The northwestern part of Pierce County, Washington, is undergoing growth in population and in urban development, creating increased demands for municipal and domestic water supplies. Because most surface waters are already appropriated, ground-water resources are expected to meet the increasing demands. This report describes the ground-water system in the Quaternary deposits of this area.

The 88-square-mile study area is underlain by unconsolidated Quaternary deposits as much as 1,800 feet thick. Subsurface stratigraphy was delineated by extrapolating information from published surficial geologic maps and from 255 drillers' lithologic logs. The preparation of 17 sections aided in defining 10 hydrogeologic units--5 aquifers 5 semiconfining units--and 1 undifferentiated unit. Maps of the five uppermost units show the extent and topography of the tops of those units. The two major aquifers are units Qc1 and Qc2.

Precipitation over the study area averages an estimated 38 inches per year. Of this, 14 inches per year enters the ground-water system as recharge. Ground water generally moves northward to Puget Sound and east and northeast to the Puyallup River. Locally, the ground water discharges to streams, creeks, and springs within the study area. Discharge from these areas is estimated at 11 inches per year. Another 4.5 inches per year is withdrawn from wells.

During the 1996 water year, 22 billion gallons of water were used to supply the study area's needs. Of this, 15 billion gallons were imported from surface-water sources outside the study area. Approximately 9 billion gallons of water were used for commercial and industrial supplies and 8 billion gallons for public supplies. Much of the remainder was used for domestic supplies, agriculture, aquaculture, and irrigation.

The overall quality of the ground water in the study area is good based on information from the sampled site. Four constituents were found at concentrations above primary drinking water standards or guidelines. Two of the four constituents, the pesticide dieldrin in water from one well and total coliform bacteria in water from four wells or springs, were at levels exceeding standards or guidelines related to human health. Concentrations of iron or manganese in water from eight wells and springs exceeded secondary drinking water standards. Concentrations of iron or manganese in ground water above secondary drinking water standards are common for the Puget Sound region. Concentrations of the other constituents were below drinking water standards or guidelines.