STABLE ISOTOPE STUDIES OF SURFACE WATER/GROUNDWATER INTERACTIONS IN CENTRAL WASHINGTON, USA

The isotopic composition of a groundwater or surface water sample reflects its meteoric water source(s) and the mixing and isotopic fractionation that have occurred since that sample fell as precipitation. Comparison of isotopic trends in precipitation, snowmelt, soil water, groundwater and streams can help delineate the relationships between these different water types. In this study, stable isotope compositions of precipitation, snowmelt, and soil water were monitored along a climate gradient on the east slope of the Cascade Mountains in central Washington state. Annual precipitation ranges from 266 cm to 23 cm, occurring mostly as snow in the winter. These data are compared to stable isotope compositions of groundwater samples collected from local and municipal wells along the gradient and surface waters from the Yakima River and its tributaries.

For precipitation and snowmelt, the seasonal and annual averages of the isotopic composition become progressively depleted moving eastward (leeward) in the climate gradient as might be expected due to rainout from moist air masses. For soil water, the isotopic data indicate that a significant component of immobile soil water, isotopically heavy due to evaporation, resides in the shallow soil throughout the summer dry season, particularly at the driest sites. During the spring snowmelt, immobile soil water is largely flushed from the soil.

Isotopic compositions of surface waters reflect the drainage area from which their waters are derived and its precipitation distribution. The Yakima River is isotopically similar to precipitation and snowmelt from near the crest of the Cascades, the source of most of its water. In contrast, local north-south trending tributaries on the east side of the study area are isotopically lighter than Yakima River water because they drain catchments to the east of the crest.

Groundwaters appear to be mixtures of three end-member groups: 1) shallow wells that are isotopically similar to the Yakima River, perhaps recharged by irrigation; 2) more typical groundwaters within basin fill sediments that are isotopically lighter and appear to be a mixture of spring snowmelt and local tributary waters; 3) ancient groundwaters that reside in deeper aquifers and are isotopically lighter than almost all modern waters.