1

97 ̊29' 97 ̊28' 97 ̊27' 97 ̊26' 97 ̊25'

32 ̊46'

32 ̊47'

32 ̊48'

32 ̊49'

32 ̊50'

EXPLANATION

Area enlarged

in figure 5

Box-core sampling

site/gravity-core

sampling site and

identifier

Box-core sampling

site

Fort

 Worth

Study area

TEXAS

LOCATION MAP

0 0.25 0.5 MILE

0 0.5 KILOMETER LWT.UB

LWT.WD

LWT.DM

IH–820

Carswell

Air Force Field

Plant 4

Woods

Inlet

Meandering

Road Creek

Landfill

No. 3

Landfill

No. 4

Live Oak Creek

Silver Creek

Building 181

West Fork 

Trinity River

Dam

Lake Worth

White

Settlement Naval Air

Station

LWT.DM

Base modified from U.S. Geological Survey

Digital orthophoto quarter-quadrangle,

Universal Transverse Mercator projection, 

Zone 14, Datum NAD 1983, 1-foot resolution

Figure 1. Location of study area and approximate sampling locations in Lake Worth, Fort Worth, Texas, 2000–

2001.

concentrations of polychlorinated biphenyls (PCBs) in fish (Texas 

Department of Health, 2000). In response to the advisory and in cooperation with the U.S. Air Force, the U.S. Geological Survey (USGS) 

collected 21 surficial samples and three deeper gravity core samples 

from the sediment deposited at the bottom of Lake Worth. The purpose 

of that study was to assess the spatial distribution and historical trends 

of selected hydrophobic contaminants, including PCBs, and to determine, to the extent possible, sources of selected metals and hydrophobic organic contaminants (HOCs) to Lake Worth. Hydrophobic 

(literally “water fearing”) contaminants tend to chemically adsorb to 

soils and sediments. Fifteen of the top 20 contaminants on the Agency 

mulate in lakes and reservoirs, they generate a partial his

of water quality. This fact sheet describes the collection 

cores, age-dating methods, and historical trends in PCB

Worth sediments. The fact sheet also describes the spatia

of PCBs in surficial sediments and concludes with objec

second phase of data collection and the approach that w

achieve these objectives. The USGS published a compreh

on the first phase of the study (Harwell and others, 2003

Lake Worth is a reservoir on the West Fork Trinity 

western edge of Fort Worth in Tarrant County. In 1914, 

Fort Worth completed the reservoir to serve as a munici

supply. Lake Worth has a surface are

square kilometers and a storage capac

lion cubic meters. The drainage area to 

is 5,350 square kilometers(Ruddy an

The surrou

the south a

marily urb

area to the 

northwest 

residentia

Collect

Sedime

Surf

ment sam

centimete

lected at 2

a box core

were distr

throughou

(fig. 1) to 

along the 

adjacent t

Plant 4 (A

Naval Air 

Reserve B

Field, the 

major trib

areas near 

are influen

sources of 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S
2

(c)

(a) (b)

Detection

EXPLANATION

Concentration less than reporting limit

1932

1914

1914

1932

1964

1964

1932

1914

1964

CESIUM-137 ACTIVITY, IN PICOCURIES PER GRAM

CESIUM-137 ACTIVITY, IN PICOCURIES PER GRAM CESIUM-137 ACTIVITY, IN PICOCURIES PER GRAM

-0.1 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0

CORE DEPTH, IN CENTIMETERS 200

180

160

140

120

100

80

60

40

20

0

LWT.WD–Woods Inlet LWT.DM–Downstream from IH–820

bridge (near dam)

LWT.UB–Upstream from IH–820

bridge

to the lake, and areas in the upper part of the lake not affected by AFP4 

and Carswell Field and less affected by urbanization.

At three sites where surficial sediment samples were collected, gravity 

cores were collected to investigate historical trends in selected contaminants. 

