Phytoremediation of Contaminated Soil and Water - Chapter 3 potx

3
FACTORS INFLUENCING
FIELD
PHYTOREMEDIATION OF
SELENIUM-LADEN SOILS
Gary S. Bañuelos
CONTENTS
Introduction
General Observations on Reported Field Studies
Influential Factors in Field Phytoremediation
Crop Rotation
Water Management
Water Reuse
Predators (Insects and Wildlife)
Selenium Inventory
Utilization of Plants in Phytoremediation
Conclusion
References
INTRODUCTION
In California’s San Joaquin Valley and in numerous other irrigated agricultural areas
in the western U.S., irrigation effluent may accumulate in confined shallow aquifers,
eventually rising to levels that adversely affect crops (Ayars et al., 1994). To sustain
long-term agricultural productivity in these regions, subsurface drainage systems for
the removal of this effluent must be installed (Mercer and Morgan, 1991; Ayars,
1996). On the western side of San Joaquin Valley, there are several thousand hectares
of irrigated land (possessing subsurface drains) with high water tables resulting from
over irrigation. Because the drainage system was never completed, the saline effluent
produced in this region was eventually routed and discharged into Kesterson National
Wildlife Refuge. The wetlands receiving the drainage water, in the course of being
used as wildlife habitat, were also operated as evaporation ponds to reduce the
volume of agricultural wastewater. Deleterious effects on birds and fish were doc-

umented on biological systems inhabitating or frequenting Kesterson Reservoir
(Ohlendorf and Hothem, 1995). Selenium was identified as the element of primary
Copyright © 2000 by Taylor & Francis
concern. Studies showed exposure to high Se diets resulted in high tissue Se con-
centrations in waterfowl (Presser and Ohlendorf, 1987; Tanji et al., 1986; Ohlendorf
and Hothem, 1995; Sylvester et al., 1991; U.S. Department of the Interior, 1986).
These findings resulted in closing of the Kesterson Reservoir for receiving drainage
effluent from agricultural lands.
Growers on the west side of the San Joaquin Valley have tried alternative
practices to reduce their production of Se-laden effluent and thus sustain their
agricultural land. After more than a decade of extensive research on Se remediation
in California, several strategies to reduce loads of Se from entering the effluent have
been proposed by the Salinity Drainage Task Force Committee in California (UC
Salinity/Drainage Program, 1993). Some of these include improvement of irrigation
and drainage management practices (Grattan, 1994; Mercer and Morgan, 1991),
microbial volatilization (Frankenberger and Karlson, 1994), and vegetation manage-
ment (phytoremediation) with perennial and annual crops (Bañuelos and Meek,
1990; Wu et al., 1988; Parker et al., 1991; Parker and Page, 1994).
Phytoremediation is a plant-based technology that is being considered for man-
aging Se in central California soils (Bañuelos and Meek, 1990; Terry and Zayed,
1994, 1998; Parker and Page, 1994; Wu and Huang, 1991) and for removing other
toxic trace elements in soils (Baker et al., 1994; Chaney et al., 1994; McGrath et
al., 1993; Salt et al., 1995; Cunningham and Lee, 1995; Blaylock et al., 1997). The
phytoremediation technology for Se implies the use of plants in conjunction with
microbial activity to extract, accumulate, and volatilize Se. Any one or a combination
of these plant responses may lead to lower concentrations of soluble Se in the soil
and thus lower amounts entering the effluent. Most greenhouse studies on phytore-
mediation of Se have shown that Se added as soluble selenate can be extracted and/or
volatilized from soil, translocated to shoot tissue, and removed as Se-laden plant
material. Studies are needed, however, that demonstrate the effectiveness of phy-

toremediation for reducing the amount of naturally occurring Se entering effluent
(Martens and Suarez, 1997), since options for disposing of Se-laden effluent are still
unclear.
Field research on Se phytoremediation is still in the nascent stage (Bañuelos et
al., 1993); however, field studies are crucial to develop sound phytoremediation
strategies for remediating soils and sediments (Schnoor et al., 1995). Growing crops
to manage soluble Se by field phytoremediation requires the application of a wide
range of knowledge about the chemistry and transformation of Se in soil, Se uptake
and its toxicity in plants and animals, and sustainable agronomical practices neces-
sary for long-term crop production. The successful implementation of phytoreme-
diation requires growing selected crops in Se-containing soils as part of a crop
rotation and simultaneously reducing amounts of soil Se primarily by plant uptake.
Factors to consider for phytoremediation under field conditions in central California
include: (1) soil salinity and high concentrations of toxic elements, (2) presence of
competitive ions affecting Se uptake, (3) adverse climatic conditions, (4) water
management strategies that produce less effluent, (5) unwanted consumption of high
Se plants by wildlife and insects, and (6) acceptance of phytoremediation as a
remediation technology by the public and growers in regions known to have Se.
Copyright © 2000 by Taylor & Francis
The objective of this chapter is to summarize results from recent field studies
conducted by the USDA–ARS, Fresno, and the University of California on managing
levels of naturally occurring Se in west side soils of central California.
GENERAL OBSERVATIONS ON REPORTED FIELD STUDIES
Field sites with moderate levels of naturally occurring Se near Los Baños, CA were
selected for evaluating crops used in phytoremediation. The soils at these sites
contained very little plant-available Se (soluble forms of Se). Because of regulatory
restrictions placed upon growers in water districts of the west side of the San Joaquin
Valley, the load of soluble Se leaving these soils via drainage effluent is closely
controlled. Thus, Se concentrations in drainage water produced from all west side
soils must be constantly monitored.

