Introduction to Phytoremediation - CLU-IN

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3.7.5 Applicable Contaminants/ Concentrations Water-soluble leachable organics and inorganics are used at concentrations that are not phytotoxic. Poplar trees were used to form a barrier to groundwater movement at a site contaminated with gasoline and diesel (Nelson 1996). 3.7.6 Root Depth Hydraulic control by plants occurs within the root zone or within a depth influenced by roots, for example: • The effective rooting depth of most crops is 1 to 4 feet. Trees and other vegetation can be used to remediate groundwater in water table depths of 30 feet or less (Gatliff 1994). • Plant roots above the water table can influence contaminants in the groundwater by interfacing through the capillary fringe. Fe, Tc, U, and P diffused upward from the water table and were absorbed by barley roots that were 10 cm (3.9 in) above the water table interface (Sheppard and Evenden 1985). • The placement depth of roots during planting can be varied. Root depth, early tree growth, and nitrogen accumulation were enhanced by placing poplar tree root balls closer to shallow groundwater during planting (Gatliff 1994). 3.7.7 Plants The following plants are used in hydraulic control: • Cottonwood and hybrid poplar trees were used at seven sites in the East and Midwest to contain and treat shallow groundwater contaminated with heavy metals, nutrients, or pesticides (Gatliff 1994). Poplars were used at a site in Utah to contain groundwater contaminated with gasoline and diesel (Nelson 1996). Passive gradient control was studied at the French Limited Superfund site using a variety of phreatophyte trees; native nondeciduous trees were found to perform the best (Sloan and Woodward 1996). 3.7.8 Site Considerations The establishment of trees or other vegetation is likely to require a larger area than would be required for the installation of a pumping well. 3.7.8.1 Soil Conditions The primary considerations for selecting hydraulic control as the method of choice are the depth and concentration of contaminants that affect plant growth. Soil texture and degree of saturation are influential factors. Planting technique and materials can extend the influence of plants through non-saturated zones to water-bearing layers. 3.7.8.2 Ground and Surface Water The amount of water transpired by a tree depends on many factors, especially the size of the tree. Some esti- 35 mates of the rate of water withdrawal by plants are given below. • Poplar trees on a landfill in Oregon transpired 70 acreinches of water per acre of trees (Wright and Roe 1996). • Two 40-foot-tall cottonwood trees in southwestern Ohio pumped 50 to 350 gallons per day (gpd) per tree, based on calculations using observed water-table drawdown (Gatliff 1994). • A 5-year-old poplar tree can transpire between 100 and 200 L water per day (Newman et al. 1997a). • Young poplars were estimated to transpire about 8 gpd per tree, based on the observed water table drawdown (Nelson 1996). • Mature phreatophyte trees were estimated to use 200 to 400 gpd (Sloan and Woodward 1996). 3.7.8.3 Climatic Conditions The amount of precipitation, temperature, and wind may affect the transpiration rate of vegetation. 3.7.9 Current Status Several U.S. companies have installed phytoremediation systems that have successfully incorporated hydraulic control. 3.7.10 System Cost Estimated costs for remediating an unspecified contaminant in a 20-foot-deep aquifer at a 1-acre site were $660,00 for conventional pump-and-treat, and $250,000 for phytoremediation using trees (Gatliff 1994). 3.7.11 Selected References Gatliff, E. G. 1994. Vegetative Remediation Process Offers Advantages Over Traditional Pump-and-Treat Technologies. Remed. Summer. 4(3):343-352. A summary is presented of the impact of poplar or cottonwood trees to influence a shallow water table at sites along the East Coast and in the Midwest that were contaminated with pesticides, nutrients, or heavy metals. The contribution of the trees to water table drawdown was measured at some sites. Information is presented on the decrease in contaminant concentrations at some of the sites. Wright, A. G., and A. Roe. 1996. It’s Back to Nature for Waste Cleanup. ENR. July 15. pp. 28-29. A poplar tree system for landfill leachate collection and treatment is described. The trees use up to 70 inches of water per acre per year. A proposed project at another landfill is presented.
