Sara Young

ABSTRACT

Soils of the eastern Palouse region commonly contain fragipans or argillic horizons within the subsoil. These horizons have been shown to perch water during the cool, wet months of the year. Occurrence of perched water tables within the sloping Palouse landscape may lead to the rapid downslope transport of surface-applied agrichemicals, causing increased application requirements and raising the risk of surface and subsurface water contamination. These hydraulically restrictive horizons may also present significant barriers to aquifer recharge via deep percolation on loessial uplands. The objectives of this study were to investigate temporal and spatial patterns of perched water tables on a landscape-scale and to monitor water movement within and below a fragipan. A study site was chosen for this research that is representative of a typical Palouse north-facing slope. The site contains a well-developed fragipan, and has well-defined catchment structure. Soil hydraulic properties including saturated hydraulic conductivity, bulk density, porosity, and the moisture release characteristic, were determined. Perched water table monitoring was done using 63 observation wells equipped with pressure transducers, spaced on a 10-x-15-m grid. Automated readings of the saturated zone thickness were taken every 12 hours during the perched water table season using a datalogger. Hydraulic heads within and below the fragipan were monitored using banks of 3 piezometers installed to depths of 1, 1.5, and 2 m at upper, mid and lower slope positions. Manual readings were taken approximately every month during the 1996-1997 season. Ground water was monitored at the site using 2 wells at the base of the site installed to depths of ~ 4 m. Automated readings of ground water levels were taken every 12 hours during the 1996-1997 season. Results of hydraulic property measurements show an abrupt decrease in saturated hydraulic conductivity and porosity and an abrupt increase in bulk density at the upper fragipan boundary. Results of perched water table monitoring show that temporal patterns of perched water tables closely follow precipitation patterns in the area. Patterns of fluctuation in perched water tables vary from year to year based on yearly precipitation patterns. Spatial patterns of perched water tables are related to fragipan depth as well as site terrain attributes. Positions with shallow fragipans and convex curvature tend to experience more fluctuation in perched water table levels. Positions with high specific catchment areas and low slope angles tend to incur more overall saturation and less fluctuation. Fragipan piezometer monitoring demonstrated a hydraulic connnection between the perched water tables and an underlying hydrologic system in the lower and mid slope positions. At the mid slope position, the perched water table appeared to be receiving upward inputs from this system. The upper position showed no evidence of gradual recharge throughout the season, indicating that perched water may be available for deep percolation in the low-lying drainageways between hillslopes. The results of this research show that perched tables in soils of the eastern Palouse are highly susceptible to climate fluxes. Patterns of fluctuation differ across the landscape based on the orientation of the fragipan within the soil as well as site terrain attributes. Recharge to the underlying hydrologic system is occurring by some mechanism. This research will aid in future efforts to model near-surface hydrology in the area and will provide initial insights into the connection between near-surface hydrology and regional hydrogeology.