Cropping, irrigation, cultivation of the soil, and organic matter (Neal and Sposito,
1991) contribute to the solubilization and movement of immobile or complexed soil
Se. For this reason, some growers are committed to planting crops considered for
phytoremediation as a preventative measure for reducing the amount of soluble Se
entering their effluent. Information regarding growth performance and uptake of
naturally occurring Se under field conditions, e.g., high salinity, boron, and sulfate,
for these crops is limited, however. Moreover, there is no information available on
managing Se in the soil with selected phytoremediation crops.
INFLUENTIAL FACTORS IN FIELD PHYTOREMEDIATION
Crop Rotation
Crop selection is an important factor for successful field phytoremediation of Se.
Phytoremediation strategies should initially consider rotations among phytoremedi-
ation crops under field conditions. This practice will likely contribute to a constant
production of biomass and to a reduction of plant disease (e.g., Fusarium, Rhizoc-
tonia root rot, Alternaria block spot), insect population, and weed buildup. Different
crops used in rotation may extract Se from different zones of the soil profile and
deposit it at more accessible depths for eventual uptake by subsequent crops used
in phytoremediation. For long-term maintenance of Se-containing soils in Se-sensi-
tive areas of the west side of central California, selected crops should be tried in
rotation with other agronomic crops, e.g., cotton, wheat, tomatoes, etc., typically
grown in these saline soils (Shennan et al., 1995).
Bañuelos and colleagues (1997) evaluated a rotation of selected crops as a
preventative measure for reducing amounts of naturally occurring Se entering efflu-
ent from soils located near Kesterson Reservoir. The following crops that can reduce
soil Se levels were evaluated near Los Baños, CA on 15 10 x 10 m plots: Indian
mustard (Brassica juncea Czern L.), tall fescue (Festuca arundinacae), birdsfoot
trefoil (Lotus corniculatus), kenaf (Hibiscus cannibinus), and bare plots (without
plants). The four different phytorotations used from 1992 to 1995 consisted of the
following: (I) bare plots; (II) Indian mustard, Indian mustard, tall fescue, tall fescue;
Copyright © 2000 by Taylor & Francis

(III) birdsfoot trefoil, birdsfoot trefoil/tall fescue mixture, birdsfoot trefoil/tall fescue
mixture, tall fescue; and (IV) kenaf, kenaf, tall fescue, tall fescue.
Table 3.1 presents the dry matter of the above-ground biomass production for
the tested crops, including two annual clippings for tall fescue, birdsfoot trefoil, tall
fescue, and birdsfoot trefoil mixture on an area (m
2
) basis for each year. Kenaf
produced the greatest amount of biomass among the tested species. Tissue Se
concentrations were all under 1 mg Se kg
-1
DM, except for Indian mustard which
exceeded 2 mg Se kg
-1
DM (Table 3.1). Plots from crop rotation II, which had Indian
mustard for the first 2 years, had the greatest reduction of total soil Se compared to
all plots after 4 years (Table 3.1). Total soil Se concentrations between 0 to 60 cm
were lower in all cropped plots than the bare plots after 4 years. Overall, the cropped
plots were more effective in lowering total soil Se in years 1992 and 1993 than in
1994 and 1995. The percentage changes between preplant and post-harvest soil Se
concentrations (lost Se) after 4 years for each crop rotation is as follows: I – 17%,
II – 60%, III – 34%, and IV – 41%.
In another multiyear field study conducted near Los Baños, Bañuelos and col-
leagues (1995) planted tall fescue on six 17 x 17 m plots and left six plots bare.
Table 3.2 shows that, despite the low tissue Se concentrations, the cropped plots had
25% lower soil Se concentrations after 4 years from 0 to 45 cm vs. 11% in bare
plots and 25% lower in cropped plots from 45 to 90 cm vs. 3% in bare plots,
respectively. Tall fescue is a perennial grass with an extensive root system and a
high transpiration rate. The species appears to be moderately effective at reducing
soil Se concentrations near the soil surface, as well as in the subsurface profiles.
Moreover, tall fescue is salt tolerant and thus a likely candidate for use on soils with