One gravity core was collected in the 

upper part of the lake upstream from the 

IH–820 bridge (LWT.UB, fig. 1) to represent historical trends in a reference 

site upstream from AFP4 and much of 

the urban influence to the lake. A second gravity core was collected in 

Woods Inlet (LWT.WD, fig. 1) near the 

mouth of Meandering Road Creek to 

represent a site influenced by runoff 

from AFP4. A third gravity core was 

collected in the lower part of the lake 

near the dam (LWT.DM, fig. 1) to represent a site influenced by all inputs of 

contaminants to the lake. 

One gravity core from each site 

was split lengthwise and described 

visually on site (fig. 2a). A second core 

from each site was extruded vertically 

(fig. 2b), and discrete intervals were 

subsampled for chemical analysis (fig. 2c). Box-core samples were 

analyzed for the same constituents as gravity-core samples except for 

cesium-137 (137Cs), which was used in age dating the gravity cores. 

Results of all chemical analyses are presented in Harwell and others 

(2003).

Age-Dating Methods

The pre-reservoir boundary (dated as 1914, when the dam was 

completed), the sampling date in 2000, and profiles of radioactive 

137Cs were used in age dating the gravity cores. In reservoir sediment 

cores, sediment deposited after the lake formed is easy to distinguish 

from pre-reservoir soils because pre-reservoir soils are relatively dry 

and can contain sand and root hairs. Because 137Cs sorbs strongly to 

fine-grained sediments, it is useful for dating sediments exposed to 

atmospheric fallout, such as those found in lakes (Durrance, 1986). 

Beginning in about 1952, large-scale nuclear weapons testing released 

detectable amounts of 137Cs to the atmosphere. Concentrations of 

137Cs in the atmosphere peaked between 1963 and 1964. Profiles 

of 137Cs activity in gravity cores can provide two date markers in 

reservoirs built before about 1950—the first occurrence between 1952 

and 1953 and the peak in 1964. Other chemical indicators also 

can be used to corroborate age assignments made using 137Cs; these 

include peaks in DDT and lead concentrations. Total DDT (the sum of 

DDT and its breakdown products) concentrations tend to peak in the 

early-to-mid-1960s, coincident with peak use of DDT in the United 

States (Van Metre and Callender, 1997). Lead concentrations in urban 

lake and reservoir sediment cores consistently peaked in the mid1970s coinciding with the switch from leaded to unleaded gasoline 

(Callender and Van Metre, 1997). All of these indicators were used to 

assign dates to core samples and to investigate the reasonableness of 

date assignments. Ages for samples at depth intervals between known 

depth-date markers were assigned on the basis of a mass accumulation 

rate (MAR) because MARs automatically adjust for compaction in a 

core.

The 137Cs activity profiles in each of the three gravity cores 

indicate different sedimentation rates for each of the sites (fig. 3). 

Core LWT.UB has an unusual 137Cs profile with nondetections in 

Figure 2. U.S. Geological Survey personnel (a) describing a 

gravity core, (b) placing gravity core in core extrusion stand, 

and (c) slicing a subsample from a gravity core (photographs 

by Barbara J. Mahler, U.S. Geological Survey).

Figure 3. Cesium-137 profiles in gravity-core samples from Lake Worth, Fort Worth, Texas, 

2000–2001.
3

Box-core sampling

site and total PCB

concentration, in

micrograms per

kilogram—Nondetection (ND) is 

less than reporting 

level, which varies 

from less than 15 to 

less than 75 micrograms per kilogram. 

Concentrations in 

gravity core shown 

in graph

EXPLANATION

12

97 ̊29' 97 ̊28' 97 ̊27' 97 ̊26' 97 ̊25'

32 ̊46'

32 ̊47'

32 ̊48'

32 ̊49'

32 ̊50'

