Binational Study Regarding
the Presence of Toxic Substances in the Rio Grande/Rio Bravo
and its Tributaries Along the Boundary Portion
Between the United States and Mexico
Estudio Binacional sobre
la Presencia de Sustandas Toxicas en el Rio Bravo/Rio Grande
y sus Afluentes, en su Porcion Fronteriza
Entre Mexico y Estados Unidos
Final Report, September 1994
Informe Final, Septiembre de 1994
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AUTHORITY
This study and report were undertaken by the United States and Mexico pursuant to the
International Boundary and Water Commission Minute 289 entitled, "Observation of the Quality
of the Waters Along the United States and Mexico Border", dated November 13, 1992.
PARTICIPATING AGENCIES
United States
Mexico
Environmental Protection Agency
Texas Natural Resources Conservation
Commission
Texas Parks & Wildlife Department
Texas Department of Health
National Water Commission
Secretaria de Desarrollo Social
International
International Boundary and Water Commission, United States and Mexico
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LEST OF PARTICIPANTS
Texas Natural Resource Conservation Commission
Jack R. Davis
Don Ottmers
Steve Twidwell
Jeff Kirkpatrick
Cassie Shaukat
Charlie Webster
Augustine de la Cruz
Greg Larson
Sergio Mendez
Jim Bard
Texas Parks & Wildlife Department
Leroy KLeinsasser
Roxie Cantu
Kenny Saunders
Gordon Linam
Kevin Mayes
Ken Rice
Randy Moss
Texas Department of Health
Jim Boyer
Gary Fest
Sharon Dubose
Robert Leshber
U.S. Environmental Protection Agency
Philip Crocker
AbelEuresti
Terry Hollister
Evan Hornig
Charlie Howell
Carl Young
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International Boundary & Water Commission - U.S. Section
Yusuf Farran
Ozzie Linguist
Sylvia Andrade
Doug Echlin
Robin Smith
John Lee
Richard Peace
Efren Romero
Jesus Robio
John Muse
BUI Harris
Reyes Ortiz
Pablo Diaz
Roy Cooley
Carlos Marin
Robert Ramos
Raul Garcia
International Boundary & Water Commission - Mexican Section
Alberto Ramirez Lopez
Jesus Navarro Lopez
David Negrete Arroyos
Roberto Enriquez
Modesto de la Torre
Sergio Lopez
Rogelio Esquivel Rangel
Guadalupe G6mez Hernandez
Comisidn Nacoa l Aia
Dolores Guerra Alvarez
Rosario Ledezma Vera
Roberto Morales Gonzalez
Julio Vazquez Soriano
Monica Perez Carrillo
Evangelina Mancinas Mena
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Forward
This report is issued by the Governments of Mexico and the United States through their sections
of the International Boundary and Water Commission, the National Water Commission of
Mexico and the U.S. Environmental Protection Agency. The governments of both countries
thank the State of Texas, specifically the Texas Natural Resource Conservation Commission, the
Texas Parks and Wildlife Department and the Texas Department of Health, for their
participation in the study.
Copies of this report in English may be obtained from the Environmental Protection Agency
Region 6 Office, 1445 Ross Avenue, Suite 1200, Dallas, Texas 75202-2733 or the International
Boundary and Water Commission, 4171 North Mesa Street, Suite 310, El Paso, Texas 79902-
1422.
Copies of this report in Spanish may be obtained from the Comisidn Internacional de Limites
y Aguas, Ave. Universidad No. 2180, Zona Chamizal, C.P. 32310 Cd. Juarez, Chih. or the
following agencies of the Comisi6n Nacional del Agua: Gerencia Regional Norte, Sub Gerencia
de Administracion del Agua, Comisidn Nacional del Agua, Boulevard Revolucion No. 2343
Ote., C. P. 27000 Torreon, Coah., Tel. 18-9939, 18-9945; Gerencia de Calidad, Reuso del
Agua e Impacto Ambiental, Ave. San Bernabe #549, Col. San Jeronimo Licice, Mexico, D.F.,
C.P. 10200, tel. 595-2344, 683-1740.
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EXECUTIVE SUMMARY
Over the last 15 years development has flourished along the Me*xico/U.S. border. 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, a 200 km
(124 mile) wide strip centered on the international boundary, has doubled to more than six
million people (Emerson and Bourbon, 1991).
Economic growth, partially fueled by over 1,400 maquiladora (product assembly) plants that now
exist along the border, has been accompanied by an increased potential for water quality
degradation. Sewage treatment is inadequate in many communities on both sides of the border.
In addition to potential impacts from oxygen-demanding substances, pathogenic microorganisms,
and toxicants associated with sewage, other water quality concerns exist. One relates to the
potential for pesticide contamination in farming regions around H Paso/Ciudad Juarez,
Presidio/Ojinaga, Eagle Pass/Piedras Negras, and the lower Rio Grande/Rio Bravo valley.
Another is the threat of toxic chemical contamination posed by operation of the maquiladoras
(Lewis etal.r 1991) 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/Rio Bravo, particularly the
potential for toxic chemical contamination associated with the proliferation of maquiladoras. At
a 1991 public hearing on the proposed Integrated Border Environmental Plan, much public
concern was voiced regarding environmental conditions along the river, and especially over the
limited amount of toxic substances data available for the Rio Grande/Rio Bravo.
In February 1992 the United States and Mexico issued the Integrated Environmental Plan for the
Mexican-U.S. Border Area (First Sfage. 1992-19941. The plan calls for the two countries to
work together to solve environmental problems in the border area. Specifically, the plan calls
for the two countries to identify areas where any transboundary water source or potential
transboundary water source is contaminated 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/Rib Bravo from El Paso/Ciudad Juarez to
Brownsville/Matamoros. Coordination between the two countries was conducted by the Mexican
and U.S. sections of the International Boundary and Water Commission (IBWC). The IBWC
developed IBWC Minute number 289, dated November 13, 1992, which approved the study
design and addressed binational cooperation for the water quality investigation. Study
participants included the Texas Natural Resource Conservation Commission, Texas Parks and
Wildlife Department, Texas Department of Health, U.S. Environmental Protection Agency, U.S.
Fish and Wildlife Service, U.S. National Park Service, International Boundary and Water
Commission - U.S. & Mexico Sections, Comisidn Nacional del Agua, and Secretaria de
Desarrollo Social.
The main objective of the study was to screen the system for the occurrence and impact of toxic
chemicals. The goals were to clarify concerns about present conditions in the river, and to
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determine if existing water quality controls are adequate. The study was conducted during 1992-
93 and involved sampling at 19 mainstem sites and 26 tributaries along the reach of the river
which forms the international boundary between the U.S. and Mexico (see attached map and
station descriptions). This study did not include sample collection from International Amistad
and Falcon International Reservoirs.
Sampling and analysis were conducted by each country according to their respective analytical
capabilities. Thus, the U.S. evaluation included determinations of toxic chemical concentrations
in water (45 sites), sediment (45 sites), and fish tissue (18 mainstem sites, 6 tributaries);
toxicity testing of water and sediment (45 sites); and bioassessments of fish communities (18
mainstem sites, 7 tributaries) and benthic macroinvertebrate communities (18 mainstem sites).
Mexico concurrently collected samples of water and sediment and conducted analyses for
conventional parameters and heavy metals (45 sites).
Valid analytical results were obtained by the U.S. for 153 toxic chemicals in water, 145 in
sediment, and 140 in tissue. A total of 48 toxic chemicals were detected, 30 of which exceeded
the screening levels established by U.S. investigators. Valid results were obtained by Mexico
for 9 conventional parameters in water and 12 heavy metals in both water and sediment. A total
of nine toxicants were identified, all of which exceeded Mexican standards.
Few potential toxic chemical-related problems were observed in the mainstem. Only 5 toxic
chemicals exceeded U.S. screening levels in water, 8 in sediment, and 12 in tissue. A total of
six toxic chemicals were identified by Mexico in the mainstem, exceeding Mexican standards
for aquatic life. In the toxicity tests, significant adverse effects occurred in just 2 of 114
determinations, from samples collected upstream from El Paso/Ciudad Juarez and downstream
from Laredo/Nuevo Laredo. Fish and macrobenthic communities generally were healthy;
however, 5 of 36 stations, listed below, exhibited aquatic community characteristics reflecting
a moderate or high probability of toxic chemical impact (numbers in parentheses are station
identifiers).
Downstream from El Paso/Ciudad Juarez (2)
Downstream from Eagle Pass/Piedras Negras (10)
Downstream from Laredo/Nuevo Laredo (12)
Downstream from Anhelo Drain South of Las Milpas (16)
Arroyo Los Olmos (12d)
Biotic integrity at the main stem sites indicated that if toxic impacts were occurring, the effects
were relatively slight. No instances of severe aquatic life use impairment were observed.
Potential problems were more prevalent in tributaries, which was not surprising since some of
them transport wastewater in relatively undiluted form. According to U.S. results, 17 toxic
chemicals exceeded screening levels in water, 15 in sediment, and 8 in tissue. In addition,
samples from 14 of the 26 tributaries produced significant adverse effects in at least one phase
of the toxicity tests. Results from Mexico's analyses indicated eight potentially toxic chemicals
that exceeded their water quality standards.
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Regarding human health issues, no short-term risks were indicated for the 24 sites that were
subjected to edible fish tissue analysis, in that there were no exceedances of U.S. Food & Drug
Administration action levels. However, low-level human health criteria were exceeded in water
and/or edible fish tissue 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 and/or fish. Significant
risks were observed for the other five sites. However, because all five are sewage effluent-
dominated tributaries, these waters are nonpotable and conventional water quality is not
conducive for support of viable fish populations.
All available information was used to identify sites and chemicals of potential concern, to
facilitate water quality management decisions and future monitoring efforts. The 30 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 residual chlorine, methylene chloride, toluene, arsenic, cadmium, chromium,
copper, lead, mercury, nickel, selenium, silver, zinc, chlordane, p,p' DDE, dieldrin, gamma-
bhc (lindane), total PCB's, and cyanide. A medium priority group consisted of non-ionized
ammonia, parachlorometa cresol, phenol, and diazinon. A low priority group was comprised
of phenolics recoverable, chloroform, antimony, thallium, bis(2-ethylhexyl) phthalate, diethyl
phthalate, and di-n-butyl phthalate. Results obtained by Mexico were in agreement with the
aforementioned priorities.
Regarding sites of potential concern, mainstem stations and tributary stations were addressed
separately. The following stations include those that 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 Ju&ez (2)
Downstream from Laredo/Nuevo Laredo (12)
Slight to Moderate Potential for Toxic Chemical Impacts
Upstream from Rib 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 (la)
Ciudad Juarez Discharge Canal (2a)
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Manadas Creek (Ida)
Zacate Creek (1 la)
Arroyo £1 Coyote (lie)
Anhelo Drain (ISa)
Slight to Moderate Potential for Toxic Chemical Impacts
Rio Conchos (3a)
Arroyo de Las Vacas (7b)
Unnamed Tributary South of Eagle Pass/Piedras Negras (9a)
Arroyo Los Olmos (12d)
Based on the degree of toxic chemical contamination and volume of inflow, the Haskell R. Street
Wastewater Treatment Plant (la) and Ciudad Juarez sewage discharge canal (2a) appeared to
have a high potential for adversely affecting the Rio Grande/Rfo Bravo. The Rfo Conchos (3a),
San Felipe Creek (7b), Zacate Creek (Ha), and Anhelo Drain (ISa) have a slight to moderate
potential for adversely affecting the river. Remaining tributaries present little or no potential
for significant impacts to the Rio Grande/Rfo Bravo based on data collected during the study.
Also, a binational study is proposed during 1994 and 1995 that will examine the prevalence and
magnitude of toxic chemicals in fish tissue collected from International Falcon and Amistad
Reservoirs.
Follow-up binational studies were recommended for the purposes of better defining the degree
of impact, assessing temporal variation, and further identifying sources of toxic chemicals. The
studies, listed below, would be conducted during 1994 and 1995, pending international
agreement through the U.S. and Mexico sections of IBWC.
