1

TEXAS

LOCATION MAP

26o30'

98o30' 97o

30' 98o

26o

0 50 100 KILOMETERS

McAllen

Harlingen

Brownsville

Donna

Edinburg

Alamo

MEXICO

UNITED STATES

Donna Canal

 study area

Laguna

 Madre GULF OF MEXICO

RIO

Intracoastal Waterway

Donna Arroyo Reservoir

Colorado

GRANDE

Figure 1. Location of the Donna Canal study area.

HIDALGO WILLACY

CAMERON

The Donna Canal is a popular fishing spot 

for residents of Hidalgo County. The 11.3-kilometer-long irrigation canal and water-supply system is home to some of the best 

bass and catfish angling in the Rio Grande Valley of South Texas, 

and fish from the Donna Canal often end up on dinner tables. The 

fish, however, might be contaminated with PCBs, a group of toxic 

and carcinogenic (cancer-causing) compounds. PCBs are hydrophobic (meaning “water fearing”). These kinds of chemicals do 

not dissolve in water but instead adsorb to sediment and become 

incorporated into animal tissue. Small animals living in or around 

sediment contaminated with PCBs accumulate these toxic chemicals in their bodies. These creatures are eaten by other animals, 

which concentrate the PCBs in their tissue, and in this way, PCBs 

work their way up the food chain. Often the final consumers and 

concentrators of PCBs are humans.

Water is pumped from the Rio Grande into 

the Donna Canal at an average rate of about 3.4 cubic meters per 

second. The Donna Canal carries the water north by simple gravity flow. The water from the canal is used for irrigation of nearby 

farmland. On its way north, the canal carries the water underneath 

a perennial stream, the Arroyo Colorado, by way of an underground siphon. The Donna Canal ultimately flows into Donna 

Reservoir, which supplies drinking water to the nearby municipalities of Donna and Alamo.

PCBs in the Donna Canal were first detected in 1993 

by the U.S. Environmental Protection Agency (USEPA) during an 

environmental study of the Lower Rio Grande Valley. As part of 

the study, the USEPA tested samples of cooked fish from nine representative households, as well as samples of blood and urine from 

the individuals who consumed the fish. One carp fillet from a fish 

reportedly caught in the Donna Canal had a PCB concentration of 

399 milligrams per kilogram, more than 1,500 times higher than 

the concentration thought to pose a health risk to an adult (U.S. 

Environmental Protection Agency, 1994). The individuals who 

consumed the fish had elevated levels of PCBs in their blood. 

During 1994–2000, the Texas Department of Health (TDH) 

and the Texas Natural Resource Conservation Commission 

(TNRCC) sampled more fish and found many with elevated 

concentrations of PCBs, although none were as high as those in 

Occurrence of Polychlorinated Biphenyls (PCBs) 

on Suspended Sediment in the Donna Canal, 

Hidalgo County, Texas, 1999–2001

In cooperation with the Texas Natural Resource Conservation Commission

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 016–02

April 2002

Some fish in the Donna Canal contain PCBs 

at levels that might pose a risk to human 

health if the fish are eaten. Early attempts 

to locate the source of PCBs in the canal 

were unsuccessful. An innovative method of 

sampling and analyzing suspended sediment 

helped scientists detect PCBs in suspended 

sediment and narrowed the probable PCB 

source area(s) from the entire 11-kilometer 

canal to a 600-meter reach.

Local fishermen at the Donna Canal pumphouse.
2

the first fish analyzed in 1993 by the 

USEPA. The risk of cancer was calculated 

to be 1 in 174 for adults consuming two 

8-ounce meals per week of fish caught 

from the Donna Canal (Buchanan, 1997). 

Possession of fish from the Donna Canal 

was banned while the TDH and the 

TNRCC tried to find the source of the 

PCBs. 

During 1994–97, more than 75 samples 

of water and bed sediment from the canal, 

the reservoir, surrounding reservoirs, the 

Rio Grande, public water supplies, and a 

ground-water monitoring well were analyzed. The only PCB detection was in a 

sample from a drainage ditch 0.3 kilometer 

from the canal (Webster and others, 1998). 

In 1997, the TNRCC asked the U.S. 

Geological Survey (USGS) for assistance 

in detecting the source of the PCBs. This 

report summarizes the results of the USGS 

investigation. 

Sampling of Suspended 

Sediment

Water in the Donna Canal is pumped 

directly from the Rio Grande and flows 

north about 11 kilometers to Donna Reservoir. The pumping rate is set at the beginning of each day. On the days samples 

were collected, the pumping rate ranged 

from 1,420 to 4,530 liters per second, and 

the flow rate ranged from about 0.07 to 

0.14 meter per second. The water in the 

canal looks cloudy or murky because the 

constant flow of water keeps sediment in 

suspension in the water. Some of the suspended sediment is pumped into the canal 

from the Rio Grande, and some of the 

sediment comes from erosion of the sides 

of the canal. 

