Llano Grande Lake Bottom Sediments--A Chronicle of Water-Quality Changes
in the Arroyo Colorado, South Texas, 1989-2001
By B.J. Mahler, and P.C. Van Metre
Fact Sheet 065-02
In cooperation with the U.S. Environmental Protection Agency
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The
Arroyo Colorado, an ancient channel of the
Rio Grande, extends 90 miles from Mission, Tex., to the Laguna
Madre. The Arroyo Colorado flows through areas of intense agricultural
cultivation and through important habitat for migrating birds and other
wildlife, including several wildlife sanctuaries and refuges. The
above-tidal segment of the Arroyo Colorado is included in the State of
Texas 2000 Clean Water Act 303(d)1
list in part because of elevated concentrations of the hydrophobic
legacy pollutants DDE (a DDT breakdown product), chlordane, and
toxaphene in fish tissue. This report addresses three questions: |
| Llano
Grande Lake and nearby agriculture (photograph by Roger M. Miranda, Texas
Natural Resource Conservation Commission). |
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Do legacy
pollutants (organochlorine compounds, major and trace elements) occur in
the Arroyo Colorado at present and at what concentrations?
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How has the
occurrence of selected legacy pollutants in the Arroyo Colorado changed
over time?
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Are current
concentrations of legacy pollutants in bottom sediments at levels of
concern for the health of aquatic biota?
To answer these questions, the U.S. Geological Survey (USGS),
in cooperation with the U.S. Environmental Protection Agency (USEPA), collected
and analyzed a sediment core from Llano Grande Lake on the Arroyo Colorado (fig.
1). Sediment cores can be used to reconstruct historical trends in
concentrations of hydrophobic contaminants (Eisenreich and others, 1989; Van
Metre and others, 1997, 2000). The lake is part of the Rio Grande delta drainage
system (fig. 1). The lake is 6 miles long and has a maximum width of 600 feet.
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Concentrations
of chlordane and toxaphene are below minimum reporting levels in
sediments deposited since 1989.
-
Concentrations
of DDE have decreased since 1989 but remain at levels that might
present a threat to the health of aquatic biota.
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To find as thick a sequence of undisturbed
sediments as possible, the sediment core was collected from a protected arm of
Llano Grande Lake, where scouring of sediment was assumed not to occur except
in the case of very large floods. A 114-centimeter (cm)-long push core was
obtained by pushing a plastic cylinder directly into the mud. The sediment was
fairly homogeneous, with light brownish-gray clay throughout the length of the
core and some fine sand between the 80- and 90-cm depth.
The core was subsampled by pushing the
sediment up through the plastic liner and slicing 4- to 6-cm-thick layers of
sediment off the top (see photo below). Each sample was analyzed for
cesium-137 (137Cs), organochlorine compounds, major and trace
elements, and organic carbon at the USGS National Water Quality Laboratory in
Denver, Colo., using the published USGS methods cited: 137Cs was
measured by radioactive counting (Van Metre and others, 1998); organochlorine
compounds were measured by dual capillary-column gas chromatography with
electron capture detection (GC-ECD) (Foreman and others, 1995); major and
trace elements were measured by inductively-coupled plasma/mass spectrometry (ICP/MS)
(Arbogast, 1996); and organic carbon was measured by combustion (Arbogast,
1996). Grain size was not measured, but (unpublished) data from other coring
studies by the authors indicate that it is typically in the silt- and
clay-size range.
The sediment core was age-dated on the
basis of the 137Cs profile and records of historical flooding in
the Arroyo Colorado. A byproduct of nuclear weapons testing, 137Cs
is a radioactive isotope with a half-life of 30.1 years. Concentrations
reached their peak in the atmosphere in 1963 and have been decreasing
exponentially ever since. Thus an undisturbed sequence of sediments whose
lowest parts predate 1963 should contain a 137Cs peak and then an
exponential decrease toward the top of the core. The 137Cs pattern
in the Llano Grande Lake core is very erratic in the lower part of the core
and then decreases smoothly from 62 cm to the top of the core (fig. 2).
