Environ. Sci. Techno/. 1995, 29, 1495- 1503
History of Trace Metal Pollution
in Sabine-Neches Estuary,
Beaumont, Texas
MAHALINGAM R A V I C H A N D R A N , * , t j t
MAHALINGAM B A S K A R A N , I
P E T E R H . SANTSCHI,+A N D
T H O M A S S . BIANCHIg
Department of Marine Sciences, Texas A M University,
Galveston, Texas 77553, and Department of Ecology,
Evolution and Organismal Biology, Tulane University,
New Orleans, Louisiana 70118
~~
~~
Introduction
The distribution of metals in sediment cores sampled from
industrialized estuaries has been used as an indicator of
past and present pollution events (1,Z). Such reconstruction studies are useful to improve management strategies
as well as to assess the success of recent pollution controls.
Trace metal contamination in sediments has received more
attention in the last two decades, and a recent issue of
Estuaries (Vol. 16 issue 3B, September 1993) has been
dedicated to the study of dated sediment cores for
environmental research. Recently, Valette-Silver (3) reviewed the availableliterature on the reconstruction studies
and concluded that, in general, heavy metal pollution of
sediments began in the early 1800s, became more prominent in the 19OOs,and increased sharply between 1940 and
1970. In the last two decades, generally a decreasing trend
in heavy metal inputs into the environment is observed (2,
'
~
~~
Sabine-Neches Estuary, near Beaumont,TX, receives
wastewater effluents from over 160 industrial and
municipal treatment plants. The concentrations of trace
metals (Co, Cr, Cu, Ni, Pb, and Zn), AI, Fe, Mn, and
organic carbon were determined in four dated
sediment cores. A reliable geochronology and
reconstruction of the history of trace metal inputs of
these sediments was possible because the 239,240Pu
profiles closely trace the bomb fallout history into the
environment. Down-core variations of aluminumnormalized enrichment factors for these metals demonstrate that the sediments of this estuary have
remained relatively 'pristine' with respect to trace
metal contamination since 1860. While the concentrations of Pb and Zn at various depths in the sediment
column are slightly enriched, Co, Cr, Cu, and Ni are
depleted. The sedimentary and biogenic particles
that are presently being deposited are also depleted
in trace metals. Lack of strong enrichment for trace
metals like Cu can be attributed to the short residence
time of water, low salinity conditions, and possibly
strong complexation of these metals with organic matter.
3).
The most commonly used methods of dating recent
sediments is based on the concentration gradient of natural
radionuclides such as 210Pband the occurrence of bomb
fallout radioisotopes such as 137Csand239,240P~
(4). Because
of its 22.3 yr half-life, 210Pbhas been widely used to
determine sedimentation rates over the last 100-150 yr (5,
6). Establishing reliable sediment chronology in shallow
estuarine environments using 210Pbis complicated by the
post-depositional mixing of particles by physical and
biological (bioturbation) processes. Hence, a second
particle-reactive tracer, such as 239,240Pu,
is used in conjunction with 210Pbto deconvolute mixing from sedimentation (refs 4 and 7 and references cited therein). 239~240Pu
is a byproduct of atmospheric testing of nuclear weapons,
introduced into the environment around 1952 with a
maximum fallout in 1963. Plutonium is an excellent
analogue for studies of heavy metal transport through the
environment, because it is particle reactive, it is effectively
retained in aquatic and marine sediments, and it has a
well-defined input function (4, 7, 8).
The Sabine-Neches Estuary is strategically located in
the golden triangle area of Beaumont, Port-Arthur, and
Orange, which constitute the principal industrial areas in
the region. Since the beginning of petroleum related
industry in the study area in 1901 (91,the industrial activity
has steadily grown, and now almost all of the major oil and
chemical companies have plants in the vicinity of the
estuary. The combined ports of these three cities comprise
the fourth largest port facility in the nation. As of 1980,
there were over 160 permitted wastewater dischargers into
the Neches and Sabine Rivers, apart from other diffuse
sources such as urban runoff (10).
Despite the concentration of industries and the consequent discharges of wastewater effluents into the estuary,
very few studies have been carried out on the levels of heavy
metal concentrations in the sediments of the SabineNeches Estuary. The Texas Department ofwater Resources
t Texas A&M University.
4 Present address: Department of Civil, Environmental and
Architectural Engineering, University of Colorado at Boulder, Campus
Box 428, Boulder, CO 80309-0428; Fax: (303) 492-7317; e-mail
address: ravicham@ucsub.colorado.edu.
Tulane University.