To try to find the source of the PCBs, 

the USGS used a different type of sampling 

approach—collection and analysis of the 

suspended sediment. Because PCBs do not 

dissolve readily in water but instead stick 

to sediment, they usually are not detected 

in water samples, even in contaminated 

environments. By removing the suspended 

sediment from the water and analyzing 

it directly, the PCBs are more likely to 

be detected. In other words, the USGS 

approach was to look for the PCBs where 

they were expected to be. 

Suspended sediment was collected 

for analysis by filtering. At each sampling 

site, tubing was suspended in the canal 

about 2.5 meters from the bank at a depth 

of about 1 meter, and water was pumped 

from the canal with a peristaltic pump; at 

bridges and at the mouth of a siphon in the 

canal, the tubing was suspended in the center of the canal. For all but the final round 

• What are PCBs? 

PCBs are synthetic compounds that were used in the United States in the 1950s to 

1970s for many industrial purposes. They were used mostly as coolants and lubricants in transformers, capacitors, and other electrical equipment. PCBs also were 

used as plasticizers in paints, plastics, and rubber products; in pigments, dyes, and 

carbonless copy paper; and in many other products (U.S. Environmental Protection 

Agency, 2001). Because of the threat they pose to human health, the U.S. Government banned PCB production and use in 1976. PCBs break down to other, lessharmful compounds extremely slowly, and PCBs that were released to the environment decades ago are still a threat to human health today. The fate of many PCBs 

used before the ban is largely unknown. PCBs have been found in a variety of residential and industrial locations and dumpsites throughout the United States and 

Canada. Some PCBs were illegally dumped or buried after their use was banned. 

The exact location of the PCB source contaminating the Donna Canal is unknown, 

but the effect of these chemicals on the local environment is seen in contaminated 

fish in the Donna Canal.

• How does eating PCB-contaminated fish affect humans? 

PCBs concentrate in the skin and fatty tissue of human consumers and most animals and can affect the skin, liver, stomach, and thyroid gland. It also can affect the 

nervous system causing severe degenerative conditions. Other effects of PCBs in 

animals include changes in the immune system, behavioral alterations, and impaired 

reproduction. PCBs are not known to cause birth defects. Rats that ate food containing high levels of PCBs for 2 years developed liver cancer. The EPA and the International Agency for Research on Cancer have determined that PCBs are probably 

carcinogenic to humans. Studies have shown that babies born to women who ate 

PCB-contaminated fish have shown abnormal responses in tests of infant behavior 

Suspended sediment from Donna (Agency for Toxic Substances and Disease Registry, 2001).

Canal collected on a filter.

Collecting a suspended sediment sample from the Donna Canal.

Filter Pump

Donna

Canal

apparatus
3

Rio Grande

Pumphouse

Siphon

Donna

Reservoir

<14

42

35

37

102, 52, 77

130, 35 54

41

72

80 23

37, 11

80, <195

29

26

56

50, 53

<19, 12

25

<26

<32, <25

Flow

Flow

Siphon

outlet

0 50 100 METERS

0 1 2 KILOMETERS

0 50 100 METERS

A

B B

A

of sampling, water was pumped through a 298-

millimeter-diameter glass fiber filter with a nominal 

pore size of 0.5 micrometer. For the final round of 

sampling, water was pumped through similarly sized 

PTFE (Teflon) filters. Water was pumped through the 

filter until the filter clogged; three filters were used at 

each site, and a total of 25 to 140 liters of water was 

filtered. The filters were put inside a baked glass jar 

and chilled until sent to the laboratory for analysis. 

Unfiltered water samples also were submitted for 

analysis of total suspended sediment concentration. 

PCBs were analyzed by gas chromatography/mass 

spectrometry (GC/MS) following the method of 

Foreman and others (1995).

Sampling Results and Conclusion

Five separate sampling trips to the Donna Canal 

were made to collect suspended sediment. The overall 

strategy was to narrow the search for the PCB source 

area(s) from the entire 11-kilometer length to a much 

shorter distance by collecting samples at more closely 

spaced intervals. The sampling sites and spacing were 

chosen on the basis of the results from the previous 

event (fig. 2). 

Round 1: February 1999

 No PCBs were detected during the first round of 

sampling when samples were collected from sites near 

the pumphouse, at the siphon inlet, at the siphon outlet, and in the Arroyo Colorado (results shown in 

white, fig. 2). These results suggested that the source 

of the PCBs must be downstream (north) of the siphon 

outlet. 

