Phytoremediation of a Petroleum-Hydrocarbon Contaminated Shallow Aquifer, Elizabeth City, NC: Planting Methods and Preliminary Results

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Title: Phytoremediation of a Petroleum-Hydrocarbon Contaminated Shallow Aquifer, Elizabeth City, NC: Planting Methods and Preliminary Results
Author: Cook, Rachel Louise
Advisors: Chris Hofelt, Committee Member
Ted Shear, Committee Member
Elizabeth Guthrie Nichols, Committee Chair
Abstract: The US Coast Guard Support Center former fuel facility is a phytoremediation demonstration site for the US EPA and North Carolina Department of Environmental and Natural Resource’s 319 Program. The primary project goal is to prevent petroleum contamination in soil and ground water from entering the adjacent Pasquotank River. Gasoline, diesel fuel, and jet fuel released from aboveground and underground storage tanks lie 1.2-2.1 meters below land surface. A free product recovery system, operated since 1991, was replaced beginning in 2006 with a phytoremediation system. Over 3,000 bare root or unrooted cuttings of hybrid poplars (Populus spp.) and willows (Salix spp.) were planted across the five acre site. Here we report the effect of three different planting methods on tree growth and mortality from 2006 to 2008. Method 1 in April and June 2006 used a direct-push rig to auger 8 cm diameter, 1.2 m holes that were backfilled with the excavated, in situ soil; Method 2 in 2007 and 2008 used a Bobcat rig to auger 23 cm diameter, 1.2 m deep holes that were backfilled with clean offsite topsoil; and Method 3 in 2007 used a 1.3 cm diameter rod to punch shallow holes between 15 to 30 cm deep with no backfill. Plant mortality was determined for each method after each growing season. In early 2008, total stem length was measured for all planted trees (n=2,984). This information was incorporated with global position system (GPS) locations into a geographic information system (GIS) for analysis and monitoring. Trees planted using Method 1 in June 2006 experienced higher percent mortality and did not grow as well as trees planted using Method 1 in April 2006. Trees planted in April 2007 using Method 2 demonstrated better survival and growth than trees planted using Method 3. Some differences in mortality and growth were observed between the four different hybrid poplar clones planted with Method 2. Overall, planting early in the growing season, augering a larger diameter hole, and backfilling with uncontaminated offsite topsoil allowed for greater survival and growth. Preliminary results regarding concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX) and methyl tert-butyl ether (MTBE) in ground water and masses of total petroleum hydrocarbons (TPH), BTEX, naphthalene, and other constituents in soil gas indicate dissolved contaminants have slowed their migration towards the river and show significantly decreased levels after planting. The effect of planted trees on residual petroleum contaminants in the vadose zone will continue to be assessed over time by monitoring soils for all 42 alkylated and non-alkylated polycyclic aromatic hydrocarbons (PAHs). Specific ratios between select alkylated PAHs will be used to continue to monitor PAH-contaminant weathering. Ground water wells, soil-gas wells, and soil collection locations as well as all planted trees were spatially referenced using GPS. Once integrated into GIS, we used interpolated values between contaminant sampling points to evaluate interactions between trees and subsurface hydrocarbon contamination. Several patterns are evident in the spatial analysis. Areas with the greatest subsurface contamination appear to have smaller trees and higher mortality, while cleaner areas have distinctly taller trees and greater survival. This observation is reinforced by comparison of soil total polycyclic aromatic hydrocarbon concentrations and tree total stem length. However, field conditions complicate correlations between contaminants and tree performance likely due to such variables as depth to water table, soil heterogeneity, tree viability, and differing planting methods.
Date: 2008-11-24
Degree: MS
Discipline: Natural Resources

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