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Direct Recharge

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WA382 - Direct Recharge from Precipitation through Glacial Till in the Puget Sound Lowland, Northwestern Washington - Completed FY1997

Problem - Rapid population growth in the Puget Sound Lowland in recent years has increased demand for new sources of water. Ground water is the preferred source of new supplies. An estimated 85 percent of the Puget Sound Lowland is mantled by the poorly permeable Vashon till that is underlain, in part, by outwash deposits that comprise the principal aquifers in the region. Although direct percolation of precipitation through the Vashon till is likely an important component of recharge in the lowland, there are no reliable estimates of the magnitude of this component. Also, the relative amounts of lateral and vertical soil-moisture movement have not been adequately quantified, but are of great importance to the nature of movement of contaminants such as those which may originate from septic tanks. Efficient long-term protection and allocation of the ground-water resources requires reliable estimates of ground-water recharge and an understanding of the flow processes occurring in the till.

Objectives - The principal objectives of this study are to: (1) quantify the amount of precipitation that percolates directly through surficial glacial till to underlying aquifers; (2) describe the generalized flow paths through the till; and (3) identify whether the predominant contribution to streamflow is from surface or shallow-subsurface runoff, for forested, till mantled catchments where there is no ground-water discharge to the streams.

Approach - Inflows and outflows will be monitored for three catchments that are completely underlain by till in the headwaters of Puget Sound Lowland streams. In order to avoid the complications of estimating ground-water discharge to streamflow in calculating water budgets, the catchments will be situated where no ground-water discharge from underlying aquifers contributes to the streamflow. Stream discharge and precipitation will be measured for two water years. Evapotranspiration will be estimated from meteorological data and soil moisture data collected at the catchment. A simple mass-balance equation of moisture will be solved to determine the amount of recharge that has occurred through the data collection period. A variably saturated cross-sectional model will be constructed to simulate flow paths of water through the till. Input to the model will be compiled from the soil moisture data, well levels in the aquifer permeameter data, and till matric potential data. At one of the catchments, a simple dye infiltration study will be performed to visually identify flow paths in the soil and upper zone of the till. In the same catchment, a bromide tracer will be applied to the surface. Water samples taken over time from suction lysimeters at various depths will be analyzed for bromide concentrations Results from these samples will give estimates of vertical and horizontal rates of water movement in the soils and upper zone of the till. In addition, rainwater, stream water, and soil water samples will be collected and analyzed for the stable isotopes of oxygen in order to apply a mixing model that may help to reveal the components of a storm hydrograph in this region.

WA292 - A Method for Estimating the Precipitation Component of Recharge through Poorly Permeable Materials--A Trial Application in the Puget Sound Lowlands - Completed FY1988

Problem - Estimation of recharge is critical to many ground-water studies. Recharge can be estimated by either basinwide water-budget or soil-moisture-budget methods. To estimate using the basinwide method, the total ground-water discharge must be known--this may be difficult to estimate, especially in coastal lowlands where a large but unknown quantity of ground-water may discharge directly into a large body of water. To estimate recharge using the soil-moisture budget method, the amount of precipitation that runs off as surface or shallow subsurface flow must be known--and although streamflow is relatively easy to measure, it is difficult to separate it into its proper components of ground-water discharge, surface runoff, and subsurface flow and the proportion of each.

Objectives - To develop methodology to estimate the direct-precipitation component of recharge to aquifers that are overlain by poorly permeable layers and thin permeable soils and to test and apply the above methodology to the problem of estimating the direct-precipitation component of recharge into the glacial deposits of the Puget Sound lowlands.

Approach - Because of its physical characteristics and previous work done in the area, the Puget Sound lowlands provide an ideal place to study how precipitation divides between surface and subsurface runoff, and recharge to aquifers. Two soil-moisture models will be used to compute the quantity of water that infiltrates the land surface and moves downward through the Vashon till into underlying aquifers. We will investigate the hydraulic properties of the materials comprising the till and overlying soil to test several hypotheses being used in application of the above models. Shallow subsurface flow rates and ground water levels in the till will be measured using water collectors in soil pits and piezometers at one or two sites in each of five catchments. Natural isotopic and chemical tracers may be used in tracer mass-balance equations to separate one or more storm hydrographs in at least three catchments.

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