relatively high levels of salinity. Although such perennial crops as tall fescue may
take up less Se, their compatibility with conventional fodder crop equipment (e.g.,
mechanical swather and baler) make them ideal candidates as low-maintenance crops
used for phytoremediation. Selenium inventory of lost Se under field conditions will
be discussed later in this chapter.
Water Management
Water requirements are not known for crops used in phytoremediation, yet water
management strategies are essential for growing Se-accumulating plants in irrigated
regions of the west side of central California regions to produce the greatest amount
of biomass with the minimum application of water. Efficient irrigation reduces
percolation losses and the production of Se-laden effluent. Bañuelos et al. (1993,
1995) used data collected weekly by neutron probe to a depth of 1.5 m and data
collected from the California Irrigation Management Information System (CIMIS)
weather station (Howell et al., 1983) to estimate crop water use and schedule
irrigation on crops used in phytoremediation. Responses of such crops as canola and
kenaf cultivars to different regimens of irrigation are presently being evaluated in a
multiyear field study conducted in central California (Tables 3.3 and 3.4). Based on
the preliminary data, production of biomass increased with the amount of water
applied up to reference evapotranspiration (Et
r
). The greater the yields, the more Se
Copyright © 2000 by Taylor & Francis
TABLE 3.1
Changes in Naturally Occurring Se Concentrations from 0 to 60 cm Depth and Mean Tissue Concentrations
of Se in Crops Grown in Different Rotations for Phytoremediation
Total Soil Se Concentration at:
Year Plant Species
a
Rotation # Preplant Post-Harvest Change
b

Shoot Se DM Yield
(mg kg
-1
soil) (%) (mg kg
-1
DM) (g m
-2
)
1992 K° (bare plot) I 1.32(.08)
c
1.27(.06) 4 NA NA
Indian mustard II 1.20(.06) 0.90(.09) 25 2.15(.06) 1328
Birdsfoot trefoil III 1.18(.12) 1.06(.09) 10 0.58(.01) 435
Kenaf IV 1.41(.09) 1.29(.10) 9 0.70(.02) 3125
1993 K° (bare plot) I 1.22(.10) 1.16(.08) 5 NA NA
Indian mustard II 0.85(.09) 0.69(.06) 19 1.70(.06) 1212
Birdsfoot trefoil/ III 1.09(.06) 0.92(.05) 16 0.61(.02) 721
tall fescue
Kenaf IV 1.26(.13) 1.09(.10) 13 0.59(.03) 3450
1994 K° (bare plot) I 1.18(.09) 1.13(.09) 4 NA NA
Tall fescue II 0.66(.07) 0.58(.07) 12 0.41(.01) 400
Birdsfoot trefoil/ III 0.86(.07) 0.77(.09) 10 0.56(.02) 902
tall fescue
Tall fescue IV 1.13(.09) 1.01(.08) 11 0.52(.03) 350
1995 K° (bare plot) I 1.15(.10) 1.10(.12) 4 NA NA
Tall fescue II 0.56(.06) 0.51(.08) 9 0.39(.01) 705
Birdsfoot trefoil/
tall fescue III 0.81(.07) 0.78(.07) 4 0.36(.01) 1121
Tall fescue IV 0.95(.08) 0.83(.06) 13 0.42(.01) 802
Note: NA = Not applicable.

a
Indian mustard and kenaf were planted and harvested and then replanted the following year. Tall fescue and birdsfoot trefoil were only planted
once in their respective plots.
b
Percent change of total Se concentrations between preplant and post-harvest soil sampling for each respective year.
c
Values represent the mean from six replicates followed by the standard error in parenthesis.
Copyright © 2000 by Taylor & Francis
TABLE 3.2
Annual Changes in Naturally Occurring Se Concentrations from 0 to 90 cm Depth Planted to Tall Fescue
or Allowed to Remain as Bare Plot
a,b
Total Se Concentrations
Year Treatment Soil Depth Preplant Post-Harvest Change Shoot Se DM Yield
(cm) (mg kg
-1
soil) (%) (mg kg
-1
DM) (kg m
-2
)
1992 Bare Plot 0–45 1.65(.12) 1.61(.14) 2 NA
45–90 1.59(.09) 1.65(.13) 4
Planted 0–45 1.47(.10) 1.32(.09) 10 1.02 4.4
45–90 1.63(.04) 1.49(.09) 9
1993 Bare Plot 0–45 1.71(.14) 1.63(.09) 5 NA
45–90 1.72(.12) 1.67(.13) 3
Planted 0–45 1.37(.10) 1.24(.08) 9 1.25 4.5
45–90 1.43(.11) 1.41(.09) 1
1994 Bare Plot 0–45 1.58(.13) 1.55(.12) 3 NA

