Figure 1. The Hydrologic Cycle
Audeen W. Fentiman
P. Andrew Karam
Ronald B. Meyers
Hydrogeology is the study of water, especially groundwater, as it moves through the subsurface environment. Groundwater is water that is underground. In Ohio, groundwater is a source of drinking water for nearly 40 percent of the population. Ohio and Federal law include a number of hydrological requirements for siting a low-level radioactive waste disposal facility. These requirements are found in Ohio statute and in Title 10 of the Code of Federal Regulations, Part 61 (10CFR61). This fact sheet discusses the hydrologic cycle, surface water and groundwater, how they affect selection of a potential low-level waste disposal site, and how they can be monitored for radioactive material.
Water continually changes from one form to another in what is called the hydrologic cycle. Water falls to earth in the form of rain, snow or sleet. Some rainfall flows into rivers, lakes, or oceans and is surface water. Other precipitation seeps through the soil and becomes groundwater. Some of the water in the soil evaporates; some is used by plants and passes into the air through the plant's leaves by a process called transpiration. Both evaporation and transpiration return water to the air where it forms clouds and falls back onto the ground as rain, completing the hydrologic cycle. The hydrologic cycle is illustrated in Figure 1.
Figure 1. The Hydrologic Cycle
Precipitation that runs off the land directly into rivers, lakes, or streams is called surface water runoff. Surface water runoff flows downhill through a drainage basin. A drainage basin is the land area which drains the surface water runoff and stream water into a larger water body. An example of a drainage basin is shown in Figure 2. Ridges or hills which divide surface water runoff into two directions are called surface water divides. Each stream or river is surrounded by surface water divides which define the boundaries of the drainage basin. Because rivers may be fed by a number of streams, the drainage basin of a river includes the drainage basins of all of its tributary streams.
Figure 2. Typical Drainage Basin
Rivers periodically flood the areas near them after heavy rains or when large amounts of snow melt. The flooded areas are called floodplains and are classified by how often floodwaters are expected to cover them. It is not possible to predict exactly how high a river will rise in any one year, but historical records indicate how often a river has risen to a certain level. If records for the past 100 years show that a river has reached a certain level 10 times (an average of once every 10 years), that level is the 10-year flood level. In the same way, the 100 year floodplain is the area that has flooded an average of once every 100 years, and the 500 year floodplain is expected to be covered with water an average of once every 500 years. It is important to note that designating an area as a 500 year floodplain does not mean that it is flooded every 500 years. It means that, on average, the area is flooded once in every 500 year time period. Since records for the past 500 years do not exist, the Ohio 500 year floodplains are estimated based on records that exist for the past 50 to 100 years.
Water that does not run off seeps into the ground. Once below the surface, water is technically classified as groundwater. Groundwater below the water table is in the saturated zone while groundwater above the water table is in the unsaturated zone. The depth of the water table can be determined by digging or drilling through the soil or rock until water is found. Holes drilled in the same area will normally reach water at about the same depth.
The time required for water to travel from the surface to the water table depends on local conditions and may take from hours to years. Soil type is an important factor in the rate of water movement. For example, clay soils are relatively impermeable (water does not move easily through them), so water may stand on clay after a rain. This is called ponding, and it is a characteristic of poorly-drained soils. When water moves easily through soils, as it does when soils contain a lot of sand and gravel, the soils are well-drained.
State and federal regulations require low-level radioactive waste disposal facilities to be sited in areas with well-drained soils. Contamination on the ground surface can move more rapidly into the groundwater in well-drained soils, but water that drains quickly has less chance to become contaminated. By preventing ponding, well-drained soils help prevent water damage to waste containers and buildings, thus extending their lives.
Beneath the water table, the ground is saturated. That is, all of the pores in the rock and soil are filled with water. When that saturated area can transmit enough clean water to wells and springs to be useful for water supply, it is called an aquifer. Aquifers sandwiched between layers of clay or rock that do not let water pass through very easily are called confined aquifers. Water in confined aquifers is usually under pressure. Unconfined aquifers generally consist of soils or rocks that allow water to pass through them easily and are not under pressure. Another type of aquifer can exist when a local pool of water gathers on a small amount of less permeable material, such as clay, which lies above the water table. This is called perched water. Perched water pockets can be found in Ohio where glaciers created pockets of clay surrounded by sand or gravel.
