PHYTOREMEDIATION THE NATURAL PUMP-AND-TREAT AND HYDRAULIC BARRIER SYSTEM

Transcription

1 PHYTOREMEDIATION THE NATURAL PUMP-AND-TREAT AND HYDRAULIC BARRIER SYSTEM By A. Basel Al-Yousfi, 1 P.E., Member, ASCE, Richard J. Chapin, 2 Timothy A. King, 3 P.E., and Sunil I. Shah 4 ABSTRACT: The use of water-loving trees as a supplemental remedy to conventional ground-water pumpand-treat systems was employed at a chemical manufacturing site in Texas. The approach entailed taking advantage of the extensive root systems of trees to extract contaminated ground water from the uppermost ground-water bearing zone located ft below ground surface (bgs). Forty poplar and mulberry trees were planted along the southeastern property line of this site in order to control, stop, and/or retreat the contaminant plume from migrating off-site. This planting project, phytofence, is intended to complement and fortify the operation of an existing 10-well recovery system within that vicinity. The uptake of water by the mature trees in the future is expected to change the subsurface hydraulic gradient and create an adequate ground-water capture zone. In order to protect the underground utilities piping networks from the tree roots near the ground surface, 10-in. HDPE casings that are 5 1/2 ft long were driven into the ground to surround each plant. This should also induce a deeper growth into the affected area. INTRODUCTION Deep-rooted and fast-growing trees such as poplar and mulberry are well suited for extracting contaminants from shallow and moderate depth aquifers (Strand et al). The high transpiration rates and the common knowledge concerning their physiology and culture make them a very popular choice. Plant species are selected for phytoremediation based upon their potential to evapotranspire subsurface water, the degradative enzymes they produce, their growth rates and biomass yield (leaves and canopy), the depth and distribution of their root zone, and their ability to bioaccumulate contaminants. Rhizospheric microorganisms in plants root systems can remove, degrade, or contain chemical constituents found in the soils, sediments, surface water, vadose zones, ground water, and even in the atmosphere. Research studies and practical experiences have shown that plants can be used to treat most classes of contaminants, including petroleum hydrocarbons, chlorinated solvents, pesticides, metals, radionuclides, explosives (TNT), and excess nutrients. Because of the diverse nature of chemical contaminants and the diversity of plants with inherent capabilities to treat them, a wide variety of phytoremediation techniques can be selected for site cleanup. Hybrid poplars (phreatophytic plants), for example which can grow almost anywhere to more than 3 m per year are found to act like a 100-ft straw that draw up contamination from soil and ground water. In addition, hybrid poplars have been demonstrated to extract and degrade chlorinated compounds as well, e.g., TCE (Newman et al. 1997). A five-yearold poplar tree (about 13 cm in diameter) is capable of transpiring L/day(ibid). Poplar trees are considered ideal phytoremediation specie. Mulberry trees have also been proven to clean up sites contaminated with such compounds 1 PhD, DEE, Sr. Envir. Engr., Health, Safety & Envir. Technol. (HS&ET), Union Carbide Corp., 3200 Kanawha Tpke., PO Box 8361, South Charleston, WV PG, Sr. Hydrogeologist, HS&ET, Union Carbide Corp., 3200 Kanawha Tpke., PO Box 8361, South Charleston, WV. 3 PG, Prog. Mgr., HS&ET, Union Carbide Corp., 3200 Kanawha Tpke., PO Box 8361, South Charleston, WV. 4 MS, Remediation Technol. Mgr., HS&ET, Union Carbide Corp., 3200 Kanawha Tpke., South Charleston, WV. Note. Discussion open until September 1, To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on December 8, This paper is part of the Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, Vol. 4, No. 2, April, ASCE, ISSN X/00/ /$8.00 $.50 per page. Paper No as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs) (Fletcher et al. 1990). Trees in arid and semiarid areas have been reported to lower aquifer levels by 5 ft during the growing season. Moreover, a single willow tree has been shown to consume over 5,000 gal. per one summer day Gatliff (1991). Wright (1997) reported that in one phytoremediation project, trees consumed a tremendous volume of water, up to 70 acre-in. per acre annually. A bioremediation process denoted as TreeMediation, developed by E.G. Gatliff in the 1990s, has been promoted to replace traditional ground-water pump-and-treat in numerous sites with less than or equal to 30-ft-deep aquifers. In this low-tech, low-maintenance, and environmentally compatible technology, a large number of trees are planted in close proximity over the contaminated aquifer or as a barrier surrounding affected areas. Trees pumping rates are directly correlated with evapotranspiration, which varies with the total leaf surface area, whether it is for an individual tree canopy or an entire plot. Accordingly, tree density is an important consideration for phytoremediation. A dense arrangement will have less leaf-surface area per tree, but the combined leaf-surface area of a dense plantation will be greater than the combined surface area of a thin and sparsely distributed planting. It is important to caution that Phytoremediation-using