Chloride distributions as indicators of vadose zone stratigraphy in Palouse loess deposits

Chloride is often used as a conservative tracer to estimate groundwater recharge rates in arid and semiarid regions. Relationships between Cl- depth profiles and vadose zone stratigraphy have revealed new information on the behavior of this dissolved constituent in pore waters of heterogeneous materials. We measured pore-water Cl- in loess deposits of the eastern Palouse region in northern Idaho, where multiple sequences of buried soils extend to 20 m depth. Three cores were collected to bedrock at summit, side slope, and valley positions. Pore-water Cl- distribution, clay content, soil strength, and secondary Mnd/Fed ratios were measured to identify relationships between natural tracer migration and vadose zone stratigraphy. Characterization of deep strata revealed complex sequences of extremely dense paleosol fragipans interstratified with less dense leached horizons. Abrupt changes in Cl- concentration reflect boundaries between these stratigraphic units that display contrasting physical and morphological properties. Results illustrate that loess stratigraphy influences vadose zone water movement in the Palouse. In addition, Cl- depth profiles can be used as indicators of deep stratigraphy across various landscape positions.