Washington Groundwater: A Vital Resource

Groundwater is an underground resource stored in bedrock pores and between water saturated soil and rock particles. The world's most abundant and economical source of domestic water, groundwater supplies well water and springs.

Although only a fraction of all groundwater is available through wells and springs, the total resource is vast.

Washington State depends on groundwater for drinking, irrigation, livestock, fish propagation, hydroelectric power, industry, and replenishing surface water (Fig. 1.). Groundwater is the source of drinking water for two-thirds of Washington residents. The rural one-fifth of Washington's population relies completely on groundwater. In Washington, groundwater is widely available and dependable for domestic use. This valuable resource is available at modest cost for many uses. Groundwater does not need the costly dam and reservoir construction for storage that surface water requires.

Figure 1. Washington groundwater use. Source: U.S. Geological Survey. National Water Summary, 1984. U.S. Geological Survey Water-Supply Paper 2275.

Most of earth's water is unavailable for water supplies. The oceans (about 97.1% of the earth's water) are salty; glaciers and ice caps (2.25%) are frozen and largely remote. Groundwater (0.60%) and lakes, streams, and the atmosphere (0.05%) make up the rest. Salty and frozen sources are not feasible water supplies. Further, people can pump out only a small part of the groundwater because most of it is bound tightly to soil and rock. Even in this tiny fraction of the earth's water resource, a tremendous volume of groundwater is available.

Where surface water is available, industry, transportation, irrigation, recreation, or wildlife protection factions lay claim to it. It would be difficult to increase our surface water supply significantly. Dams to store surface water are incredibly expensive to build and maintain. Reservoirs flood productive valleys and lose storage to evaporation and sedimentation. Further, most suitable sites have been used.

About 90% of the fresh water supply, excluding ice caps and glaciers, is groundwater. About half of all groundwater, and 95% of that used, is within one-half mile of the surface. In the United States, that fairly shallow groundwater resource would fill the five Great Lakes four times.

In Washington, only 9% of the fresh water taken from surface water and groundwater sources in 1980 was groundwater. Most was surface water from ponds, lakes, and streams. The state's greatest single fresh water use is for irrigation, largely in eastern Washington. Washington uses 34% of its total groundwater withdrawals for irrigation, but groundwater supplies only 4% of all irrigation water.

Despite scientific progress, groundwater is not well understood by many people. To some, it is a mysterious resource: magical, pure, and inexhaustible. Groundwater has no magic; it follows natural laws. In answer to some myths about this unseen resource:

• There are no vast underground lakes and rivers.
• Groundwater can be contaminated, even when it is deep underground.
• Groundwater is related to surface water, because groundwater was originally surface water.
• Groundwater is extremely important to Washington state.
• Soil does filter water as it seeps toward groundwater resources, but it does not remove many contaminating chemicals.

Learning more about groundwater properties and behavior will help residents use this vital resource more wisely and protect its quantity and quality more fully.

Groundwater comes from precipitation that infiltrates the soil surface and percolates to the water table. Surface water and groundwater connect through gravity-powered water flow. In streams and lakes, water that is above groundwater elevation flows toward and into groundwater. Groundwater above the level of lakes and streams flows into these surface waters. Streams not being fed runoff from rain or snowmelt would be dry if it were not for supplemental flows of groundwater. Baseflow is the name for low-water streamflow derived from groundwater.

Some water is beneath the earth's surface almost everywhere, but it may be difficult to find or extract. It may need to be treated before use. Groundwater can be near the surface, such as water under a marsh. Or, it may be several hundred feet below the surface, as in arid regions.

The water table forms the upper surface of the water-saturated soil and permeable or fractured rock. The water table is the upper limit of groundwater; in a local area this reaches about the level of water in shallow wells. The water table tends to follow the shape of the overlying land surface, but is typically deeper under a hill than under a valley.

The hydrologic cycle (water cycle) is the continuous circulation of water from ocean to sky to earth. Groundwater is one important part of the cycle (Fig. 2.).

Figure 2. Hydrologic cycle.

The sun evaporates water from oceans (the greatest source), lakes, rivers, and from soil and plant surfaces. Water forms clouds in the atmosphere. Precipitation as rain and snow falls by gravity back to earth's surface.

At the surface and using energy from the sun, some rain or melted snow evaporates. Under the force of gravity, the rest runs off as surface water or infiltrates into soil and rock pores.

Some of the water that infiltrates the soil transpires from plants or moves to the soil surface and evaporates back into the atmosphere. When the soil is saturated, some water percolates to the water table, where it recharges the groundwater.

Frozen soil reduces or can prevent infiltration. Groundwater recharge is slowed or delayed with intermittent freezing and can stop for long periods in frozen soils. Surface paving, compacting, and some other practices also slow or stop infiltration.

