1

Concentratio

TCE, in micro

per liter

5 to 50

50 to 500

500 to 1,000

1,000 to 5,00

5,000 to 15,0

More than 15

AFP4 and NA

boundary

Study area s

figure 2

EXPLANATION

Bomber Road

Base from U.S. Air Force, Aeronautical Systems Center 0 0.25 0.5 MILE

Trinity River

32o46'

32o

47'

97o

26' 97o

25' 97o

24'

Meandering

Road

Creek

West Fork

LAKE WORTH

Creek

Farmers

Branch

Fort Worth

AFP4 an

TEXAS

LOCATION MA

Air Force Plant 4

(AFP4)

Naval Air Station–Joint

Reserve Base

(NAS–JRB)

Carswell Field

Phytoremediation

demonstration

site

Golf course

Northern lobe 

 of TCE Plume

Central lobe 

 of TCE Plume

Southern lobe 

 of TCE Plume

Figure 1. Extent of trichloroethene (TCE) plume in the alluvial aquifer, AFP4 and NAS–JRB, Fort Worth, Texas, fall 2002, an

location of phytoremediation demonstration site, NAS–JRB. 

Joint Reserve Base Carswell Field (NAS–JRB) adjacent to Air 

Force Plant 4 (AFP4) in Fort Worth, Tex. (fig. 1). Trichloroethene 

(TCE) has been used at AFP4 in aircraft manufacturing processes 

for decades; spills and leaks from tanks in the manufacturing 

building have resulted in shallow ground-water contamination 

on-site and downgradient from the facility (Eberts and others, 

2003). The objective of the study was to determine the effectiveness of eastern cottonwoods (Populus deltoides) in decreasing the 

biological means. Before development of the phytore

demonstration site, natural attenuation of TCE at the 

by sorption, dispersion, dilution, and possibly volatil

(Eberts and others, 2003). 

Long-term, field-scale monitoring and evaluatio

site contribute to the understanding of the processes a

with phytoremediation and provide practical informa

field-scale applications of the method. This fact shee

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S
2

Approximate area of plantation

Well and number

EXPLANATION

Well nest and numbers

Well with pressure transducer

and number

N

Roaring Springs Road

Cottonwood tree

518

501

502

503

515

012

014

013

018

009

513

518

501

502

503

517 514

519

526

525

524

520 522

523

528

515

527

529

516

521 504

505

506

001

021 011

P10

017

510

511

512

020

010

022

023 008

019

P11

P12

P14

P07

P04

P09

P13

P15

P06

P05 P01

P03

P02

507

508

509

P08

whip plantation

caliper plantation

Golf cart path

General

 direction of

 ground-water

 flow

Farmers Branch Creek

0 50 100 FEET

summarizes the development of the phytoremediation demonstration site at NAS–JRB and describes some of the physical and 

chemical processes associated with phytoremediation. 

The phytoremediation demonstration site is on the southern 

edge of the central lobe of a TCE plume in the surficial (alluvial) 

aquifer. The plume originates at AFP4 about 0.9 mile upgradient 

from the site (fig. 1). The 9.5-acre site is in the northwestern corner 

of the golf course on NAS–JRB. The saturated thickness of the 

alluvial aquifer, which is composed of clay, silt, sand, and gravel, 

ranges from about 1.5 to 5 feet at the site. The total thickness of 

the alluvial aquifer ranges from about 6 to 15 feet. The GoodlandWalnut confining unit, composed of massively bedded shaley 

limestone, underlies the alluvial aquifer. The general direction of 

ground-water flow in the study area (fig. 2) is from northwest to 

southeast, approximately perpendicular to the long sides of the 

cottonwood plantations. Ground water flows toward Farmers 

Branch Creek in the area southwest of the golf cart path. At the 

time of site characterization in August 1996, depth to water ranged 

from 8 to 13 feet below land surface.

Site Development

When the study was initiated in 1996, the site was planted 

with two separate plantations of 1-year-old stems (whips) harvested from branches of eastern cottonwoods during the dormant 

season and 1-year-old eastern cottonwood seedlings (calipers). 

Each plantation is about 235 feet long and 50 feet wide and contains seven rows of trees. About 60 trees were planted in each row 

of the whip plantation, and about 30 trees were planted in each row 

of the caliper plantation.

