1

How We Sampled and Age-Dated Lake Sed

Cores of bottom sediments in Lorence Creek La

collected in August 1996 at the center of the lake, a

meters upstream of the dam. The site cored was in t

part of the lake (about 1 meter). Two cores were co

using a hand corer. The cores were 6.3 centimeters i

and about 27 and 38 centimeters long. Subsamples f

the cores were collected every 2 or 3 centimeters fo

of cesium-137 (137Cs) and major and trace element

samples similarly collected from the other core wer

for organochlorine compounds and PAHs.

Ages of core sediments were dated using a comb

physical and chemical markers. The top of a core w

the year of sampling, 1996; the bottom of the core w

assigned 1962, the estimated completion date for co

of the lake. Pre-reservoir sediment, which was iden

core by the presence of coarse sand and pebbles, w

depth of 27 centimeters. Depths of core subsamples 

related with the 137Cs content of the subsamples. M

concentrations of 137Cs, a radioactive by-product o

weapons testing with a half-life of about 30 years, f

appeared in the atmosphere about 1952, peaked dur

1963–64, and decreased to nearly zero by the mid-

The rate of sedimentation was assumed constant bet

(the assigned 137Cs peak in the lake sediments) and 

sampling year).

Lake

98o 98 27'30" o

30' 29o

37'30"

29o

35'30"

US 281

Loop

1604

Lorence Creek

0 1 2 KILOMETERS

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S

g pg() C

Lake is not unique; the same metals and organic compounds

are routinely found in lake sediments in urbanizing watersheds

(Van Metre and Callender, in press).

Many common agricultural and urban contaminants tend to

chemically adhere to sediments. As rain washes sediment particles

from streets and fields, these contaminants are flushed into

streams and lakes and can infiltrate ground water. In karst (dissolutioned limestone) terrain, recharge and ground-water flow are

rapid, and sediments in recharge water can remain in suspension

and be transported great distances underground (Mahler and

Lynch, 1998). When runoff containing sediment particles flows

into lakes, the velocity slows and particles settle to the bottom.

Over time, layers of sediment are deposited that leave a partial

record of historical water quality. This record can be revealed by

collecting and analyzing sediment cores from the bottom of lakes

and reservoirs (Van Metre and Callender, 1997).

This fact sheet summarizes the chemistry of sediment cores

collected from Lorence Creek Lake. The study was designed to

ppyCw

drains to Salado Creek about 6.4 kilometers south-so

the lake. Lorence Creek Lake has a surface area of ab

ares (about 4 acres) and has a depth of 1 meter or les

age area of the lake is about 5 square kilometers.

Land use in the Lorence Creek watershed upstream 

has gone through a series of changes since the early 1

was predominately rangeland. By the late 1960s, abo

of the land was urban, mostly single-family housing. 

1970s, urban land had increased to 25 percent of the 

upstream of the lake, and by 1992, 55 percent of the 

urban—including a major divided highway (Loop 16

church, and several commercial properties in addition 

dential areas.

Figure 1. Lorence Creek watershed upstream of Lorence Creek

Lake (USGS National Aerial Photography Program photograph,

January 1995).
2

0

100

200

Lead

0

0.2

0.1

Mercury

0

200

100

Zinc

0

200

100

Arsenic

30 5 25 20 15 10 0

DEPTH, IN CENTIMETERS

CONCENTRATION, IN MICROGRAMS PER GRAM

1996

YEAR OF SEDIMENT DEPOSITION

1962

1964

1970

1980

1990

What We Found—In the Context of What We Know

Concentrations of metals, organochlorine compounds, and PAHs in Lorence Creek Lake sediments change over time and reflect the 

historical concentrations of those constituents in the water that has entered the lake since its construction. The information on constituent sources discussed below, unless otherwise referenced, is from the Agency for Toxic Substances and Disease Registry (ATSDR) 

(1999a).

