USGS Water-Resources Investigations Report 99-4013

Demands grow for ground water under Tacoma-Puyallup;

new USGS report describes the ground-water system


generalized cross section of hydrogeologic units

In 1996 northwestern Pierce County, Washington, consumed 22 billion gallons of water, 15 billion gallons of it being imported from surface-water sources outside the area (table, 14198 bytes). The people of this 88-square-mile area near Puget Sound, where the population is growing vigorously and the land is being rapidly developed, have reached the limits of their surface-water supply and are turning to its ground water.

But what is the nature of the little-understood ground-water system that people are coming to depend on? How much water can it supply? What is the quality of the water? How will greater demands for the ground water affect nearby wells, springs, lakes, and wetlands? A new report from the U.S. Geological Survey presents a portrait of the ground-water system under Tacoma and Puyallup that will point the way to answers.

"The ground-water system we've described will help state and local water managers know what to expect of the aquifers--how much water they can produce and where the better supplies are," explained M.A. Jones, the lead USGS investigator of the project.

The new report presents the results of a half-million-dollar investigation begun in 1995 by the USGS in cooperation with the Tacoma-Pierce County Health Department, the Washington Department of Ecology, and the Cities of Puyallup and Tacoma.

USGS investigators worked from 255 drillers' logs, published maps of the surficial geology, and cross sections of the area's thick deposits of unconsolidated material laid down during the Quaternary Period, from 1.6 million years ago to the present. The USGS delineated a layer-cake local system of five coarse-grained aquifers alternating with five fine-grained semiconfining units and underlain by deep undifferentiated deposits (figure, 42589 bytes). Ground water in the two most important aquifers, dubbed Qc1 and Qc2, generally moves east and northeast into the Puyallup River and north to Commencement Bay, and Puget Sound.

Of the 6,890 million gallons of ground water withdrawn from the area in 1996 for all uses, aquifers Qc1 and Qc2 supplied by far the most--2,790 and 2,320 million gallons (table, 14198 bytes). A ground-water budget constructed for the shallow units, aquifer Qc1 and above, revealed that 86 percent of the quantified recharge in the system discharges through these shallow units. The USGS concluded that these shallow units make up the local ground water system and that aquifer Qc2 and the units below are more properly a part of a regional system.

The investigators found that the quality of the ground water in the 33 wells and springs they sampled was generally good (figure, 61466 bytes). In only one well did the concentration of the pesticide dieldrin in the water exceed drinking-water standards for human health, and in water from only four wells or springs did total coliform exceed drinking-water health standards. In eight wells or springs, water contained iron or manganese concentrations above secondary drinking-water standards, which are chiefly aesthetic, not health-related standards. The samples from all 33 wells and springs were analyzed for nitrite-plus-nitrate, major ions, arsenic, iron, manganese, bacteria, and checked for pH, temperature, specific conductance, alkalinity, and dissolved oxygen (table, 19114 bytes) and (figure, 43591 bytes). Only some samples were analyzed for trace elements, methylene blue active substances, boron, radon, pesticides, total organic carbon, and volatile organic compounds.

The report, Ground-Water Hydrology of the Tacoma-Puyallup Area, Pierce County, Washington, by M.A. Jones, L.A. Orr, J.C. Ebbert, and S.S. Sumioka, is published as U.S. Geological Survey Water-Resources Investigations Report 99-4013. The report is available for reading at the U.S. Geological Survey, WRD, 934 Broadway, Suite 300, Tacoma, Washington 98402, telephone (253) 552-1600. (abstract, 3191 bytes) and (table of contents, 27846 bytes).

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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