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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
Executive SummaryOver the last 15 years development has flourished along the United States/México border region. Immigration to the area has led to substantial population growth in the cities, and given rise to many small, unincorporated communities. During this period, the population of the border region, 124 miles (200 km) wide strip centered on the international boundary, has doubled to more than six million people.
Economic growth, partially fueled by more than 1,400 maquiladoras (product assembly) plants that now exist along the border, has been accomplished by an increased potential for water quality degradation. Wastewater treatment is inadequate in many communities on both sides of the border. In addition to potential impacts from oxygen-demanding substances, pathogenic microorganisms, and chemical substances associated with sewage, other water quality concern exists. One relates to the potential for pesticide contamination in farming regions around El Paso/Ciudad Juárez, Presidio/Ojinaga, Eagle Pass/Piedras Negras, and the lower Rio Grande/Río Bravo Valley. Another is the threat of chemical contamination posed by operation of the maquiladoras and other industries located on both sides of the border.
In the past few years, much local, state and national media attention from both countries has focused on purported water quality problems in the Rio Grande/Río Bravo, particularly the potential for chemical contamination associated with the proliferation of maquiladoras. At a 1991 public hearing on the proposed Integrated Border Environmental Plan (Border XXI), much public concern was voiced regarding environmental conditions along the river, and especially over the limited amount of chemical substances data available for the Rio Grande/Río Bravo.
In February 1992 the United States and México issued the Integrated Environmental Plan for the Mexican-U.S. Border Area (First Stage, 1992-1994). The plan called for the two countries to work together to solve environmental problems in the border area. Specifically, the plan called for the two countries to identify areas of transboundary water source or potential transboundary water source contamination, or where there is an identifiable threat of contamination.
In response to the need for comprehensive information, the two countries agreed to an intensive water quality investigation of the Rio Grande/Río Bravo from El Paso/Ciudad Juárez to Brownsville/Matamoros. The Phase 1 investigation was completed in 1994 resulting in a final Phase 1 report released jointly by both countries on September 15, 1994. Coordination between the two countries was conducted by the Mexican and U.S. Sections of the International Boundary and Water Commission (IBWC). Study participants included the Texas Natural Resource Conservation Commission (TNRCC), Texas Parks and Wildlife (TPW), Texas Department of Health (TDH), U.S. Environmental Protection Agency (USEPA), International Boundary and Water Commission - U.S. and Mexican Sections, National Water Commission (Comisión Nacional del Agua [CNA]), CNA State Laboratory in Tamaulipas (Laboratorio Estatal en Tamaulipas de la CNA), CNA North Regional Office Laboratory (Laboratorio de la Gerencia Regional Norte de la CNA) and Wastewater Management and Water Quality Office Laboratory (Laboratorio de la Gerencia de Saneamiento y Calidad del Agua).
The Phase 2 study was conducted during May to December 1995 and involved sampling at 27 mainstem and 19 tributary sites along the river which forms the international boundary between the U.S. and México. The main objective of the multi-phase studies was to screen the system for the occurrence and impact of chemical substances. The goals were to clarify concerns about present conditions in the river, and to determine if existing water quality controls are adequate. Sites from Phase 1 which indicated a low potential for impact were excluded from Phase 2. Sixteen new sites were added to Phase 2 to include areas not covered in Phase 1. Four of these new sites were located in Amistad and Falcon International Reservoirs. Additional work was performed in areas where chemical effects were found in Phase 1 to develop a better understanding of contamination and associated effects.
Sampling and analysis were conducted by each country according to their respective analytical capabilities. Thus, the U.S. evaluations included determinations of 161 chemical substances [such as volatile organic compounds (VOCs), semivolatile compounds (SVOCs), pesticides and metals] in water (37 sites), sediments (33 sites), fish tissue (24 sites), bioassessment of benthic macroinvertebrate communities (16 sites) and bioassessment of fish communities (24 sites). México concurrently collected samples of water and sediment and conducted analyses for chemical substances such as metals (46 sites).
The sites of concern were determined using data collected jointly by both countries but assessed independently using two classification methods. The Potential for Toxic Substance Effects classification system was developed by the U.S. investigators to synthesize water, sediment, fish tissue, toxicity and biological data. The Potential for Conventional Pollutant Effects classification was based on the Water Quality Index (WQI) used by México to evaluate water quality which considers the combined effects of various water pollutants.
During Phase 2, valid analytical results were obtained by the U.S. for 161 chemical substances in water, sediment and fish tissue. A total of 38 chemical substances were detected, 28 of which exceeded criteria/screening levels established by the U.S. investigators. Valid results were obtained by México for 34 chemical substances in water.
