HR: 1330h
AN: NB33F-01    [Abstracts]
TI: Influence of Hydrogeology on Nitrogen Transformations and Ground Water-Surface Water Interactions in a Coastal Plain Watershed
AU: * Tesoriero, A J
EM: tesorier@usgs.gov
AF: U.S. Geological Survey, 3916 Sunset Ridge Road, Raleigh, NC 27607 United States
AU: Spruill, T B
EM: tspruill@usgs.gov
AF: U.S. Geological Survey, 3916 Sunset Ridge Road, Raleigh, NC 27607 United States
AU: Mew, H E
EM: ted.mew@ncmail.net
AF: North Carolina Department of Environment and Natural Resources, 1636 Mail Service Center, Raleigh, NC 27699 United States
AU: Farrell, K M
EM: kathleen.farrell@nc.net
AF: North Carolina Geological Survey, 1620 Mail Service Center, Raleigh, NC 27699 United States
AB: Nitrogen transport and ground water-surface water interactions were examined in both first- and third-order streams in the Contentnea Creek basin, a coastal plain watershed in the southeastern United States. Nitrogen transport and transformations in the ground-water system were determined by analyzing ground water along 1- to 2-km flow paths for nitrogen and other redox-active species and by using age-dating indicators. Nitrate was typically found only in recently recharged water in the upper few meters of saturated thickness in the upland portion of an unconfined surficial aquifer. Ground water with a residence time between 10 and 30 years typically had low nitrate concentrations and N₂ concentrations in excess of that expected from atmospheric sources, indications that denitrification has reduced nitrate concentrations. Ground water older than 30 years also had low nitrate concentrations but contained little or no excess N₂, suggesting that this water never contained elevated concentrations of nitrate along its flow path. Nitrate transport to the first-order stream differed from transport to third-order streams. Flood-plain geomorphology and the presence or absence of a confining unit at shallow depth affected the distribution of oxic conditions and residence times of ground water in the near-stream environment. Beneath the third-order stream and its broad alluvial valley, an areally extensive shallow confining unit has been removed by stream erosion. This process has created a thick, unconfined alluvial aquifer with oxic conditions and nitrate stability extending to greater depths. Age-dating and chemical information suggest that water in the alluvial aquifer is derived from short flow paths through the riparian zone and (or) from adjacent streams during high surface-water discharge periods when a hydrologic gradient reversal occurs. Pore waters from the bed and banks of these streams are strongly reducing (e.g., iron-reducing, methanogenic). As a result, nitrate in water that moves from streams to ground water during high-flow periods may be denitrified prior to discharging back to streams when flows recede. In contrast to the third-order stream, the first-order stream is steeply incised into the landscape, lacks a broad alluvial valley and is underlain by the areally extensive confining unit. These factors result in very little bank storage as indicated by older ground water adjacent to this stream than in the third-order streams. Redox conditions suggest that direct ground-water discharge of nitrate to this stream is unlikely; rather, artificial drainages (e.g., tile drains) and storm driven flow are indicated as major sources of nitrogen.
DE: 1045 Low-temperature geochemistry
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly