Introduction
The governments of Mexico and the United States performed a
joint study on toxic substances in the Rio Grande (called the
Río Bravo in Mexico) from 1992 until 1994 in a major
binational cooperation in scientific investigation into common
environmental concerns. The results of the study were released to
the public in September 1995.
The study was prompted by a widely held public concern that the
river was being contaminated by toxic substances, possibly from
industrial and agricultural sources near the border. The concern
with industrial sources was intensified by the large increase in
the number of industrial plants in recent years. There are
currently over 1,400 such plants in the border region.
Additionally, many people are concerned that the quality of the
Rio Grande/Río Bravo may be affecting public health in ways
other than pollution by toxic substances. Some of these potential
problems, which were not a part of this study, include the
above-average rate of neural-tube defects in newborn infants and
contamination by untreated sewage and by disease-causing bacteria.
Due to resource limitations and U.S. Environmental Protection
Agency guidelines for this particular grant effort, a decision was
made to limit the study to the detection of toxic chemicals and any
effects they may be having on the environment.
Information from prior studies found some evidence of
contamination from toxic substances such as pesticides and heavy
metals, but data on any effects of these on the environment were
limited.
Goals of the Study
The main goal of the study was to see if the suspected
contamination of the river by toxic substances was occurring and,
if so, to assess damage from the toxic substances to the
river’s aquatic animals. Finally, if toxic chemicals were
found, to identify their sources.
Types of Analyses
Sampling was conducted during 1992–93; lab work and report
writing were completed in 1994. Samples were taken at 19 main stem
sites and 26 tributaries along the portion of the river that forms
the boundary between Texas and Mexico. These sites were chosen
because it was thought that they had a potential to be contaminated
by industrial or agricultural sources. (See attached map and
station descriptions.)
At each site, samples were collected of water, sediment, fish,
and benthos (bottom-dwelling animals such as clams and worms).
Three distinct types of analyses were done:
- Laboratory analysis—to measure the
presence and amount of each of approximately 150 different toxic
chemicals in each of the water, sediment, and fish tissue
samples;
- Toxicity tests—to see if the water
and sediment had any effects on the survival or reproduction of
very sensitive laboratory test animals; and
- "Bioassessments" of fish and
benthos—evaluations of the number and types of fish and
benthos to see if the populations were as plentiful and diverse as
they should be in a healthy ecosystem, or if they have
characteristics that indicate they are being potentially impacted.
(Note: bioassessment data were gathered at only 18 of the 45
sampling locations.)
Laboratory Analysis
The results of the first type, laboratory analysis, were
then compared to “screening levels” to see if a problem
had been detected. These screening levels can be thought of as
alert levels, levels above which the chemical being measured
may have the potential to cause some sort of problem.
Significantly, because the effects of any single chemical can be
very different in each type of sample (water, sediment, and fish
tissue), these screening levels can be very different depending on
the problem being evaluated. For example, a chemical concentration
limit necessary to protect human health from the eating of
contaminated fish tissue is likely to be very different than the
concentration limit needed to protect human health from the
drinking of water containing the same chemical. In turn, that is
likely to be a different level from the limit that required to
protect a fish swimming in water contaminated with the chemical. In
this study data were screened using state and federal criteria for
protection of human health and aquatic life. If any of these
various screening levels were exceeded by any of the 150 chemicals,
an exceedance was reported.
Of the more than 15,000 possible toxic chemicals evaluated in
the first type of analysis, a total of 48 were detected, 30 of
which exceeded screening levels. In the main stem of the Rio
Grande/Río Bravo, five toxic chemicals exceeded screening
levels in water, eight in sediment, and 13 in fish tissue. In the
tributaries 17 toxic chemicals exceeded screening levels in water,
15 in sediment, and eight in tissue.
Toxicity Testing
In the toxicity tests, the second type of analysis,
significant adverse effects occurred on the main stem in just two
of 114 tests. These occurred at station 1 at El
Paso–Juárez and station 12 at Laredo–Nuevo Laredo.
In the tributaries, samples from 14 of the 26 stations produced
significant adverse effects in at least one phase of the toxicity
tests.
Bioassessments
Bioassessments, the third type of analysis, showed that
the fish and benthos communities in the main stem were generally
healthy. However, two stations (2 and 12) showed a high probability
of toxic chemical impact and four (3, 10, 14, 16) showed a moderate
impact. Even so, biotic integrity at the affected sites indicated
that any effects from toxic chemicals were relatively slight. No
instances of severe impairment were observed. In the tributaries,
the probability of toxic impacts was high for stations 1a, 2a, 10a,
11a, 11c, and 15a; slight to moderate for stations 3a, 7b, 9a, and
12d; and negligible for the other stations. This greater potential
for problems in tributaries is probably occurring because some of
them transport untreated sewage in relatively undiluted form.
Sites of Concern
All of these analyses were considered together to identify sites
and chemicals of potential concern for future monitoring efforts.
The 30 chemicals that exceeded screening levels were considered to
be of potential concern, and were assigned an approximate level of
importance based on occurrence. A high-priority group included
residual chlorine, methylene chloride, toluene, arsenic, cadmium,
chromium, copper, lead, mercury, nickel, selenium, silver, zinc,
chlordane, p,p'-DDE, dieldrin, gamma-BHC (lindane), total
PCBs, and cyanide. A medium-priority group consisted of non-ionized
ammonia, para-chloro-meta cresol, phenol, and diazinon. A low
priority group was composed of phenolics recoverable, chloroform,
antimony, thallium, bis(2-ethylhexyl) phthalate, diethyl phthalate,
and di-n-butyl phthalate.
Based on this analysis, the following stations exhibited either
high potential or slight to moderate potential for toxic chemical
impacts. Unlisted sites exhibited negligible evidence of toxic
chemical impacts.
Mainstem Sites
High Potential for Toxic Chemical Impacts
- Downstream from El Paso–Ciudad Juárez (2)
- Downstream from Laredo–Nuevo Laredo (12)
Slight to Moderate Potential for Toxic Chemical
Impacts
- Upstream from Rio Conchos confluence near Presidio/Ojinaga
(3)
- Downstream from Eagle Pass/Piedras Negras (10)
- Downstream from Anzalduas Dam (14)
- Below Anhelo Drain South of Las Milpas (16)
Tributaries
High Potential for Toxic Chemical Impacts
- El Paso Public Service Board Haskell R. Street Wastewater
Treatment Plant (1a)
- Ciudad Juárez Discharge Canal (2a)
- Manadas Creek (10a)
- Zacate Creek (11a)
- Arroyo El Coyote (11c)
- Anhelo Drain (15a)
Slight to Moderate Potential for Toxic Chemical
Impacts
- Rio Conchos (3a)
- San Felipe Creek (7b)
- Unnamed Tributary South of Eagle Pass–Piedras Negras
(9a)
- Arroyo Los Olmos (12d)
Implications for Human Health
Although the goal of this study was not to answer all of the
questions that exist pertaining to the health of residents in the
Rio Grande/Río Bravo area, it is natural to ask just how
much insight this study gives into that subject. For now we can
only look to the “screening levels” that have to do with
human health. Because there were no exceedances of FDA action
levels, no short-term risks were found for the 24 sites where fish
tissues were taken for analysis. However, low-level human-health
criteria, which are screening levels for long-term exposure, were
exceeded in water, in edible fish tissue, or both at 22 of the 45
sites. For 17 of these sites, slight human health risks could
result from regular, long-term consumption of untreated water or
fish. Although risks were found for the other five sites, which are
tributaries dominated by sewage effluent, they are not likely to
affect human health because their waters are nonpotable and the
water quality is not conducive for support of viable fish
populations due to pollution from untreated sewage. All of the data
gathered in this study have been made available to various state
and federal agencies with health-related responsibilities for
possible further analysis by specialists.
Future Study Efforts
This study has identified areas with the highest probability of
toxic contamination. A second round of sampling and analysis is in
progress to gain a better understanding of the location of toxic
contamination and, something this first study was unable to do, to
try to better determine the source of the toxic substances that
were found. The second study will also provide insight into whether
the problem is worsening over time.
The second study will contain the elements listed below, pending
agreement between the U.S. and Mexico.
- Additional sampling will be done at the six main stem and ten
tributary sites, where a slight-to-moderate or high potential for
toxic chemical impact was indicated, including expanded monitoring
in the vicinities of El Paso–Ciudad Juárez and
Laredo–Nuevo Laredo.
- Intensive surveys will be performed on tributaries of potential
concern that support significant aquatic life habitat, i.e., Rio
Conchos (3a) and San Felipe Creek.
- Toxic chemical concentrations in fish tissue will be reassessed
in the Rio Grande–Río Bravo stations at Foster Ranch,
upstream from Del Rio–Ciudad Acuña, upstream from Eagle
Pass–Piedras Negras, and upstream from the old
Laredo–Nuevo Laredo International Bridge.
- Toxic chemical concentrations in fish tissue will be assessed
in the headwaters of the International Amistad and Falcon
Reservoirs.
Conclusion
The Rio Grande/Río Bravo and its biological communities
are affected by toxic contamination in relatively few locations.
Not all of the potential implications for human health implications
can be addressed by this study. The source of the toxic substances
was not identified, but the problem is most severe downstream of
the largest cities. Samples will be taken in areas that were not
previously studied and additional work will be done in those areas
where toxic impacts were found.
