Hydrology of a first-order riparian zone and stream, mid-Atlantic coastal plain, Maryland
Riparian buffer strips are considered to provide natural remediation for groundwater contaminants, but this function is partly based on a relatively 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 mid-Atlantic 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 170 nested piezometers (mostly in transects) for evaluating groundwater hydrology and geochemistry. Subsurface microstratigraphy and macroporosity are largely responsible for observed spatial and temporal variations in groundwater contributions to the stream. The portion of the stream that shows the highest rate of flow increase per stream length contains discrete zones of enhanced groundwater discharge (upwelling) to the surface, which supply most of the additional stream flow. These upwelling zones display high (positive) vertical hydraulic heads, which relate to the amount of groundwater discharged from those points. One particular area of intense groundwater upwelling supplies approximately 4% of the total stream outflow, yet comprises only 0.006% of the riparian zone. The upwelling zones also supply most of the NO-3 to the surface. Zones of focused groundwater exfiltration that sustain stream baseflow do not correspond with erosional features, and have a great impact on stream flow characteristics and NO-3 behavior. Few studies to date have elucidated and quantified contributions from specific surface features (e.g. upwelling zones, discharging macropores). More emphasis and research needs to be directed to the importance of focused groundwater exfiltration points for stream flow generation and NO-3 transport, with the goal of devising more effective management regulations for preserving and enhancing riparian zone mitigation function.
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