Full text of DOI:10.1002/etc.5620201130

Environmental Toxicology and Chemistry, Vol. 20, No. 11, pp. 2617–2626, 2001
2001 SETAC
Printed in the USA
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BIOACCUMULATION OF METALS IN PLANTS, ARTHROPODS, AND MICE AT A
SEASONAL WETLAND
K
EVIN C. TORRES*† and MICHAEL L. JOHNSON‡
†Tetra Tech, 3746 Mount Diablo Boulevard, Suite 300, Lafayette, California 94549, USA
‡University of California, John Muir Institute of the Environment, Davis, California 95616, USA
(
Received 19 September 2000; Accepted 24 April 2001)
Abstract—Concentrations of arsenic, cadmium, copper, lead, and nickel were measured in soils, house mice (Mus musculus), and
the main food items of this omnivorous mouse to examine the occurrence of these metals in selected components of a seasonal
wetland. Soil concentrations of copper, lead, and (in some areas) nickel were elevated, but extractable soil concentrations indicated
low bioavailability of metals. Levels of most metals in mice and composited arthropods were consistent with reference site
concentrations from other studies. However, copper was found to be particularly mobile within the local ecosystem and accumulated
in house mouse carcasses and composited arthropods at substantial levels. Metal residues in Scirpus robustus (alkali bulrush) roots
exceeded those in seeds, consistent with patterns of bioaccumulation commonly observed in plants. Uptake and bioaccumulation
factors for S. robustus seeds and roots, arthropods, and mouse carcasses and livers are reported. Concentrations of lead and nickel
in S. robustus roots exhibited significant linear relationships with levels in soils. Copper levels in S. robustus seeds varied significantly
with those in house mouse livers, suggesting that trophic transfer of copper from this food source to mice occurred. However, other
spatial patterns of bioaccumulation in S. robustus and house mice relative to soil/seed concentrations were absent. Metal levels in
house mice bore no relation to body weight or estimated age.
Keywords—Metal bioaccumulation
Bioavailability
Ecotoxicology
Small mammals
Terrestrial biota
INTRODUCTION
bioavailability of metals in soils and sediments, interpret the
magnitudes of metal accumulation in house mice and other
biota, derive site-specific tissue:soil uptake and bioaccumu-
lation factors for organisms at the site, and examine potential
bioaccumulation and trophic transfer relationships through re-
gressions of metal concentrations in organisms versus those
in soils/sediments. Possible relationships between metal con-
centrations in house mice and their body weight or age were
also examined.
Concentrations of chemicals in soils and organisms provide
information about their movement through the environment,
bioaccumulation, trophic transfer, and potential toxicological
effects. Many studies have examined the prevalence and dis-
tribution of trace metals in terrestrial food webs [1–5]. Patterns
of uptake and bioaccumulation have been investigated by
studying relationships between metal concentrations in soils
and plants [6–8] and in soils and tissues of co-occurring an-
imals [9,10]. A variety of physical and chemical soil charac-
teristics mediate the release of metal species bound to clays,
metal oxides, or organic matter in the soil [11], affecting up-
take of metals by biota. These influences can complicate the
identification of patterns of uptake and bioaccumulation.
The relative concentrations of metals among tissues of
plants [12] or animals [1,13] can reveal general trends of ex-
posure, uptake, translocation, and assimilation of metals within
organisms. Trophic transfer of metals within the food web may
be demonstrated by relating metal levels in dietary components
with those assimilated by an animal. In addition, tissue residues
in animals can indicate potential adverse effects posed by met-
als when linkages between toxicity test results and critical body
residues are established [14].
METHODS
Site description
The study was performed at the former Mare Island Naval
Shipyard on San Pablo Bay in Vallejo, California, USA. Ships
and submarines were built, maintained, or repaired at the fa-
cility from the 19th century until the base was decommissioned
in 1996. The 0.6-ha study site was within installation resto-
ration site 1 (IR01), encompassing the northern portion of
Wetland X [15]. This site consisted of a nontidal, seasonal,
brackish wetland partially surrounded by disturbed nonnative
grassland with scattered shrubs (Fig. 1). The site becomes
partially inundated each year from late fall to early summer
with seasonal rainwater and rising groundwater. During the
summer and fall, the lowest-lying areas are barren salt pans.
The wetland plant community is spatially heterogeneous main-
ly because of variations in microtopography and is dominated
by Salicornia virginica (pickleweed), Scirpus robustus (alkali
bulrush), Atriplex patula var. hastata (fat hen), and annual
grasses. The most abundant small mammal at the site is the
house mouse, which occurs year-round. California voles (Mi-
crotus californicus) are less abundant; they increase in num-
bers when green herbaceous vegetation is available but are
nearly absent during dry months (K. Torres and M. Johnson,
This paper describes the occurrence and distribution of met-
als in selected components of a terrestrial food web at a sea-
sonal wetland. Concentrations of arsenic, cadmium, copper,
lead, and nickel were measured in soils and sediments, tissues
of house mice (Mus musculus), and the mouse’s principal food
items. The study had five objectives: characterize the concen-
trations of these metals in soils and sediments, evaluate the
* To whom correspondence may be addressed
(kevin.torres@tetratech.com).