Seven rows were trenched in each plantation to a depth of 3 

feet with a spacing of 8 feet between rows. The whips, which were 

about 1.5 feet long, were placed 4 feet apart within the rows, and 

the calipers, which were more than 6.5 feet tall, were placed 8 feet 

apart within the rows. Irrigation lines were placed in the trenches 

along with vegetative material. The trees were watered liberally 

during the first two growing seasons to encourage deep root development. After the trenches were backfilled, the whips extended 

about 2 inches above land surface.

The whips were closely spaced to allow for a natural selfdesign of the phytoremediation system. In other words, tree growth 

and survival were predicted to be dependent upon subsurface 

heterogeneities. However, closely spaced cottonwoods in other 

studies have been shown to have smaller diameters and lesser survival rates than widely spaced cottonwoods (Krinard, 1985).

The rate of tree growth above and below land surface at the 

demonstration site was measured to assess the progression of the 

phytoremediation system over time. It was important to evaluate 

the growth rate of the planted trees because differences in cottonwood growth can take time to appear (Cooper and Ferguson, 

1979). Such documentation provides insight into the benefits and 

limitations of the planting strategy used in this demonstration, as 

well as the ability of the phytoremediation system to influence the 

ground-water hydrology of the site.

The monitoring system for the site was designed to measure 

incremental changes in site conditions because phytoremediation 

processes can require years to develop (Eberts and others, 2003). 

Ground-water levels were measured in 59 wells, surface-water 

stage (water level) was measured in Farmers Branch Creek, and 

rainfall was measured at NAS–JRB. The wells were installed 

upgradient and downgradient from, within, and surrounding the 

plantations. Several of the wells were installed adjacent to mature 

Figure 2. Location of cottonwood plantations and monitor wells at phytoremediation demonstration site, NAS–JRB, Fort 

Worth, Texas.
3

TCE TCE

TCE

TCE

TCE

TCE TCE

TCE

TCE

TCE

TCE

TCE TCE TCE

TCE

TCE

TCE

TCE

TCE

TCE TCE TCE

TCE TCE TCE

TCE TCE TCE TCE

TCE TCE TCE TCE TCE TCE TCE TCETCE

TCE

Phytostimulation of microbial degradation: Plant roots 

excrete compounds that contain organic carbon, which 

microorganisms use as a food source. 

Phytostabilization: Plants bind 

contaminated soils in place, which results in

the immobilization of toxic contaminants.

Phytovolatilizatio Phytovolatilization: Th e pla h e p ant's abil an ity to remove TCE from the leaf 

has trav as traveled through the plant.

Phytodegradation: Trees have natural substances in 

their roots, stems, and leaves that can help them to 

 break down TCE.

Transpiration and phytocontainment effects: 

Transpiration from the trees induces a cone of

depression.

1

2

3

4

5

trees, including a 19-year-old (in 1996) mature cottonwood tree 

(fig. 2). Many of the 59 wells were used primarily for measuring 

water levels. Pressure transducers were placed in three of the wells 

to measure and record water levels on an hourly basis, including 

one upgradient from the tree plantations, one in the whip plantation, and one in the caliper plantation.

Processes Associated With Phytoremediation and the 

Methods Used to Quantify Their Contributions

The primary objective of the demonstration study was to 

determine how effectively eastern cottonwood trees decrease the 

mass of dissolved TCE that is transported within an aquifer across 

the downgradient end of a planted area. It was hypothesized that 

tree physiological processes would decrease the mass of TCE in 

the alluvial aquifer by a combination of hydraulic control of the 

contaminant plume and in-place biodegradation of the contaminant 

mass (natural pump and treat). Trees were predicted to remove 

contaminated water from the aquifer by means of their root systems, followed by the biological alteration of TCE within the trees 

or the transpiration and volatilization of TCE into the atmosphere. 

The trees also would promote microbial reductive dechlorination 

of dissolved TCE within the originally aerobic aquifer (Eberts and 

others, 2003). Five specific processes (summarized in fig. 3) and 

methods used to quantify their contributions are described below.

Transpiration and Phytocontainment (Hydraulic Control)

Transpiration from trees changes horizontal aquifer gradients 

or vertical soil-moisture pressure gradients, which alters the 

direction of ground-water flow and might help to contain shallow 

contaminant plumes or contaminated soil leaching (McCutcheon 

and Schnoor, 2003); this process is called phytocontainment. The 

amount of water the planted trees can transpire throughout their life 

cycle can help to determine, in part, the effectiveness of a phytoremediation system (Eberts and others, 2003). Transpired water 

can be derived from the nearsurface soils and the saturated zone. 

