Water Quality in the Puget Sound Basin

Ground Water Studies


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Purpose

Assess the basin's ground-water quality, with particular emphasis on identifying and investigating areas that are most vulnerable to contamination. Four major tasks are listed, in the order in which they will be conducted, with each task becoming geographically and topically more focused:

Retrospective Studies (1995)

Provide a first-cut analysis of general water-quality conditions within the study unit using existing water-quality data. The Puget NAWQA staff analyzed information on the occurrence and distribution of nitrate, pesticides, and volatile organic carbon compounds (VOCs) in the basin's ground water.

Nitrate was found to be the most widespread contaminant. In-depth analyses were conducted of aquifer vulnerability to nitrate contamination. Factors of well depth, surficial geology, and land use were found to be the most statistically significant predictors of nitrate contamination in ground water, and this information was used to develop basin-wide aquifer vulnerability maps. Results are reported in a journal article (Tesoriero and Voss, 1997; see Publications page) and a fact sheet, Erwin, M.L., and Tesoriero, A.J., 1997, Predicting ground-water vulnerability to nitrate in the Puget Sound Basin: U.S. Geological Survey Fact Sheet 061-97, 4 p.

Study-Unit Survey (1996)

Broad-scale study designed to describe general water quality over a subregion of interest. Based on the regional hydrogeology, the Puget Sound Basin was divided into five hydrogeologic environments (subunits):

Unconfined Fraser aquifer
Semiconfined Fraser aquifer
Alluvial aquifers
Puget aquifer
Bedrock

The unconfined Fraser aquifer was selected for the first subunit water-quality survey based on its importance as a public and domestic water supply and its high susceptibility to contamination. Thirty existing wells that are randomly distributed over a range of land-use conditions and draw water from the unconfined Fraser aquifer were sampled in August 1996 for nutrients, pesticides, and VOCs to determine the ambient quality of water in the aquifer. Click here for a list of chemicals that were analyzed in ground water.

Land Use Surveys

Focused investigations designed to evaluate the effects of specific land uses on shallow ground-water quality. By focusing on shallow ground water that is most susceptible to contamination, these studies may serve as an early warning of future widespread impacts on ground-water quality, and they allow for a more direct evaluation of the impact of land use on ground-water quality. The retrospective studies identified the unconfined Fraser aquifer where it is overlain by urban and agricultural land uses to be among the areas that are most vulnerable to ground-water contamination, and these areas will serve as the focus of the land-use surveys.

Urban Land-Use Survey (1996 and 1997) Focused on the unconfined Fraser aquifer where it is overlain by urban land use in the Olympia-Lacey area of Thurston County and the Puyallup-Parkland area of Pierce County. About 30 monitoring wells were installed, screened within 10-20 feet of the water table, and sampled during the fall-winter recharge period.
Agricultural Land-Use Survey (1997 and 1998) Focused on the unconfined Fraser aquifer in the Lynden-Sumas area of Whatcom County. About 20 shallow observation wells were installed and sampled in a similar manner to the urban-land use study described above.

The results of the ground-water sampling during the Study Unit Survey and the Land Use Surveys were published in a Water-Resources Investigations Report in 2000.

Flow Path Studies (1997 and 1998)

Local studies of ground-water flow from recharge areas to discharge areas (usually streams) to evaluate the transformation of water-quality constituents over time and distance. These studies provide information on the importance of biogeochemical processes such as denitrification in altering ground-water quality, and they provide insight into ground-water travel times and ground-water-surface-water relations. An agricultural flow-path study was conducted in the Fishtrap Creek area of Whatcom County.

For information about Ground Water in the Puget Sound Basin, please contact Sandra Embrey, Hydrologist, at (253) 428-3600 (x2644)