0 0.25 0.5 MILE

0 0.5 KILOMETER

LWT.UB–Upstream from 

IH–820 bridge

TOTAL PCB CONCENTRATION,

IN MICROGRAMS PER KILOGRAM 

0 10 20 30 40 50 60 1910

1930

1950

1970

1990

2010

YEAR

LWT.WD–Woods Inlet

YEAR 1910

1930

1950

1970

1990

2010

0 10 20 30 40 50 60

TOTAL PCB CONCENTRATION,

IN MICROGRAMS PER KILOGRAM 

LWT.WD–Woods Inlet

YEAR 1910

1930

1950

1970

1990

2010

0 10 20 30 40 50 60

TOTAL PCB CONCENTRATION,

IN MICROGRAMS PER KILOGRAM 

LWT.WD–Woods Inlet

YEAR 1910

1930

1950

1970

1990

2010

0 10 20 30 40 50 60

TOTAL PCB CONCENTRATION,

IN MICROGRAMS PER KILOGRAM 

LWT.DM–Downstream from 

IH–820 bridge (near dam)

TOTAL PCB CONCENTRATION,

IN MICROGRAMS PER KILOGRAM 

0 10 20 30 40 50 60 1910

1930

1950

1970

1990

2010

YEAR

ND

ND

ND

ND ND

9

ND

20

15

140

ND

10 12

ND

ND

ND

12 9 ND 3

ND

most of the lower part of the core and a low-activity, rounded peak in 

the top 20 centimeters of the core. The construction of Eagle Mountain 

Lake immediately upstream from Lake Worth on the West Fork 

Trinity River in 1932 provides a logical explanation for this seemingly 

anomalous profile. Eagle Mountain Lake has a conservation storage 

capacity about five times greater than that of Lake Worth and captures 

nearly all of the runoff from the watershed of Lake Worth. As a 

result, sedimentation rates probably declined substantially after 1932, 

especially in the upper part of the lake upstream from the larger tributary inflows. The 137Cs activity profile in core LWT.DM indicated a 

similar temporal pattern, but with a much less pronounced difference 

in MAR before and after Eagle Mountain Lake was formed. Core 

LWT.WD has a 137Cs profile characterized by a distinct first occurrence and sharp peak, a smooth exponential decrease above the 

peak, and a large peak-to-top-of-core concentration ratio (Van Metre 

and Callender, 1997).

Historical Trends in PCBs

PCBs were widely used in both industrial and urban applications 

in the United States from the 1950s to the 1970s. Because of their toxicity and environmental persistence, PCB manufacture and use was 

restricted by the U.S. Environmental Protection Agency in 1971. PCB 

concentrations from urban 

lakes peaked at the height of 

their use in the mid-to-late 

1960s, then decreased following the restrictions imposed in 

1971 (Van Metre, Callender, 

and Fuller, 1997; Van Metre 

and others, 1998). Because of 

variable numbers and positions 

of chlorine atoms within their 

structure, 209 individual PCB 

compounds, or congeners, are 

possible. PCBs were marketed 

as Aroclors, mixtures of many 

congeners. The analytical 

method used for this report 

quantifies PCBs as one of three 

common Aroclors (1242, 1254, 

and 1260); the sum of these 

Aroclors is referred to as total 

PCBs.

Total PCBs were not detected in any of the upper-lake surficial 

(box-core) samples (fig. 4). Seven of the 13 surficial samples from the 

middle and lower lake had detectable total PCB concentrations (all 

less than the laboratory reporting level) with a maximum estimated 

concentration of 12 micrograms per kilogram (μg/kg). The three surficial samples from Woods Inlet had estimated total PCB concentrations of 15, 20, and 140 μg/kg, with the highest concentration at the 

farthest upstream site in Woods Inlet.

The gravity cores indicate a clear picture of temporal trends 

in total PCB concentrations (fig. 4). No detections occurred in samples 

in the upper-lake core (LWT.UB) or in samples deposited before the 

late 1940s in either the near-dam core (LWT.DM) or the Woods Inlet 

core (LWT.WD). The peak concentration in both the near-dam core 

and the Woods Inlet core occurred in the 1960s, followed by an exponential decrease to the top of the core. In both cores, the peak concentration was about three times greater than the estimated concentration 

at the top of the core. Despite decreases in recent years, these trends 

suggest that PCBs continue to enter the lake, probably attached to suspended sediment during runoff. These trends also suggest that larger 

concentrations could occur in deeper sediments farther upstream from 

the most upstream site in Woods Inlet, where the surface concentration 

was 140 μg/kg.

