HR: 17:00h
AN: H24F-05 [Abstracts]
TI: Spatial and Temporal Variability in Groundwater-Based Nitrogen Input to a Stream in an Agricultural Watershed in North Carolina
AU: * Kennedy, C D
EM: cdkenned@ncsu.edu
AF: Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box
8208, Raleigh, NC, Raleigh, NC 27695-8208, United States
AU: Genereux, D P
EM: genereux@ncsu.edu
AF: Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box
8208, Raleigh, NC, Raleigh, NC 27695-8208, United States
AU: Corbett, D R
EM: CORBETTD@ecu.edu
AF: Department of Geological Sciences, Institute for Interdisciplinary Coastal Science and
Policy, East Carolina University, Greenville, NC 27858-4353, United States
AU: Mitasova, H
EM: hmitaso@unity.ncsu.edu
AF: Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box
8208, Raleigh, NC, Raleigh, NC 27695-8208, United States
AU: Elkins, J B
EM: JBE1130@ECU.EDU
AF: Department of Geological Sciences, Institute for Interdisciplinary Coastal Science and
Policy, East Carolina University, Greenville, NC 27858-4353, United States
AU: Leahy, S T
EM: stleahy@yahoo.com
AF: Marine, Earth, and Atmospheric Sciences, North Carolina State University, Campus Box
8208, Raleigh, NC, Raleigh, NC 27695-8208, United States
AB:
We used point measurements of streambed hydraulic head gradient (J), hydraulic conductivity (K), and nitrogen
(N) concentrations to quantify groundwater-based N input to a large stream in an agricultural watershed. We
focused on fluxes across the sediment-water interface by making all field measurements directly in the
streambed (J and K apply to the top 36 cm, water samples for N measurement were drawn from a depth of 34
cm). Measurements were made at 38 points (Dec 2005) or 46 points (Feb, Apr, Jun, Aug, Oct, and Dec 2006) over
a 263 m reach of West Bear Creek in the North Carolina Coastal Plain. Point values of groundwater flux (v) and
total dissolved N flux (fTDN, mostly in the form of NO3-) were highly variable, with v ranging from -
0.80 to 3.1 m day-1 (mean v = 0.44 m day-1) and fTDN from -0.17 to 2.6 mol m-2 day-1
(mean fTDN = 0.18 mol m-2 day-1). On average, higher K and v, and lower groundwater
concentration and fTDN, were measured in the center of the channel. Values of fTDN were about 10x
larger than reported for uncontaminated forest streams and other agricultural streams. All point measurements
showed groundwater seepage into the stream, except for the 2 or 3 points on the upstream side of a beaver dam
that was present from Dec 2005-April 2006. Only 8 of 45 NO3- samples collected within 37 m
downstream of the dam had NO3- concentrations above the detection level of 0.007 mM. This area of the
streambed was likely a discharge zone for stream water that had infiltrated the streambed upstream of the dam,
suggesting that the beaver dam may have contributed to NO3- loss. Analysis of dissolved gases (Ar,
N2, CO2, CH4, O2) in streambed groundwater indicated significant lateral variability in redox
potential and excess N2 (likely from denitrification), and suggests a difference in average groundwater
recharge temperature between groundwater discharging into the left and right sides of the channel.
Water and N fluxes through the ~1800 m2 streambed were spatial integrated from a 10-cm resolution
grid interpolated from point measurements using the multiquadratic radial basis function method. Groundwater-
based input of TDN to the reach (QTDN) was 186, 251, 248, 188, 368, 637, and 109 mol day-1 in Dec
(2005), Feb, Apr, Jun, Aug, Oct, and Dec (2006) respectively, the majority of which was in the form of NO3-,
with little contribution from dissolved organic N (25, 29, 5, 3, 55, 51, 13 mol day-1 in Dec 2005, Feb, Apr, Jun,
Aug, Oct, and Dec 2006, respectively) and ammonium (<5 mol day-1). Changes in groundwater seepage
explained much of the temporal variability in QTDN (r2=0.80) and QNO3-, the groundwater-based
input of NO3- to the reach (r2=0.79). This relationship indicates strong control of these streambed N
fluxes, at the scale of a large reach, by the groundwater flux through the streambed (even though some of the
points of largest NO3- flux had among the lowest groundwater seepage rates but high NO3-
concentrations). At the reach scale, groundwater seepage may be a reasonable predictor of QTDN and
QNO3-. About 30% of the streambed accounted for 56% to 70% of QNO3- within the reach.
DE: 1830 Groundwater/surface water interaction
DE: 1831 Groundwater quality
DE: 1860 Streamflow
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2007 Fall Meeting