45–90 1.60(.10) 1.63(.09) 2
Planted 0–45 1.31(.14) 1.20(.13) 8 2.10 5.4
45–90 1.30(.10) 1.34(.09) 3
1995 Bare Plot 0–45 1.57(.10) 1.53(.10) 3 NA
45–90 1.58(.13) 1.62(.12) 2
Planted 0–45 1.19(.08) 1.15(.09) 3 1.95 6.3
45–90 1.25(.12) 1.17(.09) 6
1996 Bare Plot 0–45 1.49(.13) 1.47(.11) 5 NA
45–90 1.49(.14) 1.55(.15) 4
Planted 0–45 1.20(.12) 1.09(.11) 9 2.41 6.9
45–90 1.34(.10) 1.23(.09) 8
Note: NA = not applicable.
a
Values are means from 24 samples collected each year followed by standard error in parenthesis.
b
Values are means from three clippings collected each year.
Copyright © 2000 by Taylor & Francis
that can be extracted from the soil. If a minimum amount of water will grow a
successful crop, then the acceptance and use of phytoremediation by growers in
water districts where water supplies are stringently regulated might be greater.
Other studies have shown the effect of field irrigation practices on biomass and
Se accumulation using various plant species on a two-acre field plot by Lawrence
Berkeley Laboratory and University of California (Wu, 1994). Generally, water
applied efficiently contributed to an increase in biomass. Wu reported that irrigation
increased biomass plant tissue Se concentrations in Brassica hyssopifola Kuntze
(summer weed) and in Melilotus indica (winter weed). Irrigation scheduling influ-
ences growth of the rooting system. Thus, encouraging deeper root development
with planned water deficits may permit some plant species to access bioavailable
Se in the deeper subsoil horizons. Information is needed to evaluate the influence
of soil water levels on promoting volatilization of Se by the plant or soil microbes,

as well as whether sequential drying and rewetting cycles promote microbial activity
(Frankenberger and Karlson, 1994).
Water Reuse
Se-laden drainage effluent that contains dissolved salts and other constituents must
be managed to minimize its detrimental effects on the ecology of the water or land
where it is discharged (van Schilfgaarde, 1990; Shennan et al., 1995). Field studies
were conducted in central California by Watson and colleagues (Watson and
O’Leary, 1993; Watson et al., 1994; Bañuelos et al., 1993) with Se-laden effluent
used as a source of irrigation water on Atriplex (saltbush) spp. and B. juncea (Indian
mustard). The Atriplex species were grown on 30 x 124 m plots, while the B. juncea
was planted three successive times on 5 x 15 m plots and replicated 4 times,
respectively. The agricultural effluent used as the source of irrigation water had an
electrical conductivity (EC) of 19 dS m
-1
and a Se concentration ranging from 150
to 200 μg l
-1
. Mustard plants accumulated Se up to 3.1 mg Se kg
-1
DM, whereas the
Atriplex species did not exceed 0.6 mg Se kg
-1
DM (Table 3.5). If water reuse is to
be considered as a disposal option for Se-laden effluent, long-term feasibility of
reusing Se-laden drainage water is dependent not only on crop selection (e.g., Indian
mustard, saltbush), but also on well planned strategies related to managing chemical
and physical changes of the soil (Shennan et al 1995; Ayars et al., 1994; Grattan,
1994). Management strategies should include: (1) maintaining a favorable salt bal-
ance — the mass of salts leaving the area must be greater than or equal to that
entering the area; (2) maintaining good soil physical conditions; (3) considering the

