Assessment of hydrologic processes across multiple scales in soil/paleosol sequences using environmental tracers

In the Palouse Basin of eastern Washington and northern Idaho, ground water is the principal municipal-water resource. Reliance on ground water continues to increase in the region, yet recharge mechanisms and rates are poorly understood in thick loess deposits that blanket the Basin. The main objective of this study was to assess the ability of soil/paleosol sequences in loessial hillslopes to accommodate deep percolation using environmental tracers. Patterns between vadose zone stratigraphy and Cl− tracer profiles were established in heterogeneous regolith. Characterization of deep strata revealed sequences of extremely dense paleosol fragipans interstratified with less-dense eluvial horizons. Abrupt changes in [Cl−] reflect boundaries between stratigraphic units that display contrasting physical and morphological properties. Results illustrate that Cl− depth profiles can be used as indicators of deep stratigraphy across various landscape positions. To assess regional recharge rates and mechanisms, environmental tracer distributions were measured in three catchments reflecting a regional climosequence. Tracer profiles and stratigraphy illustrate three major hydrostratigraphic units: (1) uplands with homogeneous regolith and short pore-water residence times; (2) uplands with heterogeneous regolith and long pore-water residence times; and (3) valleys with heterogeneous regolith that display dynamic hydraulic processes. Regional relationships between deep regolith and surface soils were established in order to use the Soil Survey Geographic database to estimate spatial extent of hydrostratigraphic units. Results indicate that recharge is less than 3 mm yr−1 in 33% of the loess-covered Basin where precipitation is greatest, and regolith is heterogeneous. Recharge is 10 mm yr−1 in 37% of the Basin where loessial regolith is homogeneous. Hydraulically dynamic valley positions constitute 10% of the study area. Hydrometric measurements and natural tracers were used to assess hydrologic processes active in vadose zones, near-surface perched water tables, and streams of valley positions. Deep tensiometer readings and secondary Mn distributions indicate that ground-water recharge occurs as bypass flow. Indirect evidence from δ18O signatures of water samples suggests that near-surface perched water tables are a major source of stream flow. Representative Cl− profiles of valley soilscapes reflect differences in pore-water residence time that are governed by regional patterns of soil development.