2617

2618
Environ. Toxicol. Chem. 20, 2001
K.C. Torres and M.L. Johnson
mice. During the fifth month of trapping, 11 mice identified
as long-term residents of the site were sacrificed by carbon
dioxide asphyxiation. Individuals were selected to maximize
time since first capture, capture frequency, and discreteness of
home range. Approximate home ranges of these mice were
plotted on a map as capture locations (mean of 10.7 captures
each) (Fig. 1). Two additional adults (L and M) that were
trapped infrequently were also collected for analysis. All mice
were weighed and frozen for subsequent dissection and chem-
ical analysis.
Soil samples were apportioned so that home ranges of the
11 mice each contained two sampling locations and at least
one location was assigned to capture locations of the other
two mice (Fig. 1). The probability of each location sampled
within a home range was weighted by the relative number of
captures. To characterize other portions of the site, three soil
samples were collected outside the capture locations. The col-
lective area across sample locations was representative of those
parts of the site utilized by the population at large, which
excluded the more disturbed and barren areas. At each of 20
sampling locations, two soil samples were collected at random
directions and distances, yielding a total of 40 sampling points.
Soils were collected with a stainless-steel trowel from the sur-
face to a depth of 6 in.
Fig. 1. Capture locations of sampled house mice and soil and plant
sampling locations at study site, Mare Island, Vallejo, California,
To determine which foods were consumed by house mice
and consequently were potential pathways for trophic transfer
of metals, dietary analyses were conducted on fecal samples
from eight individuals trapped during the study. Feces were
analyzed with microhistological techniques at Composition
Analysis Laboratory (Fort Collins, CO, USA). Percentage rel-
ative densities of discernible food items were converted from
frequencies of occurrence in microscope fields [17]. Relative
densities were assumed to correspond to diet composition as
percentage dry weight. Based on dietary data (see Results),
seeds of the wetland plant S. robustus and arthropods were
selected for chemical analysis. Scirpus robustus seeds, the
primary food of local house mice, were present and collected
from plants at 12 of the 20 sampling locations (Fig. 1). At
eight of those locations, mature S. robustus roots were also
sampled for chemical analysis to help evaluate the distribution
of metals within the plant. Single composite samples of both
seeds and roots were collected across the two soil sampling
points at each location. In an upland area west of the dirt road
containing no S. robustus, house mouse diets consisted pri-
marily of grass seeds of the Bromus sp. type. For bioaccu-
mulation modeling in Torres and Johnson [18], Hordeum mar-
inum seeds were collected in a composite sample as a surrogate
for all grass seeds in that area, as they were more available at
the time of sampling (Fig. 1).
USA.
D,
sample,
●
E,
Trap location; capture locations:
⅜
K,
Mouse A,
L, and
#
u
B,
M;
ࡗ
⅜
C,
soil
Ⅲ
z
F,
y
G,
᭡
H,
᭝
I,
✙
J,
□
:
□
S. robustus sample,
᭝
H. marinum sample.
unpublished data). The federal and state endangered salt marsh
harvest mouse (Reithrodontomys raviventris) was live-trapped
only rarely during the study.
Potential sources of contaminants at or near the site include
buried waste oil sumps; materials discarded at a former dump-
ing ground (e.g., metal, concrete, and plastic construction de-
bris); the adjacent facility landfill (active from 1965 to 1978);
the historic landfill (utilized from 1942 to 1966) to the north-
west; submarine batteries, which may have released lead oxide
into the ground; and relatively high ambient levels of metals
in the artificial fill that constitute much of the surface layer of
Mare Island [15,16]. Past soil and sediment analyses at the
site and in adjacent areas revealed the presence of elevated
metals, polycyclic aromatic hydrocarbons, solvents, polychlor-
inated biphenyls, and petroleum hydrocarbons [16]. In the pre-
sent study arsenic, cadmium, copper, lead, and nickel were
evaluated because they were among the most elevated metals
in soils and sediments [16] or exhibit potential for uptake and
bioaccumulation.
Sample collection
Individual arthropod taxa in house mouse diets were not
identified through fecal analysis. Consequently, three of the
most common and readily available species of terrestrial ar-
thropods were sampled from the 20 locations. Armadillidium
Soils and sediments, plant materials, terrestrial arthropods,
and mice were sampled from the study site for chemical anal-
ysis to evaluate aspects of metal bioaccumulation for each of
the study objectives. Surface soils and sediments were not
differentiated in sampling or data analysis and are hereafter
referred to collectively as soils.
Small mammals were live-trapped from July to December
1997 in Sherman traps (Tallahassee, FL, USA) baited with
wild birdseed. Traps were spaced at 25-ft (7.6-m) intervals
within a 250- by 250-ft grid. Trapping was conducted during
two-night periods occurring every other week, with 2 of the
11 periods consisting of four nights. Over a total of 3,146 trap-
nights, 672 total captures were made of 199 individual house
vulgare (pillbug, Isopoda) (
(jumping spider, Arachnida) (
spider Araneus sp. (Arachnida) (
n
ϭ
n
67 individuals), Phidippus sp.