mobility and retention of specific elements within the soil that can be toxic to the
plant (e.g., B) or to biological consumers (e.g., Cd, Se); and (4) finding economically
viable salt-tolerant crops that accumulate Se. Brassica species may be a suitable
candidate to receive Se-laden effluent. However, unless plants take up Se faster than
evapotranspiration, the net effect of irrigating land with Se-enriched water may
increase the soil Se level (Parker and Page, 1994), unless leaching or volatilization
by plants and microorganisms is occurring.
Copyright © 2000 by Taylor & Francis
Predators (Insects and Wildlife)
Identification of insects frequenting crops and soils used in phytoremediation is
important in agriculture-producing regions like central California, especially with
flowering plant species, i.e., Indian mustard, birdsfoot trefoil; Tables 3.6 and 3.7.
Flowering plants tend to attract greater numbers of potential predators which could
be harmful or beneficial to other near-growing agricultural crops. In addition, the
transfer of Se from soil to crop, from crop to insect, from insect to insect, and from
insect to animal, is a biological Se cycle (bioaccumulation) that should be monitored
in long-term field phytoremediation of Se-laden soils. There has been contradictory
evidence as to whether biomagnification of Se exists in the food chain (Kay, 1984;
Lemly, 1985). Research is needed to examine the dynamics of Se bioaccumulation
in insects frequenting field sites. Factors affecting bioaccumulation depend upon the
availability of soil Se, plant species, the feeding behavior of the food chain consum-
ers, and mobility of insects. A herbivore may have a choice in the quality of its diet.
For example, grasshoppers (Dissosteria pictipennis Brunner) may reject plants that
accumulated higher tissue Se concentrations, whereas mantises (Litaneutria minor
Scudder) have little choice if they rely on grasshoppers (Table 3.6; Wu et al., 1995).
Unpublished data by Bañuelos and colleagues have shown that aphids (Aphididae
spp.) prefer feeding upon non-Se-containing Brassica species compared to Se-
containing plants. Plants that have accumulated high concentrations of Se may
inadvertantly discourage the infestation by some insect species. High tissue Se
concentrations by Brassica plants and subsequent bioaccumulation by insects and

animals are concerns because of the deleterious effects Se may exert on birds and
mammals that eat the insects. Ohlendorf and Santolo (1994) have illustrated in great
detail exposure pathways and projected Se concentrations in biota at Kesterson
Reservoir. Strategies that are used in fruit production (e.g., metal reflectors and noise
guns) may be useful in frightening off certain animals from field-grown Brassica
planted for phytoremediation. In field experiments, Bañuelos and colleagues (1997;
TABLE 3.3
Parameters Used for Approximating Water Application Rate for Kenaf
with Subsurface Irrigation Under Field Conditions
Treatment
Total Applied
Water
a
Net Soil
Water
Depletion
Calculated
Crop Et
c
Potential Et
r
Actual
(% Et
c
) (mm) (mm) (mm) (mm) Et
c
/Et
r
25 95 125 220 418 0.23
50 190 141 331 418 0.45

100 379 175 554 418 0.91
125 473 49 522 418 1.13
150 593 78 671 418 1.47
a
Effective precipitation was 0 mm during crop year and assumes no deep percolation losses.
Copyright © 2000 by Taylor & Francis
TABLE 3.4
Biomass of Canola and Cultivars of Kenaf Exposed to Different Water Application Rates Given in Table 3.3 with Subsurface
Irrigation Under Field Conditions
a
Treatment
% Et
c
7-N C-531
Kenaf cultivars:
C-533 EU-41 TA-2 Canola
Leaves Stem Leaves Stem Leaves Stem Leaves Stem Leaves Stem Leaves Stem
(Mg ha
-1
) (Mg ha
-1
)
25 9.4 12.7 11.4 17.4 10.1 14.8 8.4 13.0 10.5 17.7 0.86 3.9
50 10.7 13.2 12.0 24.0 10.9 22.3 10.9 23.9 17.9 21.6 1.83 5.3
100 12.9 23.5 15.5 36.6 16.2 28.9 12.9 26.6 13.9 28.3 2.36 8.3
125 12.2 2.25 15.6 30.1 15.2 26.7 11.3 21.3 12.3 26.5 4.33 7.5
150 13.7 26.2 16.0 28.8 12.2 27.6 11.2 25.4 19.2 28.2 3.10 9.3
a
Based on a plant population of 160,000 plants ha
-1

.
Copyright © 2000 by Taylor & Francis
TABLE 3.5
Shoot Tissue Concentrations of Se and Other Selected Elements in Brassica juncea and Atriplex
Species Irrigated with Se-Laden Saline Effluent
Tissue Concentrations of
Plant Species Se B Fe Mn Zn Cu S
(mg kg
-1
DM)
Brassica juncea
a
3.1(.10) 275(.10) 250(.04) 177(.08) 55(.03) 2(.01) 20400(.11)
Atriplex canescens
b
0.5(00) 126(.07) 411(.09) 60(.02) 36(.04) 6(.01) 10400(.12)
A. undulata 0.6(00) 131(.05) 348(.09) 68(.01) 37(.02) 7(.01) 7500(.10)
A. deserticola 0.6(00) 121(.03) 388(.07) 53(.02) 35(.03) 6(.01) 10900(.09)
A. nummularia 0.6(00) 142(.04) 391(.05) 44(.01) 30(.01) 5(.01) 9260(1.1)
A. polycarpa 0.6(00) 135(.03) 401(.08) 70(.02) 39(.02) 6(.01) 9960(.07)
a
Values for B. juncea are the means from three plantings followed by the coefficients of variation.
b
Values from the Atriplex species are the means from three harvests followed by the coefficients of variation. (From Watson
et al., J. Environ. Qual. 48: 157-162, 1994.)
Copyright © 2000 by Taylor & Francis
data not shown) have observed that grown Brassica seedlings are vulnerable to
vertebrae herbivores. Ground squirrels that forage throughout the winter were espe-
cially fond of eating Brassica seedlings.
Selenium Inventory