This pattern suggests that the top 62 cm of sediment is an undisturbed
sequence, that is, it post-dates any flooding events that could have caused
scouring and redeposition of sediments. The last major flood in the Arroyo
Colorado occurred in September 1988 when the maximum flow measured 2.4 channel
miles upstream from the branching of the Arroyo Colorado and the North
Floodway was 11,400 cubic feet per second (International Boundary and Water
Commission, 1988). Therefore a depth of 62 cm in the core was assigned a date
of 1989, and a constant mass accumulation rate was assumed from 1989 to
present. It is not known when the sediments from 62 cm to the bottom of
the core were deposited. The average sedimentation rate for the upper part of
the core of about 5 cm per year, although high, is not unusual for a water
body in an agricultural setting.
Slicing a subsample from a core for laboratory analysis.
|
 |
| Figure 2. Concentrations
of Cesium-137, total DDT, and normalized lead from a sediment core collected
from Llano Grande Lake. Sediments from the bottom of the core up to about
62 cm are assumed to have been disturbed, and no dates were assigned to
this interval. |
Of the 15 organochlorine compounds for which the
sediment core was analyzed, only dieldrin, DDT, DDE, and polychlorinated biphenyls
(PCBs) occurred at concentrations above their minimum reporting levels. Gamma-HCH
(lindane), heptachlor, aldrin, heptachlor epoxide, endosulfan, DDD (another
breakdown product of DDT), and mirex were not detected in any of the core samples
at a minimum reporting level of 1.0 microgram per kilogram (µg/kg) (top core
sample) and 0.5 µg/kg (all other samples). Methoxychlor was not detected at
a minimum reporting level of 4.0 µg/kg (top core sample) and 2.0 µg/kg (all
other samples). Chlordane and toxaphene, two of the legacy pollutants for which
the Arroyo Colorado was placed on the 303(d) list, were not detected in any
of the core samples at minimum reporting levels of 10 and 100 µg/kg, respectively
(top core sample), and 5 and 50 µg/kg, respectively (all other samples).
Most of the total DDT (equal to the sum of DDT, DDE,
and DDD) occurs in the form of DDE, the concentrations of which are 50 to 100
times that of parent DDT (table 1). In general, concentrations of total DDT
decrease in the part of the core assumed to represent undisturbed sedimentation
from a high of 121 µg/kg deposited in the early 1990s to 51 µg/kg in the most
recently deposited sediments (fig. 2). However, the concentration of DDE in
the most recently deposited sediments (49 µg/kg) is about 60 percent higher
than the Probable Effect Concentration (PEC), the concentration above which
adverse effects on benthic biota are expected to occur; the PEC is a consensus-based
sediment quality guideline developed by MacDonald and others (2000). The elevated
concentration of DDE in recently deposited sediments indicates that DDE is still
present in the Arroyo Colorado at concentrations of environmental concern. The
slight increasing trend in total DDT at the top of the core probably is related
to any increase in organic carbon content, and thus the trend probably reflects
partitioning rather than increased loading.
Concentrations of the two other organochlorine compounds
detected in the core, dieldrin and PCBs, were below the Threshold Effect Concentration
(TEC) in all cases (table 1). The TEC is the concentration below which adverse
effects on benthic biota are not expected to occur (MacDonald and others, 2000).
This finding indicates that direct exposure to dieldrin and PCBs at this site
probably does not pose a threat to the health of aquatic biota. PCBs were not
detected in the upper 40 cm of sediments. Nationwide, concentrations of PCBs
in lake and reservoir sediments generally peaked in the mid- to late-1960s,
then decreased following restrictions on PCB use imposed in 1971 (Van Metre
and others, 1997, 1998). The lack of detection of PCBs in the upper part of
the Llano Grande core is consistent with the hypothesis that the upper part
of the core represents sediments deposited after the 1970s.
Concentrations of major elements (table 1), an indication
of the mineralogic origin of the sediment, are fairly constant from a depth
of about 93 cm to the top of the core. From the bottom of the core to about
93 cm, concentrations of aluminum and titanium increase, and concentrations
of calcium decrease, suggesting an increase in clay content.
There is no evidence in the core that trace elements
in sediments in Llano Grande Lake are likely to cause adverse effects to benthic
biota, according to the consensus-based sediment quality guidelines (MacDonald
and others, 2000) (table 1). Concentrations of trace elements at the top of
the core are below the TEC. Concentrations of most trace elements decrease at
the bottom of the core, coincident with decreased clay content as represented
by aluminum. This concentration decrease probably is because many trace elements
tend to be associated with the clay minerals in sediment.
