1

... investigators in Austin noticed an odor associated with the insecticides .... The manager of the (chemical) plant admitted that quantitie

insecticide had been washed into the storm sewer recently and, more significantly, he acknowledged that such disposal of insecticide sp

residues had been common practice for the past 10 years.

.... For 140 miles downstream from the lake the kill of fish must have been almost complete, for when seines were used later in an effort 

whether any fish had escaped they came up empty. Dead fish of 27 species were observed, totaling about 1000 pounds to a mile of river

Rachel Carson, 1962

Town Lake, the last in the chain of Highland Lakes on the Colorado River, runs through the cent

tin, Texas. On any given day, grebes and coots dot the water, rowers skim alongside, and the sparkle o

on the water can be admired from the adjacent hike and bike path and from the windows of nearby off

ings. During the summer months, crowds gather along the shores of Town Lake to watch as many as 1 

Mexican Free-Tail bats emerge from under the Congress Avenue bridge. But below the lazily moving 

water lies a bed of sediment about 1 meter (m) thick—sediment that has been deposited gradually sinc

voir was formed in 1959 and that has been recording changes in water quality since that time. What ca

iment tell us about the history of water quality in the Colorado River? And what does it say about the 

the rapid urbanization of Austin and the future health of our environment?

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S
2

a large DDT peak in the early 

(Van Metre and others, 1998). 

in Town Lake any different? O

find out is to compare the DDT 

the Town Lake core with DDT 

other urban lakes and reservoi

The peak concentration in Tow

about 10 times greater than the 

tion in newer sediment at the t

core. The average peak in the f

lakes compared on figure 1b is 

greater than concentrations in 

ment. This comparison indicat

spill in Town Lake left behind 

centrations that are two to thre

concentrations that might be e

the basis of “normal” DDT use 

1950s and 1960s.

Historical use of other hydrophobic (incapable of dissolving in water; literally, “water fearing”) contaminant

recorded in the Town Lake sediments—in particular, lead and the organic compounds polychlorinated biphenyl

chlordane, and polycyclic aromatic hydrocarbons (PAHs) (fig. 2). Like DDT, use of lead and PCBs was widesp

the early years of Town Lake but subsequently was reduced or eliminated by regulatory action. Chlordane, alth

restricted by regulatory action, remains at relatively high concentrations in recent sediments. In contrast, PAHs, 

inate from numerous urban sources, are not regulated.

Lead concentrations in the Town Lake sediment core follow a trend pattern similar to that shown by other 

U.S. lakes. Lead in gasoline caused large increases in lead concentrations in urban lakes and streams in the 1960

1970s. Lead concentrations peaked in urban reservoir and lake cores in the mid-1970s and then decreased with 

duction of unleaded gasoline (Callender and Van Metre, 1997). Concentrations in the Town Lake core have dec

about 70 percent since 1970 (fig. 2a), a direct and encouraging response to the elimination of lead in gasoline.

PCBs had widespread industrial and commercial uses, primarily as insulation fluids for transformers and i

ances. PCB concentrations in sediment from most urban lakes peaked in the mid-to-late 1960s, at the height of 

then decreased following restrictions imposed in 1971 (Van Metre and others, 1997, 1998). The pattern in Tow

similar, with a peak concentration of 96 g/kg in the early 1960s decreasing by about 70 percent to 31 g/kg in 

(fig. 2b). Thus, for these persistent contaminants (DDT, lead, PCBs), restricting their use has resulted in large d

Town Lake.

Chlordane is an organochlorine pesticide of the same general class as DDT. Use of most organochlorin

was restricted or banned in the 1970s. Agricultural use of chlordane was restricted in 1974; however, urban use 

for termites and ants) was permitted until 1988. Even after 1988, use of existing stocks in the possession of hom

was allowed. On the basis of a nationwide survey of pesticide use in 1990, this continued use was substantial, r

1950

1960

1970

1980

1990

SEDIMENT DEPOSITION DATE

Total DDT (DDT

+ DDD + DDE)

DDE

DDD

DDT

Town Lake, Tex.

Lake Harriet, Minn.

Palmer Lake, Minn.

Lake Anne, Va.

White Rock Lake, Tex.