29), and the web-spinning
9) were collected with
ϭ
n
ϭ
forceps from the ground or vegetation and frozen. Because of
the limited amount of sample biomass, each arthropod taxon
was analyzed as a single sitewide composite sample.
Chemical analysis
Soil samples were oven dried, ground, and analyzed at the
University of California Division of Agriculture and Natural

Bioavailability and bioaccumulation of metals
Environ. Toxicol. Chem. 20, 2001
2619
Resources (DANR) Analytical Laboratory (Davis, CA, USA)
for total metal content, partially extractable metal content, and
several physical and chemical properties.
For each total metal analysis, a 0.5-g soil sample was com-
bined with 0.5 ml of nitric acid and 2 ml of 30% hydrogen
USA). Arsenic was analyzed by hydride generation. Lead was
measured with a Perkin-Elmer Model 5100 Zeeman-corrected
atomic absorption spectrometer. The quality assurance/quality
control measures included analysis of method blanks, dupli-
cates, spike recoveries, and SRMs. The standard reference ma-
terials were NIST 1577b bovine liver (Gaithersburg, MD,
USA), National Research Council of Canada (NRCC) TORT-
2 lobster hepatopancreas (Ottawa, ON, Canada), NRCC
DORM-2 dogfish muscle, and NRCC DOLT-2 dogfish liver.
Recoveries from SRMs were 87 to 111, 96 to 103, 95 to 107,
82 to 103, and 87 to 112% for arsenic, cadmium, copper, lead,
and nickel, respectively.
peroxide in a Teflon௡ PFA vessel. The mixture was micro-
waved for 14 min and diluted to 15 ml with deionized water.
Soils analyzed for extractable metals were first processed with
the diethylene triamine pentaacetic acid (DTPA) extraction
method [19]. Analytical samples were prepared by adding 20
ml of 0.005 M DTPA extraction reagent to 10 g soil. The
mixture was shaken for 2 h at 25ЊC and filtered. Totally di-
gested and DTPA-treated soils were analyzed for arsenic, cad-
mium, lead, and nickel in a thermo Jarrell-Ash atom scan 25
inductively coupled plasma atomic emission spectrometer
(ICP-AES; Franklin, MA, USA). Copper was measured with
a Perkin-Elmer 2380 atomic absorption spectrometer (AAS;
Norwalk, CT, USA). A method blank, duplicates, and National
Institute of Standards and Technology (NIST) and in-house
standard reference materials (SRMs) were analyzed for each
metal. Recoveries from IRM 020 soil and IRM 012 ash (Ultra
Scientific, North Kensingtown, RI, USA) and UCD 150 alfalfa
(DANR Analytical Lab) SRMs were 94 to 100, 99, 94, 107,
and 103% for arsenic, cadmium, copper, lead, and nickel, re-
spectively.
To evaluate the potential bioavailability of metals from
soils, four physical and chemical soil properties were deter-
mined: pH, cation exchange capacity, soil organic matter (cal-
culated on the basis of an assumed 58% organic matter carbon
content), and particle size (sand, silt, and clay content). One
sample from each pair (20 of 40 samples) was analyzed for
soil properties. One duplicate and one standard reference ma-
terial were analyzed for each soil parameter, and a method
blank was analyzed for percentage organic matter.
Mouse carcasses and arthropod composites were analyzed
in duplicates where sufficient material was available. The spi-
der Phidippus sp. was analyzed only for lead because of lim-
ited sample size. For data reporting, carcass results were ad-
justed on the basis of the amount of water added during ho-
mogenization. Copper levels in mouse livers were calculated
into the separately measured carcass concentrations, but results
for other metals were not combined in this way because they
were undetected in carcasses and/or livers.
Uptake and bioaccumulation factors
Dry-weight, soil-to-plant uptake factors (PUFs) were cal-
culated for S. robustus roots and seeds relative to soil con-
centrations and averaged across colocated samples. Soil-to-
animal tissue bioaccumulation factors (BAFs) were calculated
for arthropods and house mouse carcasses/livers on the basis
of composited or average sitewide tissue concentrations, re-
spectively. Measured wet-weight concentrations were con-
verted to dry weight on the basis of assumed water contents
of 65% in arthropods [20], 68% in small mammal whole bodies
[20], and 72% in small mammal livers [21].
Bioaccumulation regressions
Mice were thawed, weighed, washed externally, and rinsed
with deionized water. During dissection the liver was removed,
weighed, and refrozen for separate analysis. The skin, pelage,
feet, tail, and gastrointestinal tract were removed and discard-
ed, and the carcass was homogenized with a Polytron tissue
grinder (Brinkmann Instruments, Westbury, NY, USA) in a
measured amount of double-deionized water. Seeds and S. ro-
bustus roots were air dried and cleaned of visible soil particles,
and roots were rinsed and gently scrubbed with deionized wa-
ter. Roots and seeds were ground to homogeneous powders.
