Browsing by Author "Owen Duckworth, Committee Member"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • No Thumbnail Available
    Hydrologic Effects on Subsurface Fates and Transport of Contaminants
    (2009-06-19) Abit, Sergio Manacpo, Jr.; Aziz Amoozegar, Committee Co-Chair; Michael Vepraskas, Committee Co-Chair; Wei Shi, Committee Member; Owen Duckworth, Committee Member; William Showers, Committee Member
    Concerns over contamination of ground water (GW) and its subsequent effect on surface water quality underscore the need for an improved understanding of the fate and transport of the contaminants in the subsurface. Among the contaminants that are harmful to humans and the environment are nutrient pollutants [e.g., nitrogen (N) and phosphorus (P)] and microbes. The general goal of this research was to evaluate the subsurface fates and transport of contaminants in a vadose zone-GW continuum under various simulated hydrologic conditions through a series of laboratory-scale studies. The first study, which aimed to visually evaluate the effects of GW velocity and water table (WT) fluctuation on the fate and extent of horizontal transport of solutes and microbes in the capillary fringe (CF) and GW, was conducted in a sand-packed flow cell. Subsurface transport of surface-applied solutes and microbes tended to be isolated in the CF at a higher pore-water velocity. A rise in WT resulting from surface recharge of contaminated water occurred without the contaminants reaching the GW. Subsequent drainage did not effectively leach contaminants that were initially in the CF into the GW. The second study assessed the effect of pore-water velocity on the development of reduced conditions in a vadose zone-GW continuum. Reduction potential (Eh) was monitored at various locations in flow cells packed with Ponzer (Terric Haplosaprists), Lynchburg (Aeric Paleaquult), and Leon (Aeric Alaquod) soil materials that were subjected to different lateral pore-water velocities. Regardless of organic carbon (OC) content of the soil materials (12.4 to 195 g kg-1), locations close to the WT became reduced within 14 days. In contrast, the upper portions of the CF remained oxic. Increasing the pore-water velocity also slowed the development of reducing conditions especially in soils with low OC content. The third study was conducted to evaluate the effect of pore-water velocity on the fate and transport of nitrate (NO3-) in a simulated vadose zone-GW continuum. This was conducted in flow cells packed with soils of various OC content (0.3 to 35 g kg-1) that were subjected to different horizontal-water velocities. Nitrate and bromide (Br) concentrations as well as Eh at various locations along the flow path of an applied NO3- and Br- solution were monitored. Results show that in the presence of sufficient OC, NO3- was lost under reducing conditions below the WT but persisted while in transport in aerobic regions in the CF. Increasing GW flow pore-water velocity from 3.5 to 28 cm d-1 reduced the degree of NO3- removal from solution. High flow velocity also tended to limit the horizontal transport of surface-applied NO3- only in the upper regions of the CF. The fourth study was conducted to evaluate the dissolution of phosphorus (P) in pore-water flowing through the vadose zone-GW continuum. Distilled water was allowed to flow horizontally at different pore-water velocities through flow cells packed with an organic soil material (from Ponzer series). Extensive P dissolution was detected below and just above the WT. Phosphorus dissolution at the upper portion of the CF was relatively limited. These results suggest the following: a) the non-detection of contaminants below the WT down-gradient from a source does not definitively indicate that contaminants are not being transported horizontally in the subsurface as they can be transported in the CF, b) collection of samples from the CF should be considered when monitoring the subsurface transport of contaminants, and c) the hydrology of a system could be managed to improve nitrate removal from solution or to limit P dissolution.
  • No Thumbnail Available
    Riparian Buffer Effectiveness at Removal of NO3-N from Groundwater in the Middle Coastal Plain of North Carolina
    (2009-12-04) Knies, Sara Victoria; Deanna Osmond, Committee Chair; Owen Duckworth, Committee Member; Michael Burchell, Committee Member
    Non-point source pollution from agriculture is one of the causes of surface water quality degradation in the Coastal Plain of North Carolina. Riparian buffers are an important best management practice for reducing NO3 concentrations in natural waters, predominantly by vegetation uptake and denitrification. However, there continues to be debate over the optimal design of buffers, specifically buffer width, and vegetation type. This project was designed to investigate the effects of vegetation type, groundwater depth, and buffer width on NO3 removal from groundwater. Four buffers have been established at a research farm in the Middle Coastal Plain of North Carolina to investigate these factors; individual buffers are comprised of five vegetation types, two buffer widths, and two well depths. The influence of vegetation type on NO3-N groundwater decreases were as follows: revegetation had a decrease of 14% (5.75 mg N/L to 4.97 mg N/L); switchgrass had a decrease of 40% (9.19 mg N/L to 5.48 mg N/L); trees had a decrease of 32% (9.18 mg N/L to 6.20 mg N/L); native vegetation had a decrease of 35% (8.36 mg N/L to 5.41 mg N/L); fescue had a decrease of 23% (7.34 mg N/L to 5.67 mg N/L); the control had a decrease of 0% (5.85 mg N/L to 5.86 mg N/L). Influence of width and depth on NO3-N decreases were as follows: deep wells in 15 m buffers had a NO3-N decrease of 77% (5.76 mg N/L to 1.34 mg N/L), deep wells in 8 m buffers had a decrease of 53% (4.55 mg N/L to 2.13 mg N/L), intermediate wells in 15 m buffers had a decrease of 47% (7.51 mg N/L to 4.00 mg N/L), and intermediate wells in 8 m buffers had a decrease of 14% (8.38 mg N/L to 7.19 mg N/L) There was a significant three-way interaction (p = 0.001) between vegetation type, buffer width, and well depth. This interaction was desegregated by depth: at the deep depth, the effect of switchgrass was significant (p=0.0120) in removal of NO3-N in both the narrow and wide buffer widths. The effect of the revegetation treatment was significant (p=0.0093) at removal of NO3-N in the narrow width. The ratio of NO3-N/Cl was evaluated to determine if dilution of groundwater was responsible for observed NO3-N concentration decreases. Dilution was slight and did not significantly account for any observed NO3-N decreases. Reduction potential (Eh) values indicated reducing conditions at the deep well depth in three of the four buffers, suggesting denitrification was most likely responsible for observed NO3-N decreases in groundwater. Inhibition of denitrification rates could be occurring in buffers due to low levels of organic C (≈3.4 ± 0.6 mg C/L). To test this hypothesis, a laboratory study was designed to complement the field study. Flow-thru soil columns were constructed to determine the effect of dissolved organic carbon (DOC) concentration on denitrification rates and products in buffer soils. Three DOC concentrations (2.0 mg DOC/L, 4.0 mg DOC/L, 8.0 mg DOC/L, and 16.0 mg DOC/L) and a control (0.0 mg DOC/L) were utilized to study this relationship between DOC and denitrification. There was no trend between DOC concentration and rate of NO3-N loss. DOC concentrations > 4.0 mg DOC/L increased up until 12.0 mg DOC/L, after which rates leveled off. There was a linear relationship between DOC concentration and rate of N2O-N production with the exception of 12.0 mg DOC/L, with the rate of N2O-N production increased with increasing concentrations of DOC.