Figure 1. DDT trends in (a) the Town Lake sediment core and (b) five 

(a) (b)

urban lake cores.
3

1950

early-1970s agricultural use (Whitmore and others, 1992). Chlordane is of particular concern in Town Lake; hi

of chlordane in fish have resulted in a fish consumption advisory imposed by the Texas Department of Health (

Chlordane concentrations in Town Lake sediment were relatively high in the 1960s and early 1970s, then decre

1980s (fig. 2c). A small peak in the early 1960s, coincident with the DDT spill, possibly could indicate some ch

released by the spill. A second peak in the early 1970s was coincident with substantial agricultural use in the U

and with chlordane peaks in some other urban (Van Metre and others, 1998) and agricultural (Van Metre and ot

reservoir and lake cores. More surprising is the increase at the top of the core, indicating continued input of chl

Town Lake in the 1990s. Recent input was confirmed by analysis of suspended-sediment samples collected by 

in Shoal Creek (tributary to Town Lake) in November 1997 and March 1999. Chlordane concentrations in 10 su

sediment samples ranged from 9 to 340 g/kg (U.S. Geological Survey, Austin, Tex., unpub. data), compared to 

tration of 47 g/kg at the top of the Town Lake sediment core.

PAHs, in contrast to trends in the regulated or banned contaminants, show dramatic increases during the 

years in the Town Lake core (fig. 2d). This pattern is repeated, to varying degrees, in lakes with urbanizing wat

across the United States (Van Metre and Callender, 1999). PAHs are trace contaminants that occur naturally in 

coal, and other hydrocarbons. They also are produced by combustion of hydrocarbons, resulting in many urban 

including: industrial and power plant emissions; car and truck exhaust; tires; and asphalt roads and roofs. Elimin

from urban runoff is difficult because of their varied sources. PAHs are an environmental concern because they 

to aquatic life and because several are susp

cinogens (Björseth and Ramdahl, 1985; L

Morgan, 1990). 

Total PAH concentrations in recently de

sediment in Town Lake are about 16 times 

centrations in 1960, increasing from about 

in 1960 to 11,400 g/kg in 1998 (fig. 2d). 

increase corresponds with increases in traf

greater Austin (Schrank and Lomax, 1999

From 1982 to 1996, PAH concentrations i

ment core and automobile use (expressed 

miles driven on Austin roads) both increas

2.5 times (fig. 3). The relation of PAH con

to Austin traffic is evidence of the importa

industrial sources of PAHs to streams and 

indicates that vehicle emissions, road and 

and engine oil leaks could be major source

Figure 3. Comparison of Town Lake PAHs and traffic trends.

Figure 2. Trends in concentrations of four hydrophobic contaminants in the Town Lake sediment core.

TOTAL PAHs (MICROGRAMS PER KILOGRAM)

AUSTIN TRAFFIC 

(SCHRANK AND LOMAX, 1999)

(a) (b) (c) 

1960 2000 1970 1980 1990

DATE

0

12,000

4,000

8,000

0

15,000

5,000

10,000
4

trends occur. The different groups of contaminants were compared by normalizing (dividing) all contaminant c

tions by the Canadian Sediment Quality Guidelines for the Protection of Aquatic Life (Environment Canada, 19

lower level guidelines currently known as the interim sediment quality guidelines were used. Individual metals 

compounds were normalized to yield a value called a toxicity index. These toxicity indexes were then summed 

groups of contaminants (metals, organochlorine compounds, and PAHs), then plotted (fig. 4). These guidelines 

broadly accepted approach for identifying levels of contaminants in sediment that are expected to adversely aff

life. The guidelines are not related directly to human health. This normalization allows comparison of the relati

of metals, organochlorine compounds, and PAHs.

The graph (fig. 4) shows trends for the three major groups of contaminants measured in the sediment core. T

the toxicity index, the more toxic the sediment is expected to be—thus, a decrease over time means sediment q

improving. This graph indicates that the worst overall sediment quality in the past 40 years is associated with th

in 1961. Overall sediment quality improved from 1961 until about the mid-1980s, mostly because of large decr

DDT and PCBs, both organochlorine compounds. Since the mid-1980s, however, the sediment toxicity index h

increasing, in large part, because of the large increase in PAHs. This increase in toxicity indicates that improvem

iment quality caused by reductions in the use of many organochlorine pesticides and PCBs are being offset by i

PAHs, probably traffic related, as Austin grows. 

Figure 4. Trends in overall sediment quality in Town Lake. 

1Toxicity index is computed by dividing contaminant concentrations 

1960 2000 1960 1970 1980 1990

DATE

0

250

50

100

150

200

TOXICITY INDEX1

PAHs

ORGANOCHLORINES

METALS

by the Canadian Sediment Quality Guidelines for the Protection of 

Aquatic Life (Environment Canada, 1995).
5

measured by inductively-coupled plasma/atomic emission spectroscopy and graphite-furnace atom

adsorption (Fishman and Friedman, 1989). Organic compounds are measured by gas chromatogra

mass spectrometry (Furlong and others, 1994; Foreman and others, 1995). Ages of sediment layers 

Town Lake core were assigned on the basis of the cesium-137 profile and core lithology similar to 

approach used by Van Metre and others (1997).