A complete soil and plant Se inventory was not attempted in our field studies because
of the inability to accurately account for (1) the variability of soil Se; (2) the
transformation of naturally occurring Se into forms of Se not measured (e.g., volatile
Se); (3) the movement of Se below sampling depth (90 cm); and (4) the inability to
completely recover the root system of the specific crop. An estimated amount of
soil Se removed in only the above-ground portion of a crop as a function of shoot
tissue Se concentrations (on a DM basis) did not exceed 10% of the lost soil Se
reported in the field studies presented in Tables 3.1 and 3.2 (calculation used 4 x
10
6
kg soil/ ha/30 cm soil). This small percentage implies that a substantial amount
of Se was also lost or removed through other processes, e.g., volatilization, leaching.
Limited attempts at constructing Se inventories with vegetation management have
been made by Parker and Page (1994). However, because of the above factors, an
accurate mass-balance is difficult to construct. Caution should be taken when extrap-
olating greenhouse data to field scenarios because measurements and cultural prac-
tices in the field are not as precise as those in pot experiments and, more importantly,
plant tissue concentrations of Se are always lower under field conditions. This
assumption is illustrated in the following scenario with Se-laden soil collected from
Kesterson Reservoir. Bañuelos and colleagues (1998) collected Se-laden soil from
different depths ranging from 0 to 90 cm. Concentrations of total Se concentrations
were as high as 112 mg kg
-1
soil (at the surface) and as low as 10 mg kg
-1
soil (at
the deepest depth). Tissue concentrations in canola (B. napus) grown in the collected
TABLE 3.6
Tissue Selenium Concentrations Found in Grasshoppers and
Mantis from Different Sites at Kesterson Reservoir

Soil Se Range of Se in
Total Extractable Grasshopper
a
Mantis
b
Site # (mg kg
-1
soil) (mg l
-1
)(mg kg
-1
insect)
1 0.6 ± .03
c
0.05 ± .02 1.2–9.8 9–22
2 53.7 ± 17.5 0.72 ± .63 9.1–27.5 31–52
3 0.1 ± .09 0.02 ± .01 3.1–7.0 10.2–18.0
4 4.2 ± 2.3 0.16 ± .04 1.0–4.6 5.5–10.3
a
Dissosteria pictipennis Brunner.
b
Litaneutria minor Scudder.
c
Selected values are the means followed by the standard deviation. (From Wu et al.,
Environ. Toxicol. Chem. 14: 733-742,1995.)
Copyright © 2000 by Taylor & Francis
soil were as high as 200 mg kg
-1
DM. When a mass-balance was attempted after
harvest and absolute amounts of lost soil Se were calculated (based upon differences

between preplant and post-harvest soil Se concentrations), approximately 54 mg of
TABLE 3.7
General Survey of the Predominate Insects Habitating Different
Crop Species Used for Phytoremediation of Selenium Soils in
Central California
Insects
a
Crop Species Family Genus sp. or Common Name
Birdsfoot trefoil Miridae Lygus sp.
Thripidae Frankliniella sp. (western flower thrips)
Cicadellidae Empoasca sp. (leafhopper)
Aceratagallia sp.
Colladonus mountanus
Indian mustard Miridae Lygus sp.
Thripidea Frankliniella sp. (western flower thrips)
Chrysomelidae Subfamily: Alticinae (flea beetle)
Rhopalidae Liorhyssus hyalinus (hyaline grass bug);
adults and nymphs were found.
Pentatomidae Stink bugs
Cicadellidae Empoasca sp. (leafhoppers)
Tall fescue Thripidae Frankliniella sp. (western flower thrips)
Aeolothripidae Banded thrips
Chrysomelidae Subfamily: Alticinae (flea beetles)
Rhopalidae Liorhyssus hyalinus (hyaline grass bug)
Cicadellidae Amblysellus grex (hopper)
Euscelis obsoletus
Other
Delphacidae Plant hoppers
Coccincellidae Ladybird beetles and other
Kenaf Miridae Lygus sp.