0013-936W95/0929-1495$09.00/0
0 1995 American Chemical Society
VOL. 29, NO. 6, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1495
TABLE 1
Selected Metal Concentrations in Average Shale
(37),Average Soil (38),and Screening Criteria
(IO)for Metal Pollution in Sediments
elements
av shale av shale
concn [metal/Al]
ev soil
concn
av soil
[metal/Al]
screening
criteria
a
C
is the smallest (surface area = 259 km2),well mixed, and
the freshest estuary (average rainfall = 140 cm). It has the
highest annual freshwater inflow (1.63 x 1O1O m3/yr) with
an average salinity of just 2 . 3 0 ~(14). Due to the large
freshwater inflow, it has the highest areal loadingof carbon,
nitrogen, and phosphorus. Short residence time of water
(-10 days) and high freshwaterinflow have resulted in the
decline in productivity of both primary producers and
fisheries (15).
Trace Metals
Co (ppm)
Cr (ppm)
Cu (ppm)
Ni (ppm)
Pb(ppm)
Zn (ppm)
19.0
90.0
45.0
68.0
20.0
95.0
1.26
5.95
2.98
4.50
1.32
6.28
9.00
70.0
30.0
50.0
19.0
90.0
NA
NA
100
0.672
5.22
2.24
3.73
1.42
6.72
60
52
44
110
170
50
50
50
75
1.00
0.427
41.0
NA
NA
NA
NA
NA
NA
Major Elements
A1203 (%)
15.1
FezOB(%)
6.75
Mn (ppm) 850
a
1.00
0.446
56.2
13.4
5.72
550
National 85th percentile (70). Dredge disposal criteria ( 1 0 ) .
(TDWR) conducted screening-type studies in the Sabine
and Neches Rivers in 1980 to idenufy areas of organic and
inorganic pollution in surface sediments and their results
indicated that the concentrations of Cr and Cu were
significantly enriched above the national 85th percentile
screening criteria and that Pb concentration was highest
below the Mobil effluent discharge site (10). The followup work at selected effluent sites in 1987 revealed that Cr,
Cu, Pb, Ni, and Zn concentrations increased temporally
between 1980 and 1987 in most of the effluent sites (11).
During 1974- 1978,TDWR also collected surface sediments
at six study sites within the estuary (12)for trace metal
analyses, and they found that only Zn was slightly enriched
above the dredge disposal criteria (ca. 79 ppm vs Table 11,
while the measured sediment concentrations of Co, Cu,
Pb, and Zn were always below average soil concentrations.
Apart from the above-mentioned reports, no known prior
work has been done on the trace metal concentrations in
the sediments of the Sabine-Neches Estuary. The major
objective of this study was to reconstruct the historical
variation of trace metal concentrations in sediment cores
since the beginning of industrial activity in the early 1900s.
We have measured the concentrations of both 210Pband
239,240Pu
in four sediment cores, and it was found that
239,240Pu
was a reliable geochronometer in these estuarine
sediments (13). For the first time, we have established the
geochronology of these estuarine sediments and measured
the down-core variation of trace metal concentrations in
the sediment cores of the Sabine-Neches Estuary. An
attempt was made to separate the naturalvariation of trace
metal concentrations from changes due to anthropogenic
sources. This study is one of the very few studies that have
been carried out in shallow estuaries that are also highly
industrialized. The concentrationof metals in the sediment
cores was compared with commonly used screening criteria,
namely, dredge disposable criteria and national 85th
percentile criteria (10). The possible causes for the observed
concentrations are also discussed.
Setting of the Study Area
The Sabhe-Neches Estuary, situated on the border ofTexas
and Louisiana (Figure l),is the northernmost estuary of
Texas. Of the seven estuarine systems on the Texas coast,
the Sabine-Neches Estuary is unique in several ways: it
1496 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6,1995
Materials and Methods
Collectionof Samples. Four sediment cores were collected
across the upper, mid, and lower regions of Sabine-Neches
Estuary (Sabine Lake region) over a period of 10 months
in 1992-1993. The locations of the sampling sites of
sediment cores are shown in Figure 1. The mean water
depth for the four sampling stations are 1.5 m at station 2,
2.0 m at station 4, and 2.2 mat stations 6 and 7, respectively.
Sediment cores were collected using hand-held corers and
transported vertically to the lab, where they were sectioned,
bagged, and frozen until chemical analyses. The sections
were shaved off to avoid possible smearing of near-wall
parts of the cores. A portion of the sediment was weighed
and dried at 105 "C for 24 h to determine the water content
and porosity. Parts of the sediment sampleswere also wetsieved with an ASTM 63-pm sieve to determine the mass
of the 163-pm size fraction (Le., silt plus clay). To ensure
quantitative size separation, the portion retained on the
sieve was placed in an ultrasonic bath and sieved again.