vegetation to treat contaminated soil and groundwater is not a cure-all for hazardous waste, but its low-cost, aesthetics and political correctness have helped it blossom into a technology with firm roots in the cleanup industry (Matso 1995). Phytoremediation can work only at sites that are suitable for plant growth. This means that the levels of pollutants cannot be toxic to the plants and that the pollution cannot be so deep in the subsurface that plant roots cannot reach it. Therefore, phytoremediation may be the right strategy for sites conducive to plant growth with relatively shallow contamination. If a pollutant is tightly bound to the solid media (with high affinity to the organic fraction of soil), then it would not be available to plants or to microorganisms in the rhizosphere. On the other hand, if a pollutant is too water soluble and highly mobile, it will quickly pass by the root system with minimum uptake. Another unique application of phytoremediation includes phytocovers, using trees, shrubs, and crops to lock waste into landfills. The technique can create barriers that are safer, more pleasant, and less costly than traditional membranes (Stack et al. 1999). SITE BACKGROUND The case study site is located within the Union Carbide Corporation (UCC) chemical manufacturing plant, in Texas PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT / APRIL 2000 / 73

2 City, Texas. The plant occupies 440 acres. It is bordered by largely industrial and commercial properties, and to a lesser extent by residential property (a portion of which is owned by UCC). Plant operations began in 1941, and all intentions are for this area to remain industrial. The plant produces or processes a variety of petrochemical products. The general area of interest herein is the In Plant Disposal Area (IPDA), which is a group of solid waste management units (SWMUs) located in the eastern central part of the plant. The affected ground-water aquifer, known as Zone II, consists of sands and silty sands, and is divided into upper and lower sections because of the presence of an interbeded clay. The top of the clay occurs at an approximate elevation of 20 ft MSL (about ft below grade). This clay interbed ranges from about 2 to 10 ft in thickness. The overall thickness of the Zone II aquifer varies from approximately 20 to 40 ft. The potentiometric surface in Zone II across the study area indicates that ground water flows radially away from the central part of the site because of a ground-water mound around an unlined emergency fire-water storage pond. Zone II hydraulic conductivity at the study area is estimated at cm/s, and ground-water flow rates range from 1.3 to about 16.2 ft/year. Flow from the IPDA is also influenced by a ground-water divide related to the operation of a recovery system along Grant Avenue (described next). Field sampling investigations suggested the following observations (Fig. 1): The IPDA ground-water plume is comprised of two major chemical constituents, Bis (2-chloroethyl) ether (BCEE), and 1,2-dichloroethane (DCA). DCA is mainly present on-site as well as off-site in two separate areas, north and south of the IPDA. The northern DCA plume is possibly related to a local spill that may have taken place at one time in the tank farm facilities north of the IPDA. The south DCA plume covers a northwest-southeast trending area along the UCC property on the side of Grand Avenue, extending about 400 ft from the UCC fenceline. The specific source of the south DCA plume is probably related to a historic spill(s) or other local releases, but not associated with the IPDA SWMUs. The BCEE plume across the subject area is continuous; and its off-site extent is limited to a southern area on the plant s eastern boundary. The southern portion of the ground-water plume, comprising overlapping DCA and BCEE constituents and migrating off-site to a neighboring industrial manufacturing facility, is the sole focus of this project. A ground-water recovery system composed of 10 extraction wells had previously been placed in the IPDA area to pump (and treat) underground contamination at a rate of about 7 gpm. The recovery system captures the off-site plume in the immediate vicinity of the IPDA. The groundwater capture zone created by the recovery system extends approximately 400 ft east outside the fenceline. Therefore the system is adequate only in the middle section of IPDA. A new recovery well has just been recently installed to the north to capture the northern off-site plume. However, the southeast extent of the plume is currently beyond the effects of the IPDA 10-well recovery system. Our own experience at the site showed that twenty-plus years of natural growth of vegetation led to a diverse community of trees, bushes, and grasses, including hybrid Poplar and Mulberry species. Naturally occurring substances (sugars and amino acids) released by plants were also discovered in the vicinity of root systems (which could have served as a cometabolite for bacterial degradation of PAHs). In a single gram of rhizospheric soil at one location, 10,000 different microbial species were found. Ground-water modeling and measurements showed that the extreme southeast corner of the plume was at the limits of the recovery system, and the plume could not be retreated or prevented from migrating