Browsing by Author "Michael R. Burchell II, Committee Member"

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Constructed Wetlands as Remediation Tools for Shallow Groundwater Contaminated by Swine Lagoon Seepage
(2005-02-04) Hathaway, Jon Michael; Robert O. Evans, Committee Chair; Michael R. Burchell II, Committee Member; Stephen W. Broome, Committee Member
Swine waste is typically flushed from beneath confinement houses into anaerobic lagoons for temporary storage and partial treatment. When improperly constructed, studies have shown that there is potential for the high strength wastewater to leak from the lagoon into the surrounding groundwater. Constructed wetlands have been implemented as treatment systems for wastewater. Biogeochemical reactions in wetlands make them viable options for wastewater treatment. Wetlands may remove pollutants such as nutrients through adsorption, nitrification, denitrification, and plant uptake. Studies have shown that constructed wetlands are able to attenuate substantial amounts of nitrogen and phosphorous from wastewater. Denitrification is the primary mechanism by which nitrogen is removed from wastewater in constructed wetlands. Research has shown that nitrification of ammonium nitrogen to nitrate may indirectly limit denitrification. One practice that may increase the nitrogen assimilation in a constructed wetland is the use of a nitrification pretreatment system such as a trickling filter. A trickling filter contains media on which nitrifying bacteria can attach, and provides an aerobic environment in which the nitrification process can take place. This study evaluated a site where a swine lagoon had leaked into the surrounding groundwater. The swine lagoon was eventually closed-out and a plan to pump out the contaminated groundwater was initiated. The goal was to pump at a rate that would change the subsurface gradient causing the contaminated water to stop flowing towards a nearby stream. The water was pumped into a constructed wetland where nutrients could be removed. After three years of pumping the contaminated groundwater, a trickling filter was implemented to nitrify the wastewater before it was discharged into the wetland. Over the course of the study, the average NH4-N concentration in wells located down gradient of the former lagoon decreased by more than 60%, and the hydraulic gradient between the former lagoon and the nearby stream was reduced by greater than 65%. The constructed wetland assimilated greater than 76% of the total nitrogen and more than 22% of the total phosphorous that it received, resulting in the assimilation greater than 915 kg of total nitrogen and more than 145 kg of total phosphorous. The nitrification pretreatment system converted 20% of the NH4-N it received to NO3-N on a mass basis. In the samples taken from the trickling filter, the arithmetic average amount of NH4-N that was converted to NO3-N was 37%. Although the nitrification pretreatment system was functional, there was no identified increase in total nitrogen assimilation within the constructed wetland. This may have been due to influent ammonium nitrogen concentrations becoming too low for the trickling filter to function at maximum efficiency by the time it was incorporated into the system.
Effects of Subsurface Flows on Wetland Restoration at Juniper Bay and Surrounding Area.
(2006-08-21) Pati, Swamy; Rodney L. Huffman, Committee Chair; R. Wayne Skaggs, Committee Member; Michael J. Vepraskas, Committee Member; Michael R. Burchell II, Committee Member; Gary T. Roberson, Committee Member
The North Carolina Department of Transportation purchased a 270-hectare, roughly elliptical tract of agricultural land, known as Juniper Bay (a Carolina Bay), to convert to wetlands as part of their wetlands mitigation program. Preliminary water balance work suggested that there are significant flows of groundwater entering and leaving the tract. This study was initiated to examine the subsurface potentials and determine the degree to which a ditch around the perimeter of the tract controls the lateral fluxes of groundwater in the surficial aquifer. Five nests of piezometers were installed along each of four 150-m transects crossing the perimeter ditch at approximately the major and minor axes of the tract, which correspond to the suspected maxima of influx and efflux. Deep soil cores (up to 13 m) were collected along each transect to guide placement of piezometers for monitoring hydraulic heads. Piezometer water levels were recorded at 15-minute intervals. Meteorological data were collected with an on-site weather station. Models were developed for the four transects using Visual MODFLOW. Models were calibrated with observed groundwater pressure heads. Maximum absolute error in the calibration process was 0.5 m. The modeling results suggested that the ditch drained water from the surficial system from both sides. In the deeper sand layers, there was an indication of groundwater flowing into the bay at NW and NE transects. Groundwater flows in the SW transect indicated outflows. The SE transects showed water draining into the ditch from both sides. The models were extended to 800-m inside the bay to simulate conditions after the interior ditch system was blocked. Simulation results showed groundwater inflows through the NW, NE, and SE transect, and groundwater outflows through SW transect. The lateral influence of the perimeter ditch had a maximum of approximately of 100 m, observed at the SW transect, and a minimum of 30 m, observed at the SE transect. The extent of influence of the perimeter ditch was also dependent on the weather conditions, showing more influence in summer months compared to winter months. Influence of the perimeter ditch was entirely in the upper sands at the NE and SE transects, but some influence was seen in the middle sand layers at the NW and SW transects. Groundwater flow estimates from the transects were extrapolated over the whole perimeter of Juniper Bay to obtain net groundwater inflow. Net groundwater inflow was approximately 125 mm for the time period of 1 January 2004 to 30 June 2004. To develop recommendations for maintaining the perimeter ditch, the models were run for various scenarios focused on water levels in the perimeter ditch. Control levels were imposed on the ditch and options were investigated. A water level of 35.9 m MSL was identified as a critical point of control of the perimeter ditch. Controlling the water level in the perimeter ditch at 35.9 m will minimize offsite impacts and result in maximum wetland area.
A North Carolina field study to evaluate the effect of a coastal stormwater wetland on water quality and quantity and nitrogen accumulation in five wetland plants in two constructed stormwater wetlands.
(2008-08-27) Lenhart, Hayes Austin; Dennis J. Werner, Committee Member; William F. Hunt III, Committee Chair; Michael R. Burchell II, Committee Member