Groundwater usually moves underground only a few feet per year. It can move as little as inches per year, but as much as hundreds of feet per year in very permeable materials. It eventually seeps into streams, lakes, and the ocean. Some of the runoff and groundwater that enters the surface water system evaporates back into the air. Ultimately, most surface and groundwater flow to the ocean to complete the hydrologic cycle.

Climate, topography, and geology determine the amount and location of groundwater. The Cascade Range divides Washington State into humid western and semiarid eastern regions. Precipitation varies from about 8 inches per year in the drier central area to about 200 inches per year in the Olympic rain forests. Resulting groundwater recharge may be no more than 1 inch per year in the arid parts, but may be up to several inches per year in the humid parts of the state.

Bedrock is the solid rock that underlies the soil, sand, clay, gravel, and loose material at the earth's surface. Bedrock usually contains vertical fractures that intersect other fractures to conduct water. It also has pores that can hold water. Interconnecting pores and fractures permit water flow into and out of bedrock by the force of gravity.

Bedrock, such as basalt or granite, may crop out at the land surface or be several hundred feet below unconsolidated material. Consolidated bedrock may have connected pores and fractures that hold and transmit water. The size and number of those passages determine whether bedrock can yield abundant or very little well water. Fractured basalt and bedded sedimentary rock, such as slate or shale, can conduct water great distances through spaces between the layers.

Groundwater is slightly acidic, so it dissolves limestone and dolomite bedrock it passes through. The resulting solution channels carry water vertically and horizontally, sometimes at great rates through large channels. Limestone caves offer dramatic examples of solution channels and perhaps account for the notion of underground rivers. Because limestone and dolomite occur in only a small part of the state, major solution channels are relatively unimportant in Washington.

Unconsolidated material, such as sand or gravel, lies over bedrock and includes:

• rock debris and soil particles transported by glaciers or streams, or deposited in lakes,
• weathered bedrock particles forming a loose granular or clay soil, and
• wind-blown (loess) soils.

Well-sorted unconsolidated materials can store large volumes of water. Sand and gravel are coarse materials that readily yield water to wells and springs.

Aquifers are an important part of the groundwater resource. Water fills most of the voids in rock and unconsolidated material below the water table. However, aquifers are those water-bearing rocks and sediments that readily supply economic quantities of water to wells or springs (Fig. 3.). Precipitation and surface water infiltrate through permeable materials to recharge aquifers. Where the water table intersects the earth's surface, aquifers discharge to springs, streams, lakes, or wetlands. Aquifers extend from near the surface to more than 1,000 feet below, can range in thickness from a few feet to more than 1,000 feet, and underlie from a few acres to thousands of square miles.

Water that moves between two low-permeability layers, such as clay, is confined and usually is under pressure. A confined (or artesian) aquifer exists. The pressure in most confined aquifers raises well water above the confining layer. When pressure is sufficient, water flows from the well at the ground surface with no pumping needed. All users value flowing wells highly for the energy savings.

Figure 3. Groundwater sources. Adapted from D.K. Todd. 1959. Ground Water Hydrology. Wiley.

About 60% of the state has aquifers that usually yield 50 gallons per minute (gpm) or more. There are three principal aquifers: Columbia River basalt, glacial drift, and terrace and valley-fill (Fig. 4.).

Figure 4. Principal Washington aquifers. Adapted from U.S. Geologic Survey. 1984. National Water Summary, 1984. U.S. Geologic survey Water Supply Paper 2275.

The Columbia River basalt aquifer has lava flows and interbeds of sand and gravel between the flows. In the plateau near Pasco, some of the aquifer is more than 6,000 feet thick. Water storage occurs largely in fractures, rubble, and sand and gravel interbeds. It is confined in some areas. Because of variations in the aquifer materials and recharge sources, yields vary widely. Some wells produce 3,000 to 6,000 gpm and are very good irrigation supplies.

The glacial drift type of aquifer arises from glacial outwash and the more permeable materials within glacial till. It has mostly unconsolidated sand and gravel, but also has silt, clay, and somewhat consolidated till (hardpan). The highly variable aquifer material results in variable yields­usually less than 700 gpm in the Puget Sound lowlands.

This aquifer type provides most of the domestic, public, and industrial water in the Puget Sound region and Spokane Valley. In the Columbia Plateau region, the glacial drift aquifer primarily fills single family domestic needs; the underlying basalt meets the greater demands for municipal, industrial, and irrigation use. Wells in thick layers of extremely permeable sand or gravel yield up to 10,000 gpm.

The terrace and valley-fill type of aquifer has mostly sand and gravel, providing yields of only a few gpm, which is enough for single family domestic use. But near Vancouver, industrial supplies from the aquifer reach 4,500 gpm.

Two other aquifers, while less common, are important to some users. Alluvial aquifers consist of silt, sand, gravel, and cobbles along streams, deltas, and coastal beaches. These aquifers provide primarily domestic water supplies. Igneous, sedimentary, and metamorphic rocks under the Olympic, Cascade, and Northern Rocky Mountains form crystalline rock aquifers. These rock aquifers are neither very productive nor reliable.