Each arthropod species was homogenized in a sitewide com-
posite sample.
Scatter plots and regressions of S. robustus, mouse carcass,
and liver concentrations on soil concentrations were created
from colocated data (e.g., within home ranges of mice). To
ascertain whether metals accumulated in house mouse tissues
in proportion to age or body weight, carcass and liver con-
centrations were plotted against the estimated age and final
body weight of each individual. The age of individual mice
was estimated from body weight at first capture [22] increased
by the time interval preceding final capture.
Data in each graph were fit to simple linear regression
models using NCSS
linear relationships was evaluated with the
௢
(Kaysville, UT, USA). Significance of
test (H0: B1 ϭ 0).
t
Plant, arthropod, and mouse tissues were analyzed for met-
als at the California Veterinary Diagnostic Laboratory System
Toxicology Laboratory (Davis, CA, USA). Arthropod or
mouse tissue was digested by combining 1 g homogenized
tissue with 3 ml nitric acid; 0.5 g dried plant tissue was digested
with 4 ml nitric acid. Samples analyzed for arsenic were
amended with 1 ml perchloric acid and 1 ml sulfuric acid.
Samples were digested in a Tecator digestion system (Hoganas,
Sweden). Next, mixtures analyzed for arsenic were combined
with 7 ml 5 M hydrochloric acid and 1 ml 10% potassium
iodide solution; these samples were digested and diluted to 10
ml with deionized water. Digested samples analyzed for lead
were also diluted with deionized water. To tissue samples an-
alyzed for cadmium, copper, and nickel were added 2 or 5 ml
hydrochloric acid; samples were diluted with deionized water.
Arsenic, cadmium, copper, and nickel were measured with
ICP-AES on a Fison’s Model Accuris ICP (Waltham, MA,
Data for metals without any detected residues were not ana-
lyzed. However, individual nondetects were included in graphs
as one-half of the detection limits [23].
RESULTS
Metals in soils
Total concentrations of arsenic, cadmium, copper, lead, and
nickel in soils and sediments (soils) are shown in Table 1.
Levels of copper and lead were the most variable across the
site, as indicated by the wide confidence intervals of the means.
Arsenic, cadmium, and nickel were distributed more homo-
geneously in soils (Table 1).
Physical and chemical characteristics of soils varied rela-
tively little across the site despite marked differences in mi-
crotopography, inundation, and vegetation types (Table 2).
Soils were characterized by a neutral pH (mean of 7.1), low

2620
Environ. Toxicol. Chem. 20, 2001
K.C. Torres and M.L. Johnson
organic matter and sand content, and a relatively high cation
exchange capacity and silt and clay contents. The DTPA-ex-
tractable concentrations of metals and the percentages of total
metal concentrations extractable by DTPA suggest limited po-
tential for release of bound metals in soils; extractable metals
ranged from 4.5% for nickel to 24.3% for copper (Table 1).
Diet composition of house mice
Analysis of fecal samples collected from house mice re-
vealed a total of nine discernible dietary components. Scirpus
robustus seeds and arthropod parts (not identified to taxa) were
the two most abundant components (relative densities of 74.0
and 8.9%, respectively). Other plants (Salicornia virginica,
Bromus sp. type, Evolvulus sp., and two unidentified plants)
collectively totaled 17.1% of the house mouse diet. Setaria
italica (millet) seeds used in trap bait averaged 17.7% of the
relative density of identified fecal materials. Relative densities
of other dietary items were retabulated excluding S. italica
seeds for bioaccumulation modeling [18].
Metals in organisms
In S. robustus metals accumulated at substantially higher
levels in roots than in seeds (Table 3). Copper exhibited the
highest root concentrations, averaging 287 mg/kg. Seed con-
centrations were also highest for copper, followed by nickel;
the remaining metals were present at much lower or undetected
levels in seeds. Copper and nickel also showed the highest
seed:root concentration ratios in S. robustus (Table 3). Lead
was measured at 0.376 mg/kg in composited H. marinum
seeds.
The sitewide composite of the detritivorous isopod A. vul-
gare contained higher concentrations of all metals than did
the composited spider Araneus sp. (Table 3). In both species,
copper levels were highest, while lead and arsenic were among
the least accumulated metals. The composited spiders Araneus
sp. and Phidippus sp. contained similar levels of lead.
Among the metals detected in house mouse carcasses, cop-
per occurred at the highest levels, followed by lead, with ar-
senic found at the lowest levels (Table 3). Cadmium and nickel
were undetected in all carcasses. Copper, which was the only
metal detected in livers, was measured at lower concentrations
in livers than in carcasses.
Uptake and bioaccumulation factors
Both PUFs and BAFs summarize the bioaccumulation of
metals in specific biota and tissues relative to soil levels (Table
4). The largest S. robustus root:soil PUFs were calculated for
cadmium (3.5), arsenic (1.7), and copper (1.1); lead and nickel
had PUFs lower than 1. The S. robustus seed:soil PUFs for
copper and nickel were higher than those for arsenic and lead.