Arsenic

Arsenic concentrations in Lorence Creek Lake sediments 

peaked in the mid-1960s at a relatively high concentration (fig. 2) 

compared to those in other urban lake sediments (Van Metre and 

Callender, 1997), before much urban development occurred in the 

watershed upstream of the lake. The source of the arsenic during 

the 1960s is unknown, although a possible source is pesticides 

containing arsenic. Organic arsenic is the active ingredient in 

some pesticides, mostly weed killers. The decrease in arsenic 

concentrations to a low level in recent years is a notable trend, 

as arsenic is the number-one-ranked substance on the ATSDR/

U.S. Environmental Protection Agency (EPA) list of top 20 hazardous substances for 1997 (Agency for Toxic Substances and 

Disease Registry, 1999b).

Lead

Concentrations of lead in Lorence Creek Lake sediments 

peaked in the mid-1970s and decreased about 66 percent by 1996. 

Peaks in lead concentrations in urban reservoir sediment cores in 

the United States consistently occur in the 1970s and have been 

attributed primarily to the use of lead in gasoline until the earlyto-mid 1970s (Callender and Van Metre, 1997). In addition to 

lead’s elimination from gasoline, lead concentrations in paints, 

ceramics, caulk, and pipe solder have been reduced greatly in 

recent years—but fairly stable sediment concentrations since the 

mid-1980s reflect its continued, although reduced, presence in 

urban runoff. 

Mercury

Mercury concentrations in Lorence Creek Lake sediments 

peaked in the late 1970s and, like lead, decreased to fairly stable 

levels through 1996. The specific factors responsible for the 

temporal pattern of mercury in the lake sediments are unknown; 

although similar trends in mercury concentrations, with increases 

coincident with initial urban development in the watershed 

upstream of the lake and then decreases in concentration more 

recently, have been reported for lake sediments in Minneapolis, 

Minn. (Engstrom and Swain, 1997). Mercury can enter the air 

from the burning of coal and waste and from manufacturing 

plants. It can enter water and soil at waste-disposal sites. Some 

types of pesticides and household products contain mercury 

(Pais and Jones, 1997). 

Zinc

Zinc concentrations in lake sediments were fairly constant 

through the 1980s but increased in the 1990s to their maximum in 

1996. The increasing trend in zinc concentrations probably is 

associated with increasing urbanization in the watershed upstream 

of the lake because zinc has many commercial uses. Automobile 

tires contain zinc (Van Assche and Martin, 1999) and could be a 

source in Lorence Creek and other urban watersheds, where 

similar increasing zinc trends have been reported (Van Metre and 

Callender, 1997). Figure 2. Concentrations of metals in sediment cores.

A dry Lorence Creek at Loop 1604, September 1999 (photograph 

by M.B. Nyman, U.S. Geological Survey).
3

0

4

2

30 25 20 15 10 5 0

PAHs

DEPTH, IN CENTIMETERS

CONCENTRATION, IN 

MICROGRAMS PER GRAM

YEAR OF SEDIMENT DEPOSITION

1962

1964

1970

1980

1990

1996

0

80

20

40

60 PCBs

30 5 25 20 15 10 0

DEPTH, IN CENTIMETERS

0

100

50

Chlordane

0

200

100

Total DDT

CONCENTRATION, IN MICROGRAMS PER KILOGRAM

1962

1964

1970

1980

1990

1996

YEAR OF SEDIMENT DEPOSITION

Implications

The implications of trends in contaminants in urban runoff in 

the Lorence Creek watershed upstream of the lake are mixed. For 

some regulated contaminants like lead, DDT, and PCBs, the news 

is good—restrictions on their use have apparently led to substantial decreases in concentrations in water entering Lorence Creek 

Lake. For zinc, PAHs, and chlordane, the news is not good. As 

evident in other watersheds, concentrations of zinc and PAHs continue to increase as urbanization increases. Eight years after 

almost all uses of chlordane were banned, chlordane concentrations in the Lorence Creek Lake sediments are still increasing, 

suggesting continuing use in the watershed upstream of the lake.