U.S. results indicated few potential chemical related problems were observed in the Rio Grande/Río Bravo. Of the 161 potential chemicals tested, 21 were above detection limits. Of the 21 detected, 14 exceeded a criterion and/or screening level. Twelve of the 14 were found in Rio Grande/Río Bravo tributaries and five of the 14 were found in the Rio Grande/Río Bravo. All of the contaminants, with the exception of arsenic, were detected in less than five samples, and exceeded criteria/screening levels a maximum of three times. Overall, arsenic was detected in 33 of 37 samples analyzed and exceeded criteria/screening levels in all 33. Mexican results identified a total of nine chemicals in water, of which one (chromium) exceeded Mexican Standards.
In the toxicity tests, of the 37 stations from which water samples were tested, toxicity was found in twelve instances. Affected sites included two mainstem stations between Presidio/Ojinaga and Big Bend National Park, and ten tributary stations. Fish and macrobenthic (aquatic insects) communities generally were healthy. However, 10 of 27 stations, listed below, exhibited aquatic community characteristics reflecting a Potential Concern to Concern of toxic chemical impact.
CONCERN
Upstream of Presidio/Ojinaga (Mainstem Station 3)
Rio Conchos near the Mouth (Tributary Station 3a)
Downstream of Brownsville/Matamoros (Mainstem Station 18)
POTENTIAL CONCERN
Upstream of El Paso/Ciudad Juárez (Mainstem Station 1)
Downstream of El Paso/Ciudad Juárez (Mainstem Station 2)
Downstream of Presidio/Ojinaga (Mainstem Station 4)
Downstream of Santa Elena Canyon in Big Bend National Park (Mainstem
Station 5)
San Felipe Creek near the Mouth (Tributary Station 7b)
Downstream of Laredo/Nuevo Laredo (Mainstem Stations 12 and 12.1)
Downstream of Anzalduas Dam (Mainstem Station 14)
Biotic integrity at the mainstem sites indicated that if toxic impacts were occurring, the effects were relatively slight. No instances of severe aquatic use impairment were documented. México concluded that in general terms, the concentrations of metals in the mainstem are below their water quality standards thus posing no contamination threats to aquatic life and water uses.
Potential problems were more prevalent in tributaries, which should not be too surprising since some of them transport untreated/partially treated wastewater. According to U.S. results, 14 chemical substances exceeded screening levels in water, 10 in sediment and five in fish tissue as compared to seven in water, eight in sediment and 11 in fish tissue in the mainstem. Results from México's analyses indicated that one potential chemical substance (chromium) exceeded their water quality standards.
Human health criteria relates to the potential effects of regular long-term consumption of fish and/or untreated drinking water. Short-term risk was observed at possibly one of 26 sites that were subjected to edible tissue analysis, in that there was one exceedance of U.S. Food and Drug Administration action level for mercury at the Mainstem Station 11 which is located in the Laredo-Nuevo Laredo vicinity. In addition, low-level human health criteria were exceeded in water and/or edible fish tissue at 37 of 47 stations with arsenic the predominate chemical substance found at these locations.
All available information was used to identify sites and chemicals of potential concern, to facilitate water quality management decisions and future monitoring efforts. The 28 chemicals identified by the U.S. 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: N-nitrosodi-n-propylamine, Bis(2-ethylhexyl) phthalate, phenolics recoverable, Chlordane, Aroclor 1260, selenium, mercury, DDE, cadmium, unionized ammonia, silver, chromium, chloride, nickel, lead, zinc, copper and arsenic. A Medium Priority group consisted of DDT and Chloroform. A Low Priority group consisted of phenol single compound, 1,4-dichlorobenzene, dibromodichloromethane, bromodichloromethane, xylene, toluene, benzene and antimony.
Regarding sites of potential concern, mainstem stations and tributary stations were addressed separately. The following stations include those that exhibited either high or moderate Potential for Toxic Substance and/or Conventional Pollutant Effects. Unlisted sites exhibited negligible evidence of adverse impacts (Low to Slight Effects).
HIGH POTENTIAL MAINSTEM SITES
Toxic Substance Effects
Rio Grande/Río Bravo at Zaragosa Bridge in El Paso/Ciudad
Juárez (Station 2)
Rio Grande/Río Bravo Upstream of Presidio/Ojinaga (Station
3)
Rio Grande/Río Bravo Downstream of Presidio/Ojinaga (Station
4)
International Amistad Reservoir in the Rio Grande Arm (Station 6.1)
Rio Grande/Río Bravo 25km Downstream of Laredo/Nuevo Laredo
(Station 12.1)
Conventional Pollutant Effects
There were no mainstem stations listed in this category.
