Recharge from Precipitation in Three Small Glacial-Till-Mantled Catchments in thePuget Sound Lowland, Washington

摘要

This report documents an investigation to estimate ground-water recharge throughglacial till in three small catchments in the Puget Sound Lowland. It also describes interpretations of the likely pathways for the movement of water from precipitation to streams and ground water in these catchments. In this investigation, two methods were used to estimate recharge through glacial till. A water-budget method was used in three till-mantled catchments in which estimates of evapotranspiration, direct runoff (surface runoff, and shallow-soil-water seepage to streams) are subtracted from precipitation to give an estimate of percolation into the subsoil, which is then assumed to equal recharge. A tritium-tracer method was used in the same three catchments. The tritiumtracer method involved sampling and analyzing for tritiated water from the unsaturated zone above the permanent water table. Measurements of the vertical distribution of tritium and soil-moisture content were used to calculate recharge. The source of streamflow during storms and the movement of soil water in till-mantled areas were also investigated. Different isotopic compositions of oxygen in soil water, streamflow, and precipitation were used to identify the contributions to streamflow. A dye tracer was also used to give visual information on the movement of soil water.