GROUNDWATER-SURFACE WATER INTERACTION NEAR THE TEANAWAY RIVER (CENTRAL WASHINGTON STATE) BASED ON DATA FROM TEN MONITORING WELLS

The Teanaway River, a tributary of the Yakima River in central Washington, drains the east slopes of the Cascade Mountains and has been the subject of a large restoration effort. A large part of this restoration effort occurs in the Teanaway Community Forest, a state-owned community forest, that was acquired to enhance the water supply and protect the watershed. Within the Teanaway Community Forest, there is an 85-hectare parcel, on which a stand of cottonwood trees has recently died off, perhaps due to changes in water distribution to nearby farms. To inform restoration at this site, ten groundwater monitoring wells have been installed in the floodplain, penetrating 3 to 8 meters through the floodplain sediments to the sandstone bedrock of the region. Well tailings reveal that much of the northern side of the floodplain is occupied by a thick sequence of glacially-derived clays that both serve as a confining layer and reduce the storage potential of the floodplain aquifer system. In this project, groundwater levels and temperatures in these monitoring wells are combined with stable isotope measurements of groundwater samples collected from these wells.

Groundwater level data over a two-year period indicate that wells to the north are hydraulically disconnected from the lower part of the floodplain, likely representing a confined aquifer below the clay. In the rest of the floodplain aquifer, water flows roughly parallel to the Teanaway River for much of the year. Recharge occurs over a 1-month period in mid-winter at which time water flows towards the river and gradually relaxes back to the baseflow conditions over the course of four months. Stable isotope data indicates that groundwater in the riparian zone in the 300 meters closest to the river exchanges with the river rapidly. Water further up gradient appears to be a mixture of river water and isotopically lighter groundwater; fluctuations of this intermediate water are dampened and delayed compared to the river and riparian groundwater. The combination of pressure transducer and stable isotope data allow for the distinction between the transport of a pressure pulse and actual mixing of river water and groundwater.