Armadillidium vulgare:soil BAFs derived from individuals
composited across the site indicate that copper (5.6) and cad-
mium (2.7) accumulated more than arsenic (0.12), nickel
(0.07), and lead (0.04) relative to ambient soil levels. A similar
pattern held for cadmium (2.2), copper (0.82), arsenic (0.09),
nickel (0.02), and lead (0.01) in Araneus sp.
In house mouse carcasses, copper accumulated at the high-
est levels with respect to soil concentrations; on a dry-weight
basis, copper was present in carcasses at half of the mean soil
concentration (Table 4). Copper accumulated in livers at less
than six times the accumulation in carcasses relative to soil
concentrations. Liver:soil BAFs for other metals were not
quantified because of undetected residues in livers.

Bioavailability and bioaccumulation of metals
Environ. Toxicol. Chem. 20, 2001
2621
Table 2. Soil characteristics
Particle size (%)
CEC
(meq/100g)b
Organic
matter (%)
pHa
Sand
Silt
Clay
Value
7.1
Ϯ
0.1 (6.5–7.7)
(20)
39.5
Ϯ
1.3 (31.5–42.5)
(20)
2.8
Ϯ
0.2 (1.9–3.8)
(20)
12
Ϯ
3 (6–26)
(20)
45
Ϯ
2 (36–50)
(20)
43
Ϯ
2 (35–52)
(20)
a Values are mean
b CEC
cation exchange capacity.
Ϯ 95% confidence interval (p Յ 0.05). Ranges in parentheses above. Sample sizes in parentheses below.
ϭ
Bioaccumulation and trophic transfer relationships
availability to plants has been demonstrated for zinc, copper,
manganese, and iron [19] and also exhibits potential for use
with cadmium, lead, and nickel [24,25]. However, the suit-
ability of using the DTPA test to estimate arsenic bioavail-
ability is questionable [26].
A statistically significant relationship was observed be-
tween lead concentrations in S. robustus roots (ln transformed
to meet the assumption of constant error variance) and total
lead concentrations in soil (r2
ϭ
0.59; Fig. 2a). A similar
relationship between nickel levels in roots and total nickel
levels in soil was also found (r2
0.68; Fig. 2b). No such
Metal uptake and bioaccumulation
ϭ
patterns between root and total soil levels of other metals
existed. When concentrations in S. robustus roots were plotted
against DTPA-extractable soil concentrations, no significant
relationships held for cadmium, copper, or nickel. Lead levels
in S. robustus roots, however, varied significantly with DTPA-
Scirpus robustus exhibited substantial enrichment of metals
in roots relative to those in seeds (Table 3). This pattern is in
agreement with the observed tendency of many metals to be-
come immobilized in root and other below-ground storage
tissues and undergo limited translocation to aboveground
structures [27,28]. Seed:root concentration ratios were rela-
tively low for all metals (Table 3), consistent with findings
that seeds typically contain much lower concentrations of
heavy metals than other plant parts [12].
extractable soil concentrations (r2
ϭ 0.79; Fig. 3). Root con-
centrations of lead were ln transformed to meet the assumption
of constant error variance. No significant relationships were
observed between metal concentrations in S. robustus seeds
and total soil, extractable soil, or root concentrations.
Concentrations of metals in house mouse carcasses exhib-
ited no significant relationships with total soil, extractable soil,
or S. robustus seed concentrations averaged over individual
home ranges, nor were relationships found between carcass
concentrations of metals and either final body weights or es-
timated ages of mice.
As for differences among metals, concentrations in roots
and seeds were highest for copper, which was the metal also
present at the greatest soil concentrations. The metal with the
highest root:soil PUF was cadmium, with nickel and lead ex-
hibiting the lowest accumulation in roots relative to ambient
soil concentrations (Table 4). This finding was consistent with
the greater soil bioavailability and root uptake observed for
cadmium compared to most other heavy metals [3,28]. The
lower root:soil PUF for lead (Table 4) was in agreement with
the limited uptake of lead by roots [12,27]. Lead accumulation
in roots is commonly overestimated since much of the lead
measured in roots is bound tightly on surface deposits and is
not biologically assimilated [8,29]. Copper levels in S. ro-
bustus seeds were clearly greater than levels of other metals
in seeds, followed by nickel (Table 3). This is consistent with
findings that copper and nickel accumulate to a greater extent
than cadmium and lead in grains of wheats and oats [12,30].
By their trophic status the terrestrial arthropods (a detriti-
vorous isopod and two carnivorous arachnids) are expected to
have greater potential for metal accumulation than most other
invertebrates occurring in the area. Concentrations of copper,
lead, and nickel in composite samples of the isopod A. vulgare
exceeded those in the two spiders (Table 3). Terrestrial isopods
accumulate metals to a high degree as decomposers in intimate
contact with soil and plant litter and have been promoted as
biomonitors for this reason [4].
