Submitted to: The Scientific World
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 9, 2001
Publication Date: N/A
Interpretive Summary: Agricultural nitrogen is a major form of groundwater and surface water pollution. Excess nitrogen is linked to oxygen depletion in rivers, lakes, and coastal oceans (leading to large-scale destruction of aquatic life), outbreaks of water-borne bacteria (such as Pfisteria piscida), and contamination of drinking water supplies. A need for practical and economical solutions to the problem of excess nitrogen currently exists. Riparian zones (vegetated stream corridors) are considered natural sites for removal of agricultural nitrogen. This study is part of a broader project that includes assessing the effectiveness of a riparian zone to remove nitrogen. The nitrogen-removing capabilities of the system appear to be highly variable and largely related to total stream flow. Results from this study should aid in designing regulations to enhance the nitrogen-removing potential of riparian corridors. The capacity for nitrogen removal appears to be more closely related to groundwater and surface water behavior than to total riparian zone width, which is the currently accepted regulatory standard.
Technical Abstract: Riparian zones are reputed to be effective at preventing export of agricultural groundwater nitrogen from local ecosystems. This is one impetus behind riparian zone regulations and initiatives. However, riparian zone function can vary under different conditions, with varying impacts on the regional (and ultimately global) environment. Rates of groundwater delivery to the surface appear to have significant effects on the nitrogen-removing capabilities of a riparian zone. Research conducted at this site, a first-order agricultural watershed with a well-defined riparian zone in the Maryland coastal plain, indicates that significant amounts of nitrogen can be exported under moderate-to-high stream baseflow conditions. The total nitrogen load that exits the system increases with increasing flow not simply because of the greater volume of water export. Stream water nitrate-N concentrations also increase as flow increases, at least during baseflow. This appears to be largely the result of changes in dominant groundwater delivery mechanisms. Higher rates of groundwater exfiltration lessen the contact time between nitrogen-carrying groundwater and potentially reducing riparian soils. Subsurface preferential flow paths, in the wetland and adjacent field, also strongly influence nitrogen removal. Simple assumptions regarding riparian zone function may be inadequate because of complexities observed in response to changing hydrologic conditions.