Sediment Digestion. In order to measure the concentrations of trace metals in the bulk sediments, the dried
sedimentsampleshad to be completelydissolved. Recently,
microwave digestion has been shown to be an effective
method for the complete dissolution of sediment samples
for the analyses of most of the trace metals and major
elements (16). This method is advantageous in several
ways: it needs only one-third of the time and amount of
acids required by any open digestion method; it is much
easier and safer to use than alkali-fusion methods; and it
results in very low blank levels and low yields of total
dissolved solids (TDS), enabling low limits of quantitation.
Our method is a slight modificationof that used by Totland
et al. (16). For this study, a commercial microwave digestion
system, the CEM-MDS-81D, was used. About 250 mg of
dry, pulverized sediment was transferred into microwave
digestion vessels to which 5 mL of concentrated HF and 5
mL of concentrated "03
acids of trace metal grade were
added. The contents were heated in the microwave at a
maximum pressure of 90 psi for about 3 h. The solution
was then transferred to acid-cleaned Teflon beakers with
2 mL of concentrated HCIOl and dried on a hot plate. We
differed from the method outlined by Totland et al. (16)by
not adding HC1O4into the microwave digestion vessels as
this tends to corrode the vessels. To further increase the
temperature of volatilization and completely remove
fluoride ions, two 2-mL aliquots of HC104 were added to
the dried residue and evaporated to incipient dryness. The
resulting residue was dissolved in 25 mL of 0.8 M trace
metal grade "03,
which was further diluted before
measurement. For calibration purposes, the USGS sediment standard, Cody Shale (SCo-11, was digested along
with the samples. Analytical blanks were also prepared
with each digestion.
Analysis of Trace Metals and Major Elements. Trace
metals (Co, Cr, Cu, Ni, Pb, Zn) and major elements (A,
Fe,
I
94'00'
PORT ARTHUR
b
k-
Gulf of Mexico
0
9
KM
FIGURE 1. Map of the Sabine-Neches Estuary, in southeast Texas, showing the location of the sampling stations.
Mn) were measured using a Sciex Elan-500 inductively
coupled plasma mass spectrometer (ICP-MS). The blank
levels were very small, often below detection limits, and
were subtracted from the samples. The concentrations of
most of the metals in sediment standard (SCo-1) were
accurately determeed (17 ) within the reference values (16,
18). The exceptionswere Zn,which was consistentlyhigher
by about 20%, and Mn, which was consistently lower by
about 10%than the reference values. The relative standard
deviation was <5% for most of the metals except for Cr.
The RSD of 15%for Cr could be attributed to its resistance
to acid attack (16). The precision (10)oftriplicate analyses
of the sediment standard (SCo-1)was better than 8% (17).
Analysis of u9*240Pu.The procedures used for
239,240P~
analysis are similar to the methods of Wong et al.
(19) and Krishnaswami and Sarin (20). A detailed description of the methods are described elsewhere (13,17).
Analysis of Organic Carbon. Organic carbon content
in sediment samples was determined with a Leco carbon
analyzer (Model CR-121,and a detailed description of the
analytical procedures are given separately (17 ) .
Results and Discussion
Geochronology. The depth distribution of 239,240Pu
isotopes in sediment cores collected from stations 2, 4,and
7 is shown in Figure 2. A constant sedimentation rate (cm/
yr) was calculated from the distinctive peak of the Pu profile
assuming it to be the 1963 fallout horizon (4, 7 ) . The time
interval between the maximum fallout of 239,240Pu
(in
1963) and the sampling period (in 1992)is 29 yr. At station
VOL. 29, NO. 6 , 1 9 9 5 I ENVIRONMENTAL SCIENCE &TECHNOLOGY 1 1497
a
o
10
n
5
U
20
c
c,
30
n
ST 2
10
E
1t
n
U
1
201
1
ST4
50'.
0
" ' ' ' ' ~ L " ' ' ' ' " "
1
2
3
"
4
I
"
'
,
5
'
6
Pu (dpm/kg)
10
n
E
20
3
30
v
i
n
40
fi
0
2
4
6
8
Pu (dpm/kg)
ST 7
1
10
12
FIGURE 2. Vertical distribution of 2J924PPu in sediment cores from
stations 2 (a), 4 (a), and 7 (c) plotted against depth.