The amount of water transpired 

directly from the aquifer is of interest in this study because TCE and its 

breakdown products at the site are 

dissolved in the ground water.

Sapflow was measured in a 

statistical sampling of trees; from 

these results an estimate of transpiration for each of the plantations 

was extrapolated (Vose and others, 

2000). Long-term ground-waterlevel hydrographs also were used 

to estimate transpiration directly 

from the aquifer (Eberts and others, 

2003) using a method developed by 

White (1932).

Phytostimulation of Microbial 

Degradation

A symbiotic relation occurs 

between plant root systems and 

microorganisms in the root zone 

(Motoyama and others, 2001). Plant 

roots excrete sugars, alcohols, and acids that contain organic carbon, which microorganisms use as a food source. This leads 

to enhanced microbial activity in the root zone (phytostimulation), 

which results in an increase in microbial degradation of soil 

contaminants.

Ground-water samples have been collected about twice 

each year since 1996 upgradient, downgradient, and within the 

plantations, as well as beneath the mature cottonwood tree. These 

samples were analyzed for breakdown products of TCE to assess 

microbial degradation mechanisms and rates (Eberts and others, 

2003). 

The spatial distribution of the various microbial populations 

in the pore water and on the sediment of an aquifer also can be used 

to assess the mechanisms and rates of microbial degradation in that 

aquifer. A reconnaissance study of microbial activity in soil and 

ground water at the site was done in February and June 1998; the 

results of this study are discussed in Eberts and others (2003).

Phytodegradation

The uptake of contaminated ground water by the planted trees 

results in the translocation of contaminants into plant organs (roots, 

stems, and leaves). TCE might be metabolized within the plant 

organs; this process is called phytodegradation.

To assess the presence and magnitude of contaminant uptake 

and translocation of TCE at the phytoremediation demonstration 

site, samples of roots, stems, and leaves were collected and analyzed for TCE and its breakdown products (Eberts and others, 

2003).

Phytovolatilization

Phytovolatilization is a multimedia transfer of contaminants 

from water or soil to the atmosphere. Volatile organic compounds 

are taken up and transpired with water vapor or diffused out of 

leaves, stems, and roots (McCutcheon and Schnoor, 2003). 

Figure 3. Processes associated with phytoremediation.
4

Transpired vapors were not sampled and analyzed from the 

trees at the demonstration site, but data from a previous study 

(Newman and others, 1997) indicate that phytovolatilization likely 

is not a major process associated with phytoremediation of TCEcontaminated ground water.

Phytostabilization

Phytostabilization refers to the binding of contaminated soils 

in place by vegetation and the subsequent immobilization of toxic 

contaminants in soil (McCutcheon and Schnoor, 2003). The establishment of rooted vegetation in the shallow subsurface helps to 

prevent contaminant migration that occurs as a result of wind and 

water erosion, soil dispersion, and leaching (U.S. Environmental 

Protection Agency, 2000). Phytostabilization of organic pollutants 

that are foreign to living organisms is based on sequestration processes, such as humification (McCutcheon and Schnoor, 2003). 

Humification refers to the incorporation of organic contaminants 

into soil organic matter; this reduces bioavailability.

Rising-head slug tests (Bouwer and Rice, 1976) were done at 

10 of the wells in 1996 soon after the trees were planted. The tests 

were repeated in 2003 at most of the 10 wells to help quantify any 

effects of biological clogging on the aquifer (N.A. Houston, U.S. 

Geological Survey, written commun., 2004). Phytostabilization 

effects are difficult to quantify, and sequential slug testing is the 

only method that has been used to assess these effects at the site.

Interim Findings

The findings from the 7-year demonstration project support 

the initial hypothesis that planted eastern cottonwood trees can 

decrease the mass of TCE in ground water by means of their root 

systems, followed by the biological alteration of TCE within the 

trees (Eberts and others, 2003). In addition, the trees can promote 

in-place microbial degradation of dissolved TCE within the 

aquifer.

Historical water-quality data collected at the site during the 

first 7 years of the study indicate that microbial degradation of TCE 

began to occur within the aquifer during this period, but after 7 

years, the aquifer was still in a state of geochemical transition. 

