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Second Phase of the Binational Study Regarding the Presence of Toxic Substances in the Rio Grande/Río Bravo and its Tributaries Along the Boundary Portion Between the United States and México
QUESTIONS AND ANSWERS
1. QUESTION: What was the objective of the study?
ANSWER: Results of the Phase 1 Binational study were published in September 1994 and recommended follow-up studies to better define chemical impacts on water quality of the Rio Grande/Rio Bravo. The Phase 1 and 2 studies were both designed to see what chemicals are present in the Rio Grande/Rio Bravo and its tributaries, and to find out the potential impact these substances have on water quality along the U.S./Mexico border.
2. QUESTION: Are the Phase 2 results generally better, worse, or the same as the Phase 1 results?
ANSWER: Neither study was designed to quantify chemical trends along the Rio Grande/Rio Bravo and its tributaries, but rather the relative occurrence and impact that these chemicals were having on water quality. There were differences in the types and concentrations of chemical substances found in the Phase 1 and 2 studies. However, this should not be too surprising. Water samples, unless collected as a composite sample over time, give only a relative indication of what water quality was at the time of collection.
3. QUESTION: Who conducted the study and how long did it take?
ANSWER: The U.S. Environmental Protection Agency (USEPA) provided cooperative grant monies to the Texas Natural Resource Conservation Commission (TNRCC) to conduct Phase 2 of the multi-phase study for the United States. The study was a binational multi-agency effort which included the following U.S. and Mexican agencies: TNRCC, U.S. and Mexican Sections of the International Boundary and Water Commission (IBWC), Texas Department of Health, and the National Water Commission (Comisin Nacional del Agua [CNA]). International coordination of the study was handled by the IBWC.
The Phase 2 study was conducted between the months of May 1995 through December 1995 and took approximately three years to complete.
4. QUESTION: What was studied?
ANSWER: Samples of water, fish tissue and sediment were collected from 27 sites along the Rio Grande/Rio Bravo, and 19 sites along its tributaries. These samples were analyzed for 161 chemicals (or substances) such as volatile organic compounds (VOCs), semivolatile compounds (SVOCs), pesticides and metals. Only one sample was collected at each site. The results can be described as a snapshot of the water quality of the river and may not reflect actual conditions of the river.
5. QUESTION: Was this a thorough examination of the water quality in the Rio Grande/Rio Bravo?
ANSWER: The Phase 2 study is considered thorough because it analyzed more than 161 chemicals in three types of media [water, sediment and fish tissue]. However, results can only be compared to the Phase 1 study. Therefore, it is difficult to come to definitive conclusions concerning water quality trends along the Rio Grande/Rio Bravo and its tributaries. It can tell us what was found at each sampling site at that point in time.
6. QUESTION: What is the conclusion of the study?
ANSWER: The results of the Phase 2 study did not indicate any immediate threat to human health or aquatic life. The study identified seven chemicals present in the water, sediment and fish of the Rio Grande/Rio Bravo and its tributaries. Many of these chemicals are metals, some of which are naturally occurring. The concentration and type of chemicals is consistent with the known population, land-use and industrial activities along the Rio Grande/Rio Bravo. The greatest benefit of the Phase 2 study is that it will serve as a definitive basis for future studies. The study will also serve to set priorities for wastewater treatment upgrades, nonpoint source pollution best management practices, and other potential practices to reduce environmental impact.
7. QUESTION: What were sample results compared to?
ANSWER: Criteria and screening levels were used to determine whether measured concentrations of substances had the potential to cause long-term health problems for humans and aquatic life. Criteria and screening levels are comparison values established by the TNRCC Surface Water Quality Standards and State 85th Percentiles, USEPA Criteria for Protection of Aquatic Life and Human Health and National 85th Percentiles.
The criteria used for the Mexican report were the Ecological Water Quality Criteria established by SEDUE [December 1989]. In addition, the Water Quality Index was used, which is an average of the effects caused by varying levels of parameters measured in a flowing body of water.
8. QUESTION: What is a percentile?
ANSWER: Percentiles are the relative standing of a measurement. For example, suppose you scored an 80 on an examination and you want to know how you fared in comparison with others in your class. If the instructor told you that you scored in the 85th percentile, it means that 85% of the examination grades were less than yours and 15% were greater. Likewise, State and National 85th percentiles are screening values for given compounds that are higher than 85% of the values found regionally or nationally.
9. QUESTION: What are criteria and screening levels?
ANSWER: In general terms, criteria are levels above which long term exposure (or consumption in the case of humans eating fish) could cause health problems for humans, or harm to aquatic life. In specific terms, criteria refer to specific numerical based concentrations for the protection of aquatic life and human health. Screening levels are more general, and are mainly based on State and National 85th percentiles.
10. QUESTION: What did the study find?
ANSWER: Of the 161 chemicals that were tested for, in the Rio Grande/Rio Bravo, only seven chemicals exceeded criteria/screening levels in water, eight in sediment and eleven in fish tissue. In the Rio Grande/Rio Bravo tributaries, 14 chemicals exceeded criteria/screening levels in water, ten in sediment and five in fish tissue. From a statistical standpoint then, toxic chemicals were detected more frequently in the tributaries than in the mainstem. In addition, relatively few chemicals exceeded screening levels.
11. QUESTION: What were some of the chemicals exceeding criteria/screening levels in water, sediment and fish tissue in the Rio Grande/Rio Bravo?
