| Agricultural nitrate can have a negative environmental impact on surface waters, such as rivers that drain into the Chesapeake Bay. Riparian buffer strips are considered to provide natural remediation for groundwater nitrate, but this function is based on relatively simple models of riparian zone hydrology and stratigraphy. The purpose of this study was to assess the combined influences of hydrology and geochemistry on groundwater nitrate in a riparian zone. The study site, at the USDA-Beltsville Agricultural Research Center, is in the mid-Atlantic coastal plain of Maryland. The site contains a small 1 st-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. The portion of the stream that shows the highest rate of flow increase per area contains discrete zones of enhanced groundwater discharge to the surface. These 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% of the total stream outflow, yet comprises only 0.006% of the riparian zone (or 0.001% of total catchment area). The upwelling zones also supply most of the nitrate to the surface. Dissolved oxygen concentrations are high in the underlying aquifer, and in the overlying soil where hydraulic heads are high. Areas where upwelling groundwater is absent display anaerobic conditions throughout the soil. Oxic conditions, rapid groundwater movement, and limited contact area inhibit nitrate removal in upwelling zones. Chloride, sulfate, and dinitrogen data confirm that the direction of groundwater flow in upwelling areas is vertical. Valley morphology, combined with subsurface stratigraphy and macroporosity, are largely responsible for deviation from more commonly assumed horizontal flow in the subsurface. The geomorphological and stratigraphic setting, where valley fill consists of fine-grained 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 a larger extent, and should be viewed in the context of the wider problem of surface water nitrate loading. |