Additional surveillance would be conducted at the six mainstem 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 Juarez (2) and
Laredo/Nuevo Laredo (12)
Intensive surveys would be performed on tributaries of potential concern that support
significant aquatic life habitat, i.e., Rfo Conchos (3a) and San Felipe Creek (7b)
Toxic chemical concentrations in fish tissue would be reassessed in the Rio Grande/Rfo
Bravo stations at Foster Ranch (6), upstream from Del Rio/Ciudad Acuna (7), upstream
from Eagle Pass/Piedras Negras (9), and upstream from the old Laredo/Nuevo Laredo
International Bridge (11), and
Toxic chemical concentrations in fish tissue would be assessed in the headwaters of
International Amistad and Falcon Reservoirs.
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TABLE OF CONTENTS
case
INTRODUCTION 1
Historical Information 2
Study Area 3
STUDY DESCRIPTION 8
Quality Assurance 8
Sampling Sites 8
Types of Analyses 8
Parametric Coverage 9
METHODS 10
Physicochemical Techniques 10
Field Procedures 10
Water Sampling 10
Sediment Sampling 10
Tissue Sampling 11
Sampling Handling 12
Laboratory Analyses 12
Data Evaluation 12
Biological Techniques 13
Toxicity Testing 13
Macrobenthic Community Assessment 14
Fish Community Assessment 15
RESULTS AND DISCUSSION 19
Conventional Water Quality 19
Mainstem 19
Tributaries 20
Toxic Chemicals in Water 21
Mainstem 21
Tributaries 23
Toxic Chemicals in Sediment 27
Mainstem 27
Tributaries 30
Toxic Chemicals in Fish Tissue 32
Edible Fish Tissue 34
Whole Fish Tissue 35
Body Burdens 35
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TABLE OF CONTENTS (continued)
cage.
Predator Protection Limits 38
Toxicity Testing 39
Mainstem 40
Tributaries. 41
Macrobenthic Community Assessment 43
Evaluation of Collecting Techniques 44
Comparison of Data Evajugfjflff Methods 45
Macrobenthic Integrity 45
Fish Community Assessment 51
Fish Community Measures - 0 Paso/Ju3rez to Falcon Reservoir 53
Species Richness, Composition, and Similarity 53
Index ofBiotic Integrity 54
Fish Community McSfflires • Falcon Reservoir to Brownsville/MaitajniPrPS--- 56
Species Richness, Composition, and Similarity 56
Index ofBiotic Integrity 57
Fish Community Measures - Middle Reach Tributarily 58
Species Richness, Composition, and Similarity 58
Index ofBiotic Integrity 58
Integration of Data. 59
Sites of Potential Concern 59
Mainstem 60
Tributaries 62
Toxic Chemicals of Potential Concern 64
RECOMMENDATIONS 65
REFERENCES CITED 67
APPENDK A - Tables 77
APPENDIX B - Figures 227
APPENDK C - Quality Assurance Measures 239
APPENDIX D - Evaluation of Water Quality Data by Comisidn Nacional del Agua
(1973 to 1993) 246
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TABLES
page
1. List of Sampling Stations 79
2. Toxic Chemicals Targeted for Analysis in Water, Sediment, and Fish Tissue... 82
3. Sample Specifications 85
4. Methods Utilized by Texas Department of Health Environmental Chemistry
Laboratory and the Comision National del Agua Laboratory 87
5. Modified Index of Biotic Integrity Rating Criteria for Sites on the Rio
Grande/Rfo Bravo and Tributaries 90
6. Status and Preferred Habitat of Fish Species Collected in the
Rio Grande/Rfo Bravo and Tributaries 91
7. Screening Level Concentrations 92
8. Site-Specific Screening Level Concentrations for Water 96
9. Site-Specific Screening Level Concentrations for Sediment 100
10. Analytical Data - Water 104
11. Analytical Data - Sediment 124
12. Analytical Data - Tissue 144
13. Toxic Chemicals That Occurred at Detectable Levels 184
14. Summary of Screening Level Exceedances, by Parameter 186
IS. Summary of Screening Level Exceedances, by Station 192
16. Toxicity Testing Results - Ceriodaphnia dubia 198
17. Toxicity Testing Results - Pimephales promelas 201
18. Summary of Toxicity Testing Results from USEPA/TNRCC TOXNET Program 204
19. Benthic Macroinvertebrate Data 205
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TABLES (continued)
cage.
20. Fishes Collected at Selected Sites in the Rio Grande/Rfo Bravo Basin 219
21. Similarity Index Calculated for Fishes Collected from the Rio Grande/Rfo Bravo
and Tributaries 220
22. Ratings of Sites on the Rio Grande/Rfo Bravo Upstream of Falcon Reservoir
and on the Rfo Conchos Using a Modified Index of Biotic Integrity 221
23. Ratings of Sites on the Rio Grande/Rfo Bravo Downstream of Falcon Reservoir
Including Arroyo Los Olmos Using an Index of Biotic Integrity 222
24. Ratings of Middle Reach Tributaries on the Rio Grande/Rfo Bravo Using a
Modified Index of Biotic Integrity 223
25. Ranking of Mainstem Sites Based on Seventeen Components of the Toxic
Chemical Evaluation 224
26. Ranking of Tributary Sites Based on Twelve Components of the Toxic
Chemical Evaluation 225
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FIGURES
cage
1. Study Area and Sampling Stations 229
2. Sites with Contaminant Concentrations in Fish Fillets Exceeding Human
Health Screening Levels 231
3. Sites with Contaminant Concentrations in Whole Fish Exceeding 85th
Percentiles or Mean Concentrations 233
4. Sites with Contaminant Concentrations in Whole Fish Exceeding Predator
Protection Limits 235
5. Number of Fishes Collected at Selected Sites 237
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INTRODUCTION
Over the last IS years development has flourished along the Mgxico/U.S. border. Immigration
to the area has led to substantial population growth in the cities, and given rise to many small,
unincorporated communities, or colonias. During this period, the population of the border
region, a 200 km (124 mile) wide strip centered on the international boundary extending from
the Pacific Ocean to the Gulf of Mexico, has doubled to more than six million people (Emerson
and Bourbon, 1991).
Economic growth, partially fueled by over 1,400 maquiladora (product assembly) plants that now
exist along the border, has been accompanied by an increased potential for water quality
degradation. Sewage treatment is inadequate in many communities on both sides of the border.
In addition to potential impacts from oxygen-demanding substances, pathogenic microorganisms,
and toxicants associated with sewage, other water quality concerns exist. One relates to the
potential for pesticide contamination in farming regions around El Paso/Ciudad Jufrez,
Presidio/Ojinaga, Eagle Pass/Piedras Negras, and the lower Rio Grande/Rfo Bravo valley.
Another is the threat of toxic chemical contamination posed by operation of the maquiladoras
(Lewis etal.. 1991) 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/Rio Bravo, particularly the
potential for toxic chemical contamination associated with the proliferation of maquiladoras. At
a 1991 public hearing on the proposed Integrated Border Environmental Plan, much public
concern was voiced regarding environmental conditions along the river, and especially over the
limited amount of toxic substances data available for the Rio Grande/Rfo Bravo.
In February 1992 the United States and Mexico issued the Integrated Environmental Plan for the
Mexican-U.S. Border Area (First Stage. 1992-1994^. The plan calls for the two countries to
work together to solve environmental problems in the border area. Specifically, the plan calls
for the two countries to identify areas where any transboundary water source or potential
transboundary water source is contaminated 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/Rfo Bravo from El Paso/Ciudad Juarez to
Brownsville/Matamoros. Coordination between the two countries was conducted by the Mexican
and U.S. sections of the International Boundary and Water Commission (IBWC). The IBWC
developed IBWC Minute number 289, dated November 13, 1992, which approved the study
design and addressed binational cooperation for the water quality investigation. Study
participants included the Texas Natural Resource Conservation Commission, Texas Parks and
Wildlife Department, Texas Department of Health, U.S. Environmental Protection Agency, U.S.
Fish and Wildlife Service, U.S. National Park Service, International Boundary and Water
Commission - U.S. & Mexico Sections, Comisidn Nacional del Agua, and Secretaria de
Desarrollo Social.
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The main objective of the study was to screen the system for the occurrence and impact of toxic
chemicals. The goals were to clarify concerns about present conditions in the river, and to
determine if existing water quality controls are adequate. The study was conducted during 1992-
93 and involved sampling at 19 mainstem sites and 26 tributaries along the reach of the river
which forms the international boundary between the United States and Mexico (Table 1). This
study did not include sample collection from International Amistad and Falcon Reservoirs.
Historical Information
Water quality and biological data for the U.S. portion of the Rio Grande/Rio Bravo basin were
summarized by TNRCC (1992a). The Comisidn Nacional del Agua (CNA) also has conducted
water quality analyses based on physical, chemical, and bacteriological parameters from 1976
to 1993. An evaluation of this data is found in Appendix D. Conclusions regarding
conventional water quality are addressed elsewhere in the present report (see "Conventional
Water Quality" under "RESULTS AND DISCUSSION").
Literature relative to biotic integrity in the Rio Grande/Rfo Bravo, some of which addressed
influential environmental factors, also was reviewed in the 1992 report. Possible toxic chemical
impacts were mentioned for several locations, but little supporting evidence was provided.
Regarding toxic chemicals, the 1992 evaluation included all available information generated by
U.S. agencies. For the international portion of the river, the data base was described, data were
evaluated, toxic chemicals of potential concern were identified, potential sources of toxic
chemicals were addressed, possible impacts were considered, and conclusions were drawn.
Potential concerns were revealed for four locations.
The first was the segment of the Rio Grande/Rfo Bravo in El Paso/Ciudad Juarez (represented
by station 2 in the present study), where flow is dominated by municipal wastewater effluent
during low flow periods. Local macrobenthic community integrity was very low during a 1976-
77 study, with toxic pollutants thought to be partially responsible. In addition, periodic toxicity
testing by USEPA/TNRCC since 1992 has shown significant adverse effects in water on two
occasions, and in sediment eluate on one occasion (Table 18). Whereas there have been
indications of impacts by toxicants, the toxic chemical data base is limited, so concrete
conclusions were not possible at the time of the 1992 evaluation.
The second was the segment of the Rio Grande/Rfo Bravo downstream from the Rfo Conchos
confluence to a point 16 km (10 miles) downstream (represented by station 4 in the present
study), where elevated concentrations of DDE, DDD, DDT, endrin, dieldrin, and PCB's were
observed in sediment and/or fish tissue during special studies in the late 1970's.
Upstream/downstream sampling identified inflow from the Rfo Conchos as the primary
contributor, most clearly with respect to DDE and DDT. Data from the late 1980' s indicated
that contaminant levels, particularly for DDE and DDT, had diminished substantially. Periodic
toxicity testing since 1992 has revealed only one instance of significant adverse effects (Table
18). The 1992 report concluded that existing pesticide concentrations probably are not
significantly impairing biotic integrity in the reach, and more than likely do not pose an
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appreciable human health hazard, but that a possibility remains that predatory fish, birds, and
wildlife may be moderately affected through accumulation and biomagnification of pesticide
residues.
The third was the segment of the Rio Grande/Rio Bravo from Laredo/Nuevo Laredo to the
headwaters of International Falcon Reservoir (represented by station 12 in the present study).
The basis was chemical data and field observations from a monitoring station 14 km (9 miles)
downstream from Laredo, where copper, selenium, and chlordane in fish tissue had exceeded
screening criteria. Also, on several occasions fish collected at the site had exhibited an elevated
incidence of physical abnormalities. Whether toxic chemicals were responsible was unknown.
A potential for slight toxic impact has been shown in periodic toxicity testing since 1991, as
significant adverse effects have occurred on two occasions (Table 18). Nonetheless, recent
studies have shown that the local species assemblage is fairly diverse, indicating that
environmental conditions are reasonably healthy and that fish community integrity is not being
appreciably impaired by toxic chemicals or other ecological factors.
The fourth was the segment of the Rio Grande/Rfo Bravo immediately upstream from
International Anzalduas Dam near Mission/Reynosa (represented by station 14 in the present
study), where elevated levels of DDT, DDE, and toxaphene in fish tissue were documented by
the U.S. Fish and Wildlife Service (USFWS) during 1967-79. In a 1988 USFWS report, an
evaluation of temporal trends for tissue data from the site (1970-86) showed that DDT and DDE
steadily declined, while toxaphene exhibited a slight increase. Data for the site in a 1988 U.S.
Geological Survey report included several instances where DDE and toxaphene exceeded
screening criteria in fish tissue. TNRCC fish tissue monitoring for DDT, DDD, DDE, and
toxaphene in this segment has shown only one exceedance, by DDD. Although no specific
impacts by pesticide residues have been documented in the International Anzalduas Dam area,
a potential appears to exist for adverse effects on piscivorous fish, birds, and wildlife.
