CASE STUDY – PETROLEUM
Project Name: US Coast Guard Former Fuel Storage Facility (Guthrie Nichols et al., 2014) (Cook et al 2010)
Location: Elizabeth City, NC
Scientists: Elizabeth Guthrie Nichols (a); Rachel L. Cook (a); James E. Landmeyer (b); Brad Atkinson (c); Jean-Pierre Messier (d)
Institutions: (a) Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC
(b)U. S. Geological Survey, S.C. Water Science Center, Columbia, SC
(c) North Carolina Department of Environment and Natural Resources-Division of Waste Management, Raleigh, NC
(d) U. S. Coast Guard, Elizabeth City, NC
Date installed: 2006-2007
Number of trees/ species installed:
- Hybrid Poplar – Populus deltoides Bartram ex Marsh. x nigra Clones OP-367, DN-34, 15-29 and 49-177
- Willow – Salix nigra ‘Marsh’., Salix interior ‘Rowle’s, and Salix exigua ‘Nutt’.
- Loblolly Pine – Pinus taeda
2006: 112 bare root Poplars- 1.2 meter (4 foot) height, and 403 unrooted Poplar and Willow cuttings were installed. Plantings were placed in 8 centimeter (3 inch) diameter boreholes, 1.2 meters (4 feet) deep and backfilled with un-amended onsite soil. Trees were mulched. All plantings were spaced 3 meters (10 feet) apart. Mortality averaged 28% because the contaminant concentration was too high for plant survival in some areas.
2007: 2,176 new trees (2,123 poplars, 43 Willows, and 10 trial Loblolly Pines) were planted in 23 centimeter (9 inch) diameter boreholes, 1.2 meters (4 feet) deep. Boreholes were backfilled with clean topsoil from off site. Trees were mulched. All plantings were spaced 2 meters (6.5 feet) apart. Mortality decreased and averaged 13% because the borehole width was increased and backfilled with clean soil to aid in plant establishment.
2007: 65 Poplars and 208 Willow were planted using a dibble tool to create a 15-30 centimeter (6-12 inch) deep holes just wide enough for each cutting. No backfill was used. All plantings used a 2 meter (6.5 foot) spacing. Mortality averaged 89% because soil was too contaminated for establishment.
Contaminant: Petroleum compounds: TPH, BTEX, MTBE, PAH. Estimates from monitoring wells show 567,000–756,000 liters (150,000–200,000 U.S. gallons) of gasoline, diesel, and aviation jet fuel at the site in the groundwater. Up to 85 centimeters (33 inches) of petroleum product was floating on top of the water table at the site.
Target Media & Depth: Soil and groundwater with depth ranging from 0.9-1.2 meters (3-4 feet) below ground surface. The ground water table fluctuates from 1.2-2.7 meters (4 to 9 feet) below surface except after major precipitation events.
US Coast Guard Site Photographs
A 5 acre former US Coast Guard Fuel Farm leaked large amounts of fuel since the Second World War and it was migrating towards the Pasquotank River, 150 meters (490 feet) from the site. A viable tree community (2,984 trees) was established to prevent further ground water discharge of fuel to the nearby river. Coastal plain surface waters are particularly vulnerable to leaking underground storage tank (LUSTs) contamination due to shallow water tables, porous soils, and proximity of LUSTs to natural surface water bodies.
A phytoremediation system was installed from 2006 to 2008 (Cook et al., 2010) using Salix, Pinus taeda, and four hybrid poplar clones (Populus). During the tree installation process (April 2006 until April 2008), hydrocarbon contaminants were monitored using ground water samples and soil gas analyses. Tree plantings began in 2006, but high contamination concentrations on part of the site resulted in a tree mortality of 26%. Fuel contamination on the site was very well characterized and documented, but ground water data did not indicate the high levels of contamination found once the trees were planted. In the next round of planting in 2007, trees were backfilled with clean soil and survival rates were much higher. Within 2 years, soil gas analyses of total petroleum hydrocarbons and BTEX in soil showed a reduction of 95% and 99%, respectively. For the contaminants in the ground water, as of 2013, only one monitoring well remained above regulatory NCAC 2L standards for benzene, but three wells were above MTBE NCAC 2L standards due to hydraulic control down-gradient of the site. Removal of not only the lighter, easier to degrade hydrocarbon fractions took place, but heavier polycyclic aromatic hydrocarbons (PAHs) removal is was significant based on analyses of soil transects in 2008, 2010, and 2012. (Nichols, 2013)
Additional Lessons Learned:
- Tree height and growth rate as measured in 2010 correlated significantly with the TPH levels in the soil below. The lower the contamination, the higher the tree height, with more polluted areas having lower tree height. The plants are acting as visual indicators of the contaminant concentrations below ground.
Top: US Coast Guard Phytoremediation Planting Site Diagram- TPH Removal within 2 years
Bottom: US Coast Guard Phytoremediation Planting Site Diagram- BTEX Removal within 2 years