2, the maximum concentration of 239,240Pu
was observed
at a depth interval of 14- 16cm, resulting in a sedimentation
rate of 0.52 f 0.03 cm/yr (=0.54 g cm-' yr')at the upper
estuary. In the core collected at station 7, the peak was
found at a depth of 10- 12 cm, resulting in a sedimentation
rate of 0.38 f 0.03 cm/yr (=0.30 g cm-* yr-l) at the lower
estuary. The Pu peak at station 4 (mid estuary)was observed
much deeper, at a depth of 26-29 cm, which corresponds
to a sedimentation rate of 0.95 & 0.05 cmlyr (=0.89 g cm-2
yr-l). The higher sedimentation rate at station 4 is likely
to be the result of dumping of subaerial and subaqueous
dredge spoils into the western end of this estuary (21).The
sedimentation rate at station 6 could not be estimated since
the Pu profile did not show a clear peak. Assuming a
constant sedimentation rate, the 41 cmlong core at station
2 contained 79 yr of historical record, the 50 cm long core
1498
-
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6.1995
at station 7 contained 132 yr of record, and the 44 cm long
core at station 4 contained 46 yr of record.
The assumption of constant sediment accumulation rate
may not be entirely justified. The porosityvalues changed
up to about 25-30% from the mean values (17,which
imply that the errors associated with the estimated sediment
accumulation rates, assuminga constant accumulationrate,
could be as high as f45%. Changes in sedimentation rates
could have been influenced by recent construction of dams
in the Sabine and Neches Rivers, which is likely to have
trapped part of the sediment load and by the dumping and
reworking of dredged sedimentary material (21), which
would have accelerated the sedimentation rate in some
parts of the estuary.
Mixing of the sediments has been evaluated (13)using
a numerical sedimentation and mixing model (7) and a Pu
input function measured in Houston, TX (22). It was found
that eddy diffusive sediment mixing had a minor effect on
Pu distributions, and it is ignored here in the discussion of
major historical trends. A more detailed discussion of the
down core tailing of Pu and the possible reasons for the
disagreement between Z1oPband 239,240Pu
dating are given
elsewhere (13).
Normalization of Trace Metal Concentration. In
estuarine sediments, natural trace metal concentrations
can vary by a factor of 2-3 depending on the distribution
of grain size and concentrations of Al, Fe, organic carbon,
and Mn. Even if clay minerals do not play a direct role in
sequestering trace elements, they can act as mechanical
substrates for the precipitation and flocculation of organic
matter and hydrous iron and manganese oxides, which in
turntend to concentrate trace elements (23). In fact a strong
positive correlation was observed between trace metal
concentration, grain sue, and concentrations of geochemical substrates like organic carbon and iron oxides, with
correlation coefficients (R)varying between 0.77 and 0.97
for 90 data points (17). Because trace elements tend to
concentrate both within and onto the surfaces of finergrained sediments (23), coarser-grained material and
carbonates often act only as diluants of trace metal
concentrations in bulk sediments. Such effects can be
reduced (but not eliminated) by applying grain size
corrections. One commonly used method is to analyze a
particular grain size fraction, most often the (63-pm size
(24, 25). However, this approach not only requires a
quantitative separation of the <63-pm fraction but also
needs careful interpretation because concentrations in the
finer fraction may not reflect the concentrations in the total
sediment. Another method used is the measurement of
concentrations in bulk sediments normalized by weight
fraction of the <63-pm size fraction in the bulk sediments
(26, 27) to obtain the concentration in the fine fraction.
The results, however, may not reflect true chemical
concentrations, especiallywhen the <63-pmfraction is less
than 50% of the bulk (23). For example, it has been shown
that the measured concentrations of Pb in the finer fraction
of Columbia Slough sediments (only 18% of the bulk was
in the <63-pm fraction) was 72 ppm, while the grain size
normalized concentration (i.e., concentration in the bulk/
0.18) was 239 ppm (23). Hence, in areas where there is
drastic down-core and lateral variations in grain size, as is
the case of the Sabine-Neches Estuary (wherethe <63-pm
fraction varies between ~ 2 0 %
and >go%), this approach
could result in erroneous values. Hence, normalization to
the grain size was not applied to this data.
Other reference elements such as Fe (28,291and Al(25,
30-36) are also commonlyused for normalization of metals
from naturalvs anthropogenic sources. Though iron oxides
are excellent scavengers of trace elements from solution,
as reflected in their strong positive covariance with most
trace metals as well as with Al (R= 0.81-0.97 for n = 90;
18, they may not be the best candidate for this purpose
because Fe itself is susceptible to enrichment from anthropogenic sources (36)and post-depositionalredoxprocesses.
By far, Al is the most successful and widely used normalizer
(25, 30-36). It compensates for variations in both grain
size and composition because it represents the quantity of
aluminosilicates, the most important carrier phase for
adsorbed metals. The metal to aluminum ratios are
relatively constant in the crust and are less likely to be
affected by human activities (34). On a regional scale,
contaminated samples have been identified by plotting
metal concentrations against Al concentrations (33-35).
