Application of Phreatophytes to Remediate Contaminated Groundwater Before Discharge to Protected Surface-Water Systems

Science Center Objects

State environmental agencies are charged with the protection of groundwater and surface-water systems from water-quality degradation. Although the point-source discharge of wastes to surface waters is allowed up to permitted levels, contaminant releases from non-point sources, such as the discharge of contaminated groundwater, is not regulated. A common cause of groundwater contamination is the release of petroleum hydrocarbons from underground storage tanks (USTs). Even if the leak from a UST is contained and the tanks and piping removed, the source area typically becomes characterized by residual contamination that is bound to soil and aquifer sediments. This residual contamination provides a long-term source of contaminants to groundwater and, potentially, adjacent surface-water resources. As such, the scenario exists where groundwater contamination can become a non-point source of unregulated contaminant discharge to surface-water systems.

Problem:

Using Bobcats with solid augers to create more holes to plant more (3,500) trees.
Using Bobcats with solid augers to create more holes to plant more (3,500) trees. (Public domain.)

There are more than 690,000 permitted USTs in the U.S. (U.S. General Accounting Office, 2001), of which 29 percent may be leaking. About 10 percent of leaking USTs are found in North Carolina (NC), which has documented at least 19,300 known releases at UST sites; 5,790 sites have confirmed impact to groundwater above the NC 2L Groundwater Standards, and may affect adjacent surface-water systems (oral communication, Brad Atkinson, NC Department of Environmental and Natural Resources, Division of Waste Management (NCDENR-DWM), February 2005). In NC, assessment and remediation costs at such UST release sites are paid by Federal- and state-funded UST Trust Funds after private deductibles ranging between $20,000 and $75,000 have been met. For the period 2002-2003, the NC UST Trust Fund expended approximately $29 Million dollars to address petroleum releases from USTs, although the Trust Fund consisted of less than $28 Million dollars. To be protective of the state's ground- and surface-water resources, either additional funds need to be raised (through additional fees, taxes), or less expensive but still effective remedial alternatives be employed to perform the clean up.

A potential solution that could help resolve this dilemma would be the application relatively low-cost but effective remediation technologies that would be restorative of contaminated groundwater and protective of surface waters. The use of phreatophytes, or plants that use subsurface sources of water, has been shown to be a relatively low cost and low maintenance technology (phytoremediation). Phreatophytes, such as grasses, shrubs, or trees, can specifically be installed to affect the site groundwater hydrology and to take up contaminants dissolved in groundwater (Landmeyer, 2001). Thus, using phreatophytes in this manner could be a cost-effective strategy to remediate contaminated groundwater and be protective of downgradient surface-water systems.

Approach:

Good growth 6 months after planting.
Good growth 6 months after planting.​​​​​​​(Public domain.)

The approach of this proposed investigation consists of determining if phreatophytes can (1) decrease groundwater contamination, and (2) decrease unsaturated zone contamination to reduce the risk of transport to adjacent surface water. Although phreatophytes that use copious quantities of groundwater are most frequently associated with the arid deserts of the states of the Southwestern US, they also are found naturally along floodplains in the more humid Southeast (such as river birch and sycamore). In such areas, their influence of local water budgets is more prominent during times of drought. As would be expected, trees in these areas also can use surface water and recent rainfall to meet evapotranspirational demands. At sites characterized by the potential for contaminated groundwater discharge to surface water, the existing plants are typically not dense enough to achieve significant reductions in groundwater contaminant concentrations or flow necessary to reach remedial goals within realistic timeframes. The engineered installation of phreatophytes, however, permits sufficiently dense plantations of phreatophytes to affect groundwater flow, contaminant concentrations, and subsequent reduction in the potential for contaminants to be discharged to surface water.

