Evaluation of subsurface solute transport and its contribution to nutrient load in the drainage ditches prior to restoration of a Carolina Bay

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Title: Evaluation of subsurface solute transport and its contribution to nutrient load in the drainage ditches prior to restoration of a Carolina Bay
Author: Abit, Sergio Manacpo, Jr.
Advisors: Michael J. Vepraskas, Committee Co-Chair
Rodney L. Huffman, Committee Member
Aziz Amoozegar, Committee Co-Chair
Abstract: 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.
Date: 2006-02-16
Degree: MS
Discipline: Soil Science
URI: http://www.lib.ncsu.edu/resolver/1840.16/2312

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