Windom et al. (33) prepared a regional data base of Al and
trace metals concentrations in uncontaminated samples
from the southeastern United States. On a scatter plot, the
data points that fall within 95% confidence levels of the
data base were taken to be natural, and those points that
were above the confidence limit were considered to be
enriched (33). In site-specific studies, it can be assumed
that the natural sources of sediments are relatively constant,
and metal concentrations in average shale (37) or average
soil (38) may be taken to represent the pre-industrial
concentration (2.5).
Enrichment Factors. Salomons and Forstner (25)
suggested standardizing the metal content in sediments to
that of a standard material, such as average shale (37),Le.,
to calculate the enrichment factor (EF),which is the ratio
between metalla in the sample and metallAl in average
soil. EFs are a convenient tool for plotting geochemical
trends and aid in making comparisons between measurements made in different areas (25, 28,391. Here, we have
calculated the enrichment factors for trace metals, using
the average soil values of Bowen (38)as a normalizer (Table
1). The enrichment factors for Ni, Cr, and Cu are less than
1.0, which implies that they are depleted relative to the
average soil concentration. On the other hand, Pb, Zn,
and Co are all enriched above the average soil value.
However, the mere fact that the enrichment factors are less
than 1.0 for some metals may not imply that there was no
input from human activities. It only suggests that these
metals are not significantlyenriched above the average soil
concentrations.
Distribution of Trace Metals in Sediment Cores. The
historical trends of metal enrichment factors in four
sediment cores from the Sabine-Neches Estuary are plotted
in Figure 3a-f. Since the pre-industrial sediment horizon
could not be reached unequivocally, it is assumed here
that the trace metal concentrations in average soil represent
the pre-industrial concentration in the study area. Due to
the uncertainty of the constancy of sedimentation rates,
enrichment factors are plottedvs depth rather than against
time. In the discussion below, however, approximate
calender years (givenin parentheses) are assigned for peaks
in metal concentrations. The trace metals’ distribution in
sediment cores is discussed in the followingparagraphs, in
the order of decreasing levels of enrichments.
Lead. The concentrations of Pb in sediment cores varied
in all the sampling sites and were always enriched above
the average soil concentration (Figure 3a). At station 2,
there was a concentration maximum at a depth of 29 cm
(corresponding to year 1936) where it was 90% (Le., EF =
1.90)higher than the metalla ratio in soil. Above this depth,
the Pb content had slightly decreased to about 50-70%
above the average soil value (EF = 1.50-1.70). At station
4, a subsurface maximum was observed at a depth of 13 cm
(corresponding to year 19791, where the EF was similar to
that at station 2; above and below this depth, the EF value
decreased to 1.20. At station 6, no enrichment was observed
to a depth of 30 cm, above which it was enriched up to 70%.
A maximum concentration of Pb was observed at station
7, where the EF varied between 1.70 and 2.20 below 15 cm
(i.e., before 1953). There was no systematic variation
between 0 and 15 cm at this station. It is interesting to
note that, in this core and others, small peaks in the Pb
concentration often coincide with the enrichment of other
metals like Mn, Co, Cu, and Zn. This suggests that Pb is
derived from the same source(s) that deliver(s)some of the
other trace metals to this site and that the geochemical
affinityof these metals to the sediment particles are similar.
Lead was first added to gasoline in 1923; the most
dramatic increases in its use began in the 1950s, with its
maximum use in early 1970s, after which time the usage
of Pb in gasoline has substantially declined (40). Consequently, Pb profiles in soils and sediments often reflect the
Pb fluxes from the use of leaded gasoline, and the Pb
maximum in recent sediments has been used as a time
marker in sediment dating (35, 41). However, historical
trends in Pb profiles reported here do not likely reflect the
time-variable input of lead from the use of leaded gasoline
(32) because, as discussed earlier, the peaks of Pb concentration do not occur at the expected depths (based on
Pu dating method). Furthermore, an increase in Pb
concentration coincides with an increase in the concentration of other metals. Thus, it is more likely that the increases
in Pb concentrations are mostly derived from local industrial
discharges in addition to runoff from roads. The lack of
strong influence from atmospheric sources was similarly
observed by Bricker (42) for Narragansett Bay.
Zinc. The most common use of zinc is in electroplating
industries and the production of alloys. Zinc was enriched
up to about 50%relative to the average crustalvalue (Figure
3b). In most of these places, the EFvalues have a minimum
of about 1.10-1.15 and increase to 1.45-1.55. The true
concentration of Zn could be -20% lower than the
measured concentration (the accuracy of measurement of
Zn in the standard reference material was consistently high
by about 20%);thus, the enrichment factor for Zn would
be only about 1.25-1.35. Therefore, it can be concluded
that the anthropogenic signal of Zn was relatively small.
