|Prestegaard, Karen - UNIV. OF MARYLAND|
Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 26, 2004
Publication Date: January 1, 2005
Citation: Angier, J., McCarty, G.W., Prestegaard, K. 2005. Hydrology of a first-order riparian zone and stream, mid-atlantic coastal plain, Maryland. Journal of Hydrology. 309:149-166. Interpretive Summary: Studies have shown that excessive nutrients are linked to oxygen depletion in rivers, lakes, and coastal oceans (leading to large-scale destruction of aquatic life), outbreaks of water-borne microbes, and contamination of drinking water supplies. Practical and economical management practices are needed that can mitigate excess nutrients from agricultural fields. Riparian zones (vegetated stream corridors) are considered natural sites for nutrient mitigation. This study was part of a broader project that includes assessing the overall function of a riparian zone. The focus of this study was the impact of hydrology on nitrate delivery from groundwater to surface waters in a riparian zone and its export from the ecosystem. The nitrate-removing capabilities of the system are highly variable, and local differences were largely related to groundwater delivery patterns. Results from this work suggested that the overall ameliorative capacity of a riparian system was more closely related to groundwater and surface water behavior than to total riparian zone width, which is the currently accepted regulatory standard. These results will aid in developing guidelines for maximizing contaminant mitigation potential of riparian corridors.
Technical Abstract: Riparian buffer strips are considered to provide natural remediation for groundwater contaminants, but this function is based on a simple model of riparian zone hydrology, specifically, horizontal matrix flow through the shallow subsurface. Deviation from horizontal flow leads to asymmetrical groundwater emergence onto the surface and greater propensity for contaminant delivery to the stream. The study site, at the USDA-Beltsville Agricultural Research Center, is in the coastal plain of Maryland. The site contains a small first-order stream that is instrumented with five stations for monitoring stream flow and chemistry and with 170 piezometers for evaluating groundwater. The portion of the stream that shows the highest rate of flow increase per stream length contains discrete zones of enhanced groundwater upwelling to the surface. These upwelling zones display high vertical hydraulic heads, which relate to the amount of groundwater discharged. One particular area of intense groundwater upwelling supplies approximately 3.5 percent of the total stream outflow yet comprises only 0.006 percent of the riparian zone. The upwelling zones also supply most of the nitrate to the surface. Subsurface microstratigraphy and macroporosity are largely responsible for observed spatial and temporal variations in groundwater contributions to the stream. The geomorphological and stratigraphic setting, where valley fill consists of relatively non-transmissive wetland soils over a more hydraulically-conductive aquifer, is typical for the mid-Atlantic coastal plain. Groundwater-fed headwater wetlands are also common in this region, so the results of this research may be applicable to other sites and should be viewed in the context of the wider problem of surface water nitrate loading.