Trends in lead concentrations are consistent with
the assumption of undisturbed deposition of the upper part of the core since
1989. Concentrations of lead, when normalized to aluminum concentrations to
account for differences in clay content down the core, decrease from 62 cm to
the top of the core (fig. 2). This is consistent with the decreasing trends
in lead in numerous other reservoir cores that were caused by the elimination
of lead in gasoline in the 1970s (Callender and Van Metre, 1997). The lack of
a sharp lead peak supports the hypothesis that these sediments were deposited
after the 1970s.
Information on contaminants associated with sediments
from the Arroyo Colorado since 1989 can be reconstructed from a sediment core
collected from Llano Grande Lake. Major floods that moved and redeposited sediments
prevented obtaining an undisturbed sequence of sediments that represents deposition
earlier than 1989. Analyses of discrete intervals within the sediment core indicate
that two of the legacy pollutants of interest in the Arroyo Colorado, chlordane
and toxaphene, are not detectable in sediments deposited at the coring site
after 1989. Concentrations of a third legacy pollutant, DDE, have decreased
by more than one-half since the early 1990s. Concentrations of DDE in the most
recently deposited sediments, however, still exceed concentrations thought to
be toxic to benthic biota by a factor of about 1.5. Other organochlorine compounds
analyzed occurred at extremely low concentrations (dieldrin), occurred only
in older sediments (PCBs), or were not detected.
Trace elements occurred in a range of concentrations
at which adverse effects on benthic biota are not expected. Some chromium concentrations
only slightly exceed the upper level of this range and were less than one-half
the concentration above which adverse effects on benthic biota are expected
to occur.
This project was funded by the U.S. Environmental
Protection Agency. We thank Sylvia Ritzky and Philip Crocker (USEPA), Jennifer
Wilson and Edward Callender (USGS), and Roger Miranda (Texas Natural Resource
Conservation Commission) for their helpful comments on the manuscript.
Arbogast, B.F., ed., 1996, Analytical methods manual for the Mineral
Resource Surveys Program: U.S. Geological Survey Open-File Report 96-525, 248
p.
Callender, Edward, and Van Metre, P.C., 1997, Reservoir sediment
cores show U.S. lead declines: Environmental Science and Technology, v. 31,
no. 9, p. 424A-428A.
Eisenreich, S.J., Capel, P.D., Robbins, J.A., and Boubonniere,
R.A., 1989, Accumulation and diagenesis of chlorinated hydrocarbons in lacustrine
sediments: Environmental Science and Technology, v. 23, no. 9, p. 1,116-1,126.
Foreman, W.T., Connor, B.G., Furlong, E.T., Vaught, D.G., and Merten,
L.M., 1995, Methods of analysis by the U.S. Geological Survey National Water
Quality Laboratory—Determination of organochlorine pesticides and polychlorinated
biphenyls in bottom sediment by dual capillary-column gas chromatography with
electron-capture detection: U.S. Geological Survey Open-File Report 94-140,
78 p.
International Boundary and Water Commission, 1988, Flow of the
Rio Grande and related data: Water Bulletin no. 58, 144 p.
MacDonald, D.D., Ingersol, C.G., and Berger, T.A., 2000, Development
and evaluation of consensus-based sediment quality guidelines for freshwater
ecosystems: Archives of Environmental Contamination and Toxicology, v. 39, p.
20-31.
Van Metre, P.C., Callender, Edward, and Fuller, C.C., 1997, Historical
trends in organochlorine compounds in river basins identified using sediment
cores from reservoirs: Environmental Science and Technology, v. 31, no. 8, p.
2,339-2,344.
Van Metre, P.C., Mahler, B.J., and Furlong, E.T., 2000, Urban sprawl
leaves its PAH signature: Environmental Science and Technology, v. 34, no. 19,
p. 4,064-4,070.
Van Metre, P.C., Wilson, J.T., Callender, Edward, and Fuller, C.C.,
1998, Similar rates of decrease of persistent, hydrophobic and particle-reactive
contaminants in riverine systems: Environmental Science and Technology, v. 32,
no. 21, p. 3,312-3,317.
Any use of trade, product, or firm names is for descriptive purposes only and
does not imply endorsement by the
U.S. Government.
Information on technical reports and hydrologic data related
to this study can be obtained from:
District Chief
U.S. Geological
Survey
Phone: (512) 927-3500
8027 Exchange
Dr.
FAX: (512) 927-3590
Austin, TX 78754-4733
World Wide Web:
E-mail: dc_tx@usgs.gov
http://tx.usgs.gov/
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