Figure 4. Spatial and temporal distribution of total polychlorinated biphenyls (PCBs) in sediments 

from Lake Worth, Fort Worth, Texas, 2000–2001.
4

Gravity-core

sampling site

Box-core

sampling site

Large-volume

suspendedsediment

sampling site

EXPLANATION

97 ̊28' 97 ̊27'

32 ̊46'

32 ̊47'

Study Continued

The results from this study provided much 

information but raised new questions about the extent 

and sources of PCB contamination in Lake Worth 

sediments. To answer these new questions, a second 

round of sampling was done. The second round of 

sampling had two principal objectives: The first 

objective was to map the extent of sediments with 

elevated concentrations of PCBs (greater than about 

20 μg/kg) in Woods Inlet. The second was to determine likely sources of PCBs to sediments entering Woods Inlet from 

AFP4 and from Meandering Road Creek and other small tributaries. 

To address these objectives, additional sediment samples were 

collected. Surficial sediment samples from 17 new sites and gravity 

cores from three new sites were collected in Woods Inlet (fig. 5). 

A congener-specific PCB analysis was done on both the bottomsediment samples and on suspended-sediment samples in an attempt to 

chemically “fingerprint” PCBs from different sources. The USGS has 

pioneered the use of large-volume suspended-sediment (LVSS) sampling for HOCs, such as PCBs (B.J. Mahler, U.S. Geological Survey, 

written commun., 2003). The technique is useful to detect and quantify 

HOCs in stormwater where traditional water sampling and streambedsediment sampling have yielded poor results. LVSS sampling sites 

were established at five locations (fig. 5) where flow enters Woods 

Inlet from AFP4 and the surrounding (upstream) urban area. The 

LVSS sampling was done using passive samplers, a low-tech but 

effective and inexpensive way to get large-volume (25-liter), firstflush stormwater samples. LVSS samples were collected for three 

storms at each site. Bed sediments were collected from the creek bottoms at four of the five tributaries adjacent to LVSS sampling sites. 

PCB concentrations and congener mixtures at each site relative to the 

drainage area and to the occurrence of PCBs in lake-bottom sediment 

samples will be used to try to determine sources of PCBs to Lake 

Worth.

References

Agency for Toxic Substances and Disease Registry, 2001, Top 20 hazardous 

substances—ATSDR/EPA priority list for 2001: accessed January 23, 

2002, 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.

Durrance, E.M., 1986, Radioactivity in geology—Principles and applications: 

Chichester, West Sussex, England, Ellis Horwood Limited, 441 p.

This fact sheet is based on the report

Harwell, G.R., Van Metre, P.C., Wilson, J.T., and Mahler, B.J., 2003, 

Spatial distribution and trends in trace elements, polycyclic aromatic 

hydrocarbons, organochlorine pesticides, and polychlorinated 

biphenyls in Lake Worth sediment, Fort Worth, Texas: U.S. Geological Survey Water-Resources Investigations Report 03–4269, 56 p.

Ruddy, B.C., and Hitt K.J., 1990, Summary of selected characteristics of large 

reservoirs in the United States and Puerto Rico, 1988: U.S. Geological 

Survey Open-File Report 90–163, 295 p.

Texas Department of Health, 2000, TDH warns against eating fish from Lake 

Worth: Texas Department of Health news release, April 19, 2000, 

accessed July 15, 2003, at URL 

http://www.tdh.state.tx.us/news/b_new308.htm

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

Rock Creek Basin from 1912–1994 identified using sediment cores from 

White Rock Lake reservoir, Dallas, Texas: Journal of Paleolimnology, 

v. 17, p. 239–249.

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., 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.

— Christopher L. Braun and Glenn R. Harwell

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/

Figure 5. Approximate sampling locations for second round of 

sampling in and near Woods Inlet of Lake Worth, Fort Worth, 

Texas, 2000–2001.