Retrieving a gravity core 

from the bottom of Town 

Lake.

Sampling on a lake in California from the pontoon boat.

Slicing a 5-ce

long subsamp

a core for labo

analysis.
6

3

3

Non-urban lake; number

 of lakes sampled

Urban lake; number

 of lakes sampled

References

Björseth, Alf, and Ramdahl, Thomas, 1985, eds., Handbook of polycyclic 

aromatic hydrocarbons—v. 2, Emission sources and recent progress 

in analytical chemistry: New York, Marcel Dekker Inc.

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.

Carson, Rachel, 1962, Silent spring: Boston, Houghton Mifflin, 386 p.

Environment Canada, 1995, Interim sediment quality guidelines: Ottowa, 

Ontario, Ecosystem Conservation Directorate, Evaluation and Interpretation Branch, 63 p.

Fishman, M.J., and Friedman, L.C., eds., 1989, Methods for determination of inorganic substances in water and fluvial sediments: U.S. 

Geological Survey Techniques of Water-Resources Investigations, 

book 5, chap. A1, 545 p.

Foreman, W.T., Connor, B.F., 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.

Furlong, E.T., Vaught, D.G., Merten, L.M., Foreman, W.T., Gates, P.M., 

and Koleis, J.C., 1994, Methods of analysis by the U.S. Geological 

Survey National Water Quality Laboratory—Determination of 

semivolatile organic compounds in bottom sediment by solvent 

extraction, gel permeation chromatographic fractionation, and 

capillary-column chromatography/mass spectrometry: U.S. 

Geological Survey Open-File Report 95–719, 67 p.

Long, E.R., and Morgan, L.G., 1990, The potential for biological effects 

of sediment-sorbed contaminants tested in the National Status and 

Trends Program: National Oceanic and Atmospheric Administration, Technical Memorandum NOS OMA 52, 175 p.

Schrank, D.L., and Lomax, T.J., 1999, The 1999 annual mobility report—

Information for urban America: College Station, Tex., Texas 

Transportation Institute, Texas A&M University, accessed 

December 28, 1999, at URL 

http://mobility.tamu.edu/study/report.stm

Texas Department of Health, 1996, Fish advisories and ban

Department of Health, Seafood Safety Division, 18 p.

Van Metre, P.C., and Callender, Edward, 1999, Trends in se

in response to urbanization, in Morganwalp, D.W., and 

eds., U.S. Geological Survey Toxic Substances Hydro

gram—Proceedings of the Technical Meeting, Charle

March 8–12, 1999—v. 2—Contamination of hydrolog

and related ecosystems: U.S. Geological Survey Wate

Investigations Report 99–4018B, p. 471–475.

Van Metre, P.C., Callender, Edward, and Fuller, C.C., 1997

trends in organochlorine compounds in river basins id

sediment cores from reservoirs: Environmental Scienc

nology, v. 31, no. 8, p. 2,339–2,344.

Van Metre, P.C., Wilson, J.T., Callender, Edward, and Full

Similar rates of decrease of persistent, hydrophobic an

reactive contaminants in riverine systems: Environme

and Technology, v. 32, no. 21, p. 3,312–3,317.

Whitmore, R.W., Kelly, J.E., and Reading, P.L., 1992. Nati

garden pesticide use survey, final report: Research Tria

RTI/5100/17–01F, 140 p.

—P.C. Van Metre and B.J. Mahler

Photographs by C.E. Ranzau

Information on technical reports and hydrologic data r

study can be obtained from:

District Chief

U.S. Geological Survey

8027 Exchange Dr. 

Austin, TX 78754–4733

E-mail: dc_tx@usgs.gov

Phone: (512) 927–3500

FAX: (512) 927–3590

World Wide Web: http://tx.usgs.gov/

regionally, with continued larger concentrations in 

urban lakes in the South and upper Midwest and 

decreasing trends in the Northeast (Van Metre and 

others, 1997, 1998). Trends in some “unregulated” metals and organic compounds are also 

variable and, in some cases, continue to increase 

with urban growth. Arsenic and zinc, for example, are increasing in many lake sediment cores concurrent with u

(Van Metre and Callender, 1999). PAH trends, on the other hand, are very consistent in lakes in areas of new ur

growth—increasing in all urbanizing lakes where sediment cores have been collected for the NAWQA Program

Figure 5. Sites sampled during 1992–99 for the USGS rec

trends study.