What is the net effect of all the trends? Is sediment quality getting better or worse? To answer these questions, the ratios of contaminant concentrations to sediment quality guidelines developed 

DDT

The concentration of total DDT (sum of DDT, DDD, and DDE) 

in Lorence Creek Lake sediments (fig. 3) peaked in the mid1960s, soon after use in the United States peaked (Rapaport and 

others, 1985). DDT breaks down to DDD and DDE, which also 

are toxic and resistant to further chemical decomposition. The use 

of DDT was banned by the EPA in 1972, but DDT lasts for years 

in the soil (2 to 15 years for one-half the DDT in soil to break 

down [Agency for Toxic Substances and Disease Registry, 

1999a]). Detection of DDD and DDE in the upper parts of the core 

indicates that DDT breakdown products are continuing to enter 

Lorence Creek Lake, probably with soils eroding from the watershed upstream of the lake.

Chlordane

Chlordane concentrations in Lorence Creek Lake sediments 

increased from the 1960s to the 1990s and were highest in the 

most recently deposited sediments. A similar trend was reported 

for chlordane in sediments of White Rock Lake in Dallas 

(Van Metre and others, 1997). Chlordane has been used as an agricultural and urban insecticide since 1948. In 1983, EPA banned 

most uses of chlordane and in 1988 banned all uses except for fire 

ant control in power transformers and for depletion of existing 

stocks by homeowners (U.S. Environmental Protection Agency, 

1990). Like DDT, chlordane tends to adhere to soil particles and 

can last for years in the soil. The increasing trend in concentrations 8 years after the ban on almost all uses reflects the persistence of chlordane in the environment and the probability of 

continuing use.

PCBs

PCB concentrations in lake sediments peaked in the mid1960s, decreased substantially during the 1970s, and remained 

fairly stable during the 1980s and 1990s. PCBs are a group of 

organic chemicals that, because they are fire resistant and good 

insulators, have been widely used as coolants and lubricants in 

transformers, capacitors, and other electrical equipment. The trend 

of PCB concentrations in Lorence Creek Lake sediments generally reflects the production and regulation of PCBs in the United 

States. Production of PCBs increased in the 1960s, peaked in 

1970, and was voluntarily stopped in the 1970s (Smith and others, 

1988). Because of environmental persistence and toxicity, their 

manufacture and use in the United States was banned by EPA in 

the late 1970s.

PAHs

PAH concentrations in Lorence Creek Lake sediments (fig. 4) 

peaked in the mid-1970s and again in the mid 1990s. PAHs are a 

group of compounds formed during the incomplete burning of 

fossil fuels, garbage, or other organic substances. Sources include 

vehicle exhaust, burning coal, forest fires and agricultural burning, asphalt roads, and tar; therefore, the recent increase in PAH 

concentrations might be associated with several sources. According to trends in sediment PAH concentrations in four urban lakes 

in other parts of the United States, PAH concentrations tend to 

increase with increasing watershed urbanization (Van Metre and 

Callender, in press). Figure 4. Concentration of polycyclic aromatic hydrocarbons 

(PAHs) in sediment cores.

Figure 3. Concentrations of organochlorine compounds in 

sediment cores.
4

—Patricia B. Ging, Peter C. Van Metre, and Edward 

Callender 

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 hydrologic data related to this study can be 

obtained from:

District Chief

U.S. Geological Survey

8027 Exchange Dr. 

Austin, TX 78754–4733

Email: dc_tx@usgs.gov

Phone: (512) 927–3500

FAX: (512) 927–3590

1960 2000 1970 1980 1990

YEAR

0

140

20

40

60

80

100

120

Polycyclic aromatic 

 hydrocarbons

Metals

Organochlorine

 compounds

NORMALIZED CONCENTRATION

Engstrom, D.R., and Swain, E.B., 1997, Recent declines in 

atmospheric mercury deposition in the Upper Midwest: 

Environmental Science and Technology, v. 31, no. 4, 

p. 960–967.