ANSWER: For example, at Station 3 [Rio Grande/Rio Bravo 5km upstream of the Rio Conchos confluence near Presidio/Ojinaga]: in water, arsenic and chloride; in sediment, copper, lead, zinc and nickel; and in fish tissue, selenium and DDE. Specific results are identified in Volume II, Appendix J of the final report (available in English and Spanish).
12. QUESTION: What were some of the chemicals exceeding criteria/screening levels in water, sediment and fish tissue in the tributaries of the Rio Grande/Rio Bravo?
ANSWER: For example, at Station 3a [Rio Conchos 0.2km upstream of mouth near Ojinaga]: in water, chloride and Bis(2-ethylhexyl) phthalate; in sediment, zinc and DDE; in fish tissue, cadmium, selenium and zinc. Specific results are identified in Volume II, Appendix J.
13. QUESTION: Does this mean that these chemicals are present in the entire length of the river and tributaries?
ANSWER: No, it simply means these chemicals were found to exceed criteria/screening levels at one or more sampling sites. In many cases, the chemical may have exceeded the criteria/screening level at only one of the 46 sites sampled.
14. QUESTION: What were the most commonly detected chemicals found in water, sediment and fish tissue in the mainstem and tributaries?
ANSWER: Water: Arsenic
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Sediment: Zinc
Fish Tissue: Copper
15. QUESTION: Where do arsenic, zinc and copper come from?
ANSWER: Arsenic, zinc and copper are naturally occurring elements in soil. They can enter the environment by natural processes such as soil erosion. They can also be a component/part of air pollution. They can enter the environment through industrial activities such as mining, steel production, coal production and burning, and waste burning. Historically, arsenic was used to make pesticides.
16. QUESTION: Were there any trouble spots identified on the Rio Grande/Rio Bravo?
ANSWER: The results of the Phase 1 and 2 studies indicate a gradual increase in downstream chemical pollutants in El Paso/Jurez [Station 1] to the Santa Elena Canyon, Big Bend National Park [Station 5] reach. Similar results were observed in the reach extending from Las Milpas [Station 16] to Brownsville/Matamoros [Station 18] and the Laredo/Nuevo Laredo reach [Stations 11 to 12.2].
It was also noted that increases in chemical pollutant concentrations, mainly salinity, are strongly influenced by contributions of the tributaries and the discharges of wastewater from several drains. Salinity (salts) are dissolved minerals that water picks up as it passes through the air, over and under the ground, and as it is used by households, agriculture and industry forming metal and non-metal compounds.
17. QUESTION: Where were the trouble spots identified on the Rio Grande/Rio Bravo tributaries?
ANSWER: There were eight identified: (1) Manadas Creek [Station 10a]; (2) Chacon Creek [Station 11b]; (3) El Coyote [Station 11c]; (4) Los Olmos [Station 12d]; (5) the municipal discharge from Ciudad Jurez [Station 2a]; (6) the discharge from El Anhelo Drain [Station 15a]; (7) to a lesser extent the Pecos River [Station 6a]; and (8) the discharge from Manhole 115 of Riverside III, Nuevo Laredo [Station 11b.3].
18. QUESTION: Are there any potential human health concerns?
ANSWER: Human health criteria relate to potential effects of regular long-term consumption of fish and/or untreated drinking water. Speaking in general terms, water samples indicated that four chemical pollutants were found at levels exceeding human health criteria: arsenic, bromodichloromethane, bis (2-ethyhexyl) phthalate, and n-nitrosodi-n-propylamine.
Edible fish tissue criteria were exceeded for arsenic, mercury, chlordane and DDE. Elevated total mercury concentrations were identified at Stations 10 through 12 [Laredo/Nuevo Laredo vicinity], and in particular Station 11 indicated a mercury concentration that exceeded the U.S. Food and Drug Administration (USFDA) Action Level. For this reason and others, the Texas Natural Resource Conservation Commission (TNRCC) and the Texas Department of Health (TDH) are in the process of a follow-up study in the Laredo/Nuevo Laredo area to see if it's ok to eat fish caught near Laredo. For now, we can't really say whether it's safe to eat the fish in the Rio Grande/Rio Bravo Laredo/Nuevo Laredo vicinity, but the USFDA does recommend that pregnant women not eat fish more than once a month.
19. QUESTION: What can be done to expedite the release of data from future studies?
ANSWER: According to the LaPaz Agreement1, the U.S. and Mexican governments decided that results from joint studies would not be given to the general public without "mutual agreement."
In an effort to expedite the release of the study data in the future, the IBWC is currently in the process of amending its agreements to allow both countries to release all study data upon completion of quality assurance/quality control protocols appropriate to each country, and prior to the final report.
1Article 16 of the LaPaz Agreement of 1983.
20. QUESTION: What is the next step?
ANSWER: The final phase [Phase 3], of the multi-phase study, will focus on the region known as the El Paso/Ciudad Juárez - Big Bend National Park reach. This reach extends from upstream of the El Paso/Ciudad Juárez metropolitan area to the Big Bend National Park area and includes the area upstream and downstream of the Río Conchos confluence with the Rio Grande/Río Bravo. The primary goals of the Phase 3 project will be to:
(1) Verify the Phase 1 and Phase 2 results and better define problems using more intensive monitoring at fewer sites;
(2) Combine information on habitat, land use, physical/chemical, and biological data to determine those forces influencing [stressors] water quality along the Rio Grande/Rio Bravo; and
(3) Determine the stressors that have the greatest effects on aquatic communities and human health.
The results can be used to set priorities for wastewater treatment upgrades, nonpoint source pollution best management practices and development of other potential practices designed to reduce environmental impact and generally improve water quality.