past 200 years (about 0.6 per year). Of these, 68 have had magnitude less than 3 on the Richter scale, 45 have been in the 3-5 range, and 7 have been greater than 5.

Sometimes the stresses on the rock are so great that not only does the rock break, but the two pieces of the rock slide along the break. When this happens, the breaks are called faults. Some rocks may move only inches along a fault, while others may move hundreds of feet.

If the rock cracks but does not move much, the crack is called a fracture. Water can flow through fractures. In Ohio, fractures are more common than faults.

Glaciers

Several times during the past one million years, parts of Ohio have been covered with sheets of ice several thousand feet thick, referred to as glaciers. The glaciers moved into the state from the north, retreating to Canada for the last time about 10,000 years ago. As the glaciers moved, they changed the landscape of Ohio, filling old valleys with glacial debris and carving new valleys. The flatter northern and western parts of Ohio were covered with ice while the hilly southeast was largely unaffected (Figure 2).

Figure 2. Area of Ohio Affected by Glaciers

Soils

The types of soils found in Ohio have been significantly affected by glaciers. The glaciers scraped up and ground up soil, rock, and sand from Canada, the bottom of Lake Erie, and northern Ohio. Most of this material was carried at the base of the glacier. Some was transported on the ice, and some was pushed in front. The debris concentrated as ridges at the former sides of the glacier is called a moraine. As the glaciers melted, much of the material underneath the glacier and some of the moraine was washed downstream by melting water from the glacier. This material is called outwash. The debris comprising the ground moraine and end moraine (the material deposited beneath or at the end of a glacier) is called till. Outwash, till, and clays from lakes that formed from glacial meltwater are common in Ohio, covering large portions of the state. (The general term, drift, is sometimes used to refer to all glacial deposits: outwash, till, and lake clays.) Some of the surface features created by glaciers in Ohio are illustrated in Figure 3.

Figure 3. Surface Features Generated by Glaciers in Ohio

While rocks are made of consolidated particles, soils consist of unconsolidated material. This means that soil cannot withstand stress as well as rocks. When unconsolidated material, sand, for example, is poured onto a pile, it may build up for a little while, but it will eventually slide down the sides. In general, the sides of a pile of sand will keep the same slope, depending on the primary particle sizes,regardless of the amount of sand in the pile. If any part of the pile becomes too steep (causing too much stress), the sand will slide downhill to give the right slope. Soil behaves in a similar way. If the slope of a hillside is such that it causes the maximum stress the soil can withstand, any additional stress will cause the soil to slide. Water can provide additional stress. Rain water added to soil will increase its weight, adding to the stress and causing the soil to slide downhill to try to reach a more gradual slope. This down slope movement of soil is called creep if it is slow and a landslide if it is rapid. Soils can also compact, allowing structures to sink slowly. This is known as subsidence. Another form of subsidence occurs when a mine or cave collapses, causing the ground above to sink.

Another concern with unconsolidated materials is that, under the right conditions, they can act like a liquid. This process, called liquefaction, takes place when there is a great deal of water in the unconsolidated material, and together, they are subjected to stress such as an earthquake. The stress can cause the water to push upward against the grains of unconsolidated material, momentarily keeping them from resting against each other. Since the grains are only in contact with water, they suddenly act like a liquid and cannot support any weight. This is similar to what happens in quicksand. Water from a spring underneath a sand layer will separate the grains of sand, making the sand act like a liquid, and therefore not weight supporting.

Impact of Geology on Site Selection

When a site for a low-level radioactive waste disposal facility is selected, two of the conditions that must be considered are the stability of the ground on which the structure is to be built, and the movement of water at the site. To determine the stability of the ground, the type of rock and soil present and the likelihood of earthquakes, landslides, subsidence, and liquefaction must be studied. Movement of water at the site will depend on slope, soil and rock type, grain size, and whether fractures, faults, or karst features are present.

For More Information

If you would like to read more about geology, some of the references listed below may be helpful.

Ernest Carlson, Minerals of Ohio, Ohio Department of Natural Resources, Division of Geological Survey, Bulletin No. 69, 1991.

Aurele LaRocque and Mildred Fisher Marple, Ohio Fossils, Ohio Department of Natural Resources, Division of Geological Survey, Bulletin No. 54, 1990.

B.W. Marck, B.J. Skinner and S.C. Porter, Environmental Geology, John Wiley & Sons, 1996.

Frank Press and Raymond Siever, Earth, 4th Edition, W.H. Freeman & Co., New York, 1986.

Other fact sheets in this series:

RER-34, "What Are the Siting Criteria for a Low-Level Radioactive Waste Disposal Facility in Ohio?"

Author Notes:

Dr. Audeen W. Fentiman is an Associate Professor in Nuclear Engineering at The Ohio State University. P. Andrew Karam is a Certified Health Physicist and a Graduate Research Associate in Geology and Ronald B. Meyers is a Graduate Research Associate, Ohio State University Extension.