Cobalt. The input of cobalt has significantlyvariedsince
the beginning of the industrial era. The EF for cobalt varied
between 1.20 and 1.90 (Figure 3c). The maximum concentration of Co was observed at station 2, at 27-30 cm
(-1962) below the sediment surface. While the concentration was more variable at station 4,small peaks in the Co
concentration were observed at 5-6, 16-18, and 20-23
cm below the surface at stations 6 and 7. Though Co is
apparentlyenriched above naturalvalues, it should be noted
that the choice of reference values for normalization can
make a substantial difference (Table 1). If average shale
was used for normalization instead of average soil, then all
of the enrichment factors in these estuarine sediments will
be below unity (i.e., they will be depleted relative to the
average shale).
VOL. 29, NO. 6.1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY a 1499
a
Enrlchment Factor
1
1.5
2
Enrfchment Factor
0
0.5
1
2.5
--6
r'
1
1.5
2
n
Enrichment Factor
1.4
1.6
1.8
2
Enrichment Factor
1.4
1.6
2
1.8
20
-E
30
5$ 30
1
2.2
1.2
1
1.2
1
1.2
Enrichment Factor
1.4
1.6
1.8
2
20
n
40
40
50
50
Enrichment
Enrichment Factor
5
1.2
10
1.2
1.4
1.6
1.8
2
0
-5
1
10
1
2.5
.
C
0
Enrichment Factor
Enrichment Factor
0.5
2
0
50
0
Enrichment Factor
1.2
1.4
1.6
1.8
0
1.4
Factor
1.6
1.8
2
0
-:
-
10
10
20
20
30
30
0
40
40
50
50
2.2
Enrichment Factor
0
0.k
Enrichment Factor
0.8
1.2
1.6
0
Enrichment Factor
0.4
0.8
1.2
1.6
0
0.4
0
Enrichment Factor
0.4
0.8
1.2
0.8
1.2
1.6
so
Enrichment
Enrichment Factor
Factor
--
or-c-
10
5n 3 0
8
40
-
-
50
e
-
0
0.2
Enrichment Factor
0.4
0.6
0.8
-1
2
5 2 0 -
10
20
30
0
40
50
Enrlchment Factor
Enrichment Factor
1
0
0
0
10
10
3
20
5
$30
0.2
0.4
0.6
0.8
1
20
20
0
30
40
40
so
50
50
Enrichment Factor
0.4
0.6
1
0
0.2
Enrichment Factor
Enrichmmt Factor
0.8
0.4
0.6
0.8
1
0
n
0.2
0.2
10
3
40
0
0
0
$30
n
1.6
0
0
0.2
0.4
0.6
0
0.8
I
0.2
0.4
0.6
0.8
Enrichment
1
0.4
Factor
0.6
0.8
20
30
30
50
50
FIGURE 3. Historical trends of metal enrichment factors in sediment cores from stations 2, 4, 6, and 7: (a) lead; (b) zinc; (c) cobalt; (d)
chromium; (e) nickel; (f) copper.
Chromium. Chromium is extensivelyused as an electroplating and cleaning agent and also in cooling waters (10).
Cr concentrations showed an overall increasing trend
towards the surface in all the cores that were studied (Figure
3d). There were several pulses observed in stations 2 and
7, while the cores collected at stations 4 and 6 did not show
such rapid increases and decreases. The EF for Cr has
increased by about 60-80% above the lower values. The
average Cr/Al ratio was 0.60 !I 0.10 at its minimum
concentration and 1.00 0.05 at its maximum concentra-
*
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ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6.1995
tion. Only a few data points in stations 2 and 6 showed any
enrichment above the average soil values. This is interesting
given the fact that Cr concentrations were found to be higher
than the natural values (sometimes up to an order of
magnitude) in sediment samples collected around many
of the effluent discharge sites (10).
Nickel. Nickel enters the estuarine system from effluents
delivered by electroplating industries and atmospheric
emissions. A distinct increase in the concentration of Ni
was observed from the vertical profdes at stations 4,6,and
7, while it was nearly constant at station 2 (Figure 3e). At
station 4,there was a sharp increase in Ni concentration
in the 20-23-cm section (year 1970); at station 6 this
maximum was observed at a depth of 15 cm, and in station
7, the Ni peak was observed at 7-8 cm (1973) below the
sediment-water interface. These two peaks and a small
peak in station 2 could have resulted from the increased
input of Ni in the early 1970s. In all cores studied, the
minimum EF was about 0.45 & 0.05 and increased almost
by a factor of 2 to -0.90 EF. It is possible that a significant
amount of Ni was derived from human activities, even
though the Nil& ratio in these sediments is lower than the
average soil values.
Copper. The concentration of copper was generally
found to be higher in sewage treatment plant effluents (10)
than in average soil. In the sediment cores analyzed, Cu
concentrations closelyfollowed those of Ni, except in station
2, where there were two peaks, one at a depth of 26-29 cm
(1936-1942) and the other more recent at 2-3 cm (1987)
below the sediment surface (Figure 30. In general, Cu
concentration was not strongly enriched in the sediment
cores and was depleted relative to the average soil value.
