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Rio Grande Toxic Substances Study Summary

Summary of Phase I of a study of toxic substances in the Rio Grande, including introduction, goals, types of analyses, sites of concern, human-health implications, future study efforts, and conclusion.


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:

  1. 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;
  2. Toxicity tests—to see if the water and sediment had any effects on the survival or reproduction of very sensitive laboratory test animals; and
  3. "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, 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)


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.

  1. 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.
  2. 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.
  3. 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.
  4. Toxic chemical concentrations in fish tissue will be assessed in the headwaters of the International Amistad and Falcon Reservoirs.


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.