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News Release

U.S. Department of the Interior
U.S. Geological Survey

Release: September 16, 1999
Brian Drost, Hydrologist
(253) 428-3600, ext. 2642

New tool evaluates Thurston County ground water

A new computer model will allow water-resource managers to estimate the effects of future development, droughts, and climate changes on the ground-water system of Thurston County. The model, recently completed by the U.S. Geological Survey (USGS), simulates ground-water flow to depths of as much as 2,000 feet under more than 500 square miles of Thurston County. Among its possible uses are delineating recharge areas, estimating effects of increased pumping, and estimating effects of changing from septic systems to sewers.

"The model is a very useful tool for estimating the consequences of new development on ground-water levels and ground-water flows to rivers and creeks in Thurston County," said Brian Drost, USGS hydrologist and lead author of the report documenting the computer model.

A rapidly growing population in the county has resulted in full allocation of surface waters and a large demand for new water supplies from the ground-water system. In cooperation with the Thurston County Health Department, the USGS constructed the computer model to gain a better understanding of how the ground-water system operates and to provide the county with a management tool.

The model calculated a total annual inflow to the ground-water system of 660,000 acre-feet (215 billion gallons). Pumping from the ground-water system was estimated to equal about 9 percent of the annual recharge (62,000 acre-feet). Much of the water pumped from the ground is returned through septic systems or deep percolation of irrigation water. This type of recharge, called secondary recharge, increases ground-water availability but can result in some degradation of water quality.

The model was used to simulate the effects of pumping an additional 23,000 acre-feet per year. This amount is the county's reserved water rights. The model indicated that this amount of increased pumping would reduce ground-water flows to springs and Puget Sound by about 12,000 acre-feet per year and to streams by about 11,000 acre-feet per year. A small amount of increased leakage from streams to the ground-water system would be induced (about 700 acre-feet per year).

The recharge area for McAllister and Abbott Springs (Olympia's primary water source) was delineated using the model. Water discharging at the springs was determined to come from an area of about 20 square miles directly to the south of the springs. This water originates as precipitation recharge, secondary recharge, and leakage from Lake St. Clair and other surface-water bodies.

The report, "Conceptual Model and Numerical Simulation of the Ground-Water-Flow System in the Unconsolidated Sediments of Thurston County, Washington," by B.W. Drost, D.M. Ely, and W.E. Lum II, is published as USGS Water-Resources Investigations Report 99-4165. The report is available for reading at the USGS, Water Resources Division, 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402, telephone (253) 428-3600. The report can be purchased from the U.S. Geological Survey, Branch of Information Services, Box 25286, Denver, Colorado 80225, telephone (303) 202-4166. A limited number of free copies of the report are available from the Thurston County Health Department (contact Bob Mead at 360-754-4111 x7320, email

As the nation's largest water, earth, and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to sound economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources.

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