Browsing by Author "Aziz Amoozegar, Committee Co-Chair"

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Evaluation of subsurface solute transport and its contribution to nutrient load in the drainage ditches prior to restoration of a Carolina Bay
(2006-02-16) Abit, Sergio Manacpo, Jr.; Michael J. Vepraskas, Committee Co-Chair; Rodney L. Huffman, Committee Member; Aziz Amoozegar, Committee Co-Chair
Subsurface solute transport is a major mechanism that contributes to the contaminant load in both surface and ground waters. Among these contaminants are plant nutrients that if transported in excessive amounts to surface waters can cause adverse effects on humans and animals, as well as negative impacts on aquatic life. The general objective of this study was to conduct a field evaluation of subsurface solute transport in the capillary fringe (CF) and shallow ground water (SGW) and their contribution to nutrient load in the ditches prior to restoration of a Carolina Bay. Specifically, this study was aimed at evaluating: a) the horizontal flow of bromide (Br-) in the CF and SGW under field conditions, b) the fate of nitrate (NO3-) in the CF and SGW in a sandy field site drained by ditches, and c) the possible contribution of subsurface flow to the increased nutrient load in drainage ditches at a drained Carolina Bay following storm events. The study was conducted in Juniper Bay, a drained Carolina Bay in Robeson County, NC. A solute transport experiment was conducted at a sandy site in the Bay where a solution containing Br- and NO3- was applied into an auger hole dug to about 10 cm above the CF during the time of application. The transport of Br- and NO3- in the CF and SGW was monitored by frequently collecting soil water samples using tension lysimeters installed at depths of 45, 60, 75, 90 and 105 cm at lateral distances of 20, 60, 120, 220 and 320 cm from the auger hole along the general direction of the ground water flow. A representative monitoring site from each of the Bay's mineral and organic soil areas was also chosen for a year-long monitoring of fluctuations in nutrient concentrations in water samples from the Bay's main ditch exit as well as from the vadose zone, ground water and lateral ditches. Soil solution from the vadose zone and ground water samples were collected using tension lysimeters installed at 15-cm depth intervals from 15 to 120, and 30 to 180 cm depths at the mineral and organic soil sites, respectively. Ground water samples were collected from three fully perforated wells. Seven piezometers installed at each site also allowed collection of ground water samples from different depth intervals below the water table The direction and magnitude of the subsurface hydraulic gradient at the monitored sites were also determined using the three-point technique. Lateral transport of Br- in the CF was observed in the direction of ground water movement up to 320 cm from the auger hole where solutes were applied. The Br- plume from the unsaturated zone that entered into the CF tended to stay and move horizontally in the CF until it was partially moved into the ground water by the fluctuating WT following rain events. The normalized concentrations (concentration in soil solution/concentration in the applied solution) of both NO3- and Br- in water samples collected from CF were comparable for all distances from the application spot. However, in the groundwater, the normalized concentration of NO3- was substantially lower than the normalized Br- concentrations. We believe the reduction in NO3- concentration in the ground water was due to denitrification. Results from the nutrient monitoring experiment reveal that the sample taken from the main ditch exit following a 5 cm d-1 storm event had higher concentrations of total organic carbon (TOC), phosphates (PO4-P), calcium (Ca) and magnesium (Mg) compared to the average of samples collected during baseflow conditions. The same was also observed for samples collected from the vadose zone especially at depths closer to the soil surface where organic carbon and extractable Ca, Mg and PO4-P contents were higher. Higher concentrations of these solutes in the ditches and vadose zone coincided with observed increase in the magnitude of the groundwater hydraulic gradient. In addition, it was observed that following the storm events, the direction of the ground water hydraulic gradient tended to become more perpendicular to the nearby lateral ditch suggesting that the route taken by the water as it moves in the subsurface towards the ditch is shortened. We believe that the increase in concentration of PO4-P, Ca, Mg and TOC in the soil solution at certain depths in the soil profile coupled with their higher rate of movement in the subsurface towards the ditch following the storm event should have contributed to the increase in concentration of such nutrients in the ditches.
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.
Microbial Fate and Transport in a Seasonally Saturated North Carolina Coastal Plain Soil.
(2008-07-24) Stall, Christopher James; Alexandria Graves, Committee Member; David L. Lindbo, Committee Co-Chair; Diana Rashash, Committee Member; Aziz Amoozegar, Committee Co-Chair