Since most metals were undetected in livers, only regres-
sions with copper were modeled. No statistical trend was ob-
served between copper levels in livers and in soil (total or
extractable concentrations) or with body weights or estimated
ages. However, liver concentrations of copper did vary sig-
nificantly with copper concentrations in seeds averaged over
the home ranges of individuals (r2
ϭ 0.51; Fig. 4). Liver con-
centrations were square-root transformed to meet the assump-
tion of constant error variance.
DISCUSSION
Concentrations and bioavailability of metals in soils
Concentrations of arsenic and cadmium in surface soils and
sediments (soils) were within ambient concentrations in Mare
Island artificial fill, which generally originated from dredged
offshore sediments, and probable effects levels for freshwater
sediments (Table 1). However, concentrations of copper, lead,
and (in some areas of the wetland) nickel were clearly elevated
above these values (Table 1). Identifying the origins of metals
in sediments and upland soils at the site is complicated by the
various contributions from the island’s bay sediment substrate
and on-site and off-site activities.
While high concentrations of copper, lead, and nickel pose
potentially high exposures to local biota, conditions mediating
bioavailability also must be considered in characterizing the
transfer of metals into the food web. Relatively small fractions
of total metals in soil were extractable with DTPA (Table 1),
indicating limited availability of metals for uptake by organ-
isms. Use of DTPA soil extraction as an indicator of metal
Concentrations of cadmium in A. vulgare and the spider
Araneus sp. and levels of lead in A. vulgare, Araneus sp., and
Phidippus sp. (Table 3) were similar to concentrations found
in other terrestrial isopods and spiders from unpolluted habitats
(Table 5). The cadmium BAFs calculated for A. vulgare and
Araneus sp. were lower than BAFs reported for other isopods
and spiders (means of 21.1 and 18.2, respectively) (converted
to dry weight assuming 65% water content in arthropods) [31].
However, copper concentrations in A. vulgare and Araneus
sp. were high relative to reference concentrations (Table 5)

2622
Environ. Toxicol. Chem. 20, 2001
K.C. Torres and M.L. Johnson

Bioavailability and bioaccumulation of metals
Table 4. Plant and animal bioaccumulation factors
Environ. Toxicol. Chem. 20, 2001
2623
Plant uptake factor/bioaccumulation factora
Organism/
tissue:soil
Arsenic Cadmium Copper Lead
Nickel
Scirpus robustus
Root:soil
Seed:soil
1.7
0.003
3.5
1.1
0.06
0.42
0.001
0.24
0.01
Ͻ
0.38b
House mouse
Carcass:soil
Liver:soil
0.01
Ͻ
Ͻ
1.0b
0.53
0.08
0.06
Ͻ
Ͻ
0.02b
0.01b
Ͻ
0.36b
0.59b
Ͻ
0.03b
a Calculated from detected values only unless otherwise specified.
b Undetected in tissue; upper limit of PUF/BAF based on average
detection limits.
and similar to those in isopods and spiders from copper-pol-
luted sites [4,32]. Measured levels of arsenic and nickel in
these taxa do not appear elevated.
Carcasses of house mice contained concentrations of ar-
senic, lead, and nickel that were within the ranges of average
whole-body or carcass concentrations observed in rodents from
reference sites or other areas with low metal pollution (Table
5). Lead concentrations in carcasses were far below whole-
body concentrations (41 and 44 mg/kg wet wt) associated with
Fig. 3. Plotted relationship between concentrations of lead in Scirpus
robustus roots and in diethylene triamine pentaacetic acid-extractable
soil. Significant relationship based on
6 degrees of freedom, 0.0031.
t test for B1 ϭ 0: t ϭ 4.7659,
p
ϭ
least slightly underreported since carcass concentrations could
not be adjusted with undetected residues of these metals in
livers. Carcass concentrations of lead should not be signifi-
cantly biased since more than 90% of the total mammalian
body burden of lead is stored in bone (P. Barry, 1975, as
reported in [33]). Excluding hair and nails from carcass anal-
yses was likely to result in lower arsenic concentrations in
carcasses compared to whole-body concentrations since much
of the arsenic in mammals is stored in hair and nails. Cadmium
and nickel were undetected in all house mouse carcasses, but,
on the basis of its detection limits (0.4–0.7 mg/kg), nickel also
appeared to be within ambient levels in rodents (Table 5).
Copper concentrations in house mouse carcasses (arith-
kidney pathology in short-tailed voles (Microtus agrestis
)
[21]. Arsenic and lead concentrations in carcasses may be at
Fig. 2. Plotted relationships between concentrations of (a) lead and
(b) nickel in Scirpus robustus roots and in totally digested soils.