3.8 Vegetative Cover Systems 3.8.1 Definition/Mechanism A vegetative cover is a long-term, self-sustaining system of plants growing in and/or over materials that pose environmental risk; a vegetative cover may reduce that risk to an acceptable level and, generally, requires minimal maintenance. There are two types of vegetative covers: the Evapotranspiration (ET) Cover and the Phytoremediation Cover. • Evapotranspiration Cover: A cover composed of soil and plants engineered to maximize the available storage capacity of soil, evaporation rates, and transpiration processes of plants to minimize water infiltration. The evapotranspiration cap is a form of hydraulic control by plants. Risk reduction relies on the isolation of contaminants to prevent human or wildlife exposure and the reduction of leachate formation or movement. Fundamentally, an ET cover is a layer of monolithic soil with adequate soil thickness to retain infiltrated water until it is removed by evaporation and transpiration mechanisms. Mechanisms include the uptake and storage of water in soil and vegetation. An ET cover is one type of a water-balance cover, illustrated in Figure 3-6. • Phytoremediation Cover: A cover consisting of soil and plants to minimize infiltration of water and to aid in the Vegetative grasses and legumes soil = 6” Clay Waste up to 60’ Geomembrane Figure 3-6. Illustration of an Evapotranspiration (ET) cover. 36 degradation of underlying waste. Risk reduction relies on the degradation of contaminants, the isolation of contaminants to prevent human or wildlife exposure, and the reduction of leachate formation or movement. Mechanisms include water uptake, root-zone microbiology, and plant metabolism. The phytoremediation cover incorporates certain aspects of hydraulic control, phytodegradation, rhizodegradation, phytovolatilization, and perhaps phytoextraction. Figure 3-7 presents the evolution of a phytoremediation cover as it moves from a remediation function to a water exclusion function. In limited cases, vegetative covers may be used as an alternative to traditional covers that employ a resistive barrier (i.e., a multilayered cover with a relatively impermeable component). Vegetative covers may be appropriate to address contaminated surface soil or sludge, certain waste disposal units, waste piles, and surface impoundments. In general, the application of any cover system should provide the following functions: • isolate underlying waste from direct human or wildlife exposure (e.g., prevent burrowing animals from reaching the contaminants); • minimize the percolation of water into the underlying waste; Root system Uncompacted soil Conventional Cover Vegetative Cover
Page 1 and 2: Introduction to Phytoremediation Na
Page 3 and 4: Foreword The U.S. Environmental Pro
Page 5 and 6: Table of Contents Foreword ........
Page 7 and 8: Figures Figure 2-1. Mechanisms for
Page 9 and 10: Acronyms AAP Army Ammunition Plant
Page 11 and 12: Acknowledgements This Introduction
Page 13 and 14: • Chapter 3 provides a literature
Page 15 and 16: Figure 2-1. Mechanisms for phytorem
Page 17 and 18: Table 2-2. Root Depth for Selected
Page 19 and 20: Table 2-3. Phytoremediation at Supe
Page 21 and 22: egulated by the requirement. The Ma
Page 23 and 24: site. Plants and animals recolonize
Page 25 and 26: This chapter presents a literature
Page 27 and 28: Figure 3-1. Phytoextraction. The fo
Page 29 and 30: In this literature review of resear
Page 31 and 32: Rhizofiltration has not been evalua
Page 33 and 34: sion of metals and metal concentrat
Page 35 and 36: Figure 3-2. Rhizodegradation. • L
Page 37 and 38: - PCP at 400 to 4100 mg/kg (in soil
Page 39 and 40: This paper introduces the important
Page 41 and 42: - A qualitative study indicated tha
Page 43 and 44: Figure 3-4. Phytovolatilization. 3.
Page 45: Newman, L. A., C. Bod, N. Choe, R.
Page 49 and 50: also be used in the treatment of so
Page 51 and 52: trial habitats are greatly improved
Page 53 and 54: • Achieve objectives - Perform qu
Page 55 and 56: could be more effectively treated t
Page 57 and 58: Table 4-2. (continued) • Sterile/
Page 59 and 60: The following are examples of commo
Page 61 and 62: arrier to block other site activiti
Page 63 and 64: 5.1 Remedial Objectives Chapter 5 R
Page 65 and 66: tems should not be used to infer th
Page 67 and 68: in soil and binding to soil were al
Page 69 and 70: The six case studies presented in t
Page 71 and 72: mum concentration of 640 ppb of 112
Page 73 and 74: a comparison of the performance of
Page 75 and 76: 6.2.6 Contacts Greg Harvey US Air F
Page 77 and 78: Figure 6-4. Sampling Grid Edward Se
Page 79 and 80: gravel drainage layer, and compacte
Page 81 and 82: variety had only a 54% survival rat
Page 83 and 84: surements are being taken to produc
Page 85 and 86: Phytoaccumulation: The uptake and c
Page 87 and 88: Project Name/ Size Primary Media an
Page 95 and 96: Adler, T. 1996. Botanical Cleanup C
Page 97 and 98:
Ferro, A. M., L. Stacishin, W. J. D
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Newman, L. A., S. E. Strand, D. Dom
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Appendix D Common and Scientific Na
Page 103 and 104:
Scientific Name Listed First Scient
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