To summarize the 1992 evaluation, the overall conclusion for the basin was that toxic chemical
contamination and associated impacts were relatively insignificant at that point in time.
However, it was emphasized that the toxic chemical data base was rather limited for some
segments, mainly with regard to parametric and/or matrix coverage. Recommendations for
filling data gaps were offered, and were taken into account in the design of the present study.
Study Area
The Rio Grande/Rfo Bravo originates in the San Juan Mountains of southern Colorado, flows
southward through New Mexico, and enters Texas about 32 km (20 miles) northwest of El
Paso/Ciudad Juarez. From there to the Gulf of Mexico, the river forms the international
boundary between the United States and Mexico. The river extends for about 3,059 km (1901
miles), with the U.S./Mexico reach being about 2,053 km (1276 miles) in length. The
watershed encompasses about 924,300 square kilometers (335,500 square miles). Of that total,
about 231,317 square kilometers (88,968 square miles) in the United States and 227,149 square
kilometers (87,365 square miles) in Mexico contribute streamflow to the Rio Grande/Rfo Bravo;
the remaining area drains internal (endorheic) basins. The U.S. portion of the basin below El
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Paso/Ciudad Jufrez contains 125,580 square kilometers (48,300 square miles), of which 100,880
square kilometers (38,800 square miles) contribute streamflow to the river.
The study was conducted on the U.S./Mexico reach of the river, that portion extending from the
New Mexico/Texas/Chihuahua border to the Gulf of Mexico (Figure 1). Population along the
reach is centered in five transborder metropolitan areas: El Paso/Ciudad Juarez (1,303,130);
Eagle Pass/Piedras Negras (116,829); Laredo/Nuevo Laredo (341,312); McAllen, Edinburg,
Mission/Reynosa (416,776); and Brownsville/Matamoros (365,017). The economy of the area
is based on wholesale and retail trade, oil and gas production, agriculture, manufacturing,
tourism, and international trade.
The river is an important natural resource for industry, agriculture, domestic water supply,
recreation and aesthetic enjoyment, and wildlife and aquatic life habitat. Most of the major
tributaries, and some of the lesser ones, are also of significance in these respects. Substantial
agricultural areas where river water is diverted for irrigation include the El Paso/Ciudad Juarez
area, Eagle Pass/Piedras Negras area, and Rio Grande/Rio Bravo valley downstream from
International Falcon Dam. Through the reach from Laredo/Nuevo Laredo to the mouth, the
river constitutes the primary drinking water source for up to 98% of the population in both
countries.
In western Texas, a substantial reach extending from near Redford/H Mulato to near
Terlingua/Nuevo Lajitas forms the southern boundary of the Big Bend Ranch State Natural Area.
Immediately downstream, another long reach lies within the U.S. Big Bend National Park, and
constitutes a major feature of that facility. The U.S. portion of the reach from the eastern
boundary of the park to International Amistad Reservoir is a designated National Wild and
Scenic River segment. International Amistad Reservoir and International Falcon Reservoir, two
large mainstem impoundments constructed primarily for water conservation management and
floodwater control, are major tourist attractions. Amistad is a designated National Recreational
Area, and Falcon is the site of Falcon State Park, Texas. In the lower Rio Grande/Rio Bravo
valley, the river and its riparian environment are prominent features within a number of parks
and refuges. In Texas these include Bentsen State Park, Anzalduas Park, Santa Ana National
Wildlife Refuge, Anacua State Wildlife Management Area, Sabal Palm Sanctuary, and the Lower
Rio Grande/Rio Bravo Valley National Wildlife Refuge, the latter comprised of 30 small,
separate tracts. Cumulatively, these parks and refuges are heavily utilized for recreation, and
are inhabited by significant assemblages of plants, birds, wildlife, and aquatic life, including
various rare and endangered species, and species that occur only peripherally in the United
States. The States of Mexico that border the river are Chihuahua, Coahuila, Nuevo Leon and
Tamaulipas.
Various characteristics of the study area are important to an understanding of the river's
ecology. These include climatic, hydrologic, geologic, physiographic, and biotic features.
Climate in the northern portion of the basin generally is hot and arid, but becomes increasingly
tropical in a southward direction. Annual rainfall averages about 20 cm (8 inches) at El
Paso/Ciudad Juarez, 46 cm (18 inches) at Del Rio/Ciudad Acuna, 51 cm (20 inches) at
Laredo/Nuevo Laredo, and 65 cm (26 inches) at Brownsville/Matamoros.
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The system is complex hydrologically. At the New Mexico/Texas/Chihuahua border, low flow
in fall and winter is derived from alluvial seepage and delayed irrigation return flows. Higher
flow during the spring/summer irrigation season originates from reservoir releases in New
Mexico. Most of the flow reaching El Paso/Ciudad Juarez is diverted for irrigation at the
American Dam (U.S.) and International Dam (Mexico). Perennial flow reappears through the
lower portion of El Paso, sustained by a large municipal wastewater discharge from the El Paso
Public Service Board Haskell R. Street wastewater treatment plant. A short distance
downstream, most of the flow is diverted for irrigation at the Riverside Diversion Dam (U.S.).
The long reach from there to the Rib Conchos confluence is seasonally intermittent. Base flows
are derived mainly from irrigation returns, with small contributions from alluvial seepage and
springflow from mountain creeks and arroyos.
Inflow from the Rfo Conchos (Mexico), the largest tributary in the U.S./Mexico reach, typically
dominates flow through the next stretch, which assumes the Rfo Conchos' water quality and
biological characteristics (Davis, 1980). The volume of flow contributed by the Rfo Conchos
is dependent upon releases from Chihuahua reservoirs. Flow in the Rio Grande/Rib Bravo is
perennial from the mouth of the Rfo Conchos to the Gulf of Mexico. No overly significant
inflows or diversions exist between the Conchos and International Amistad Reservoir near Del
Rio/Ciudad Acuna. Two major U.S. tributaries, the Pecos and Devils rivers, contribute inflow
to International Amistad Reservoir. Downstream from Amistad, instream flow volume is
dependent on releases from the reservoir, and to a lesser extent on the amount of inflow
contributed by tributaries, primarily San Felipe Creek, Sycamore Creek, Pinto Creek, and Las
Moras Creek (U.S.), and the Rfo San Diego, Rfo San Rodrigo, and Rfo Escondido (Mexico).
Major diversions for irrigation and electric power generation, and return flows, result in
increasingly variable instream flow conditions downstream from Eagle Pass/Piedras Negras. In
the Laredo/Nuevo Laredo area, instream flow is augmented by substantial volumes of domestic
wastewater entering from both sides of the river. Downstream, the river is impounded by
International Falcon Dam, and below there instream flows are governed by releases from the
reservoir and by the volume of inflow from three Mexican tributaries, the Rfo Salado, Rfo
Alamo, and Rfo San Juan. At International Anzalduas Dam near Mission/Reynosa, large
volumes of water typically are diverted for domestic and agricultural usage in Mexico. From
there to the Gulf of Mexico, instream flow generally progressively decreases due to multiple
small withdrawals, but is highly variable depending on releases from International Anzalduas
Dam, operations of wastewater dischargers and municipal water supply systems, and irrigation
return flows.
Based on geologic, physiographic, climatic, and biotic characteristics, the river is divisible into
three distinctive reaches. The upper reach extends from El Paso/Ciudad Juarez to Big Bend
Village/La Linda, and lies within the Chihuahuan biotic province (analogous to the Southern
Deserts ecoregion). This reach is physiographically complex, and has three natural geological
sections.
The Bolson Section from El Paso/Ciudad Juarez to Ft. Quitman/Banderas lies in a large bolson
of Quaternary alluvial deposits, in terrain marked by arroyos, bad-land topography, dunes, and
blow sand, with thin scrub brush and grass cover. Tertiary mountainous outcrops border the
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river valley on the Mexican side. The river through this section is principally a sand-bed
stream. Channel width averages about 40 m (131 feet), and the mean gradient is 0.6 m/km (0.1
feet/mile). The river is channelized/leveed through much of the section, and there are no major
tributaries.
The Presidio/Ojinaga section, from Ft. Quitman/Banderas to 16 km (10 miles) southeast of
Redford/El Mulato, is topographically rugged, with mountains and basalt-capped mesas
exhibiting precipitous fronts along the river valley except in a bolson at the Rio Grande/Rio
Bravo-Rfo Conchos confluence. Vegetation is sparse except in the river bottom where salt cedar
and mesquite thrive. The bed load is principally coarse gravel to small boulder, although sand
and fine gravel predominate in localized areas. Riverbed width is about 40 m (131 feet) and the
mean gradient is 0.8 m/km (0.2 feet/mile). The Rfo Conchos is the only major tributary.
The complex terrain of the upper Canyon Section, from 16 km (10 miles) southeast of
Redford/El Mulato to Big Bend Village/La Linda, alternates between level bolsons and elevated
horst blocks, intrusive uplifts, and anticlinal mountains, through which the river has cut deep
canyons. Vegetation is scant, except for dense salt cedar and mesquite along the river. Bed
load material is mainly sand and gravel, ranging from fine sand to large cobbles. Stream width
varies from IS m (49 feet) in constrictive canyons to 40 m (131 feet) in bolsons, and the mean
gradient is 0.9 m/km (0.2 feet/mile). Although no major tributaries exist, discharge increases
through the section due to springflow.
The lower Canyon Section extends from Big Bend Village/La Linda to just south of Del
Rio/Ciudad Acuna, and lies in a transitional zone between three biotic provinces: the
Chihuahuan to the west, Balconian to the east, and Tamaulipan to the south (analogous to the
Southern Deserts, Central Texas Plateau, and Southern Texas Plains ecoregions, respectively).
The river throughout is incised in hilly Cretaceous limestones of the Stockton and Edwards
plateaus. Desert shrub, mesquite, oak, and juniper cover thickens downstream with increasing
rainfall, and extensive stands of cane grass and Bermuda grass grow along the stream margins.
Streambed composition is similar to that in the upper Canyon Section. Stream width is variable,
with a maximum of 100 m (328 feet) at Del Rio/Ciudad Acuna, and the mean gradient is 0.7
m/km (0.1 feet/mile). Principal tributaries are the Pecos and Devils rivers, which converge with
the Rio Grande/Rib Bravo below Langtry/San Ignacio to form International Amistad Reservoir.
Downstream of Del Rio/Ciudad Acuna, the river emerges from the Edwards Plateau and enters
the Rio Grande/Rfo Bravo embayment, a broad, mesquite-chaparral syncline plunging gently to
the south. This constitutes the lower reach, or Coastal Plains Section, which extends to the Gulf
of Mexico. The area is encompassed by the Tamaulipan biotic province (analogous to the
Southern Texas Plains ecoregion). Strong neotropical biotic influences are exerted, in contrast
to the Chihuahuan and Balconian provinces where nearctic influences predominate. The
topography is flat to gently rolling, with local relief rarely exceeding 90 m (295 feet).
Vegetative cover, predominated by thorny brush species, increases to the south as the climate
changes from semiarid near Del Rio/Ciudad Acuna to subtropical near Brownsville/Matamoros.
The river is entrenched as much as IS m (49 feet) into Tertiary formations. Bed load material
between Del Rio/Ciudad Acuna and International Falcon Reservoir is principally small gravel
and sand. Downstream from International Falcon Dam, it grades to fine sand and then to sandy
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silt as the Gulf of Mexico is approached. The channel generally is wide, in the range of 100-
150 m (328-492 feet). The mean gradient decreases from 0.7 m/km (0.1 feet/mile) near Del
Rio/Ciudad Acuna to 0.2 m/km (0.04 feet/mile) near Laredo/Nuevo Laredo, and approaches sea
level near Brownsville/Matamoros. Significant tributaries include Sycamore Creek, Pinto Creek,
and Las Moras Creek (U.S.), and the Rio San Diego, Rio San Rodrigo, Rio Escondido, Rio
Salado, Rio Alamo, and Rfo San Juan (Mexico).
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STUDY DESCRIPTION
The study was designed on the basis of several interagency planning meetings, and comments
from numerous internal and external reviews of early drafts of the work plan. Agencies with
principal involvement in project planning included: UNITED STATES - Texas Natural
Resource Conservation Commission (TNRCC), Texas Parks and Wildlife Department (TPWD),
Texas Department of Health (TDK), U.S. Environmental Protection Agency (USEPA), U.S.
