Submitted to: International Symposium on Preferential Flow
Publication Type: Proceedings
Publication Acceptance Date: September 15, 2000
Publication Date: N/A
Interpretive Summary: Management of vegetative strips along rivers and streams (riparian buffers) is important for protecting rivers, lakes, and coastal waters from agricultural pollution. The USDA-Natural Resources Conservation Service has initiated a program to develop two million miles of riparian buffers for protection of national water resources. With the great emphasis on riparian nbuffers, a need exists to better assess buffer function for water resource protection and to develop management strategies to maintain high function. Studies have shown that non-uniform movement of water (preferential flow) through riparian wetlands has a major impact on the ability of wetlands to remove agricultural pollutants. Design and management strategies for riparian systems which take preferential water movement into account will greatly increase buffer function for improved water quality.
Technical Abstract: The impact of preferential pathways for groundwater movement on wetland function was studied in a riparian system associated with a first-order stream draining an agricultural catchment. Evidence for preferential groundwater flow included open macropores and layers of wetland soil with high hydraulic conductivity. These flow pathways were found to operate on different orders of scale within the wetland ecosystem. Open channel macropores in the system typically function on a relatively small (centimeter) scale as determined by pore diameter and length. Larger orders of scale included continuous layers of high-conductivity material which can operate with meter-plus dimension. The conductivity range can span three orders of magnitude (10 e-4 to 10 e-7) within typical profiles of the wetland soil providing a strong tendency for preferential water movement through sections of high conductivity. Biological activities are also highly stratified in these profiles with the highest activity occurring in the upper portion of profile that receives organic residue from wetland vegetation. The prevalent movement of groundwater by preferential flow pathways in conjunction with strong anisotropic structure of biological activities, such as denitrification, suggests that strong spatial components within wetland ecosystems may constrain their ability to mitigate agricultural pollutants. Concentrated flow of water limits both residence time and degree of interaction with biologically active zones. The concentrated flow may also allow complete bypass of zones with greatest ability to remove pollutants.