Environment Canada, 1995, Interim sediment quality guidelines: 

Ottawa, Canada, Ecosystem Conservation Directorate, 

Evaluation and Interpretation Branch, 63 p.

Mahler, B.J., and Lynch, F.L., 1998, Muddy waters—Temporal 

variations in sediment discharging from a karst spring: 

Journal of Hydrology, v. 214, p. 165–178.

Pais, Istvan, and Jones, J.B., Jr., 1997, The handbook of trace 

elements: Boca Raton, Fla., St. Lucie Press, 223 p.

Rapaport, R.A., Urban, N.R., Capel, P.D., Baker, J.E., Looney, 

B.B., Eisenreich, S.J., and Gorham, E., 1985, “New” DDT 

inputs to North America—Atmospheric deposition: 

Chemosphere 14, no. 9, p. 1,167–1,173.

Smith, J.A., Witkowski, P.J., and Fusillo, T.V., 1988, Manmade 

[sic] organic compounds in the surface waters of the United 

States—A review of current understanding: U.S. Geological 

Survey Circular 1007, 92 p.

U.S. Environmental Protection Agency, 1990, Suspended, 

canceled, and restricted pesticides: U.S. Environmental 

Protection Agency, Office of Pesticides and Toxic 

Substances, EN–342.

Van Assche, F., and Martin, M., 1999, Zinc in the environment: 

International Zinc Association, accessed August 17, 1999, at

URL http://www.zincworld.org/zw04/zw0401/040101.htm

Van Metre, P.C., and Callender, Edward, 1997, Water-quality 

trends in White Rock Creek Basin from 1912–94 identified 

using sediment cores from White Rock Lake reservoir, 

Dallas, Texas: Journal of Paleolimnology, v. 17, p. 239–249.

______in press, Trends in sediment quality in response to 

urbanization, in Toxic Substances Hydrology Program 

Technical Meeting, Charleston, S.C., March 8–12, 1999, 

Proceedings: U.S. Geological Survey.

Van Metre, P.C., Land, L.F., and Braun, C.L., 1997, Water-quality 

trends using sediment cores from White Rock Lake, Dallas, 

Texas: U.S. Geological Survey Fact Sheet, FS–217–96, 4 p.

by Environment Canada (1995) were computed. The guidelines 

use a broadly accepted approach for identifying levels of sediment 

contaminants that are expected to adversely affect aquatic life. 

The guidelines are not directly related to human health. The computed ratios (normalized concentrations) were composited on a 

graph (fig. 5). The higher the composite normalized concentration, the more toxic the sediment. Therefore, a decrease in the 

composite normalized concentration over time indicates sediment 

quality is improving. Several conclusions are evident from the 

graph:

• Overall sediment quality has improved since the 1960s with 

large decreases in organochlorine compounds and some 

decreases in metals.

• The recent increase in chlordane (fig. 3) causes an increase in 

toxicity from organochlorine compounds.

• Of the three groups of contaminants, PAHs are the only one 

steadily increasing through the mid-1990s.

Although the overall trend in sediment quality is good news, 

steady increases in chlordane and PAHs raise long-term concerns 

for the health of aquatic life in the watershed upstream of the lake 

and for the quality of urban runoff that likely recharges the 

Edwards aquifer. Additionally, the increases in chlordane and 

PAHs are an indicator of water-quality degradation from urbanization, and their occurrence suggests other urban-related contaminants (not analyzed for in this study) could be present.

References

Agency for Toxic Substances and Disease Registry, 

1999a, ToxFAQs: Accessed August 5, 1999, at 

URL http://www.atsdr.cdc.gov/toxfaq.html

______1999b, Top 20 hazardous substances—ATSDR/EPA 

priority list for 1997: Accessed August 5, 1999, 

at URL http://www.atsdr.cdc.gov/cxcx3.html

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.

Figure 5. Contaminant concentrations normalized to Environment 

Canada (1995) sediment quality guidelines.