Fish and Wildlife Service (USFWS), U.S. National Park Service (USNPS), U.S. Section,
International Boundary and Water Commission (USIBWC); MEXICO - Seccion Mexicana de
La Comisi6n Internacional de Lfmites y Aguas (CILA), Comisidn National del Agua (CNA),
Secretaria de Desarrollo Social (SEDESOL). The binational sampling team was comprised of
representatives from TNRCC, TPWD, USEPA, USIBWC, CILA, and CNA.
Quality Assurance
The study was conducted in accordance with a USEPA-approved quality assurance project plan
(TNRCC, 1992b). Specified data quality objectives were achieved. Results of data quality
evaluations are presented in Appendix C.
Sampling Sites
A total of 45 sites were sampled (Table 1; Figure 1), 19 of which were on the mainstem.
Sixteen mainstem sites were established to bracket areas where the greatest likelihood for toxic
chemical contamination was thought to exist. These included sites upstream and downstream
from El Paso/Ciudad Juarez, Presidio/Ojinaga, Del Rio/Ciudad Acuna, Eagle Pass/Piedras
Negras, Laredo/Nuevo Laredo, International Anzalduas Dam, Hidalgo/Reynosa, and
Brownsville/Matamoros. Single stations were established in the U.S. at Big Bend National Park
and in the U.S. at Foster Ranch near Langtry/San Ignacio, to characterize conditions in remote,
ecologically important reaches. A supplemental station was established at the mouth of Lozier
Canyon to provide a baseline for future assessments of the effects of inflows from the canyon.
Twenty-six inflows to the river, collectively categorized as tributaries, were sampled (13 in the
U.S.; 13 in Mexico). These were selected based on size, geographical proximity to the
mainstem areas of principal interest, and suspected potential for contributing toxicants to the
mainstem. Each tributary was sampled in the lowermost reach, but far enough above the mouth
to avoid mainstem backwaters.
Types of Analyses
The 18 major mainstem sites were subjected to measurements of selected conventional
parameters in water; determinations of toxic chemical concentrations in water, sediment, and fish
tissue; toxicity testing of water and sediment; and bioassessments of fish and benthic
8
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macroinvertebrate communities. Analyses perfonned at the supplemental mainstem site (5b) and
all tributaries included measurements of selected conventional parameters in water;
determinations of toxic chemical concentrations in water and sediment; and toxicity testing of
water and sediment. Certain tributaries, mostly those that were large enough to permit boat
electrofishing, were also subjected to determinations of toxic chemical concentrations in fish
tissue, and/or bioassessments of fish communities (stations 3a, 6a, 6b, 7b, 8d, 9b, 12d).
Mexico conducted analyses on conventional parameters and metals on the water and sediment
samples.
Parametric Coverage
In addition to toxic chemical parameters, water samples were analyzed for ammonia, total
organic carbon, total hardness, total suspended solids, total dissolved solids, chloride, sulfate,
turbidity, pH, temperature, specific conductance, dissolved oxygen, and residual chlorine;
sediment for particle size composition, total organic carbon, and acid volatile sulfide; and fish
tissue for percent lipid content. A total of 161 toxicants were targeted for analysis in all sample
matrices (Table 2). These included priority pollutants identified in the U.S. Code of Federal
Regulations (CFR) Part 423 Appendix A, except for dioxin and asbestos, plus the following non-
priority pollutants: 11 pesticides for which numerical criteria have been established by the State
of Texas and approved by the U.S., 19 pesticides recommended for inclusion by USEPA Region
6, and three chemicals shown by Lewis et al. (1991) to have a potential for affecting the Rio
Grande/Rib Bravo. For water, the targeted list of toxicants totalled 163, due to inclusion of two
potentially toxic conventional parameters, ammonia and residual chlorine. Numbers of
parameters for which valid analytical data were obtained are summarized in the Data
Completeness section of Appendix C.
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METHODS
Physicochemical Techniques
Field Procedures
Standard TNRCC sampling protocols (Roques et al., 1991) were employed in the United States
and Mexico except where specific modifications were required. Dissolved oxygen, temperature,
pH, and conductivity were measured in the field using a Hydrolab Surveyor n, and residual
chlorine using the DPD ferrous titrimetric method (APHA, 1992). Instantaneous flow was
obtained from U.S. Section IBWC flow gages where available; otherwise, measurements were
made on-site by IBWC/CILA personnel.
Water Sampling
Water samples were collected in flowing water, generally at midstream, by boat or by wading.
Aliquots for all but one parametric group were collected directly from the stream by submerging
appropriate containers to a depth of one foot.
Aliquots for dissolved metals were obtained using ultra-clean procedures involving the use of
disposable rubber gloves and a peristaltic pump. Water was pumped directly from the stream,
through pretreated rubber tubing with a 0.45/i in-line filter in place. Metals-grade nitric acid
and type 2 deionized water were used to pretreat tubing and containers and to preserve the
samples. Volumes, specifications and pretreatment of containers, and preservation methods for
the various types of water samples are presented in Table 3.
Field blanks and duplicates were employed at a frequency of about 10%. Quality assurance
samples were collected, preserved, and handled in identical fashion to ambient water samples.
Sediment Sampling
Sediment sampling generally was performed in slack water areas near the stream banks, where
deposition was adequate to allow collection of sufficient sample volume (> 9 liters). The entire
column of fine-grained, surficial sediment was sampled, regardless of thickness. Thus, the depth
to which subsamples were taken was variable among sites, ranging from about 0.5 cm (0.2
inches) to about 8 cm (3 inches).
Most samples were taken with a stainless steel Ekman dredge, which required the collection of
10-20 bites. During one survey, the dredge became inoperable, and sampling was conducted
with a shovel, with 5-10 scoops comprising each sample. At a few tributary sites, surficial
sediment layers were so thin that sampling had to be performed by scooping a large number of
small subsamples with teflon lid liners.
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At each site, subsamples were composited in a plastic bucket and thoroughly mixed with a large
plastic spatula. The slurry was then poured into individual sample containers. Sediment
sampling equipment was scrubbed with a brush in dilute Alconox solution and thoroughly rinsed
with site water prior to collecting each sample. Types of sediment samples and descriptions of
sample containers, container pretreatment, and sample preservation are presented in Table 3.
Tissue Sampling
Tissue sampling was conducted using the study protocol, which called for the collection and
analysis of two whole body and two fillet (edible tissue) composite samples per selected site.
Each composite sample was to be comprised of five fish of similar size, except where target
species were scarce and a smaller number had to be utilized or where more individuals had to
be composited to achieve the required amount of tissue. The actual number varied from two to
18, and occasionally an individual fish was analyzed. Efforts were made to include a predatory
species and a bottom-feeding species at each site. Target species were largemouth bass
(Micropterus salmoides), channel catfish (Ictalurus punaatus), and common carp (Cyprinus
carpio). Alternate species collected included white bass (Morone chrysops), smallmouth bass
(Micropterus dolomieu) and blue catfish (Ictalurus Jurcatus).
Fishes were collected by boat electrofishing and were held in live wells until specimens were
selected for analysis. The fish selected were held on ice in clean coolers pending field
preparation of the samples. Total length and weight were recorded for each specimen and any
unusual deformities, wounds, or infections were noted. The sex of each individual was also
noted in the case of fillet samples. If fishes contacted debris during collection and handling,
they were rinsed with distilled water before being processed.
The Texas Tissue Sampling Guidelines (Appendix 18 in: Roques et al., 1991), a consensus
document prepared by Texas state and U.S. federal agencies, was followed with minor
exceptions in preparation of edible fish tissue samples (fillets). Fishes were filleted on a
polypropylene cutting board covered with aluminum foil. The dull side of the foil was placed
toward the sample on the cutting board, and when specimens were wrapped. Skinless fillets
were removed from both sides of each fish and individually double wrapped hi aluminum foil.
All coolers, stainless steel fillet knives, polypropylene cutting boards, weighing trays, and
measuring boards were cleaned between stations, or between composite samples. The cleaning
procedure was a detergent wash, followed by rinsing in ambient water and a final rinse in
distilled water. All instruments were allowed to air dry. Cutting boards were covered with
fresh foil between samples.
In processing whole fish samples, dorsal and pectoral spines, if present, were clipped (and
included in the sample) to avoid puncturing the foil wrapping. Each fish was double wrapped
in aluminum foil.
Foil packages were labeled and placed in a plastic bag with other individuals for that composite
sample. Fillet samples were split for dual analysis by the U.S. and Mexico.
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Sanrole Handling
Recommended storage, preservation, and holding time requirements were observed during
transport and analysis of water, sediment, and tissue samples (see Table 3). All samples were
stored and shipped on ice. Ice chests containing the samples and appropriate chain-of-custody
forms were sealed with tape, and shipped to the laboratories via overnight freight.
Laboratory Analyses
Split samples of water, sediment, and tissue were collected for analyses by labs in the U.S. and
Mexico. All U.S. analyses of water, sediment, and fish tissue were performed by the Texas
Department of Health (TDH) Environmental Chemistry Laboratory, Austin, Texas. TDK
performed analyses of environmental samples according to a USEPA-approved quality
assurance/quality control plan (Twidwell et al., 1991).
M&ico conducted lab analyses on its samples in the CNA Laboratories in Chihuahua, Chin.,
Torreon, Coah., Tampico, Tamps., Monterrey, N.L., and Mexico, D.F., as well as in the
laboratories of ATLATEC, S.A., in Monterrey, N.L. With regard to the evaluation of the
results, Mexico used Mexican standards for water quality.
Methods employed by both labs are presented in Table 4.
Data Evaluation
U.S. toxic chemical data were evaluated using screening level concentrations listed in Tables 7,
8, and 9. For water, screening values were derived from the following sources, in order of
priority: (1) State of Texas criteria for protection of aquatic life and human health (TNRCC,
1991); (2) U.S. federal water quality criteria (USEPA, 1986, and subsequent updates thereto);
(3) chemical concentrations that have been considered for U.S. federal criteria (USEPA, 1980a-
1980n); (4) national 85th percentile values (Greenspun and Taylor, 1979); and (5) chemical
concentrations from supplemental sources. All sources from which screening levels were
adopted are documented in footnotes to Tables 7 and 8. This array of information allowed
incorporation of at least one screening value for every toxic chemical detected in water (Table
13).
Mexico evaluated data using Mexican water quality standards. The Mexican water quality
standards are listed in Table 7.
For U.S. sediment data, preferential screening values involved national interim or draft sediment
quality criteria (USEPA, 1989, 1991). However, as these are available for only a few
chemicals, sediment data were primarily screened using contaminant threshold concentrations
for protection of aquatic biota (USEPA, 1985a). These were derived by USEPA in a manner
similar to the equilibrium partitioning approach (USEPA, 1989), employing an equilibrium
partitioning assumption and USEPA aquatic life water quality criteria. The original values are
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based on an assumption that sediment contains four percent organic carbon. For the present
study, threshold values presented by USEPA (1985) were modified using total organic carbon
data from Table 11:
' USEPA U^eshoMco^^on
For sediment contaminants for which threshold values were not developed in the 1985 USEPA
study, screening level concentrations were obtained from additional sources, as identified in
footnotes to Tables 7 and 9. National 85th percentile values (Greenspun and Taylor, 1979) were
also utilized if available.
U.S. edible fish tissue data were screened for human health risks using USFDA (1993) action
or tolerance limits, USEPA (1993) values for establishment of fish advisories, and TDK (1992)
risk assessment levels. Whole fish tissue data were primarily evaluated using national 85th
percentiles (Schmitt and Brumbaugh, 1990), state 85th percentiles (TNRCC, 1994), national
mean concentrations (Schmitt et al., 1990; USEPA, 1992), and predator protection limits
developed by various agencies. All sources from which screening levels were derived are
identified in footnotes to Table 7.
Biological Techniques
Toxicity Testing
U.S. toxicity testing was performed by the USEPA Region 6 Laboratory in Houston, Texas,
according to procedures described by Weber et al. (1989). Accuracy and precision were ensured
through conformance with standard USEPA quality assurance/quality control procedures.
U.S. sediment eluates were prepared by combining a subsample from the homogenized sediment
sample with appropriate culture water. The sediment and water were combined in a sediment-to-
water ratio of 1:4 on a volume basis by volumetric displacement. After combining, the mixture
was tumbled end-over-end for approximately 24 hours, after which the mixture was allowed to
settle for an additional 24 hours at 3-4 °C. After settling, the eluate was siphoned off and
filtered through a 1.5 p. glass fiber filter before testing was initiated.