Oxidation-reduction conditions in the aquifer beneath the planted 

area are expected to continue in the future, but overall the principal 

process for decreasing the mass of TCE in the aquifer during the 

early years of the demonstration has been hydraulic control (Eberts 

and others, 2003). The relative contributions of transpiration and 

phytocontainment (hydraulic control) and in-place microbial 

degradation to the attenuation of the contaminant plume will be 

assessed after the plantations are judged to be operating at peak 

efficiency.

These interim findings provide insight into the duration of 

time (years) necessary for a phytoremediation system to become 

fully effective. They also demonstrate the importance of long-term 

monitoring for explaining processes related to changes in an aquifer that can be attributed to planted trees.

References

Bouwer, Herman, and Rice, R.C., 1976, A slug test for determining 

hydraulic conductivity of unconfined aquifers with completely or 

partially penetrating wells: Water Resources Research, v. 12, no. 3, 

p. 423–428. 

Cooper, D.T., and Ferguson, B., 1979, Avoid early selection for growth 

rate in cottonwood, in Southern Forestry Tree Improvement Conference 15, Mississippi State University, Proceedings: Starkville, Miss., 

p. 52–58.

Eberts, S.M., Harvey, G.J., Jones, S.A., and Beckman, S.W., 2003, 

Multiple-process assessment for a chlorinated-solvent plume, in

McCutcheon, S.C., and Schnoor, J.L., eds., Phytoremediation—

Transformation and control of contaminants: Hoboken, N.J., Wiley, 

p. 589–633.

Krinard, R.M., 1985, Cottonwood development through 19 years in a 

Nelder's Design: U.S. Department of Agriculture, Forest Service, 

Southern Forest Experiment Station, Research Note SO–322, 4 p.

McCutcheon, S.C., and Schnoor, J.L., 2003, Overview of phytotransformation and control of wastes, in McCutcheon, S.C., and Schnoor, 

J.L., eds., Phytoremediation—Transformation and control of contaminants: Hoboken, N.J., Wiley, p. 3–58.

Motoyama, T., Kadokura, K., Tatsusawa, S., Arie, T., and Yamaguchi, I., 

2001, Application of plant-microbe systems to bioremediation: 

RIKEN Review, no. 42, p. 35–38.

Newman, L.A., Strand, S.E., Choe, N., Duffy, J., Ekuan, G., Ruszaj, M., 

Shurtleff, B.B., Wilmoth, J., Heilman, P., and Gordon, M.P., 1997, 

Uptake and transformation of trichloroethylene by hybrid poplars: 

Environmental Science and Technology, v. 31, no. 4, p. 1,062–1,067.

U.S. Environmental Protection Agency, 2000, Introduction to 

phytoremediation: Cincinnati, Ohio, Office of Research and 

Development, National Risk Management Research Laboratory, 

EPA/600/R-99/107 [variously paged].

Vose, J.M., Swank, W.T., Harvey, G.J., Clinton, B.D., and Sobek, C., 

2000, Leaf water relations and sapflow in eastern cottonwood 

(Populus deltoides Bartr.) trees planted for phytoremediation of a 

groundwater pollutant: International Journal of Phytoremediation, 

v. 2, no. 1, p. 53–73.

White, W.N., 1932, A method of estimating ground-water supplies based 

on discharge by plants and evaporation of soils, results of investigation in Escalante Valley, Utah: U.S. Geological Survey WaterSupply Paper 659–A, p. 1–105.

— Sachin D. Shah and Christopher L. Braun 

For additional information, please contact

District Chief or Gregory Harvey, ASC/ENVR

U.S. Geological Survey Bldg. 8, Suite 200

8027 Exchange Dr. 1800 10th St., Area B

Austin, TX 78754–4733 Wright Patterson AFB, OH 45433

E-mail: dc_tx@usgs.gov E-mail: Gregory.Harvey@wpafb.af.mil

Any use of trade, product, or firm names is for descriptive purposes only 

and does not imply endorsement by the U.S. Government.

This study was supported by the Superfund Innovative Technology 

Evaluation Program of the U.S. Environmental Protection Agency, 

the Environmental Security Technology Certification Program of the 

Department of Defense, and the Environmental Safety and Health 

Division of the Aeronautical Systems Center, Engineering Directorate.