In summary, lead was enriched by a factor of up to 2.2
and the increase does not appear to be a direct result of
input from the use of leaded gasoline. Except at station 2,
Zn concentration exceeded the dredge disposal criteria,
and in all the four cores, the Zn concentration exceeded
average soil and shale concentrations (Table 1). Cobalt
may or may not be considered enriched above the natural
level, depending on the choice of reference material for
normalization. Chromium concentration has increased by
up to -80% above the lowest EF for Cr in the sediments;
however, it did not exceed the Cr/Al ratio of average soil.
The Cr concentrations in cores collected at stations 4, 6,
and 7 were enriched above national 85th percentile
screening criteria (Table 1). Both Ni/Al ratios and C d A l
ratios did not exceed average soil values; Ni and Cu
concentrations were always lower than the average soil
concentrations and screening criteria (Table 1). Nickel
showed distinct stages of increase in the estuarine sediments, and the maximum EF was 2.0. For the most part,
copper concentration followed the distribution of nickel.
Recent Trace Metal Fluxes. Using the sedimentation
rate estimated for each core, present day fluxes (Aof metals
(in pg cm-z y r l ) can be calculated using the formula
where C, is the concentration of metals in surface sediments
(ccg g-9; At is the time between peak fallout of Pu and
period of sampling (=29 yr);#i is the porosity at each section
(cm3 of water/cm3of wet sediment); g is the density of dry
sediments (=2.5 g/cm3);Az is the thickness of each section
above the Pu peak. The recent fluxes of metals to the
sediment surface are presented in Table 2. The flux of each
metal to surface sediments was directly proportional to the
sedimentation rate: grain size also influenced the fluxes of
metals to sediments. The maximum flux was at station 4
where the sedimentation rate was also the highest, while
metal fluxes at stations 2 and 7 are comparable. The surface
sediment concentrations of trace metals were always less
than in average soil, which indicates that present day input
of metals is primarily derived from natural sources.
TABLE 2
Estimation of Trace Metal Fluxes to Surface
Sediments of Sabine-Neches Estuary
surface
concn
(ppm)
ST4b
surface
concn
(ppm)
elements
surface
concn
(ppm)
ST2’
SWC
mean
surface
concn
(ppm)
co
8.60
8.15
8.11
8.29
44.0
40.8
13.1
11.3
13.9
14.0
13.9
17.2
16.0
19.5
52.7
78.3
66.7
85.0
76.7
Cr
41.1
cu
12.1
Ni
13.7
Pb
Zn
37.4
8.73
estimated
fluxd
(pg cm-2yr-1)
4.72
(2.40-7.21)
22.6
(13.0-33.1)
6.16
(3.88-7.73)
7.80
(4.14-12.3)
9.62
(5.77-14.2)
42.1
(25.2-59.0)
a Sedimentation rate = 0.52 cm/yr; mass accumulation rate = 0.54
g c m Z yr-’. Sedimentation rate = 0.95 cm/yr; mass accumulation
rate = 0.89 g cm-2 yr-l. cSedimentation rate = 0.38 cm/yr; mass
accumulation rate = 0.30 g cm-* yr-l. Mean values calculated using
fluxes in{[(ST4 ST7)/21+ ST2}/2. Values in parentheses indicate the
range of values.
+
Factors Controlling the Retention of Trace Metals in
Sabine-Neches Estuary
As discussed earlier, concentrations of most of the trace
metals, except for Zn and Pb, are less than the natural values
(i.e., EF < 1.0). Though some trace metal concentrations
have increased in these estuarine sediments by almost a
factor of 2 duringthis century, metallAl ratios in sediments
are often less than their ratios in average soil or shale (except
for Zn and Pb). Only a slight increase in the enrichment
of metals from anthropogenic sources was observed. This
could be caused by the depletion of these metals or the
enrichment of Al in the source rocks. It is surprising that
the estuarine sediments had remained relatively ‘pristine’
over the past 100yr in terms of heavy metal pollution given
the fact that there are so many wastewater discharges into
the Sabine and Neches Rivers.