U.S. water and sediment eluate samples were evaluated using two different toxicity tests. The
first was the Ceriodaphnia dubia Seven-Day Survival and Reproduction Test (USEPA Method
1002.0). Neonates less than 24 hours old were utilized for testing. One neonate was added to
each of ten replicates for the control (culture water) and the 100% test water or sediment eluate
sample. Test chambers were 30 mL beakers containing 15 mL of test or control water. The
organisms were fed once daily. Test solutions were renewed on days two, four, and six.
Mortality, number of young produced, dissolved oxygen, and temperature were monitored and
recorded daily. At the termination of the tests, mortality and reproduction data were analyzed
statistically (p = 0.05) using Fisher's Exact Test and the t test, respectively, to determine
differences between control organisms and those exposed to the test solutions.
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The second was the Fathead Minnow, Pimephalespromelas, Seven-Day Embryo/Larval Survival
and Teratogenicity Test (USEPA Method 1001.0). Embryos less than 36 hours old were utilized
for testing. Ten embryos were added to each of three replicates for the control (culture water)
and the 100% test water or sediment eluate sample. Test chambers were 400 mL Nalgene
culture dishes containing 250 mL of test or control water. Feeding was not required during the
exposure period. Test solutions were renewed on days two, four, and six. Mortality (deformed
larvae were counted as dead), dissolved oxygen, and temperature were monitored and recorded
daily. At the termination of the tests, mortality data were analyzed statistically (p = O.OS) using
the / test, to determine differences between control organisms and those exposed to the test
solutions.
Macrobenthic Community Assessment
Macrobenthic organisms were collected using two techniques. At sites where rocky-bottomed
riffles were present, a composite of three to five subsamples were taken with a Surber square
foot sampler. Where riffles were lacking, snag (submerged woody debris) sampling was
employed. Snags of 2.54 cm (1 inch) diameter or less were cut into pieces using lopping shears,
with enough material collected to fill two 1-qt. Mason jars. At two sites, both types of samples
were collected for purposes of comparison.
Benthic samples were preserved in 5% formalin, returned to the lab, and washed in a U.S.
Standard No. 30 soil seive. Snags were scrubbed with a soft-bristle brush, and after all
organisms were removed, the surface area of each snag was determined. Organisms were
picked from debris at 12X magnification using a dissecting microscope, enumerated, and
identified to the lowest possible taxonomic level.
Macrobenthic data were evaluated using two techniques, to provide a crosscheck. The first was
the Mean Point Score (MPS), the method routinely used by TNRCC for assessing macrobenthic
community integrity. The MPS involves six metrics, five of which relate to community
structure (species richness, standing crop, Ephemeroptera-Plecoptera-Tricoptera or EPT index,
diversity, and equitability). The sixth relates to community function, and is comprised of three
submetrics (number of functional feeding groups, prevalence of the most abundant functional
feeding group, and cumulative prevalence of organisms that feed on fine particulate organic
matter). Each metric value is assigned from 1 to 4 points using criteria developed by TNRCC
(Twidwell and Davis, 1989). Point scores correspond to aquatic life use subcategories (4 =
exceptional; 3 = high; 2 = intermediate; 1 = limited). The point score assigned the
community function metric represents the lowest indicated by any of the three submetrics. The
MPS is calculated by dividing the sum of the individual point scores by six. An aquatic life use
subcategory rating is derived from the following MPS criteria ranges: >3.50 = exceptional;
2.50-3.50 = high; 1.50-2.49 = intermediate; < 1.50 = limited.
The second technique was the Invertebrate Community Index (ICI) developed by Ohio EPA
(1987), which utilizes ten community structure metrics (taxa richness, mayfly taxa richness,
caddisfly taxa richness, dipteran taxa richness, percent mayfly composition, percent caddisfly
composition, percent tribe Tanvtarsini midge composition, percent other dipteran and non-insect
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conposition, percent tolerant organisms, and EPT index). Each metric value is assigned a point
score (0, 2, 4, or 6), and the scores are summed to arrive at the ICI value. An aquatic life use
subcategory rating is derived from the following ICI criteria ranges: 45-60 = exceptional; 35-
44 = high; 11-34 = intermediate; 0-10 = limited.
Fish Community Assessment
U.S. fish community evaluations were conducted by the TPWD Freshwater Studies Program.
Boat electrofishing and seining were employed in tandem at all mainstem sites as well as at sites
on selected tributaries (Figure 1). The goal was to collect a representative sample of the fish
species present in proportion to their relative abundances. Attempts were made to sample all
major habitat types in a study reach. Electrofishing was conducted with a boat-mounted, boom
electrode powered by a 7.5 kV generator producing pulsed DC current. Duration was at least
15 minutes per site, with sampling occurring in a downstream direction. Attempts were made
to net all observed fish. As a complementary technique, seining was typically used to sample
habitats where boat electrofishing would not be as effective (e.g., shallow riffles and bars). Two
straight seines were used for most collecting: 30 feet by 6 feet by 1/4 inch delta weave mesh
and IS feet x 6 feet x 3/8 inch delta weave mesh. The number of seine hauls depended on
available habitat and varied from four to 11. All fishes collected by both methods were
examined for deformities, lesions, and tumors. Seine samples and voucher specimens of larger
fishes were fixed in 10% formalin and later transferred to 75% ethanol. Fishes were identified
or field identifications were verified in the laboratory employing a variety of references,
including Hubbs et al. (1991). Common and scientific names follow Robins et al. (1991).
Several approaches were used to evaluate the fish community for potential anthropogenic
influences, and reflect different levels of screening. An initial screen was provided by
descriptively evaluating the occurrence of species in this study and determining whether patterns
of presence or absence of fish species indicated any long-term trends. That evaluation was
followed by comparisons to historic data. A second level screen involved evaluating species
richness and composition and then employing a similarity index, a measure of the similarity of
species composition between two sampling sites (Odum, 1971). This index varies from zero,
with no species in common between sites, to 1.0, with all species in common. The equation
employed was:
S = 2C / (A + B),
where S = index of similarity, A = number of species in sample A, B = number of species
in sample B, and C = number of species common to both samples. Given the fact that the
study design employed sites upstream and downstream from major sister cities, any substantial
change in species composition between the samples could indicate impacts (in the absence of
sampling bias or physical habitat related differences). A third level of screening involved
calculating a community index derived from the Index of Biotic Integrity (IBI) presented by Karr
et al. (1986), and evaluating individual metrics as well as observing the cumulative score.
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Developing an IBI type index for the Rio Grande/Rib Bravo and its tributaries was problematic
given the wide range of habitats, fauna! changes, and hydrologic modifications encountered over
the more than 2,000 km (1,243 miles) of river covered by the study. Despite these problems,
an attempt was made to present a preliminary IBI, though much more emphasis was placed on
interpretation of individual metrics than the total score. These metrics should be considered
provisional until they can be applied to additional data sets to determine if they respond in a
predictable fashion to direct or indirect impacts to the fish community. Two different series of
IBI metrics were calculated corresponding to communities identified through preliminary analysis
of fauna! patterns. These groupings encompassed the following areas: mainstem Rio
Grande/Rib Bravo upstream of International Falcon Reservoir and tributaries including the Rfo
Conchos, Pecos River, Devils River, San Felipe Creek, Rio San Rodrigo, and Rio Escondido
(stations 1-12, 3a, 6a, 6b, 7b, 8c, and 9b ); and the mainstem Rio Grande/Rio Bravo
downstream of International Falcon Reservoir and including Arroyo Los Olmos (12-18, 12d).
More indices could have been derived given the differences noted by Hubbs et al. (1977)
upstream and downstream of the Rio Conchos and the distinct fauna found in the tributaries.
However, since the goal of this effort was to evaluate communities relative to the potential
presence of toxic chemicals, it was appropriate to simplify the criteria and employ only one set
of metrics for the entire area above International Falcon Reservoir. This approach was
reinforced by the overall study design, which emphasized differences between sites upstream and
downstream of major sister cities rather than a longitudinal comparison of all sites.
Furthermore, the middle river tributaries (3a, 6a, 6b, 7b, 8c, and 9b) comprise distinct habitats
and were analyzed separately from the mainstem sites. In summary, IBI scores were not and
should not be compared among these groups since the composition and rationale of the metrics
as well as the habitats varied widely.
This IBI was substantially modified from Karr et al. (1986) given the variation of Rio
Grande/Rio Bravo fish communities from those in the midwestern United States from which the
index was originally developed. Modifications were based upon examination of this data set,
suggestions by Karr et al. (1986) and Miller et al. (1988), and previous experience in applying
IBI to Texas streams (Linam and Kleinsasser, 1987; Kleinsasser and Linam, 1989; Homig et
al., in press). Criteria were developed using historical data (Trevino-Robinson, 1959; Hubbs
et al., 1977; Edwards and Contreras-Balderas, 1991), a summary of fauna in the Rio
Grande/Rio Bravo basin (Smith and Miller, 1986), and data from a project designed to develop
biological criteria for stream communities within the state's ecoregions (Bayer et al., 1992).
The metrics employed for the mainstem upstream of International Falcon Reservoir and the Rio
Conchos are summarized in Table 5 along with the. rating criteria. The number of metrics was
much reduced from that proposed by Karr et al. (1986), with only three of the original ones
being employed in this study. The original metrics were the total number of species, total
number of individuals, and percentage of diseased individuals. Though Miller et al. (1988)
caution against reducing the number of metrics, the Rio Grande/Rio Bravo fauna is somewhat
depauperate and cannot be appropriately evaluated using simple modifications. Consequently,
we followed the approach of Moyle et al. (1986), who used a reduced series of metrics to
evaluate the depauperate fauna of the Sacramento-San Joaquin drainage in California.
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Metrics evaluating the contribution of percids, centrarchids, and catostomids were eliminated in
favor of a single metric representing the number of minnow species other than the introduced
common carp. Centrarchids and percids were eliminated because native species from those
groups are fewer in the Rio Grande/Rfo Bravo when compared to the fauna of more eastern
drainages (Smith and Miller, 1986). Suckers are also somewhat depauperate in the mainstem
of the Rio Grande/Rfo Bravo, with only four species being commonly collected and those species
having differing responses to man-induced environmental stress. Cyprinid species have
historically been species rich throughout the Rio Grande/Rfo Bravo basin and dominated the
diversity of the Rio Grande/Rfo Bravo ichthyofauna (Smith and Miller, 1986). We have
observed them to be reliable indicators of environmental change in our other studies of Texas
streams and rivers (Unam and Kleinsasser, 1987; Kleinsasser and Linam, 1989). Hughes and
Gammon (1987) used cyprinids as a target group in an IBI study of the Willamette River in
Oregon, citing the responsiveness of that family to deterioration of habitat structure (see also
Minckley, 1973; Moyle, 1976). Ramsey (1968) proposed that many species in the minnow
family could be good indicators of water quality, though he cautioned that specific habitat
requirements for many species are unknown.
Metrics relating to tolerance were eliminated given the naturally harsh environmental conditions
in the Rio Grande/Rfo Bravo basin. The number of intolerant species and proportion of green
sunfish [=tolerant species (Karr et al., 1986)] were replaced with a single metric, the percentage
of individuals in the most abundant species, as an indication of whether a single species was
dominating the fish community at a site.
The hybrid metric (Karr et al., 1986) has rarely provided much information about degradation
in previous studies employing IBI in Texas and was replaced in this study with the percentage
of individuals as introduced species. Miller et al. (1988) indicated that the hybrid metric has
been difficult to apply in most regions and reviewed the problems associated with it. Use of a
metric dealing with introduced species provides another means of evaluating perturbations, since
these species may become populous in altered habitats. This metric has previously been used
by Crumby et al. (1990), and, as cited by Miller et al. (1988), the proportion of introduced
individuals often increases with increasing habitat degradation (see Moyle and Nichols, 1973;
Courtenay and Hensley, 1980; Leidy and Fielder, 1985). Hubbs (1982) indicates that the
survival of exotics can be enhanced by other perturbations and used impoundments as an
example. The term "introduced species" as employed here, refers to species not native to the
Rio Grande/Rfo Bravo basin, recognizing, however, that certain species native to the basin such
as inland silverside (Menidia beryllina) have increased their ranges through introductions. The
status (introduced or not) of the species we collected are listed in Table 6 and result from
consulting Hubbs (1982), Smith and Miller (1986), and Hubbs et al. (1991).
