Browsing by Author "David Genereux, Committee Chair"

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Carbon, Chloride, and Oxygen Isotopes as Tracers of Interbasin Groundwater Flow at La Selva Biological Station, Costa Rica
(2007-04-04) Webb, Mathew Douglas; David Genereux, Committee Chair; Neal Blair, Committee Member; John Fountain, Committee Member
Groundwater and surface water samples were taken at 14 locations at a lowland rainforest site (La Selva Biological Station) in Costa Rica for the analysis of DIC, DOC, 14C, 13C, 36Cl, 18O, and other geochemical parameters. The data are consistent with the mixing of two endmember groundwaters. One is a local water having low Cl concentrations (<0.07 mM), low DIC (<3.0 mM), high 14C (>100 pmc), δ13C between -22‰ and -26‰, and highly variable 36Cl⁄Cl ratios. This chemistry is consistent with locally recharged shallow groundwaters having short residence times in which the DIC originates from plant root respiration and atmospheric deposition is the only source of Cl. The other endmember is bedrock groundwater, representing interbasin groundwater flow (IGF) into La Selva and having relatively high Cl concentration (>0.9 mM), high DIC (about 14 mM), low 14C (<8 pmc), high δ13C (-3‰ to -5‰), and a low and more consistent 36Cl⁄Cl ratio. This chemistry is consistent with the expectations for bedrock groundwater recharged on the flanks of Volcan Barva to the north of La Selva, with a majority of the DIC and Cl derived from magmatic degassing and dissolution of the volcanic rocks that make up the aquifer. A 14C age of 750 — 4650 years before present was estimated for the bedrock groundwater endmember using NETPATH geochemical mass-balance modeling software, suggesting an average linear velocity of 3-20 m⁄yr for this groundwater; the actual age is probably closer to the upper limit, and velocity closer to the lower limit. The results of this study are consistent with prior work using major ion, 18O, and physical hydrologic data, suggesting that the conclusions about IGF and groundwater mixing at this site are correct. Also, new DIC data for bedrock groundwater and previous hydrologic data on bedrock groundwater inputs to the Arboleda watershed at La Selva suggest that IGF of bedrock groundwater is responsible for a large inorganic carbon flux into lowland watersheds (about 740 grams of carbon per m2 of watershed each year for the Arboleda).
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.
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 â€œcenter-highâ€ 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 â€œcenter-lowâ€ 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.
Spatial and Temporal Variability of Streambed Hydraulic Conductivity in West Bear Creek, NC
(2007-04-11) Leahy, Scott Thomas; David Genereux, Committee Chair; John Fountain, Committee Member; Aziz Amoozegar, Committee Member
Spatial and temporal variability of vertical hydraulic conductivity (KV) in a sandy streambed were evaluated from 487 field permeameter KV measurements taken during a 1-year study from December 2005 to December 2006. Bimonthly KV measurements were made at 46 locations (38 in December 2005) in a 262.5 m reach (the "large reach") of West Bear Creek in eastern North Carolina. More closely-spaced measurements were also made in two smaller (62.5 m) reaches within the large reach (Small Reach 1, measured in July 2006, and Small Reach 2, measured in August 2006). Vertical conductivity was calculated from field permeameter tests with data analysis based on Hvorslev (1951, case E, page 44). Results show significant spatial (transverse and longitudinal) and temporal variability in KV. Calculated arithmetic mean KV values for the large reach ranged from 3.85 to 21.33 m⁄day and the mean of the 7 arithmetic mean values (one for each bimonthly run) averaged 15.44 m⁄day. Overall, the range of streambed KV was 0.01 to 66.21 m⁄day. Variance in lnKV (s2lnKv) ranged from 1.88 to 4.18 for the large reach measurement runs and s2lnKv values measured from Small Reach 1 and Small Reach 2 were 1.88 and 6.04, respectively. Smaller s2lnKv values were calculated in the center of the stream channel, compared to values of s2lnKv for measurements taken from the left or right sides of the stream channel. The opposite is true for average KV values, where the highest averages come from the center measurement points. While measurement spacing in the small reach runs was about 4-5 times closer than in the large reach runs, results from the two types of reaches had similar results with respect to range of KV, s2lnKv, and distribution of KV in the stream channel. Correlation lengths calculated from exponential model curves fit to the experimental lnKV semi-variograms were 1.4 and 8.2 for Small Reach 1 and Small Reach 2, respectively. Correlation lengths from the small reaches bracket those calculated from the large reaches. Temporal changes in K are probably due to deposition and erosion of stream sediment, and possibly time-varying behavior of biofilms and/or gas bubbles in sediment, but not to changes in temperature. Contour maps of lnKV were created using three different interpolation techniques. Two of the three interpolation methods (radial basis function and kriging with a linear semi-variogram model) produced similar and fairly realistic looking maps. Large reach run maps showed a decrease in lnKV from the right bank to the left bank in some areas. However, overall the data show higher lnKV in the center of the channel. Small reach contour maps show higher lnKV in the center. Average uncertainty in KV is equal to about 16.2%. This was calculated by adding, in root mean square fashion, the uncertainty due to lack of knowledge of the transformation ratio "m" ( 5%) to each uncertainty value arising from propagation of uncertainty in the measurands, and then averaging these 430 uncertainty values. Percent uncertainty in KV is larger for smaller KV values since the slope of the linear regression needed to calculate KV is less accurately constrained from the field measurements of head with time when KV is low.