Once an aquifer becomes contaminated, few choices are available to users. Sometimes drilling a new well can locate good quality water. Or, water can be pumped, treated, and returned to the aquifer. This option is seldom economical.

The aquifer can cleanse itself because fresh water dilutes contaminated water. A large amount of water must travel large distances to flush a groundwater system. Improving water quality by dispersing and mixing contaminants with clean water is a long-term and unpredictable process. The best alternative is to protect groundwater quality.

Sole source aquifers are designated by the U.S. Environmental Protection Agency (US-EPA). The process requires a person or entity to petition the US-EPA. Then the agency must find that the aquifer or aquifer system supplies at least 50% of the area's drinking water and that contamination of the source would create a significant hazard to public health.

The designation as sole or principal source aquifer not only publicizes groundwater value, but also provides limited federal groundwater quality protection. Being named a sole source does not indicate relative groundwater value or vulnerability, nor does it provide any comprehensive quality protection.

The US-EPA has named eight sole source aquifers in the Pacific Northwest, of which seven are in Washington: Spokane Valley-Rathdrum Prairie, Camano Island, Whidbey Island, Cross Valley, Newberg Area, Lewiston Basin, and Cedar Valley.

Groundwater use in the state, including groundwater for irrigation in eastern Washington, continues to increase. Irrigation accounts for most groundwater use in Adams, Franklin, Grant, and Lincoln counties.

The water table has dropped in parts of the Columbia Plateau, largely because of extensive pumping of groundwater for irrigation. The decline has been as much as 10 feet per year for 20 years, notably in the Odessa-Lind area of Adams county. Water table decline is also significant on the Palouse, around Pullman, and Moscow, Idaho, where pumping has lowered well water levels throughout the groundwater basin.

The Washington State Department of Ecology (Ecology) regulates groundwater use by issuing permits for well drilling and water pumping. It has denied many applications for new groundwater withdrawals predicted to lower the water table more than 10 feet per year.

In contrast, many parts of the Columbia Plateau have excess groundwater levels. Water from overirrigation and from canal leakage interferes with field operations by keeping water tables near the land surface during the irrigation season. The water table has risen as much as 300 feet and caused drainage problems in the Quincy area and the Yakima River Valley. Eventually, soils will become saline (salty) and crop yields will decrease.

Groundwater is a renewable resource often taken for granted. In recent decades, users have learned the value and vulnerability of groundwater. Increased irrigation and urban and industrial demands have lowered water tables, requiring deeper wells and increased pumping costs. Only concerted water management by all users can reverse regional depletion.

As the principal reserve source of fresh water, groundwater is important to Washington. Its value to social and economic success is immeasurable. Well owner or not, all have a stake in wise groundwater use. Fortunately, the groundwater volume is great, generally easy to develop, and of good quality.

A stable supply of high quality groundwater can be assured for the future:

• If accurate information and public awareness efforts promote innovative solutions to water supply problems.
• If decisions and recommendations are based on the scientific principles of groundwater occurrence, movement, and characteristics.
• If public institutions—social, economic, political, and legal—allocate groundwater use rigorously but fairly to prevent overuse and maintain quality.
• If users pay the real cost of water.
• If every user conserves water and follows recommended practices to help improve and protect water quality.

Partial funding for publications in this series on Groundwater Protection was obtained through U.S. Environmental Protection Agency nonpoint source pollution grants administered by the Washington State Department of Ecology.

Additional Information

U.S. Geological Survey, 1201 Pacific Avenue, Suite 600 Tacoma WA 98402

Washington State Department of Ecology, Mail Stop PV-11, Olympia WA 98504-8711

Washington Water Research Center, Washington State University, Pullman WA . 99164-6120.

Selected References

Freeze, R. A. and J. A. Cherry. 1979. Groundwater. Prentice-Hall, Inc.

Todd, D. K. 1959. Ground Water Hydrology. Wiley.

U.S. Geological Survey. 1984. National Water Summary, 1984. US Geological survey Water-Supply Paper 2275.

By Ronald E. Hermanson, Ph.D., P.E., Washington State University Extension Agricultural Engineer

The author acknowledges the contributions of Christopher F. Feise, Ph.D., Washington State University Extension Western Washington Water Quality Coordinator and Groundwater Fact Sheet Project Coordinator, WSU­Puyallup Research and Extension Center; John H. Pedersen, Ph.D., Consulting Technical Editor and retired manager of the Midwest Plan Service, Iowa State University, Ames, IA; and Ronald E. Hermanson, Ph.D., P.E., WSU Extension Agricultural Engineer and Water Quality Project Leader, WSU­Pullman.

College of Agriculture and Home Economics, Pullman, Washington

Issued by Washington State Cooperative Extension and the U. S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Published August 1991. Reprinted May 1995. Subject code 376. A. EB1622