Trophic metrics—proportion of insectivorous cyprinids, proportion of top carnivores, and
proportion of omnivores—were not employed because of concerns about collecting fishes in their
relative abundances in a large, turbid river system.
Metrics and criteria used to evaluate the mainstem Rio Grande/Rfo Bravo downstream of
International Falcon Reservoir, and Arroyo Los Olmos, are listed in Table 5. They are similar
to those used on the upstream reach of the river, but were modified to take into account the
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proportionate numbers of euryhaline species that have become common downstream.
Consequently, a metric was added to account for the percentage of the sample as
estuarine/marine species. Designation of these species is listed in Table 6 and follows Edwards
and Contreras-Balderas (1991), though estuarine and marine species were combined into one
group. Criteria were adapted from historical information in Trevino (19S5) and summarized in
Edwards and Contreras-Balderas (1991). This metric measures the species shift from the native,
riverine community to one increasingly represented by euryhaline species. The number of
minnow species was eliminated, recognizing that they were historically an important group in
the lowermost reach. However, all sites downstream of International Falcon Reservoir would
have scored poorly using that metric, making it insensitive in differentiating between sites. In
addition, it was beneficial to keep the number of metrics the same in upstream and downstream
reaches. However, if the primary intent of the study had been to consider historical changes in
the fauna, it would have been included.
Though it departs from the conventions of others (Trevino-Robinson, 1959; Hubbs et al., 1977;
Smith and Miller, 1986; and the description of the Study Area in the present report), in
discussing fish communities the upper reach is defined as the area upstream of International
Amistad Reservoir; the middle reach, the area from International Amistad Dam to International
Falcon Reservoir; and the lower reach, the area from International Falcon Dam to the mouth.
Finally, it should be noted that a single sample evaluation of the fish community can provide an
indication of potential problems, but only at a screening level. In short, further sampling and
evaluation would be needed to validate and define the extent of potential problem sites noted in
this study.
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RESULTS AND DISCUSSION
Sampling was accomplished through four field surveys: El Paso/Ciudad Juarez to Big Bend
National Park/San Vicente, November 11-15, 1992; Rio Grande City/Camargo to
Brownsville/Matamoros, January 11-14, 1993; Langtry/San Ignacio to Eagle Pass/Piedras
Negras, February 8-12, 1993; and Eagle Pass/Piedras Negras to Rio Grande City/Camargo,
March 22-26, 1993. Sampling dates for individual sites are presented in Table 10. Survey
sequence was based on prevalence of favorable weather and hydrological conditions.
The study focused on instream conditions associated with low flow. As such, demonstrable
instream effects primarily reflected influences by point source discharges. Instream flows (Table
10) were within targeted ranges at 18 of the 19 Rio Grande/Rfo Bravo sampling sites. Flow at
station 13, (Rio Grande/Rio Bravo at Los Ebanos/Valadeces), was about four times the preferred
level, due to releases from International Falcon Reservoir. Despite a request, the release could
not be shut down because of irrigation needs in Mexico. The existence of high flow downstream
from International Falcon Reservoir was not considered a major detriment. Station 13, the only
sampling site in the affected reach, was an upstream control site where no appreciable toxic
chemical-related problems were anticipated. Most of the flow there would have consisted of
water released from International Falcon Reservoir, whether the discharge had been 28 or 113
cms (1,000 or 4,000 cfs). In addition, most of the water released from the reservoir was being
diverted at International Anzalduas Dam, and desired low flow conditions were prevalent at the
remaining survey sites.
There were slight differences in the analytical results between the two countries, possibly as a
result of the differences in methodology and instrumentation.
Conventional Water Quality
Previous information (TNRCC, 1992a) and data from the present study (Table 10) were
reviewed to provide an indication of conventional water quality. CNA also has conducted water
quality analyses based on physical, chemical, and bacteriological parameters from 1976 to 1993.
An evaluation of this data is found in Appendix D.
Mainstem
TNRCC (1992a) summarized ten years of U.S. water quality data (1982-1991) for 12 Rio
Grande/Rfo Bravo monitoring sites that bracket six major U.S./Me'xico sister cities. Potential
human health risks due to bacteriological contamination were evident for five of the six
downstream sites. Nutrient concentrations were somewhat elevated in these same areas. Inflows
of treated and untreated sewage and nonpoint source runoff from the sister cities were considered
responsible. Average dissolved oxygen concentrations, however, exceeded 5 mg/L throughout
the longitudinal gradient, with no major depressions at sites downstream from the sister cities.
Only 16 of 1,257 instantaneous dissolved oxygen measurements, or about 1%, were less than
5 mg/L. Most of the depressed values occurred at the site upstream from Presidio/Ojinaga, and
in the reach downstream from International Falcon Reservoir, and were attributed mainly to
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sluggish current velocity/low atmospheric reaeration rates associated with extreme low flows.
One other item of potential concern involved periodic exceedances of total dissolved solids
criteria upstream from Presidio/Ojinaga and downstream from Brownsville/Matamoros.
Data from the present study for chloride, sulfate, total dissolved solids, pH, temperature, and
dissolved oxygen were evaluated using Texas Surface Water Quality Standards (TNRCC, 1991).
Criteria were not achieved in only two instances, as chloride and total dissolved solids were
elevated at station 3, and sulfate and total dissolved solids at station 18, the same sites where
occurrences of this type have previously been documented by TNRCC (1992a). Dissolved
oxygen concentrations were greater than 5 mg/L at all sites. Total organic carbon data were
reviewed as an indicator of organic enrichment. Levels were relatively low at all sites, ranging
from 3-11 mg/L. Concentrations at ten of the 19 sites were 5 mg/L or less.
Thus, indications are that conventional water quality in the Rio Grande/Rfo Bravo is reasonably
good, except for locally elevated levels of fecal coliform bacteria, nutrients, and total dissolved
solids. The river evidently is able to assimilate the oxygen-demanding load it receives without
the development of substantial dissolved oxygen depression.
CNA evaluated water quality in the Rio Grande/Rfo Bravo from 1976 to 1993, (Appendix D).
CNA obtained results similar to TNRCC's relative to the quality of the waters of the Rio
Grande/Rfo Bravo during this period.
Tributaries
Data for the same parameters mentioned above were evaluated to provide an indication of
conventional water quality in the tributaries that were sampled (Table 10). The lower Pecos
River (6a), Devils River (6b), and San Felipe Creek (7b) are designated segments, and are
governed by specific water quality criteria (TNRCC, 1991). All conventional parameter criteria
were achieved at these sites, except in the Pecos River where chloride and sulfate levels were
slightly elevated.
Nondesignated segments are presumed to support a high aquatic life use and are expected to
meet a 5 mg/L minimum dissolved oxygen concentration (TNRCC, 1991). Of the 23 tributaries
in this category, four exhibited dissolved oxygen concentrations less than 5 mg/L (2a, Ciudad
Juarez sewage discharge canal; 9a, unnamed tributary 3.6 km (2.2 miles) downstream from
Piedras Negras; lie, Arroyo el Coyote; ISa, Anhelo Drain). The observed concentrations
were noteworthy in light of the season of occurrence and prevalence of low water temperatures,
and reflected a potential for anoxic conditions during summertime. These four tributaries
transport domestic effluent from Ciudad Juirez, Piedras Negras, Nuevo Laredo, and Reynosa,
respectively.
Five of the 23 sites exhibited total organic carbon concentrations greater than 20 mg/L, including
the four just mentioned, plus station 1 la (Zacate Creek). The concentrations ranged from 22-49
mg/L, reflecting a degree of organic enrichment Organic loading to the mainstem appears
minimal for 9a and lie, which had inflow volumes less than 0.06 cms (2 cfs), slight for ISa
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(0.45 cms or 16 cfs), and substantial for 2a (1.7 cms or 61 cfs). At the first mainstem sites
downstream from these tributaries, only the one below 2a (station 3) exhibited a total organic
carbon concentration perceptibly above baseline levels.
Four of the 23 sites exhibited total dissolved solids concentrations that were elevated compared
to the maximal level observed in the mainstem (1,820 mg/L at station 3) (lOa, Manadas Creek;
lib, Chacon Creek; 12a, Rfo Salado; 12d, Arroyo Los Olmos). The Pecos River (station 6a)
also was in this category, although the criterion for that segment was not exceeded. The
recorded levels (2,920-7,480 mg/L) would be expected to have deleterious effects on freshwater
aquatic life. Three of these tributaries (lOa, lib, 12d) appeared to contribute little total
dissolved solids to the mainstem, as corresponding flows were less than 0.06 cms (2 cfs). The
opposite appeared true for the other two (6a, 12a), as their flows were substantial (6.2 and 1.0
cms, or 218 and 37 cfs, respectively).
In conclusion, conventional water quality in tributaries generally was good, with several
exceptions.
Toxic Chemicals in Water
U.S. water samples from all 45 stations were analyzed for toxic chemicals. Thirty-five of the
153 toxic chemicals for which valid analytical results were generated occurred at detectable
levels (Tables 10 and 13). Seventeen of the 35 exhibited possible screening level exceedances
(Table 14). These, together with the number of sites involved, were: un-ionized ammonia (4);
residual chlorine (2); parachlorometa cresol (1); phenol (1); phenolics recoverable (1);
chloroform (1); antimony (1); arsenic (9); chromium (1); mercury (2); selenium (8); silver
(5); thallium (1); diazinon (2); bis(2-ethylhexyl) phthalate (1); diethyl phthalate (1); and
cyanide (2).
Mainstem
The number of toxic chemicals detected by U.S. data ranged from two at stations 9, 10, and 15
to eight at station 2. At 16 of the 19 sites, five or fewer toxic chemicals were detected (Table
13).
U.S. data indicated elevated toxic chemical concentrations were uncommon in the mainstem
(Table 15). There were only six instances where possible screening level exceedances occurred,
involving five toxic chemicals (Table 14): residual chlorine, acute and chronic, station 2;
arsenic, national 85th percentile, stations 4 and 5; selenium, chronic, station 11; silver, acute
and chronic, station 12; and cyanide, chronic, station 14. No station exhibited elevated
concentrations for more than one chemical.
The presence of residual chlorine at station 2 (downstream from El Paso/Ciudad Juarez)
probably contributed to the impoverished condition of the macrobenthic community at the site.
Although the concentration was too low to quantify, it probably was greater than the acute
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aquatic life criterion, in light of chlorine's extreme toxicity. The primary source was the El
Paso Public Service Board Haskell R. Street wastewater treatment plant discharge (station la),
which enters 13.8 km (8.6 miles) upstream. The effluent, which is chlorinated for disinfection
purposes, contained a residual chlorine concentration of 1.2 mg/L, and the discharge volume was
substantial (1.3 or 45 cfs, equivalent to 24% of the flow at station 2).
Arsenic levels at stations 4 (downstream from Presidio/Ojinaga) and 5 (mouth of Santa Elena
Canyon), 14.4 and 15.8 Mg/L, respectively, did not appear to be impacting the river. There
were no significant effects in the toxicity tests, and resident fish and macrobenthic communities
were healthy. This is not surprising, since the screening level that was exceeded, the national
85th percentile, is not based on biological effects. Both concentrations were far less than the
chronic aquatic life criterion of 190 pg/L. The principal source of arsenic evidently was the Rio
Conchos (station 3a), which enters 18.7 km (11.6 miles) upstream from station 4. That tributary
exhibited the highest arsenic level in the study (20.6 /tg/L), and contributed a large volume of
inflow (15 cms or 530 cfs, equivalent to 66% of the flow at station 4). Alamito Creek (station
3b), entering 0.6 km (0.4 miles) upstream from station 4, contained 10.6 pg/L of arsenic.
However, its contribution was negligible, in light of the low inflow volume (0.03 cms or 1.1 cfs,
equivalent to 0.1 % of the flow at station 4).
Arsenic in surface waters is primarily derived from natural processes (dissolution of arsenates
from metallic ore-bearing rocks; soil erosion), air pollution (fossil fuel combustion), industrial
wastewaters, and arsenical pesticides (Irwin, 1989; McKee and Wolf, 1963). Arsenic levels at
stations 4 and 5 appear to be naturally-derived to an extent, judging from the somewhat elevated
level in Alamito Creek, a minimally-impacted stream, plus the fact that the concentration at
station 5 was slightly higher than at station 4, despite an absence of likely anthropogenic inputs
through the intervening reach. Baseline concentrations in the Rfo Conchos may be similarly
derived; however, the presence of arsenical pesticides entering in agricultural runoff may be
responsible for the magnitude of the concentration that was observed there. The Rfo Conchos
in the past has been shown to contribute other agriculturally-derived pesticides to the Rio
Grande/Rfo Bravo (TNRCC, 1992a).
