Pierce County Groundwater

Science Center Objects

The rapid growth of population in the Tacoma-Puyallup area in Pierce County has placed increasing demands on the ground-water resource. Most domestic water needs are met by wells completed in the several hundred feet of glacial deposits that underlie the area, and about 9,000 persons are served by a single spring on the southwestern side of Puyallup. Most of the population relies on individual septic tanks.

To help local officials develop a better understanding of the characteristics and quality of the area’s ground-water resource, the USGS is describing the water-bearing and confining units, determining the quantity of water, and determining the quantity and fate of recharge to the ground-water system. The study also includes assessing the general water chemistry of the significant water-bearing units, determining any regional patterns of ground-water contamination, and analyzing the water chemistry of several major springs in the area.

WA394 - Ground-Water Resources of the Tacoma-Puyallup Area, Pierce County, Washington - Completed FY2000

Problem - The study area comprises about 75 square miles in northern Pierce County. Physiographically, the area is composed of an undulating plateau of moderate relief and the lower, flatter floodplain of the Puyallup River. The plateau is underlain by several hundred feet of unconsolidated glacial sediments; the floodplain is underlain by relatively young alluvium. The population of the area has grown very rapidly, placing increased demands on the ground-water resource. Most domestic water needs are met by wells completed in the glacial deposits. About 9,000 persons in the Puyallup area are served by a single spring on the southwestern side of the city. The majority of the population in the study area relies on individual septic tanks. Puyallup and Pierce County health officials are concerned that the high density of septic tanks and a local landfill could lead to excessive concentrations of nitrate, chloride, and bacteria in the ground-water system in general, and the drainage area of Maplewood Spring in particular.

Objectives - To (1) describe and quantify the ground-water system; (2) describe the general water chemistry of the significant geohydrologic units and any regional patterns of ground-water contamination; and (3) determine the amount and fate of recharge to the ground-water system.

Approach - Approximately 175 wells will be inventoried in the study area. Drillers' logs will be used to construct a net of cross sections to correlate aquifer and confining units. Structure (top) maps will be drawn of significant geohydrologic units. Water-level maps will be drawn for the principal aquifers, and water levels in selected wells will be monitored on a bimonthly basis for two years. Specific-capacity data will be used to calculate hydraulic conductivity for each geohydrologic unit. The major springs in the study area will be inventoried and their discharges either measured or estimated. Recharge will be calculated using regression analysis and a water-use inventory will be completed. About 30 wells and 3-4 major springs will be sampled and their waters analyzed for a variety of chemical constituents.