Cocincellidae Hypersapis sp.
Cicadellidae Empoasca sp. (leafhoppers)
Aceratagallia sp.
Lepidoptera Salt marsh caterpillar
Cotton Miridae Lygus sp.
Cicadellidae Empoasca sp. (leafhoppers)
Chrysomelidae Subfamily: Alticinae (flea beetle)
Fallow (weeds) Thripidae Frankliniella sp. (western flower thrips)
Lygaeidae Nysius raphanus (false chinch bug)
a
Insects are not ranked in order of predominance. Predominance varies with time of
year. Sampling took place 15 times during the growing season using an insect sweep
net. An in-depth identification of insects is presently being pursued.
Copyright © 2000 by Taylor & Francis
Se from the potted soil was missing. Just 23% of this missing Se was accounted for
by accumulation in plant tissues, while 77% went unaccounted. Under field condi-
tions, total soil Se concentrations were on the average of 26 mg kg
-1
soil from 0 to
90 cm depth. Canola planted in this soil at Kesterson Reservoir accumulated approx-
imately 50 mg Se kg
-1
DM in the plant tissue. When a mass-balance was attempted
based on 30-cm depth, approximately 312 mg Se m
-2
was missing from the soil
(based upon differences between preplant and post-harvest soil Se accounted for in
plant tissue). Canola accumulated less than 10% of the Se missing from the top 30
cm of soil.
The efficacy of phytoremediation is generally greater under controlled green-

house conditions than under field conditions. In the above greenhouse and field
studies conducted with Kesterson soil, it is clear that a percentage of lost soil Se
(difference measured between preplant and post-harvest Se concentrations) was not
recovered in plant tissue or in soil at post-harvest soil sampling. This “unaccounted
for Se” may have been lost through biological volatilization of Se. The phenomenon
of Se volatilization is suggested as an alternative explanation for the lower Se
concentrations in soils, especially under field conditions. Certain fungi can convert
Se species, selenite and selenate, to dimethylselenide and/or dimethyldiselenide,
which are volatile compounds of Se (Frankenberger and Karlson, 1990). Although
direct measurements of field volatilization were not made in the previously described
studies, we acknowledge the ability of microorganisms and plants (Terry et al., 1992;
Biggar and Jayaweera, 1993; Wu and Huang, 1991; Doran, 1982) to volatilize Se.
Under field conditions, Frankenberger and Karlson (1994) and Karlson and Fran-
kenberger (1989) found that volatilization of Se can be enhanced in the field with
the addition of a carbon source and by having sufficient aeration and moisture under
high temperatures. Irrigation practices (wetting and drying) may enhance the organ-
ically bound release of organic-bound Se to the methylating organisms in other field
studies. Biggar and Jayaweera (1993) found that soil planted to barley volatilized
20 times more Se than soil alone, while Wu and Huang (1991) measured rates of
180 μg Se m
-2
d
-1
from Distichlis spp. (salt grass). Although it is difficult to distin-
guish between plant and microbial volatilization, the presence of plants increases
the total rate of Se volatilization (Biggar and Jayaweera, 1993). Terry and his research
group at the University of California, Berkeley are determining the physiological,
biochemical, and microbial mechanisms involved in the volatilization process of Se.
Volatilization of Se by plants will be discussed in Chapter 4.
Utilization of Plants in Phytoremediation

Selenium, while not required by plants, is an essential trace element for adequate
nutrition and health for mammals (Se deficiencies are probably a greater problem
than Se toxicities in animals, especially on the east side of central California).
Generally, diets containing 0.1 to 0.3 mg kg
-1
Se will provide adequate Se for animal
feed (Mayland, 1994). Animal producers wishing to ensure adequate supplies of Se
to their livestock have a variety of techniques at their disposal, which include giving
Se by injection or by mouth as a feed supplement. Alternatively, harvested plant
material, e.g., Brassica, used in phytoremediation can be carefully blended with
Copyright © 2000 by Taylor & Francis
animal forage and fed to animals (pending approval by regulating agencies) in Se-
deficient areas. The quality of Brassica herbage is more comparable to a concentrate
than to a traditional forage because of the relatively low fiber and high protein
content (Wiedenfoeft and Barton, 1994). Therefore, plant species used for phytore-
mediation in Se-laden soils may not only minimize the Se load eventually entering
agricultural effluent by plant uptake and volatilization, but also the crop may become
a product of economic importance for the grower as part of an agronomic crop
rotation scheme.
Another means of improving the Se status of animals is to add plant material
with high concentrations of Se (e.g., Brassica) to soils as a source of organic Se
fertilizer for such forage plants as alfalfa, birdsfoot trefoil, or tall fescue. These plant
species will eventually absorb some of the organically added Se (Bañuelos et al.,
1993; Ajwa et al., 1998). Table 3.8 shows the accumulation of Se and selected
nutritional values in potential forage crops grown in soil with incorporated Se-laden
plant material used previously for phytoremediation. Ajwa and colleagues (1998)
have evaluated carefully the fate of Se as a mineralized product excreted from
animals and followed its subsequent uptake by plants. They found that canola and
tall fescue accumulated less than 10 mg Se kg
-1

