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Mark Mastin,
Hydrologist,
934 Broadway,
Suite 300
Tacoma, WA 98402

(mcmastin@usgs.gov)
(253) 552-1609
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Direct Recharge

  
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About 85 percent of the Puget Sound Lowland is mantled by poorly permeable Vashon till of glacial origin. The till is underlain, in part, by glacial outwash deposits that make up the principal aquifers in the region. Although an important component of recharge to the aquifers is direct percolation of precipitation through the till, there are no reliable estimates of the magnitude of this component, and little is known about the lateral and vertical movement of moisture-and possible contaminants-through the till.

Ground water is the preferred source of new water supplies as the population of the Puget Sound Lowland continues to increase rapidly. Efficient long-term allocation and protection of the ground-water resources requires reliable estimates of ground-water recharge and an understanding of the flow processes occurring in the glacial till.

To provide a better understanding of the quantity and movement of water in glacial till, the USGS is investigating the amount of precipitation that percolates directly through surficial glacial till to underlying aquifers and describing the generalized flow paths through the till. In addition, the USGS is monitoring inflow and outflow in streams in three till-mantled catchments where ground-water discharge from underlying aquifers does not contribute to streamflow. The effort will determine whether the predominant contribution to streamflow is from surface or shallow-subsurface runoff.

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