The slight exceedance of the chronic aquatic life criterion by selenium at station 11 (above
Laredo/Nuevo Laredo) did not appear ecologically important, as no adverse impacts were
indicated by toxicity testing or bioassessment results. There were no obvious sources of
selenium that would have affected the site. Although this was the only mainstem station where
selenium was elevated, seven tributaries had concencentrations that exceeded screening levels.
Interestingly, six of these eight sites, including station 11, were geographically clustered from
just upstream of Laredo/Nuevo Laredo to Rio Grande City/Camargo.
Selenium occurs at naturally high levels in soils in other parts of the United States. It has
numerous industrial applications, and may be present in industrial wastewaters. Other potential
sources include atmospheric fallout from coal-fired power plant emissions (including washoff
from land surfaces where fallout is deposited), municipal sewage from industrial communities,
and insecticide sprays (Irwin, 1989; McKee and Wolf, 1963).
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The concentration of silver at station 12 (downstream from Laredo/Nuevo Laredo) was greater
than acute and chronic aquatic life criteria; whether this was an actual exceedance is uncertain,
because the criteria are based on the free ion form while the measured concentration represented
total dissolved silver. Nevertheless, a possibility exists that the observed silver concentration
may have been partially responsible for depressed fish and macrobenthic community integrity
at the site. There were no conspicuous sources, as levels in the three proximal, upstream
tributaries that were sampled (stations 1 la, 1 Ib, 1 Ic) were below detection. However, there are
a number of other inflows between stations 11 and 12 (Buzan, 1990) that could have been
involved.
Silver generally does not occur in significant concentrations in natural waters. Various forms
of silver have industrial applications, including the production of jewelry, silverware, metallic
alloys, and ink; for electroplating; in the processing of food and beverages; and in photography.
Silver in surface waters typically is derived from wastes generated by these processes (McKee
and Wolf, 1963).
Although the cyanide concentration at station 14 (downstream from International Anzalduas
Dam) was greater than the chronic aquatic life criterion, no appreciable impacts were evident.
There were no significant effects in the toxicity tests, and resident macrobenthic and fish
community integrity was relatively high (although fish exhibited a slightly elevated incidence of
physical abnormalities). Regarding sources, there were no obvious inputs that would have
contributed cyanide to the site. Cyanide enters surface waters in effluents from gas works, coke
ovens, gas-scrubber processes of steel mills, metal cleaning/electroplating operations, and
chemical industries (McKee and Wolf, 1963).
U.S. data indicates toxic chemicals were more prevalent in tributaries than in the mainstem.
This is not surprising since some of the tributaries transport wastewater in relatively undiluted
form. The number of toxic chemicals detected ranged from one at station 8d to 17 at station la.
More than five toxic chemicals were detected at eight of the 26 sites (Table 13).
There were 37 instances where screening levels possibly were exceeded, involving 17 toxic
chemicals (Table 14): un-ionized ammonia, acute and chronic, station 2a; chronic, stations 7a,
9a, and lie; residual chlorine, acute and chronic, station la; parachlorometa cresol, national
85th percentile, station 2a; phenol, national 85th percentile, station 2a; phenolics recoverable,
national 85th percentile, station 2a; chloroform, national 85th percentile, station la; antimony,
human health and national 85th percentile, station lOa; arsenic, human health, stations la, 2a,
9a, lie, and 15a; national 85th percentile, stations 3aand 3b; chromium, chronic, station 12a;
mercury, human health, stations 2a and 15a; selenium, chronic, stations 5a and lla; chronic
and national 85th percentile, station lie; human health, chronic, and national 85th percentile,
stations 9b, 12a, 12b, and 12c; silver, acute and chronic, stations 7b, 8a, 8b, and 8e; thallium,
human health, station lOa; diazinon, acute and chronic, stations lla and 12d; bis(2-ethylhexyl)
phthalate, chronic and national 85th percentile, station lie; diethyl phthalate, chronic, station
15a; and cyanide, chronic, station 12d. The number of chemicals that occurred at elevated
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levels, by station, were: six, station 2a; four, station lie; three, stations la and ISa; two,
stations 9a, lOa, lla, 12a, and 12d; one, stations 3a, 3b, Sa, 7a, 7b, 8a, 8b, 8e, 9b, 12b, and
12c; zero, stations 6a, 6b, 8c, 8d, lib, and 12e.
U.S. data shows the four tributaries that had elevated un-ionized ammonia concentrations,
represented by stations 2a, 7a, 9a, and lie, transport effluent from Ciudad Juarez, Ciudad
Acuna, Piedras Negras, and Nuevo Laredo, respectively. These elevated levels result from
decomposition of nitrogenous organic matter introduced in domestic effluent from each of the
cities. Significant effects occurred in toxicity testing of water from all four sites, and in each
case un-ionized ammonia was considered the primary causative agent. Un-ionized ammonia
inputs associated with stations 7a, 9a, and lie probably had little or no effect on the Rio
Grande/Rio Bravo, in light of low inflow volume (Table 10). Contributions from 2a, however,
may have exerted substantial effects for some distance downstream, as the associated inflow
volume was considerable (1.7 cms or 61 cfs).
Residual chlorine at station la (El Paso Public Service Board Haskell R. Street wastewater
treatment plant outfall) was judged to be the primary cause of toxicity in water from that site.
The input adversely affected the mainstem for at least 13.8 km (8.6 miles) downstream
(discussed in more detail under "Mainstem" above).
Elevated levels of three related chemicals (parachlorometa cresol, phenol, phenolics recoverable)
at station 2a reflected an origin in Ciudad Juarez. These contaminants may emanate from the
distillation and chemical treatment of coal tar or wood tar, or from gas works, coke ovens, oil
refineries, chemical plants, livestock dips, or human and animal refuse (McKee and Wolf,
1963). Although these chemicals may have had minor involvement in toxic effects of water
from the site, the Rio Grande/Rfo Bravo appeared unaffected, as none exceeded screening levels
anywhere in the mainstem.
Exceedance of the national 85th percentile by chloroform at station la probably was of no
ecological consequence, as the concentration was well below human health and chronic aquatic
life criteria. No elevated chloroform concentrations were observed anywhere in the mainstem.
Chloroform is used as an anesthetic, counterirritant, solvent, cleansing agent, and antiseptic
(McKee and Wolf, 1963).
Elevated levels of antimony and thallium at station lOa, (Manadas Creek), did not appear
ecologically detrimental. Significant adverse effects occurred in toxicity testing of water from
the site, but were attributed to total dissolved solids, as antimony and thallium levels were well
below aquatic life criteria. Both metals exceeded human health criteria, indicating a potential
human health hazard if untreated water and/or fish from the creek were consumed on a regular,
long-term basis. Contributions by Manadas Creek inflow did not noticeably affect the Rio
Grande/Rfo Bravo, as neither metal exceeded screening levels anywhere in the mainstem. The
probable source was a nonferrous metals smelter/refinery owned by Anzon, Inc., located in the
upper portion of the Manadas Creek watershed. The operation was under enforcement action
in 1991 for an unauthorized stormwater discharge of antimony to Manadas Creek (TNRCC,
1992a).
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Arsenic levels at stations 3a (Rfo Conchos) and 3b (Alamito Creek), discussed in part under
"Mainstem" above, exceeded the national 85th percentile but were well below aquatic life
criteria. No adverse effects were observed in associated water toxicity tests, and the fishery
evaluation at 3a reflected relatively high community integrity.
Arsenic concentrations at stations la, 2a, 9a, 1 Ic, and 15a were not elevated compared to levels
observed elsewhere. There probably were no effects on aquatic life, and the amount of arsenic
contributed to the mainstem would have been negligible. However, because flow in these
tributaries is dominated by domestic effluent, their waters are considered nonpotable. Therefore,
the stringent national human health criterion based on consumption of fish, which takes into
account the carcinogenicity of arsenic, was applicable. The arsenic level at each of these five
sites exceeded the criterion, indicating a possible human health hazard if fish from these systems
were regularly consumed on a long-term basis. However, potential risks appear minimal,
because conventional water quality is not conducive for support of viable fish populations.
The chromium concentration at station 12a, (Rfo Salado), was greater than the chronic aquatic
life criterion. Whether or not an actual exceedance occurred is unknown, because the amount
of chromium present in the hexavalent state was not determined. There were no significant
effects in toxicity testing of water from the site. Chromium in surface waters generally is
derived from industrial effluent or cooling system discharges (McKee and Wolf, 1963).
Mercury levels at stations 2a (Ciudad Juarez sewage discharge canal) and 15a (Anhelo Drain)
exceeded the applicable human health criterion. Hows in these tributaries were dominated by
domestic effluent from Ciudad Juarez and Reynosa, respectively. Elemental mercury is used
in scientific and electrical instruments, dentistry, power generation, solders, and the manufacture
of lamps. Mercuric salts are used commercially and industrially as medicinal products,
disinfectants, detonators, pigments, and in photoengraving. Mercury contamination of surface
waters usually results from the disposal of wastes from these types of operations (McKee and
Wolf, 1963).
Whereas the observed mercury concentrations would not be expected to adversely affect aquatic
life, a human health hazard could exist if fish from these systems were consumed on a regular,
long-term basis. However, degraded conventional water quality probably precludes the existence
of viable fish populations in these tributaries (verified for Anhelo Drain through collecting
efforts). Mercury contributions from Anhelo Drain probably have little effect on the mainstem,
due to the small inflow volume (0.45 cms or 16 cfs). Inflow from the Ciudad Ju&ez sewage
discharge canal, on the other hand, was substantial (1.7 cms or 61 cfs), and human health
hazards resulting from associated mercury inputs could extend for some distance downstream
in the Rio Grande/Rfo Bravo.
Selenium exceeded various screening levels at stations 5a, 9b, Ha, He, 12a, 12b, and 12c. A
degree of geographical clustering was apparent, as was discussed under "Mainstem" above.
Levels at all of these sites were greater than the chronic aquatic life criterion, reflecting a
potential for minor deleterious effects on resident aquatic life. However, only stations lla and
lie exhibited significant adverse effects in toxicity testing of water, and the role of selenium
appeared to be slight or negligible in those two instances. Based on exceedances of the human
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health criterion, a potential human health hazard was indicated for the Rfo Escondido, Rfo
Salado, Rfo Alamo, and Rfo San Juan, if untreated water and/or fish from those streams were
consumed on a regular, long-term basis. Selenium inputs from the seven tributaries did not
appear to appreciably affect the Rio Grande/Rfo Bravo, as only one mainstem station (11)
exhibited a screening level exceedance.
Silver concentrations at stations 7b, 8a, 8b, and 8e were greater than acute and chronic aquatic
life criteria. It is not certain that these were actual exceedances, because the criteria are based
on the free ion form, while the data represent total dissolved silver. As for selenium, the sites
were clustered, as all four tributaries enter between Del Rio/Ciudad Acuna and Eagle
Pass/Piedras Negras. However, a factor other than geography may have been responsible in this
case, as the silver concentrations that were observed may have resulted from procedural
contamination. The four sites were sampled during the third field survey, during which silver
was detected in the field blank (see Appendix C).
In the water toxicity tests, station 7b (San Felipe Creek) was the only one of the four sites for
which significant effects were observed. The associated silver concentration was considerably
higher than anywhere else in the study, and evidently was the primary causative factor. Effects
of silver inputs from these tributaries on the Rio Grande/Rfo Bravo appeared negligible, as an
excessive concentration was observed at only one geographically removed mainstem site (station
12).
Elevated diazinon levels at stations lla (Zacate Creek) and 12d (Arroyo Los Olmos) were
considered primarily responsible for adverse effects in toxicity testing of water from those sites,
and probably were involved in depressed fish community integrity observed at the latter site.
Resident aquatic communities in Zacate Creek probably were also adversely affected. Flow in
both tributaries was minimal (Table 10), and effects of inputs on the Rio Grande/Rfo Bravo
probably were negligible, as diazinon was not detected anywhere in the mainstem. Probable
sources were urban runoff from Laredo and Rio Grande City, respectively.
