Dilution of Nonpoint‐Source Nitrate in Groundwater

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Abstract
Nonpoint‐source pollution from agriculture can cause the degradation of groundwater and surface water. Some studies conducted in Coastal Plain aquifers have shown NO−3 removal from groundwater due to assimilation by vegetation or denitrification before discharge to a stream is significant; relatively few have been conducted on other physiographic and geological regions within the Chesapeake Bay watershed. This study was conducted at the boundary of the Valley and Ridge and Appalachian Plateau physiographic provinces to understand how the hydrological and geochemical conditions in this region effect the transport and removal of NO−3. The groundwater flow system at the farm studied, located at the base of a valley, is influenced by one groundwater flow component parallel to the axis of the valley and a second flowing perpendicular to the axis having been recharged from topographic highs along the sides of the valley. The axis‐perpendicular component is transporting NO−3 and Cl− applied as fertilizers and manure on an upgradient field and also Cl− from salting of an adjacent road to the study site. The relatively nutrient‐free water from the axis‐parallel component dilutes NO−3 and Cl− concentrations at the site from seasonal averages of 11 mg N/L and 90 mg Cl/L to levels −3 removal from groundwater <3 m belowground; however, it is difficult to confirm the removal process with dilution dominating the system.

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