The most important aspect of this proposed investigation is the collection of the performance data necessary to document the affect of plants on groundwater and unsaturated zone contaminant levels. The data collection effort and subsequent interpretation will consist of the following four phases:

Phase 1:

The assessment of baseline conditions of groundwater flow and contaminant distribution, and unsaturated zone contamination will occur prior to the addition of phreatophytes. Such assessment activities are envisioned to occur in early Fiscal Year (FY) 2006. Existing wells at the site will be used to measure depths to the water table, and a water-table map will be constructed during periods that reflect low-and high-tide conditions in the Pasquotank River. This is important to document, because the probability exists that during some periods, surface water from the Pasquotank River may move toward the aquifer. An automatic water-level recorder will be temporarily installed in at least one well to document any tidal affects that may impact sitewide groundwater flow directions.

The existing plumes of benzene and MTBE will be delineated by sampling existing wells using standard sampling methods, and the contaminant concentrations in soil and groundwater will be determined using standard analytical techniques at the North Carolina State University. The redox condition of the groundwater also will be determined during field sampling, by measuring concentrations of dissolved oxygen and ferrous iron using a field spectrophotometer, and by quantifying sulfate, nitrate, and methane using standard laboratory methods.

The selection of phreatophytes to be planted will be based on a range of plants suitable to the coastal conditions of North Carolina, with hardiness to exposure to saline waters, and their resistance to petroleum hydrocarbons such as BTEX. It has been shown that hybrid poplar trees (specifically clones OP-357 and DN-34) will meet the needs of the project (Landmeyer, 2001).

Phase 2:

The hybrid poplars, willows, and other vegetation (to be determined after compilation of the results of Phase I) will be installed at the site during early spring calendar year 2006. The plants will be provided by the North Carolina State University. The plants will be installed in 10 columns, 15 rows, at 5-foot centers, across the former source area using the USGS South Atlantic Water Science Center - South Carolina direct-push drill rig. This planting density was selected in order to completely cover the former source area. The trees planted will be 6-foot bare-root whips, and planted to a depth near the mean high water table (ie, 4 ft below grade). Experience by the author at a site in Charleston, SC, suggests that these 6-foot whips will grow about 10 ft/year. Due to the 3-year nature of this project, however, there may be insufficient data from groundwater or soil monitoring to definitively conclude that the plants have affected the contaminated system.

To enhance the remediation potential for the residually contaminated soils (essentially the long-term source of contamination to both groundwater and surface water), the tree root- mass will be inoculated with a commercial preparation of endo-and ecto-mycorrhizae bacteria before being planted in the ground. These bacteria will assist in enhancing the natural biodegradation of fuel compounds in the unsaturated zone, thereby decreasing the mass of contaminants available for dissolution and transport. This will be examined by comparing contaminant reductions beneath trees that received the inoculant relative to trees that did not, after normalizing for differences in initial contaminant concentration.

Phase 3:

Phase III will start in late calendar year 2006 and consist of the collection of performance verification monitoring data, collected on at least a biannual interval, to determine and document the effectiveness of the phytoremediation system to reduce the concentrations of contaminated groundwater and soils in the planted area. The contaminant concentrations (such as benzene, naphthalene, and MTBE) collected from existing monitoring wells or from temporary wells installed with the USGS direct-push drill rig will be compared to concentrations of these contaminants measured prior to the phytoremediation system installation in Phase 1.

Groundwater levels will be collected manually as well as using the automatic water-level recorder installed in one well during Phase I to produce water-table maps. These maps will be compared to conditions documented during Phase I. Periodically, tree-tissue samples will be collected and analyzed for the presence of petroleum hydrocarbons and MTBE using methods outlined in Landmeyer and others (2000) and Vroblesky and others (2004), to provide direct evidence that the trees are capturing contaminants dissolved in groundwater.

Phase 4:

Phase 4 will consist of data synthesis and report preparation, and is envisioned to start in late calendar year 2007. The goal of the data synthesis is to assess the performance of the phreatophytes selected in decreasing the concentration of contaminated groundwater and soils adjacent to a sensitive surface-water receptor.