Once water reaches an aquifer, it moves with the groundwater in the aquifer, flowing through the small pore spaces in the rock or soil. The rate at which the water moves depends, in part, on the size of the spaces and how well they are connected. Water movement in an aquifer also depends on the type of aquifer. In an unconfined aquifer, water flows downhill. In a confined aquifer, water flows from the area of highest head to the area of lowest head and can actually flow uphill.
Several terms commonly used by hydrogeologists when describing the movement of groundwater are defined in the following sentences. Water head is a term which refers to a combination of water pressure and elevation and is reported as water "elevations". These elevations are put together in a map to form a potentiometric surface. Figure 3 shows a series of wells drilled into a confined aquifer under pressure. The height of the water in each well is determined by the pressure at the well's location. An imaginary line drawn through the top of the water in each well forms the potentiometric surface. The slope of the potentiometric surface is the gradient. Groundwater always flows downgradient - from high head to lower head. It is important to know where the groundwater is and how it moves near a potential low-level waste disposal facility site.
Figure 3. Illustration of a Potentiometric Surface.
State and Federal regulations require low-level radioactive waste disposal facilities to be sited and constructed to minimize the chance that the waste could contaminate surface water or groundwater. The site shall be a place where neither surface water nor groundwater is likely to be in contact with the waste. The facility should be constructed using multiple physical barriers to further reduce the likelihood of groundwater or surface water contamination. Barriers include the waste container and the disposal vault. Water cannot reach the waste unless all barriers are broken.
Groundwater contamination occurs when water carries dissolved wastes through the soil and into the groundwater. Because some wastes dissolve more easily than others, the potential for groundwater contamination from a leaking low-level radioactive waste disposal facility depends on the type of waste, barriers present, and the type of soil at the site. Once in an aquifer, contaminated water flows in the same direction the groundwater is flowing, carrying the waste with it. Contaminants that reach the groundwater tend to spread out, resembling plumes, and are called contaminant plumes. (See figure 4.) However, some materials in rock and soil may retard the movement of contaminants in groundwater.
Figure 4. Contaminant Plume
Ohio's law regulating the low-level radioactive waste disposal site requires that wells be drilled on the site to periodically sample (or monitor) groundwater at the site. The water samples from these monitoring wells can be checked to see if radioactive material above background levels is in the water at that location. Due to the presence of naturally occurring radioactive materials, a small amount of background radiation is always present in natural waters. Radiation levels above background could indicate a possible release at the disposal site, allowing radioactive materials to enter the environment. In addition, under state statute, Ohio's low-level waste disposal site must be surrounded by an ecological monitoring zone in which the effects of the site on surface water, plants and animals will be monitored. Together, the groundwater monitoring wells and the ecological monitoring zone are intended to detect any effect the site may have on the environment.
If you would like to read more about hydrogeology, some of the documents listed below may be useful.
C.W. Fetter, Applied Hydrogeology, 3rd Edition, Merrill Publishing, Englewood Cliffs, New Jersey, 1988.
Ralph Heath, Basic Groundwater Hydrology, Water Supply Paper 2220, United States Geological Survey, 1989.
National Geographic Society, "Special Issue on Water," National Geographic Magazine, Vol. 184, No. 5a, November 1995.
Ohio Revised Code, Section 3747.14
The Ohio State University Extension, "Groundwater Resources Fact Sheets," Columbus, Ohio. (Under development for individual counties. Contact your County Extension Agent for availability in your county.)
The Ohio State University Extension, "Water Resources Fact Sheets," Columbus, Ohio. (Under development for individual counties. Contact your County Extension Agent for availability in your county.)
U.S. Nuclear Regulatory Commission, Code of Federal Regulations, Title 10, Part 61, "Disposal Site Suitability Requirements for Land Disposal."
Other fact sheets in this series:
RER-34, "What Are the Siting Criteria for a Low-Level Radioactive Waste Disposal Facility?"
RER-45, "How Are Low-Level Radioactive Waste Disposal Facilities Monitored?"
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. Ronald B. Meyers is a Graduate Research Associate, Ohio State University Extension.
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