Geology topics

J.A. Vomocil

Publications and source records attributed to J.A. Vomocil.

2 recordsLinked to original sources

USGS1982

Perched water tables on hillsides in western Oregon: I. Some factors affecting their development and longevity

Perched water tables on hillsides located on the western border of the Willamette Valley in Oregon in some cases have the potential to transport pollutants from either domestic or agricultural sources downslope to streams, ponds, or reservoirs, resulting in the deterioration of the quality of these waters. In this paper, some factors responsible for the development and longevity of these potentially problem-causing perched water tables on three hillsides were examined. Analyses of hydraulic conductivity data and the relationships between rainfall, soil water pressure potential, and time suggest that permeable rock below 110 cm rather than clayey B horizons is mainly responsible for the development of perched water tables on upper convex slope positions on two of these hillsides. The data also suggest that subsurface flow from the upper convex regions contributed significant amounts of water to the lower convex slope positions. Further, perched water tables developed more rapidly, to a greater extent, and lasted longer in upper horizons of a lower concave region of slope where a shallow perched water table was already present at the onset of rainfall. Finally, perched water tables did not develop in the upper horizons on a third hillside that had no impermeable regions in the soil or upper rock mantle.

Oregon↗

USGS1982

Perched water tables on hillsides in western Oregon: II. Preferential downslope movement of water and anions

Perched water tables on hillsides in western Oregon potentially provide a means by which pollutants from agricultural and domestic sources may enter surface waters and consequently degrade the quality of these waters. This paper reports the results of experiments which were carried out to investigate the flow of solutes and water from buried line sources in and above perched water tables on three different hillsides in western Oregon. In the saturated soil and rock mantles of two hillsides, evidence suggests that water and solutes flow preferentially through large continuous voids. The evidence includes the very high maximum rate of anion movement downslope, the failure of pore velocities calculated from Darcy's Law to predict the rate and direction of anion movement, the short time before high levels of anions are detected downslope, and the spatial variability in the maximum rates of anion movement over the depth and width of sites. On the third hillside, tracer anion movement and soil water pressure measurements indicate that preferential flow occurs through large continuous pores during heavy rainfall while the surrounding soil and rock mantle remains unsaturated. Finally, experiments with Rodamine WT dye show that a variety of continuous voids of different origins and sizes conduct water and solutes preferentially in the soil and upper regions of rock on these hillsides.

Oregon↗