MODERATE POTENTIAL MAINSTEM SITES
Toxic Substance Effects
Rio Grande/Río Bravo at Santa Elena Canyon in Big Bend National
Park (Station 5)
International Amistad Reservoir in Devils River Arm (Station 6.2)
Rio Grande/Río Bravo Downstream of Eagle Pass/Piedras Negras
(Station 10)
International Falcon Reservoir-Headwaters (Station 12.2)
Rio Grande/Río Bravo Downstream of el Anhelo Drain (Station
16)
Conventional Pollutant Effects
Rio Grande/Río Bravo at Courchesne Bridge in El Paso (Station
1)
Rio Grande/Río Bravo Upstream of El Paso Haskell Street WWTP
(Station 1.1)
Rio Grande/Río Bravo at Zaragosa Bridge in El Paso/Ciudad
Juárez (Station 2)
Rio Grande/Río Bravo at Fort Hancock International Bridge
(Station 2.2)
Rio Grande/Río Bravo Downstream of Fort Hancock International
Bridge (Station 2.3)
Rio Grande/Río Bravo Upstream of Presidio/Ojinaga (Station
3)
Rio Grande/Río Bravo Downstream of Presidio/Ojinaga (Station
4)
Rio Grande/Río Bravo at Santa Elena Canyon in Big Bend National
Park (Station 5)
Rio Grande/Río Bravo 13.2km Downstream of Laredo/Nuevo Laredo
(Station 12)
Rio Grande/Río Bravo 25km Downstream of Laredo/Nuevo Laredo
(Station 12.1)
Rio Grande/Río Bravo Downstream of US 83/77 Brownsville/Matamoros
(Station 18)
HIGH POTENTIAL TRIBUTARY SITES
Toxic Substance Effects
Ciudad Juárez Wastewater Canal (Station 2a)
Manadas Creek in Laredo (Station 10a)
Arroyo el Coyote in Nuevo Laredo (Station 11c)
El Anhelo Drain in Reynosa (Station 15a)
Conventional Pollutant Effects
Ciudad Juárez Wastewater Canal (Station 2a)
Arroyo el Tornillo in Piedras Negras (Station 9a)
Manhole 115 of Riverside Collection System, Nuevo Laredo (Station
11b.3)
Arroyo el Coyote in Nuevo Laredo (Station 11c)
El Anhelo Drain in Reynosa (Station 15a)
MODERATE POTENTIAL TRIBUTARY SITES
Toxic Substance Effects
Montoya Drain near the Texas/New México State Line (Station
0.5a)
Arroyo el Tornillo in Piedras Negras (Station 9a)
Chacon Creek in Laredo (Station 11b)
Arroyo los Olmos near Rio Grande City (Station 12d)
Conventional Pollutant Effects
El Paso Haskell Street Wastewater Treatment Plant (Station 1a)
Río Conchos Near the Mouth (Station 3a)
Río Conchos 25km Upstream from Mouth (Station 3a.1)
Manadas Creek in Laredo (Station 10a)
Zacate Creek in Laredo (Station 11a)
Chacon Creek in Laredo (Station 11b)
Laredo Zacate Creek Wastewater Treatment Plant (Station 11b.1)
Laredo Southside Wastewater Treatment Plant (Station 11b.2)
Arroyo los Olmos Near Rio Grande City (Station 12d)
Follow-up binational studies were recommended for the purpose of
further assessing the Rio Grande/Río Bravo mainstem reach
extending from El Paso/Ciudad Juárez (Station 1) to Santa
Elena Canyon, Big Bend National Park (Station 5). The results of
the Phase 1 and 2 data indicate a gradual increase in downstream
concentrations of conventional and metal pollutants in this reach.
Similar results were observed in the mainstem reach extending from
Las Milpas (Station 16) to Brownsville /Matamoros (Station 18).
It was also noted that increases in concentrations of physicochemical
parameters in the Rio Grande/Río Bravo, mainly salinity,
are strongly influenced by the contributions of the tributaries
and the discharges of wastewater from several drains. It was determined
that the tributaries Manadas (Station 10a), Chacon (Station 11b),
El Coyote (Station 11c), Los Olmos (Station 12d), the municipal
discharge from Ciudad Juárez (Station 2a), the discharge
from El Anhelo Drain (Station 15a), and to a lesser extent the Pecos
River (Station 6a) and the discharge from Manhole 115 of Riverside
III, Nuevo Laredo (Station 11b.3) constitute sources of pollution
for the Rio Grande/Río Bravo. These sites need further assessment
to identify and implement appropriate actions to improve the water
quality of these tributaries and drains. Such assessment should
focus on those forces [stressors] that have the greatest effects
on aquatic communities and human health.