off-site without a complementary ground-water recovery system. Although at present the plume is not expanding, a supplemental system, phyto-fence, based on phytoremediation, was adopted for the site. Approximately 40 trees were planted along Zero Street between Avenue I and Avenue J, right on the property s fenceline (parallel to Grant Avenue; Fig. 2). This additional ground-water withdrawal is anticipated to supplement the IPDA 10-well recovery system, capturing the entire plume and preventing the need for additional active wells. METHODOLOGY The main objective of this project was to supplement and enhance the performance of the existing ground-water recovery system at the southeastern part of the affected area, where capture is weak and migration off-site has occurred. Phytoremediation was selected for the task for several reasons: 1. In situ nonintrusive remedial option 2. Environmental compatibility (green technology) 3. Effectiveness at shallow and moderately deep aquifers 4. Low capital and maintenance 5. Aesthetics benefits FIG. 1. Chemical Manufacturing Site Located in Industrial Zone (Not to Scale; Highlighted Areas Represent Ground-Water Plume and Recovery Well-System 74 / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT / APRIL 2000

3 FIG. 2. Phytofence Trees Planting to Recover Southeast Corner of Plume (Not to Scale; Refer to Highlighted Areas in Fig. 1) During the project, trees (mainly poplar) were placed in one row along the fenceline, at approximately 15 ft intervals (Fig. 2). Previous experiences with tree phytoremediation reported the implementation of spacing intervals in the range of 5 15 ft (Chappell 1988), depending on the site-specific conditions. It was decided that a 15-ft grid system was adequate in the first phase of the project. Ground-water monitoring data will determine the future need for additional row(s) of trees, envisioned to be planted parallel to and at the centers of the existing plantation intervals. Each tree was planted within a 10 in. HDPE casing that is 5 1/2 ft long to promote the tree roots to grow down before expanding horizontally (Fig. 3). Furthermore, organically rich top-soil, amended with fertilizers and nutrients, was also added into the earth holes where the trees were planted. This arrangement should prevent the tree roots from interfering with shallow underground pipelines or any maintenance work on the pipelines, which are located ft east of the trees. Also, the design will enhance more vertical root growth and will direct extraction from deeper ground-water bearing locations within Zones II where contaminants are known to be present. FIG. 4. Schematic of Tree Irrigation System FIG. 3. Trees HDPE Casing System An irrigation system was also provided to promote growth of trees during the initial period and dry season (Fig. 4). However, several planted trees died because of the severe drought that affected the region in spring and summer of The unsuccessful plantings were replaced by mulberry trees, which are considered native species and effective water-transpiring plant conduits (this has been done in consultation with Professor John Fletcher, of the University of Oklahoma). Native trees should be much heartier under the regional conditions and more resistant to local pathogens and parasites. As described in the following segments, it was found beneficial to choose native plants when designing phytoremediation schemes (phytostabilization processes in several waste basins at the site have been exclusively carried out by native plant species). Choosing native species (mulberry) and screening them for their ability to metabolize the specific halogenated contaminants was the key to sustaining and complementing this ground-water phytoremediation project. This latest tree planting arrangement of poplar and mulberry has been viable, stronger, and more resistant. COST/BENEFIT ANALYSIS An obvious and compelling benefit of the phytoremediation technology is that cleaning up polluted industrial sites may PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT / APRIL 2000 / 75

4 not require billion-dollar government programs ( Phytoremediation 1998). In general, phytoremediation costs vary based upon the treatment strategy. For instance, harvesting plants that accumulate contaminants can drive up the cost in comparison to treatments that do not require harvesting. In any case, phytoremediation is often rendered cheaper than comparable technologies. The costs of implementation and maintenance of the phytofence at this site were significantly less than a conventional upgrade to the existing pump-and-treat system. The installation cost for the entire phytofence was approximately $20,000, including the replanting described earlier. This included the costs of trees, installation, initial irrigation and fertilizing, and the material and installation of the irrigation system. This cost also included a limited feasibility study to compare the effects of the trees with those of additional wells. The cost of the hybrid poplar and mulberry trees typically ranges from $2 to 15 per tree, depending on the age of the tree and local availability. Mature trees can be more expensive (>$15 per tree). Experience has shown that the cost per tree has increased sharply over the last 3 5 years due to high demand, although they still remain