Plutonium is a highly particle reactive element, and
therefore Pu concentration profiles in sediments can be a
useful tracer to normalize the retention capacity of particle
reactive trace metals (like Pb) in this estuary. Though Pu
is primarily derived from atmospheric deposition, a significant contribution can come from the soil erosion in the
drainage basin. On the basis of the 90Srfallout measurements made in Houston, TX, by Environmental Measurements Laboratory (,?,?), the atmospheric source of Pu is
estimated to be about 0.20 dpmlcm‘, assuming Pu concentration = 3oSrconcentrationx 0.02 (7). The contribution
of the drainage basin depends on the surface area of the
estuary relative to that of the drainage basin and is estimated
to be 0.29 dpm/cm2 (13). Thus, if all of the Pu were
efficientlytransferred to the sediments, the maximum total
inventory of Pu expected in the sediments of SabineNeches Estuary would be 0.49 dpmlcm’. The inventories
measured in the sediment cores were 0.24 dpm/cm2 at
station2,0.14dpm/cmzatstation4,0.11
dpm/cm*atstation
6, and 0.09 dpm/cm2 at station 7. This represents only
18-49% of the total expected Pu inventories. Thus, it can
be concluded that only a fraction of the incoming trace
metals are retained in the sediments.
The partitioning pattern of trace metals between the
dissolved and particulate phases is different for various
VOL. 29, NO. 6,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1 1501
metals. Recently, it has been shown in the Sabine-Neches
Estuary that of the total (dissolved particulate) concentrations of Pb, Zn, and Cu in the water column, an average
of about 40% Pb, 66% Zn, and 85% Cu was found in the
dissolved form ( 4 3 ) . It is well known that lead is a highly
particle reactive metal ( 4 4 ) , which can explain why it is
found more in the particulate form (43) and more enriched
in the sediments of Sabine-Neches Estuary than other
metals. On the other hand, Cu tends to form strong
complexes with organic macromolecules (45). Indeed,
during many sampling trips, it was found that the estuarine
water had a tannic acid color as a result of high concentrations of DOC, much of which consisted of humic and fulvic
acids derived from wetlands that surround this estuary (15).
In an organic carbon-rich (5- 15mg DOC/L,which is similar
to the values measured in the Sabine-Neches Estuary)
Ochlockonee Estuary in north Florida, for example, about
80-90% of Fe and Hg were found to be complexed by
surface sites of colloids ( 4 6 ) . Thus, complexation reactions
of Cu and other trace metals with these aromatic compounds and subsequent flushing of DOC (15)can partially
explain the lack of strong enrichment of these metals in the
sediments.
The Sabine-Neches Estuary has the highest freshwater
inflow in Texas, which results in a short residence time for
water (-10 days) and high concentrations of total suspended
particulates (15). When the hydraulic residence time is so
short, it is likely that a significantfraction of the metals that
are adsorbed on fine suspended particles and complexed
organic matter escape the estuary into the Gulf of Mexico.
The high freshwater inflow has also resulted in low salinity
conditions, where the average annual salinity is just 2.3%0
( 1 4 ) . Under such low salinity conditions, some trace metals
are not easily removed from the water column due to slow
rates of coagulation, similar to what was observed in Clyde
Estuary in Scotland (47).
+
Conclusions
Sediment cores collected from the Sabine-Neches Estuary
provide an ongoing and historical record of pollutant
discharge into one of the most highly industrialized areas
of Texas. This is also one of the few documented case
studies of a shallowestuary. The study revealed that except
for Pb and Zn all the other trace metals were depleted
relative to AI normalized average soil or shale composition.
Though the concentrations of some metals have increased
by a factor of two in the last 100yr, the enrichments of most
of these metals were often below average soil values when
normalized to aluminum. Present fluxes of trace metals to
the surface sediments of the Sabine-Neches Estuary are
also depleted relative to the average soil.
The down-core variations in the concentrations of
239,240Pu,
a bomb-fallout nuclide (used to date sediments),
closely track the atmospheric fallout history, suggesting
that the variations in the input of the trace metals through
the rivers and atmospheric fallout also will be retained in
the sedimentary record. Since the 239,240Pu
and zloPb
inventory is only a fraction of what is expected from the
combined input from the atmospheric fallout and the
drainage basin, it is likely that only a fraction of the metal
input is retained. Low concentration of metals like Cu and
Zn in addition to metals like Pb is likely due to the
complexation of metals with dissolved organic matter in
the water column and the subsequent loss of metals through
outflow due to a relatively short hydraulic residence time.
1502
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 6,1995
For a complete understanding of the retention capacity of
sediments for trace metals, an evaluation of the concentration of these metals in the water column and in suspended
particles at the head and mouth of the estuary and a
construction of a flux balance would be needed.
Acknowledgments
This work was funded by the Advanced Research Program
of the Texas Board of Higher Education (Grant 003581).
M.R. would like to thank Dr. Joseph Ryan, University of
Colorado at Boulder, for an earlier review of this manuscript.
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Received for review July 19, 1994. Revised manuscript received February 17, 1995. Accepted February 21, 1995.@
ES940444T
@
Abstract published in Advance ACS Abstracts, April 1, 1995.
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