Round 2: July 1999

 The results of the second sampling round (shown 

in yellow, fig. 2) confirmed that the source was in or 

downstream of the siphon outlet, and that it was probably upstream (south) of the 90-degree bend. On the 

basis of these detections, samples were collected at 

more closely spaced intervals in the reach downstream 

of the siphon to try to pinpoint the PCB source. 

Round 3: January 2000

 The results of the third sampling round (shown in 

pink, fig. 2) indicated a possible PCB source in the 

200-meter reach downstream of the siphon outlet and 

a possible second source at least 150 meters upstream 

of the 90-degree bend. 

Round 4: July 2000

 On the basis of previous results, the fourth sampling round samples were collected at about 50-meter 

intervals on both banks of the canal downstream of the 

siphon outlet. The results (shown in blue, fig. 2) indicated a potential PCB source on the right bank of the 

canal, just downstream of the siphon outlet. 

Round 5: April 2001

 In an attempt to pinpoint the location of an additional source or sources upstream of the 90-degree 

bend, the fifth sampling-round samples were collected 

Figure 2. Aerial photographs of the Donna Canal (indicated by the 

dashed blue line), sampling sites, and concentrations detected. Flow is 

to the north from the pumphouse toward the reservoir. To the right are 

enlarged images of two areas of the canal chosen for more closely 

spaced sampling. Sampling sites are indicated with yellow dots, and 

results of samples collected during the same sampling trip are shown in 

the same color: white (February 1999), yellow (July 1999), pink (January 

2000), blue (July 2000), and green (April 2001). All concentrations are in 

micrograms per kilogram of sediment collected.
4

at about 50-meter intervals in the reach upstream of the 90-degree 

bend. The results (shown in green, fig. 2) confirmed the presence 

of PCBs in suspended sediment along this section of the canal but 

do not identify any one location as a probable PCB source. One or 

several sources of PCBs might be present along this stretch of the 

canal.

The TNRCC also participated in the fifth and final sampling 

round, collecting whole water and bed sediment at selected locations in the canal and extracting two soil borings from the area of 

the canal just downstream of the siphon outlet. However, no PCBs 

were detected in the TNRCC water or sediment samples, and this 

additional sampling was unsuccessful in further pinpointing the 

location of the PCB source(s).

In conclusion, the source or sources of the PCBs must be 

located between the siphon outlet and the 90-degree bend in the 

Donna Canal.

What is the future of the Donna Canal? 

Using the information gathered from the suspended sediment 

sampling during 1999–2001, the TNRCC has begun a multiphase 

project to investigate and remediate all sources of PCBs in the 

Donna Canal. Efforts will be concentrated in the 600-meter reach 

identified by the USGS as the most likely reach of the canal to 

contain the source(s) of the PCB contamination. The first phase of 

the TNRCC project began in August 2001 and involves additional 

assessment and delineation of the PCB source. Subsequent phases 

of the project will involve the development of a remediation plan 

and implementation of remedial actions in the canal.

References

Agency for Toxic Substances and Disease Registry, 2001, 

ToxFAQ for polychlorinated biphenyls (PCBs): Accessed 

August 24, 2001, at URL 

http://www.atsdr.cdc.gov/tfacts17.html 

Buchanan, T.A., 1997, Polychlorinated biphenyls in fish in the 

Donna Reservoir and Canal system and in the Lower Rio 

Grande Valley of Texas: Houston, University of Texas Health 

Science Center, Masters thesis, 84 p.

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 electroncapture detection: U.S. Geological Survey Open-File 

Report 94–140, 78 p. 

U.S. Environmental Protection Agency, 1994, Lower Rio Grande 

Valley environmental monitoring study—Report to the 

community in the pilot project: Washington, D.C., 20 p.

______2001, PCBs, polychlorinated biphenyls: Accessed August 

24, 2001, at URL 

http://www.epa.gov/Region8/toxics_poisons/pcb/pcb.html

Webster, C.F., Buchanan, T.A., Kirkpatrick, J., and Miranda, 

R.M., 1998, Polychlorinated biphenyls in Donna Reservoir 

and contiguous waters: Texas Natural Resource Conservation 

Commission, Agency Studies AS–161, 46 p.

— B.J. Mahler, P.C. Van Metre, and R.M. Miranda

Photographs by B.J. Mahler and M.P. Cordell

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 Phone: (512) 927–3500

U.S. Geological Survey FAX: (512) 927–3590

8027 Exchange Dr. World Wide Web: 

Austin, TX 78754–4733 http://tx.usgs.gov/ 

E-mail: dc_tx@usgs.gov

Fisherman at Donna Reservoir, Hidalgo County. 

Sampling suspended sediment at Donna Canal.