relatively inexpensive when compared with extraction wells. Because phytoremediation systems are primarily used as hydraulic barriers powered by the solar energy, the most closely related engineering technology is a pump-and-treat system. For this project, an addition to the existing pump-and-treat system of at least 3 to 5 pumping wells was estimated to be more than $100,000 including well installation, pumps, and associated piping. This estimate did not include an upgrade of the storage or treatment facilities, since excess capacity was available. Obviously, for more remote sites without existing infrastructure, the cost differential between these two technologies would be greater. Our own experience with pump-andtreat systems indicates an approximate cost of $10 per 1,000 gal. of ground water; the phytoremediation overall cost is expected to be at least an order of magnitude lower. Maintenance costs for the phytofence are less than $2500 per year for this project. This cost is largely irrigation but also includes pruning, mulching, and tree replacement. Three replacement, for those that die or are damaged by storms, is expected to be needed for approximately 1 in 10 plantings through the first 3 to 5 years, based on previous experience. Although not needed at this site, other maintenance costs may include routine application of pesticides to control leaf-eating pests. MONITORING The effectiveness of the remedial tree plantation will be monitored routinely by measuring ground water potentiometric surfaces and contaminant levels in the area before, during, and after growth. This monitoring plan will be finalized to demonstrate the extent of ground-water influence and capture zones. A preliminary ground-water modeling simulation in the area was performed using the Visual ModFlow computer program, which predicated notable sinks and a ground-water gradient reversal (Fig. 5). The shaded area on Fig. 5 is the inferred ground-water depression that will be produced by the trees after maturation. The trees have been in the ground for less than one growing season, thus as of now there is very little performance data available. The regular measurement and routine sampling plan in the area will determine if the current plantation is adequate or additional trees and/or monitoring wells will be needed in the future. Two or more rows of trees can be accommodated and installed if needed in the area to fortify the ground-water mitigation and integrate the recovery system. The monitoring program will continue beyond five years, the time period needed for the trees to fully mature. FIG. 5. Trees and Wells Zones of Influence (Shaded Area Represents Phytofence Zone of Influence) SUMMARY AND CONCLUSIONS Phytoremediation was selected as the remedy of choice to provide mitigation means and a hydraulic barrier against contaminant migration off-site from a chemical manufacturing plan in Texas. Forty poplar and mulberry trees were planted along the property fenceline (phytofence) as an integral part of an overall active remediation system consisting mainly of well recovery systems. A particular design configuration, the 10-in. HDPE casing, was used to surround the planted trees and enhance vertical roots growth into the affected groundwater zone without any interference with the shallow subsurface piping networks. The mature trees are anticipated to offer a natural pump-and-treat system, extracting between L/day per tree. This remedial alternative is not only self-sustaining, maintenance-free, low-cost, and technically effective, but also is environmentally compatible and offers aesthetic values. Long-term routine monitoring of the system will commence soon by measuring ground-water elevation and depicting the anticipated reversal trend in flow direction. Additional enhancement and future tree planting will be decided upon and implemented if deemed necessary. APPENDIX. REFERENCES Chappell, J. (1998). Phytoremediation of TCE in groundwater using Populus. Status Rep. prepared through National Network for Environmental Management Studies (NNEMS) by Duke University, for the U.S. EPA Technology Innovation Office (TIO) (compiled June Aug., 1997). Fletcher, J., McFarlane, C., and Pfleeger, T. (1990). Effect, uptake and disposition of nitrobenzene in several terrestrial plants. Envir. Technol. and Chem., 9, Gatliff, E. G. (1991). Vegetative remediation process offers advantages over traditional pump-and-treat technologies. Remediation, 4(3), Matso, K. (1995). Mother nature s pump and treat. Civ. Engrg., ASCE, 65(10), / PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT / APRIL 2000

5 Newman, L. A. et al. (1997). Uptake and biotransformation of trichloroethylene by hybrid poplars. Envir. Sci. and Technol., 31(4), Phytoremediation: Poplar vs. pollution. (1998). USA Today, Oct. 5. Stack, T. M., Potter, S. T., and Suthersan, S. S. (1999). Putting down roots. Civ. Engrg., ASCE, 69(4), Strand, S. E. et al. (1995). Removal of trichloroethylene from aquifers using trees. Conference Proc., Conf. Innovative Technol. for Site Remediation and Haz. Waste Mgmt., American Soc. of Civil Engineers, New York, Wright, A. G., and Roe, A. (1997). It s back to nature for waste cleanup. Engrg. News Rec., 238(17). PRACTICE PERIODICAL OF HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE MANAGEMENT / APRIL 2000 / 77