Paleosols as deep regolith: implications for ground-water recharge across a loessial climosequence

Abstract

Thick loess deposits consisting of paleosol sequences comprise the deep regolith of the Palouse region in eastern Washington and northern Idaho. Ground water is the principal water supply for the Palouse Basin, yet recharge mechanisms and rates through this regolith are poorly understood. Forecasting the sustainability of the water supply has been hampered because models that predict ground-water recharge are complicated by deep regolith that has complex stratigraphy and, in some instances, multiple buried hydraulically restrictive horizons. To assess water movement, pore-water Cl − and δ 18O distributions were measured to 6-m depths in three catchments representing a climosequence across the Basin, and interpreted using catchment and basin-wide stratigraphic relationships. Vertical facies between dense and soft horizons coincide with variations in [Cl −]. In the eastern Basin, pore-water [Cl −] ranged from 15 to 150 mg/L and increased with depth. In addition, shifts in isotopic signature of oxygen in pore water coincide with Cl −-rich horizons, and suggest long pore-water residence times. In contrast, [Cl −] in the central and western Basin ranges from 1 to 15 mg/L and changes little with depth. Tracer profiles illustrate three major hydrostratigraphic units in the Basin: (1) uplands with relatively homogeneous regolith that has short pore-water residence times; (2) uplands with heterogeneous regolith that has 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 SSURGO database to estimate the spatial extent of each hydrostratigraphic unit. The degree of paleosol development mimics that of contemporary sola across the climosequence. Results indicate that recharge is less than 3 mm year −1 in 33% of the study area where precipitation is greatest and regolith is heterogeneous. Recharge increases to 10 mm year −1 in 37% of the study area where regolith is homogeneous. Valley positions having active hydraulic regimes constitute 10% of the Basin.