A large part of the flow of streams originating in the Puget Sound Lowland consists of ground water discharged from aquifers of unconsolidated Pleistocene glacial outwash deposits. This part of streamflow is termed baseflow. The water in these streams is used for drinking water, irrigation, and industry, and is appropriated (legally "set aside") for water users through a permit system administered by Ecology. Withdrawals from many streams are limited by State regulations that prohibit users from withdrawing water when the stream has receded to a prescribed minimum acceptable flow. The minimum flows were established so that enough water remains in the stream to allow for the passage of anadromous fish (for example, salmon), the dilution of wastes, and other instream uses. In most cases, the total amount of water that has been appropriated from a stream exceeds the amount available (the amount in excess of the minimum acceptable flow) during periods of low flow, and many of these streams have been closed to further appropriation. Nevertheless, the population continues to increase in the Puget Sound Lowland, and in areas where streamflow is no longer available, water managers, developers, and individuals in need of new water supplies are requesting ground-water-withdrawal permits from the State. There is concern that development of ground water as a water supply may lower ground-water levels and consequently the baseflow of streams in some basins. This would reduce the availability of surface water to existing users and could reduce baseflows to levels below the established minimum flow during some periods. In order to allow development of ground-water resources while ensuring acceptable amounts of baseflow in regulated streams, Ecology needs to estimate the potential for a proposed withdrawal to reduce baseflows.
During the Pleistocene epoch, southward moving continental glaciers covered the lowland numerous times. Most aquifers and many of the confining layers in the lowland are composed of unconsolidated sedimentary materials deposited as a result of the glaciers' passage. The depositional processes associated with the glaciers produced the layering which is characteristic in the lowland. Periods when the glaciers were advancing or retreating are associated with layered deposits of sand or gravel and till, and periods when glaciers were not in the area, or far removed from it, are associated with fine-grained lacustrine deposits.
The topography of the lowland has been shaped by deposition and erosion that has occurred during the 12,000 to 13,000 years since the last glaciation. The lowland is generally characterized by flat, featureless drift plains that lie at altitudes of 200 to 600 ft above sea level. In places, the drift plains have been incised by major stream valleys; steep bluffs form the boundaries between the drift plains and the major stream valleys below. The effects of continental glaciation on the topography of the lowland are evident in the predominant north-south and northwest-southeast alignment of lakes, ridges, and major stream valleys that were etched by moving ice. As they cross the drift plains, streams have low hydraulic gradient, but the gradient steepens as the streams descend from the plains to the major stream valleys below.
The Puget Sound Lowland has a mid-latitude, Pacific-coast-marine type climate characterized by warm, dry summers and cool, wet winters. Mean annual precipitation ranges from about 25 to 60 in/yr, with a mean of 38 in/yr in 26 drainage basins within the Puget Sound Lowland (J.J. Vaccaro, U.S. Geological Survey, written commun., 1993). Nearly 80 percent of annual precipitation falls between October and March. Summer temperatures range from 60o F to 80o F, and winter temperatures range from 30o F to 50oF.
Where soils are poorly drained, native vegetation includes fir, cedar, alder, and madrona with an understory of huckleberry, Oregon grape, salal, and blackberry. On well-drained soils underlain by coarse-grained outwash deposits, the dominant vegetation consists of wild grasses, bracken fern, and scotch broom with patches of fir and oak.
In 1990, water use in the Puget Sound Lowland was 810 Mgal/d (J.J. Vaccaro, U.S. Geological Survey, written commun., 1993), 21 percent (174 Mgal/d) of which was ground water supplied by public water systems and 22 percent (178 Mgal/d) of which was ground water from private water systems. The total ground-water withdrawal of 352 Mgal/d in 1990 was approximately three times the amount supplied from ground-water sources in 1965.
References to the many studies of the geology and hydrology of the Puget Sound Lowland can be found in a bibliography compiled by Jones (1991) as part of a regional aquifer system analysis (RASA) carried out by the U.S. Geological Survey. Vaccaro (J.J. Vaccaro, U.S. Geological Survey, written commun., 1993) summarizes many of the results and conclusions from the RASA study, including analyses using cross-sectional numerical models in various hydrogeologic settings in the Puget Sound Lowland. This work provided much of the basis for the conceptual model used in the study. The findings of Dion and others (1994) for northern Thurston County and Woodward and others (1995) for southwestern King County also greatly influenced the conceptual model.
The criteria used in developing the conceptual model of the basin were (1) the basin had to be defined in sufficient detail to incorporate the salient features that control ground-water flow, and (2) the definition had to be general enough to be representative of a typical lowland basin. Attaining a balance between simplicity and detail in the conceptual and numerical models was key to producing useful results from the model analysis. Whereas detail was required to provide realistic boundary conditions, simplicity was essential for interpretation of cause and effect relations from model results.
A scale was chosen for the basin that would allow analysis of ground-water development scenarios ranging from single-well, local-scale withdrawals to multiple-well, basin-scale withdrawals. The physical attributes of the hypothetical basin, including topography, geology, and drainage, were then synthesized on the basis of the conceptual model. The conceptual model provided the guidelines such as the altitude and slope of the land surface, stream gradients and tortuosity, and thickness and extent of geologic layers. The spatial data describing the basin were compiled, checked, and stored in digital form using a geographic information system (GIS); this system was later used to create the data files needed by the numerical ground-water model and to store, display, and analyze the results of the model.
Initial estimates of recharge, hydraulic characteristics, and boundary conditions used in the model were based on typical values found by previous investigators in the Puget Sound Lowland. Most of these values were modified during calibration to make the hydrologic conditions simulated by the model more closely match conditions found in small Puget Sound Lowland basins; modifications were generally minor and always left the model parameters well within the range that would be expected in the Puget Sound Lowland for similar conditions or materials. The model was calibrated for predevelopment, steady-state hydrologic conditions. The parameter-adjustment process was completed when the simulated conditions matched expected conditions within tolerable limits.
The hydraulic heads and discharges to streams and springs simulated by the predevelopment, steady-state model represented the baseline hydrologic conditions used in the analysis of the hydrologic response of the hypothetical basin to ground-water development. Seven series of simulations were designed to analyze the effects of specific development variables, such as well depth or distance from a stream, on the response of the ground-water system. The response of the system to each scenario was evaluated by comparing simulated heads and ground-water discharge to streams and springs with those from the baseline model. The location and magnitude of reductions of natural ground-water discharge to streams and springs and the extent and magnitude of water-level declines were compared for each series of simulations. A total of 30 simulations were made in 7 series.