Browsing by Author "Kennedy, Casey David"

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    Pilot project on groundwater dating in confined aquifers of the North Carolina Coastal Plain
    (2004-04-21) Kennedy, Casey David; John Fountain, Committee Member; David DeMaster, Committee Member; David Genereux, Committee Chair
    This pilot project presents 14C groundwater ages in the Black Creek and Upper Cape Fear aquifers of the Coastal Plain of North Carolina, an evaluation of the relationship between He concentration and groundwater age, and 3H concentrations in groundwater. Groundwater samples were collected with a Bennett pump from 7 wells that lie along a trend roughly parallel to groundwater flow (at least, predevelopment groundwater flow). 14C, 13C, DIC, DOC, He, Ne, Ar, N2, O2, CO2, CH4, H2, 3H, S, Fe, Al, Mn, Si, Na+, K+, Ca2+, Mg2+, Cl-, SO4-, and NO3- were measured in all samples. Estimation of groundwater age involved inverse mass balance modeling with NETPATH to account for geochemical reactions (calcite dissolution, organic matter oxidation, and cation exchange) affecting 14C activity in groundwater, as well as a separate correction to account for loss of 14C by diffusion into contiguous aquitards. 14C groundwater ages were 580, 10700, 19100, and greater than 35300 years old at four wells in the Black Creek aquifer, and 15100, 26900, and 31100 years old at three wells in the Upper Cape Fear aquifer. These groundwater ages, together with falling heads, suggest that groundwater withdrawals in these aquifers represent a sort of 'mining.' He concentration in groundwater increased with 14C groundwater age with one exception (a sample very high in He concentration from the Upper Cape Fear where it directly overlies crystalline basement rocks). Groundwater from 6 of the wells had 3H concentrations that are consistent with the presence of young water, but it is uncertain whether the 3H in these wells is from relict drilling fluid, downward leakage along the well casings, or a more broadly distributed downward leakage.
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    Quantifying rates, controls, and spatiotemporal dynamics of water and nitrogen fluxes through the streambed of West Bear Creek, North Carolina, USA
    (2008-12-08) Kennedy, Casey David; Reide Corbett, Committee Member; David DeMaster, Committee Member; Helena Mitasova, Committee Member; David Genereux, Committee Chair
    This paper presents results on the rates and spatiotemporal dynamics of the coupled water flux (v) and nitrogen (N) fluxes (mainly fNO3 and fDON for nitrate and dissolved organic N) through a streambed in an agricultural watershed in North Carolina. Physical and chemical variables were measured at numerous points in the streambed of a 0.26 km reach: hydraulic conductivity (K) and head gradient (J), and concentrations of NO3- and other N species in the streambed groundwater, from which water (v=KJ) and N fluxes (e.g., fNO3=v[NO3-]) through the streambed were computed, mapped, and integrated in space. The result was a novel set of streambed maps of the linked variables (K, J, v, N concentrations and fluxes), showing their spatial variability and how it varied over a year (based on 7 bimonthly sets of maps). Mean fNO3 during the study year was 154 mmol m-2 day-1; this NO3- flux, together with that of DON (fDON = 17 mmol m-2 day-1) accounted for >99% of the total dissolved N flux through the streambed. Repeat measurements at the same locations on the streambed show significant temporal variability in fNO3, largely controlled by changes in v rather than changes in [NO3-]. One of the clearest and most temporally-persistent aspects of spatial variability was lateral variability across the channel from bank to bank. K and v had “center-high†patterns (greater values in the center of the channel); this distribution of K (ultimately a reflection of sediment dynamics in the channel) apparently focuses groundwater discharge toward the center of the channel. The opposite “center-low†pattern was found for J, [NO3-], and (to a lesser extent) fNO3. Contrary to suggestions in some prior work, J was not a good index for v. fNO3 was characterized by localized zones of high and low values that changed in size and shape over time but remained in basically the same locations (the same was true of K, J, [NO3-], though less so for v), with 70% of NO3- flux occurring through about 38% of the streambed area. Lateral distributions of the physical hydrologic attributes (K, J, v) were highly symmetrical across the channel, while those of [NO3-] and fNO3 showed higher values on the right than left, likely a reflection of different N use on opposite sides of the stream. These and other results show the streambed-based approach taken here can offer a number of insights not possible with reach mass-balance approaches in which net exchange between a stream reach and surrounding groundwater is calculated using surface water data.