Submitted to: Agriculture and Water Quality in the PAcific Northwest and Idaho Ground Water Connections Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: September 1, 2004
Publication Date: N/A
Technical Abstract: Excessive irrigation can leach agricultural chemicals and nutrients through the vadose zone to contaminate ground water. A four-year field experiment near Kimberly, ID, was designed to study the transport of water, using applied Br as a tracer, and native nitrate through a Portneuf silt loam (Durinodic Xeric Haplocalcid) cropped to dry bean (Phaseolus vulgaris L.) in growing season 1, and a mixed sward of HiMag tall fescue (Festuca arundinacea Schreb.) and ryegrass (Lolium spp.) in seasons 2 & 3. We irrigated the field using a solid-set irrigation system and a leaching fraction of 22% in season 1, 30% in season 2, and 35% in season 3. We measured native N in soil samples, precipitation and irrigation using rain gages, soil water contents using a neutron probe, and solute concentrations in soil solution samples taken from depths of 0.3 to 4.3 m. Solute movement and dispersion by depth were characterized using breakthrough curves of relative concentration (C/C0) plotted versus time. Bromide peak concentration decreased and movement slowed as the solute pulse moved downward through the profile. Nitrate-N or Br moved preferentially through 6 of 8 monitored sites and to depths from 0.9 to 4.3 m. Preferential solute transport occurred more at shallower depths (' 0.9 m) than deeper depths. Nitrate-N was transported through this irrigated and cropped silt loam by both matrix flow and spatially varying preferential flow. Leaching of mobile solutes through preferential flow paths in southern Idaho Portneuf soils is likely, even when irrigating with a judicious leaching fraction of 22%.