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United States Department of Agriculture

Agricultural Research Service

Title: Effects of Initial Water Content on Macropore/matrix Flow and Transport of Surface-Applied Chemicals

Authors
item Shipitalo, Martin
item Edwards, William

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 1995
Publication Date: N/A

Interpretive Summary: In most instances no-till farming practices reduce runoff and erosion, thus helping to preserve surface water quality. Rapid movement of water through soils via large pores (macropores) created by plant roots and burrowing earthworms accounts for part of the reduction in runoff. This has raised concern as to whether this rapid flow increases leaching losses of surface-applied agricultural chemicals. We found that a significant amount of rainwater moves through macropores in no-till soils. The concentration of chemicals in the water moving through the macropores, and potentially to the groundwater, depends on how wet the soil was when leaching occurred and the nature of the surface-applied chemicals. The concentrations of reactive chemicals (which include most pesticides) decreased with increasing soil moisture content. Only under unusual circumstances (a heavy rainstorm immediately following application), however, would soil moisture have a noticeable effect on chemical concentrations. Regardless of moisture content, only relatively small quantities of the surface-applied chemicals were leached through the soil. Therefore, the potential impact of no-till management on subsurface water quality appears to be minimal.

Technical Abstract: Soil moisture is an important factor which may influence the relative contribution of macropores and matrix porosity to water movement and chemical transport. We surface-applied SrBr2.6H2O, atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), and alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide] one hour before subjecting 30 by 30 by 30 cm, undisturbed, soil blocks from a no-till corn (Zea mays L.) field to a simulated rain of 30 mm in 0.5 h. Dry, intermediate, and wet antecedent moisture levels were investigated. In order to distinguish applied water from resident water and to assess interaction of the rain water with the soil matrix, RbCl was added to the simulated rain as a tracer. Sequential percolate samples were collected in 10 mL aliquots from 64 cells at the base of the blocks. Moisture level had no detectable effect on the volume of percolate produced or on total chemical transport, but moisture level did affect how water and solutes were transported through the blocks. As soil moisture increased, the matrix became increasingly involved in the flow processes, but the outflow from the blocks was still dominated by the macropores. Displacement of resident water and interaction of rain water with the matrix increased as soil moisture increased. As a result, percolate concentrations of the reactive, surface-applied, constituents (Sr, atrazine, alachlor) decreased with increasing soil moisture. Overall, the results suggested that the contribution of macropores to chemical transport and water movement decreases as the soil becomes wetter.

Last Modified: 7/25/2014
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