ABSTRACT
| Perched water, or
episaturation, in soils of the eastern Palouse region is a seasonal condition, which
typically occurs from December to May. Infiltrating precipitation and meltwater perches
over a hydraulically restrictive argillic horizon or a fragipan. This condition of
episaturation in the sloping soils of the region is generally indicated by the presence of
an E horizon over the restrictive horizon. The E horizon is the result of lateral,
downslope movement of perched water over the restrictive horizon, and, due to the nature
of E horizon genesis, its presence suggests perched water has transport capabilities.
Transport of agrichemicals via perched water can decrease yields and degrade water
quality. Understanding the flow properties of perched water can improve land management
and can help clarify theories of soil genesis in the region. Therefore this study was
conducted to examine the flow characteristics of perched water in eastern Palouse soils
and the potential for chemical transport via perched water.
Three sites having soils and physiography typical of the
regional landscape were chosen along a precipitation gradient in the eastern Palouse
(increasing precipitation from west to east). The 830-mm, 640-mm, and 610-mm sites are
named for their mean annual precipitation. At each site, a grid of 23 piezometers was
installed to monitor perched water and the downslope movement of an applied Br tracer.
Depth to perched water was recorded and water was withdrawn for Br analysis. Horizontal
and vertical soil cores were extracted from the described horizons and saturated hydraulic
conductivity (Ksat) measured using a falling-head permeameter. Particle-size
distribution, bulk density (rb),
and clay mineralogy were determined for the E and restrictive horizons to help explain
observed differences in measured Ksat.
Perched water was present from December 1994 to May 1995 at all three sites and sampling began on December 2. Observed rates of downslope Br movement ranged from 22 cm d–1 at the 610-mm site to 80 cm d-1 at the 830-mm site. Up to 40% of the applied Br was recovered in the sampling grid at the 830-mm site, and 22% was recovered at the 610-mm site. Examination of the Br flow patterns suggest that the topography of the restrictive horizon surface results in much of the Br being transported obliquely out of the sampling grid thereby making the Br recovery rates relatively low. Nevertheless, the observed transport of applied chemicals via perched water will result in economic losses for producers in the eastern Palouse and potential degradation of local water quality. The three sites have similar Ksat depth profiles. Average vertical Ksat for the three sites are: 100, 30, 1 and 0.01 cm d-1 for the Ap, Bw, E, and restrictive horizons, respectively. Restrictive horizons typically have ~7% more clay than E horizons. X-ray diffraction of the clay fraction indicated slightly higher amounts of smectite in the restrictive horizons compared to the E horizons. Average r b is 1.63 Mg m-3 for E horizons and 1.65 Mg m-3 for restrictive horizons. The E horizon is typically thought to be the region of active water flow, resulting in eluviation of clays and organic material. However, comparison of rates of Br tracer movement with measured Ksat indicates that the most rapid and dynamic flow in these soils is occurring in the Ap and Bw horizons. Rates of the observed Br movement suggest rapid transport of chemicals via perched water and the percent recovery shows that large amounts of applied chemicals may be moved. Finally, while the increased clay content and higher amounts of smectite in the restrictive horizons may be reducing Ksat values, the data are inconclusive. Similarities between E horizon and restrictive horizon rb suggest that other soil properties, such as pore size and continuity, are controlling Ksat. |