Fig. 4. Plotted relationship between square-root copper concentrations
in house mouse livers and concentrations in Scirpus robustus seeds
Significant relationships based on
6 degrees of freedom, 0.0261; (b)
freedom, 0.0121.
t
test for B1
ϭ
0: (a)
t
ϭ
2.9343,
p
ϭ
t
ϭ
3.5488, 6 degrees of
in individual home ranges. Significant relationship based on
B1 0: 2.7216, 7 degrees of freedom, 0.0297.
t test for
p
ϭ
ϭ
t
ϭ
p ϭ

2624
Environ. Toxicol. Chem. 20, 2001
Table 5. Average metal concentrations reported in arthropods and rodents from unpolluted reference sitesa
Concentration (mg/kg wet weight)b
K.C. Torres and M.L. Johnson
Arsenic
Cadmium
Copper
Lead
Nickel
Arthropods
Terrestrial isopods
Spiders
—
—
2.1–12.7
0.1–1.0
9.1–85
14.8–49
0.3–13.2
0.5–2.8
—
—
Rodents
Whole body/carcass
Liver
0.004–0.04
0.06–2.6
0.02–0.3
0.03–0.3
2.1–4.3
2.6–23.3
0.2–2.7
0.1–2.5
0.36–3.1
0.5–1.2
a Sources: Hunter et al. [4]; Talmage and Walton [21]; Hopkin [32]; Eisler [39]; others cited in Torres [50].
b Dry-weight concentrations were converted to wet-weight values assuming moisture contents of 65% in arthropods [20] and 68% and 72% in
whole bodies and livers, respectively, of small mammals [20, 21].
metic mean 40.1 mg/kg; geometric mean 27.6 mg/kg) were
considerably greater than those reported in rodents from both
reference sites (Table 5) and sites with identified copper pol-
lution (body burdens ranging up to 26.3 mg/kg wet wt)
[5,21,34]. A very high body burden of copper (522 mg/kg wet
wt) was also noted in one house mouse collected elsewhere
on Mare Island [15]. Although copper was found at high levels
in soil and arthropods at the site, it seems evident that the
substantial levels of copper in house mice could not occur
without disruption of internal copper regulation. Gastrointes-
tinal absorption of copper in mammals is normally regulated
by the amount of copper in the gastrointestinal tract [35].
Storage and biliary excretion of copper as coordinated by the
liver is actively regulated by blood copper levels and hormones
[36]. Copper accumulation in mammals also can be influenced
by genetic factors and dietary levels of proteins, cadmium,
silver, zinc, iron, and molybdenum [36,37]. Reports of no sig-
nificant differences in body burdens of copper in small mam-
mals among polluted and unpolluted sites have been attributed
to effective homeostatic control of this essential metal [5,21].
However, differences in the concentration of copper in livers
noted among sites with varying contamination suggest that
internal copper regulation may prove ineffective for animals
inhabiting highly polluted areas, possibly the case for mice at
this site [1,13].
Undetected levels of arsenic, cadmium, lead, and nickel
and measured levels of copper in house mouse livers were
consistent with background concentrations in rodent livers (Ta-
ble 5). Lead accumulated in livers below the toxic range for
mammals (above 1.4 mg/kg wet wt [33]), and cadmium levels
were far below those linked to liver pathology in common
shrews (Sorex araneus) at a smelter site (84 to 280 mg/kg
[38]). It is uncertain why copper was markedly elevated in
carcasses (including liver concentrations) but not in livers.
These results are at odds with observations that copper pref-
erentially accumulates in liver compared with muscle, bone,
and other tissues [1].
density of house mice varied from 49 to 105 individuals/ha,
with the population increasing at the close of the study (K.
Torres and M. Johnson, unpublished data).
A previous investigation of small mammals at Mare Island
and other pickleweed marshes along San Francisco and San
Pablo Bays (CA, USA) found variable levels of metals and
detectable levels of polychlorinated biphenyls in tissues but
was unable to conclude whether contaminants posed harmful
effects to species such as the endangered salt marsh harvest
mouse [40]. Possible risks to salt marsh harvest mice in Mare
Island wetlands at and near the study site were identified for
lead, nickel, manganese, and selenium using conservative as-
sumptions [15]. Additional research on the local diet of this
mouse would help in quantifying its chemical exposures in
potentially contaminated wetlands.
In terrestrial ecological risk assessments, increased data on
tissue residues would eliminate problems associated with bio-
availability assumptions in assessing soil contamination and
reduce uncertainties in exposure modeling. Critical whole-
body and organ residues associated with acute or chronic ef-
fects have been established for certain metals and organic com-
pounds in aquatic organisms [14,41], but data for terrestrial
wildlife are lacking. Efforts to quantitatively link tissue resi-
dues, physiological biomarkers, histopathology, and relevant
effects in wildlife species exposed in the laboratory [42], in
experimental field enclosures [43], and at contaminated sites
[44] are valuable. These data are critical in view of the com-
plexities of interspecies and seasonal exposure shifts, metal
speciation, metallothionein binding and regulation, and dose-
dependent mechanisms of toxic action.
