HYDROLOGIC PROCESSES IN VALLEY SOILSCAPES OF THE EASTERN PALOUSE BASIN IN NORTHERN IDAHO

Abstract

Vadose zone hydrology in the eastern Palouse Basin of northern Idaho is poorly understood because loess deposits often contain multiple hydraulically restrictive horizons that impede water flow. Valley soilscapes are of particular interest from a hydrologic perspective, because during the winter months, most of the precipitation is redistributed as runoff and throughflow into these landscape positions. Understanding the relationship between near-surface perched water table dynamics and vadose zone hydraulic processes in valley soilscapes is necessary to assess the sustainability of the groundwater resource. We implemented a combined approach to assess hydrologic processes in valley positions using hydrometric measurements, natural tracers, and stratigraphic observations. Hydrographs of near-surface monitoring wells indicate that valley positions maintain a thicker zone of saturation for longer duration compared with adjacent upland positions. Deep tensiometers demonstrate that multiple zones of seasonal saturation develop within the vadose zone of valley soilscapes in response to paleosol fragipan horizons and sediments of contrasting hydraulic conductivity. In some instances, the saturated thickness of vadose zone water tables was greater than 2.0 m and, because they are confined, displayed a positive pressure head. On adjacent uplands, seasonal saturation occurs only above the uppermost fragipan. Eluvial horizons having low Mn_d correspond to zones of saturation, whereas aquitards reflect Mn_d maxima. The recharge rate calculated using natural Cl⁻ mass balance was 2.4 mm y⁻¹ and did not correspond to measurements of saturated thickness by tensiometers. In addition, natural chloride profiles of other valley soilscapes display differences in recharge rates according to regional patterns in soil development. Together, deep tensiometer readings, secondary Mn distributions, and Cl⁻ profiles suggest that groundwater recharge does not occur via piston flow. Detailed stratigraphic analysis illustrates that preferential flow is a possible recharge mechanism. Results suggest that valley soilscapes play an important role in both surficial and deep regolith hydrological processes in the Palouse Basin.