Browsing by Author "Rodney L. Huffman, Committee Member"

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Application of Ground-Penetrating Radar to Map Stratigraphy of a Drained Carolina Bay and Aid Its Wetland Restoration
(2005-01-14) Szuch, Ryan Paul; C. W. Zanner, Committee Member; Rodney L. Huffman, Committee Member; Michael J. Vepraskas, Committee Co-Chair; Jeffrey G. White, Committee Co-Chair
Carolina Bays (bays) are a wetland type found along the Atlantic Coastal Plain that occur as oval-shaped depressions. Knowledge of bay stratigraphy might improve inconclusive theories on bay formation and aid attempts at wetland restoration of drained bays. Ground-penetrating radar (GPR) provides high-resolution and continuous profiles of the subsurface but has seldom been used for large-scale investigations or in Carolina Bays. A GPR survey was performed at Juniper Bay, a 300 ha drained bay in Robeson County, North Carolina. The survey included 23.2 km of GPR transects and soil borings at 174 locations. The broad objective of the survey was to map Juniper Bay's stratigraphy, particularly to determine the depth, extent, and continuity of clayey horizons likely to act as aquitards. To prevent ambiguity in identifying the ground surface on GPR transects, a 'lift-test' was developed that delineated the surface. Spatial variation in wave velocity was addressed using a "reflector-interface matching" technique to determine velocity at multiple locations and at various depths within the bay. A linear calibration equation relating travel time of GPR waves to depth of soil interface was developed. The average deviation between observed and predicted depth to clayey horizons was 0.25 m (16% error). Error was mainly attributed to the survey's large scale, subsurface complexity, presence of organic soils, and depth of the horizons. The lift-test improved accuracy by 10%, and the use of multiple calibration points limited extrapolation of the calibration equation. These calibration methods should prove valuable for future study of GPR accuracy on large-scale, complex sites. Information obtained during the GPR survey and associated coring was used to describe Juniper Bay's stratigraphy and develop theories on its formation. The bay's stratigraphy consists of alternating layers of sands and clays. Clayey layers appear continuous over much of the bay except where truncated by features that seem to be paleochannels. Historic geomorphic events at Juniper Bay have varied spatially and temporally but have included repeated lacustrine deposition, fluvial deposition, and fluvial incision. The original bay probably expanded and incorporated a smaller bay and fluvial feature. Future GPR work and integration of geologic and hydrologic studies may aid our assessment of Juniper Bay's stratigraphy and evolution. Many bays have been drained for conversion to agriculture. Clayey subsurface strata commonly occurs in bays and act as aquitards, restricting vertical water flow. Modification to drainage systems could lead to restoration of wetland conditions. The North Carolina Department of Transportation (NCDOT) intends to restore Juniper Bay for wetland mitigation credit. Ground-penetrating radar interpretation found that clayey aquitards underlay most of the bay at an average depth of 1.64 m. An anomalous GPR reflection in the southeast corner of the bay was interpreted as a fluvial deposit that does not contain aquitards until 3 to 5 m. NCDOT should consider alternative restoration plans for this area. By comparing the depths of aquitards and drainage ditches, several areas were identified as likely locations of ditch-induced aquitard discontinuity. NCDOT should fill or line suspect ditches to prevent potential water losses. Hypothetical proposals by professional firms indicated that GPR could provide large volumes of data with cost and time efficiency. Ground-penetrating radar surveys are suggested as a useful tool for determining suitability of potential wetland restoration sites.
Estimating Nitrogen Efficiency of Swine Lagoon Liquid Applied to Field Crops Using Continuously Variable Irrigation
(2002-08-19) Cobb, Chester Ray; Robert L. Mikkelsen, Committee Chair; Rodney L. Huffman, Committee Member; Larry D. King, Committee Member
Application of anaerobic swine (Sus scrofa domesticus) lagoon liquid onto cropland by irrigation is a common method of waste disposal and treatment. Currently, the application rate of swine lagoon liquid is based on the N concentration of the lagoon liquid and the N required by the receiver crop to obtain a realistic yield. In North Carolina, only 50% of the total N in the swine lagoon liquid applied by irrigation is considered available for plant use during the first year after application. Uncertainty exists as to whether this coefficient accurately predicts the amount of plant-available N. Therefore, research was conducted in the Coastal Plain of North Carolina to determine the efficiency of N uptake by corn (Zea mays L.) and soybean (Glycine max Merrill) receiving swine lagoon liquid through irrigation. The line-source sprinkler irrigation method was used to provide a continuous variable N rate, ranging from 0 to 290 kg N/ha, across the field during 1999 and 2000. Ammonia volatilization losses r anged from 6 to 22% during irrigation. Crop yield and grain N recovered were affected more by the amount of liquid than N applied in 1999. Nitrogen recovered in grain in 1999 was <15% for both corn and soybean at 168 kg N/ha of either swine lagoon liquid or ammonium nitrate. In 2000 at the 168 kg N/ha rate, grain N removal by corn, nonnodulating soybean, and nodulating soybean was 28, 25, and 39% from swine lagoon liquid and 45, 31, and 56% from ammonium nitrate. Based on yields and grain N removed by corn and nonnodulating soybean in 2000, N from applied swine lagoon liquid, accounting for N losses during irrigation, was about 70% as effective as ammonium nitrate. Symbiotic N2 fixation by the soybean was reduced by 60% when applied N reached 175 kg N/ha for both ammonium nitrate and swine lagoon liquid. While nodulating soybean removed more grain N than did either corn or nonnodulating soybean in 2000, soil inorganic N concentrations at the end of the growing season were higher for the nodulating s oybean. Therefore, it is not conclusive if soybean would be a better receiver crop than corn for swine lagoon liquid. Based on the results of this study, using the 50% available N coefficient of the lagoon liquid comes close to predicting plant-availabl e N when N losses during irrigation are around 25%. Nitrogen losses during irrigation can significantly affect plant-available N when applied N is based on the N concentrations of the lagoon liquid.
Evaluation of Evapotranspiration-based and Soil-Moisture-based Irrigation Control in Turf
(2008-01-29) Vasanth, Arjun; Rodney L. Huffman, Committee Member; Aziz Amoozegar, Committee Member; Grady L. Miller, Committee Member; Daniel C. Bowman, Committee Member; Garry L. Grabow, Committee Chair
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.