Bioaccumulation and trophic transfer relationships
The most direct bioaccumulation pattern examined, that
between metal concentrations in S. robustus roots and in am-
bient soils (uptake), exhibited significant linear relationships
for lead and nickel (Figs. 2 and 3). Indeed, metal concentra-
tions in soils are the primary factor determining concentrations
in rooted aquatic macrophytes, despite the intimate contact of
submerged plants with the water column [45]. However, levels
of the other three metals in roots did not vary significantly
with either total or extractable soil concentrations. Other stud-
ies failed to show positive relationships between levels of ar-
senic, cadmium, or lead in roots of estuarine and aquatic plants
and those in soils or sediments [7,8], and arsenic levels in the
roots of one plant were negatively correlated with soil levels
[7].
Copper was the only metal examined that was found at
excessive levels in house mice relative to literature values. In
healthy mammals exposed to copper, barriers to copper ab-
sorption and incorporation of copper ions into hepatic metal-
lothionein, nuclear proteins, and lysosomes generally protect
against copper toxicity except at very high exposures [36,39].
At high oral exposures, liver and kidney damage, anemia, re-
tarded growth, reproductive and developmental effects, and
mortality may occur [35,39]. It is unknown whether house
mice in this study were adversely affected by ingested copper.
Population data collected during trapping indicated that the
No statistically significant relationships existed between
metal concentrations in S. robustus seeds and in roots. The

Bioavailability and bioaccumulation of metals
Environ. Toxicol. Chem. 20, 2001
2625
correspondence between seed and root levels would depend
on metal translocation from roots to seeds, a transport and
storage process not necessarily directly dependent on root con-
centrations. The lack of a trend between S. robustus seed and
soil concentrations is not surprising, given the added factors
of soil availability and root uptake.
Tissue burdens of metals in terrestrial animals may vary
with ambient soil concentrations through direct ingestion of
soil particles and via transfer through the food web. Metal
bioaccumulation in house mouse carcasses and livers was not
significantly related to soil concentrations in the home ranges
of individual mice. In contrast, Shore [10] found significant
correlations between liver and kidney levels of cadmium in
field mice (Apodemus sylvaticus) and common shrews and
levels of these metals in associated soils, and between liver
and kidney concentrations of lead in field mice and short-tailed
voles and concentrations in soils. Sharma and Shupe [9] also
reported a significant relationship between cadmium levels in
rock squirrel (Spermophilus variegatus) livers and soil but
uncovered no such patterns for arsenic or lead.
available reference site concentrations from other studies.
However, copper was found to be particularly mobile within
the local ecosystem and accumulated in house mouse carcasses
and composited arthropods at substantial levels. Metal residues
in S. robustus roots exceeded those in seeds, consistent with
patterns of bioaccumulation and translocation commonly ob-
served in plants. Concentrations of lead and nickel in S. ro-
bustus roots exhibited significant correlations with levels in
ambient soils. Copper levels in S. robustus seeds varied sig-
nificantly with those in house mouse livers, suggesting that
trophic transfer of copper from this food source to house mice
occurred. However, other spatial patterns of bioaccumulation
in S. robustus with respect to soil levels and in house mice
relative to soil and seed concentrations were not demonstrated.
Finally, metal levels in house mice bore no relation to body
weight or estimated age.
Acknowledgement—We thank Daniel W. Anderson and Roger L.
Hothem for their comments on the manuscript. The assistance of
Cuong Pham, Wallace Neville, John Randell, Robin Leong, and others
at Mare Island was also appreciated.
The significant relationship between copper concentrations
in livers and S. robustus seeds (Fig. 4) indicates that trophic
transfer of copper to house mice most probably occurred. Sim-
ilarly, in a study of metal accumulation in forest plants and
animals, copper concentrations in wood mouse (Apodemus
flavicollis) livers were positively correlated with concentra-
tions in the species’ main dietary item (Fagus sylvatica nuts)
[46]. However, the present study did not demonstrate any
trends of concentrations of copper or other metals in carcasses
with increasing S. robustus seed concentrations. A possible
explanation is that copper accumulates in brain, muscle, and
bone (which constitute a large portion of the carcass biomass)
less closely in relation to environmental contamination than it
does in liver, kidneys, or hair [1].
Finally, the absence of relationships between house mouse
tissue concentrations and estimated age or body weight did
not support other findings that these time-varying factors are
associated with metal accumulation. Cadmium is known to
accrue with age in kidneys and liver, but cadmium concentra-
tions could not be regressed on age or body weight because
of lack of detection in livers and carcasses. Lead similarly
accumulates in bone over time, but no such pattern was seen
in carcasses.
In general, the lack of regression relationships involving
metal accumulation in biota may be due to effective regulation
of metals by organisms and/or spatial variation in availability
of metals from soil or food. Extractable metal concentrations
were applied as indicators of bioavailable fractions in the soil,
but soil characteristics were not considered in regression mod-
els. While some studies (e.g., [47]) have found significant
relationships through multiple regressions among metal con-
centrations in plants and various extractable soil or sediment
fractions, others (e.g., [6]) have had limited or no success
relating plant concentrations to soil properties through simple
or multiple regression models.
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