DM after three plantings in the same
soil amended with seleniferous organic materials. It is important to be aware of the
absorption and eventual fate of Se fed to animals. Different crops may store Se as
different seleno-amino acids (Abegaz, 1997) and thus affect its rate of absorption
after consumption by different animals.
Other disposal possibilities for Se-laden plant material include its use in the
production of paper products or utilization as a fuel for biogeneration power plants.
The latter option must be considered if concentrations of toxic trace metals (i.e., Cd
and As) found in the crop exceed concentrations deemed environmentally safe for
TABLE 3.8
Forage Quality of Different Plant Species Grown
Under Field Conditions in Soil Enriched with Se-
Laden Indian Mustard
Crude Digestible
Protein Dry Matter Se Content
Planted Species
a
(%) (%) (mg kg
-1
DM)
Tall fescue 19 92 1.3
Alfalfa 22 93 4.7
Birdsfoot trefoil 19 92 3.1
Canola 25 92 51.0
a
Harvested Indian mustard, previously irrigated with selenate-contain-
ing water, was incorporated into soil to achieve a preplant total soil Se
concentration of 0.7 mg kg
-1
soil.

Copyright © 2000 by Taylor & Francis
animal consumption (National Research Council, 1980). Table 3.9 shows concen-
trations of selected trace elements in four crops used in the phytoremediation of
naturally occurring Se under field conditions. Successful long-term phytoremedia-
tion strategies must consider the trace element accumulation in harvested plant
material. If the harvested plant material is to be considered for animal forage, factors
such as age and physiological status of the animal (e.g., growth and lactation),
nutritional status, levels of various dietary components, duration and rate of exposure,
and biological availability of the compound influence the level at which a mineral
element causes an adverse effect in an animal or on the environment (Oldfield, 1998).
Oil extracted from the seed of Brassica species, e.g., canola, grown for Se
phytoremediation under field conditions is presently being evaluated by Bañuelos
et al. (1999, unpublished). Like other vegetable oils, Brassica oil is composed
predominantly of fatty acid containing triacyclglycerols with lesser amounts of
phospholipids and glycolipids (Uppström, 1995). The canola oil is characterized by
a low level of monounsaturated fatty acids and contains significant amounts of oleric
and inolenic acids, which have important nutritional properties explained elsewhere
(McDonald, 1995). The meal remaining after oil extraction is high in protein as a
result of accumulation during seed development of napin and cruciferin storage
proteins and oleasin, a structural protein associated with the oil bodies. This potential
product may increase the growers’ interest in considering a Brassica species in their
crop rotation for managing soil Se, since canola oil, a product of Brassica seeds, is
already an established and viable oil product throughout the world.
CONCLUSION
Field phytoremediation requires an integrated approach, which must consider initial
selection of crops used for phytoremediation, crop rotation, irrigation and drainage
water management strategies, chemical and mineralization transformations within
the soil, pest management, harvest techniques, biomass use, economic feasibility,
and social acceptance. Field studies should be conducted to effectively evaluate
phytoremediation as a technology to reduce and manage Se levels in Se-containing

soils and thus reduce the amount of soluble Se entering drainage effluent. More
importantly, field phytoremediation requires time and must be considered as a long-
term commitment.
Copyright © 2000 by Taylor & Francis
TABLE 3.9
Mean Concentrations of Selected Trace Elements in Four Crops Used for the Phytoremediation of Se-Laden
Soils Under Field Conditions for 2 Years
Elemental Concentrations:
Species Organ Zn Cd Mn Fe Al Cu Mo B
(mg kg
-1
DM)
Kenaf Shoot 42(1)
a
22(.4)
b
222(5) 354(12) 420(15) 11(.4) 1(.1) 737(41)
Root 29(.8) 21(.6) 35(1) 1268(52)
b
1335(70)
b
6(.2) 1(.1) 138(6)
Indian mustard Shoot 26(.7) 15(.4)
b
48(2) 162(12) 177(13) 5(.2) .4(.1) 273(10)
Root 20(.8) 12(.5) 30(3) 773(39) 1045(50)
b
4(.2) .3(.1) 120(7)
Tall fescue Shoot 24(.6) <1 46(3) 84(6) 77(4) 1(.1) <.1 99(3)
Root 28(.5) <1 22(1) 497(12) 941(44) 4(.2) <.1 112(9)

Birdsfoot trefoil Shoot 20(.4) <1 37(2) 97(4) 80(3) 6(.1) <.1 117(9)
Root 20(.5) <1 36(2) 477(16) 980(57)
b
5(.1) <.1 112(8)
a
Values are the means followed by the standard error mean in parenthesis for 2 years.
b
Concentrations could be considered potentially toxic for long-term direct animal consumption. (Based upon values reported and established by the
National Research Council, 1980.)
Copyright © 2000 by Taylor & Francis
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