Riparian Buffer Landscape Effects on Groundwater Flow Paths and Nitrate Occurrences in Groundwater near Streams of the Eastern Shore of Virginia

Elevated concentrations of nitrate occur in groundwater throughout the United States due to the widespread application of nitrogenous fertilizers associated with agricultural land uses. Forested stream buffers, which separate upland agricultural areas from stream channels, are recognized for their importance as locations of groundwater nitrate removal through plant uptake and denitrification. An understanding of the groundwater hydrology of the buffer subsurface is important in being able to predict where denitrification can occur, since the groundwater hydrology determines the locations in which groundwater with elevated nitrate will intercept available supplies of organic matter. Previous studies have identified differences in the pattern of groundwater flow through riparian floodplains within stream buffers, which was related to differences in their ability to remove nitrate by denitrification. In this study, the effect of forested buffer topography on groundwater flow paths and nitrate occurrences in groundwater near streams of the Eastern Shore of Virginia was investigated. A field and groundwater modeling-based approach of five buffer-stream sites was undertaken to address the objectives of this study. Observations of groundwater nitrate, hydraulic head and soil/sediment characteristics were collected in transects oriented approximately perpendicular to the stream channel at each site. Groundwater sampling results indicate groundwater inputs to the buffer at each site generally contain elevated nitrate, with monitoring wells located near the agricultural field-buffer boundary having mean nitrate concentrations ranging from 0.0 to 7.1 mg/L. The mean nitrate concentration for the outer monitoring well at one site (4.6 mg/L) does not reflect nitrate occurrences beneath iii the streambed, where a mean nitrate concentration of 12.4 mg/L was observed at a depth of 0.88 m below the streambed. A semi-normal distribution of elevated nitrate was observed for hillslope piezometers, which contrasted with a bimodal distribution of nitrate for riparian floodplain piezometers. For the riparian floodplain piezometers, a group with relatively low nitrate concentrations was observed along with a second group having elevated nitrate concentrations. The results of the MODFLOW groundwater flow modeling indicate riparian floodplains have an effect on groundwater flow paths, with a downward deflection of the shallow groundwater flow paths generally occurring at the hillslope-floodplain transition. At one buffer site, the downward deflection of groundwater flow paths at the hillslope-floodplain transition agreed with observations of relatively low mea nitrate concentrations (≤ 0.4 mg/L) in shallow floodplain groundwater (< 2 m below ground surface) and observations of elevated mean nitrate concentrations (3.6 to 4.1 mg/L) in the relatively deep floodplain groundwater (≥ 2 m below ground surface). A clear relationship between groundwater flow paths and nitrate occurrences at sites with relatively narrow riparian floodplains was not obtained. Based on the observations and groundwater modeling results from this study, a lack of elevated nitrate may be observed in shallow groundwater (< 2 m below ground surface) within relatively extensive riparian floodplains of the Eastern Shore of Virginia due to a by-pass of groundwater beneath the riparian floodplain sediments.

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