Phthalate esters occurred at excessive levels in two effluent-dominated tributaries. The bis(2-
ethylhexyl) phthalate concentration at station lie (Arroyo el Coyote), which exceeded the
chronic aquatic life criterion, was considered partially responsible for adverse effects in toxicity
testing of water from the site. Diethyl phthalate exceeded the chronic aquatic life criterion at
station ISa (Anhelo Drain). It may have been the main cause of toxicity in the water sample,
and could have been partially responsible for the apparent absence of fish from the drain. Flow
in both tributaries was minimal (Table 10), and effects of inputs of these phthalate esters on the
Rio Grande/Rfo Bravo appeared negligible, as neither was detected in water at any mainstem
site.
The elevated cyanide concentration at station 12d (Arroyo Los Olmos) may have been marginally
involved in toxic effects of water from the site, and in reduced integrity of the local fish
community. The flow volume was extremely small (0.02 cms or 0.8 cfs), and effects on the
mainstem appeared negligible, as cyanide was not detected in water at the first Rio Grande/Rfo
Bravo site downstream (station 13).
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Toxic Chemicals in Sediment
Sediment samples from all 45 stations were analyzed for toxic chemicals. Valid analytical
results were obtained for 145 toxic chemicals, thirty of which occurred at detectable levels
(Tables 11 and 13), and 16 of which exceeded screening levels (Table 14). These, together with
the number of sites involved, were: methylene chloride (6); toluene (3); arsenic (8);
chromium (27); copper (2); lead (1); mercury (2); nickel (29); selenium (1); silver (1);
zinc (2); chlordane (4); DDE (3); dieldrin (1); bis(2-ethylhexyl) phthalate (1); and di-n-butyl
phthalate (1). Two additional chemicals for which no screening levels exist occurred at
anomalously high concentrations: parachlorometa cresol (2); and phenol (1).
Mainstem
U.S. data indicates that the number of toxic chemicals detected was relatively uniform, ranging
from 11 at stations 4, 5, 14,15, and 18, to 17 at station 12 (Table 13). There were 48 instances
where screening levels were exceeded, involving eight toxic chemicals (Table 14): methylene
chloride, threshold value, stations 1, 2, 3, and 12; toluene, threshold value, stations 2 and 12;
arsenic, threshold value, stations 2, 3, 4, 5, 5b, and 14; chromium, threshold value, stations
1, 2, 3, 4, 5, 5b, 6, 7, 8, 11, 13, 14, 15, 16, 17, and 18; copper, threshold value, station 2;
lead, threshold value, station 2; mercury, threshold value, station 2; and nickel, threshold
value, stations 1, 2, 3, 4, 5, 5b, 6, 7, 8, 11, 12, 13, 14, 15, 16, 17, and 18. The number of
toxic chemicals that exceeded screening levels, by station, were: eight, station 2; four, station
3; three, stations 1, 3, 5, 5b, 12, and 14; two, stations 6, 7, 8, 11, 13, 15, 16, 17, and 18;
zero, stations 9 and 10.
Methylene chloride, an organic solvent, exceeded aquatic life threshold values at four sites.
There was no discernible impact at stations 1 and 3. No significant effects occurred in sediment
eluate toxicity tests, and local fish and macrobenthic communities did not reflect appreciable
impacts by toxic chemicals. A possibility of slight impact was evident for station 2. Although
there were no effects in the sediment eluate toxicity test, the concentration was the highest
observed in the mainstem, and macrobenthic community integrity was relatively low. The
concentration at station 12 was regarded as a possible contributing factor to sediment eluate
toxicity and the impaired condition of local fish and macrobenthic communities. There were no
obvious sources of methylene chloride in the vicinity.
Toluene was elevated at stations 2 and 12. In both cases, implications for impact were the same
as for methylene chloride. Toluene is a constituent of coal tar, and is used in the manufacture
of organic materials and as a solvent in the extraction of various substances from plants (McKee
and Wolf, 1963). There was no obvious source of toluene upstream from station 2. For station
12, inflows from Chacon Creek (station lib) and Arroyo el Coyote (station lie) were probable
contributors. Chacon Creek had the highest toluene concentration in water in the study (9.0
/ig/L), probably derived from urban runoff from Laredo. Arroyo el Coyote, which transports
domestic effluent from Nuevo Laredo, had the third highest toluene concentration in water in
the study (6.0 j*g/L), and the highest concentration in sediment (33,000 /xg/kg).
27
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Arsenic levels exceeded threshold values at five successive sites from downstream of El
Paso/Ciudad Juarez to the mouth of Lozier Canyon (stations 2, 3, 4, 5, and Sb), then disjunctly
at station 14 (downstream from International Anzalduas Dam). However, associated ecological
effects were minimal. There were no significant effects in corresponding sediment eluate
toxicity tests, and fish and macrobenthic community characteristics reflected little or no potential
that toxic impacts were being exerted at stations 4, 5, and 14. A moderate potential for toxic
impact was indicated by the macrobenthic community at station 2, but arsenic did not appear to
be involved, as the amount by which the threshold value was exceeded was no greater than for
several unimpacted sites. The arsenic concentration at station 3, which exceeded the threshold
value by the greatest relative amount in the study, may have contributed to less-than-optimal
macrobenthic integrity at that site.
Arsenic concentrations from station 2 to station Sb appear to be naturally derived to some extent.
However, several inputs to the reach were evident. Arsenic concentrations were elevated in
water in inflows from the El Paso Public Service Board Haskell R. Street wastewater treatment
plant (la), the Ciudad Juarez sewage discharge canal (2a), the Rio Conchos (3a), and Alamito
Creek (3b), while high levels in sediment were documented for the Rfo Conchos (3a) and
Terlingua Creek (Sa). For further information on arsenic inputs to the reach, see previous
discussions for water.
Regarding station 14, Los Olmos Creek (12d) was shown to be a contributor to that reach, as
it exhibited the highest arsenic concentration in sediment in the study. However, in light of the
small volume of inflow, its role may not be significant.
Chromium exceeded threshold values at all mainstem stations except 9, 10, and 12. No
substantial impacts were apparent, as none of the 16 sites exhibited significant effects in
sediment eluate toxicity tests.
The prevalence of elevated concentrations was in sharp contrast to results for water, in which
there were no screening level exceedances in the mainstem. This, together with the lack of
appreciable impact by chromium in sediment, suggests that most of the chromium present in the
Rio Grande/Rio Bravo is in highly insoluble form, such as hydroxide or carbonate salts, and
therefore is biologically unavailable. In effect, then, the USEPA threshold value may be overly
stringent for this system.
Regarding inputs, 14 of the 26 tributaries had comparatively high chromium concentrations in
sediment. The most noteworthy were those recorded for stations la, 7a, lie, 12a, 12b, and
12d, where associated concentrations ranged from 12.9 to 45.1 mg/kg. Station 12a also
exhibited an anomalously high chromium concentration in water (15 /ig/L).
Potential industrial sources of chromium were addressed in the preceding discussion for water.
For the Rio Grande/Rio Bravo system, there is evidence that chromium levels in sediment may
be naturally elevated due to geological characteristics of the watershed, with chromium entering
the river via weathering of volcanic rock, soil erosion, and runoff from tailings from past mining
activities (Irwin, 1989).
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The elevated copper concentration at station 2 did not appear to be causing substantial impact,
as there were no significant effects in the sediment eluate toxicity tests. It is possible, though,
that it may have been partially responsible for the somewhat degraded condition of the
macrobenthic community.
Copper is one of the most common contaminants in urban runoff. Other sources include soil
erosion, corrosion of pipes and tubes, and industrial and sewage treatment plant discharges
(Irwin, 1989). Urban runoff from El Paso/Ciudad Juarez may have been involved, but the
principal source of copper at station 2 appeared to be the El Paso Public Service Board Haskell
R. Street wastewater treatment plant effluent (station la), which exhibited the highest copper
concentrations in the study. The level in water (8.8 pg/L) was more than twice the next-to-
highest value recorded, while the concentration in sludge (292 mg/kg) exceeded the second
highest concentration observed by an order of magnitude.
Lead exceeded the aquatic life threshold value at station 2; the implications for impact were the
same as for copper. Lead is introduced to surface waters in effluents from various types of
industries, sewage treatment plants, and mining operations, from dissolution of lead pipe, and
in urban runoff (McKee and Wolf, 1963). Effluent from the El Paso Public Service Board
Haskell R. Street wastewater treatment plant (station la) evidently was a primary contributor of
lead to station 2, as associated levels in water (2.8 /tg/L) and sludge (80.6 mg/kg) were the
highest in the study. Urban runoff from El Paso/Ciudad Judrez may also have been involved.
Mercury was another metal that was elevated at station 2. Potential effects were similar to those
for copper. As for the previous two metals, effluent from the El Paso Public Service Board
Haskell R. Street wastewater treatment plant appeared to be the major contributor. The
concentration in water was below detection, but the concentration in sludge (1.51 mg/kg) was
by far the highest in the study.
With regard to implications of copper, lead, and mercury concentrations in the El Paso Public
Service Board Haskell R. Street wastewater treatment plant "sediment" sample, it is important
to note that those results were based on a sample of sludge which had been removed from the
system. Whereas the observed characteristics are not directly relatable to conditions in the river,
they do reflect contaminant concentrations associated with the suspended solids fraction of the
effluent, which do have a potential for exerting instream impact.
Nickel exceeded aquatic life threshold values at all mainstem stations except 9 and 10. The
greatest margins of exceedance occurred in the reach from station 1 to station 5. Impacts
appeared relatively minor, as no significant effects were observed in sediment eluate toxicity
tests for 16 of the 17 affected sites. At the only site where toxic effects were seen, station 12,
nickel did not appear to be a contributing factor, as the margin by which the threshold value was
exceeded was much less than at many sites where no toxicity occurred.
The lack of substantial toxic effects, together with the fact that screening levels were not
exceeded in water, suggests that nickel is tightly bound in the sediments and biologically
unavailable. As for chromium, then, the USEPA threshold value may be overly stringent for
die Rio Grande/Rfo Bravo system.
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Regarding inputs, nickel concentrations in sediment either exceeded threshold values or were
over 10 rag/kg at 16 of the 26 tributary sites. The most noteworthy, eight tributaries which
exhibited concentrations from 10.6 to 18.9 mg/kg, were distributed throughout the longitudinal
gradient. As in the mainstem, nickel concentrations in water did not exceed screening levels at
any tributary site.
Nickel has a variety of industrial applications, metal-plating processes being one of the more
prominent, and may enter surface waters in industrial or municipal wastewater effluents (McKee
and Wolf, 1963). Indications are, however, that much of the nickel in the Rio Grande/Rio
Bravo system may be naturally derived. Igneous rock and associated ores and minerals typically
contain an abundance of nickel (Hem, 1970), and igneous outcrops are common in the
watershed. Dissolution of these formations, erosion of associated soils, and runoff from tailings
from past mining activities may be the principal contributor of nickel to the Rio Grande/Rfo
Bravo. That the source may be largely natural is supported by the fact that the highest
concentration in the study occurred in a remote, relatively unimpacted tributary, Terlingua Creek
(station 5a).
Based based on U.S. data the number of toxic chemicals detected ranged from nine at stations
6a and 8a to 19 at station la. More than 12 toxic chemicals were detected at 11 of the 26 sites
(Table 13).
There were 44 instances where screening levels were exceeded, involving 15 toxic chemicals
(Table 14): methylene chloride, threshold value, stations 2a and 3b; toluene, threshold value,
station 2a; arsenic, threshold value, stations 3a and Sa; chromium, threshold value, stations
2a, 3a, 3b, 5a, 7a, 8c, 8e, lOa, lib, 12a, and 12b; copper, national 85th percentile, station la;
mercury, national 85th percentile, station la; nickel, threshold value, stations 2a, 3a, 5a, 8c,
8e, lOa, lla, lib, 12a, 12b, 12c, and 12e; selenium, national 85th percentile, station la;
silver, national 85th percentile, station la; zinc, national 85th percentile, stations la and lie;
chlordane, threshold value, stations lla and 15a; threshold value and national 85th percentile,
stations lib and lie; DDE, national 85th percentile, stations lOa, lla, and 12d; dieldrin,
national 85thpercentile, station lla; bis(2-ethylhexyl)phthalate, national 85thpercentile, station
lie; and di-n-butyl phthalate, national 85th percentile, station 12d. The number of toxic
chemicals that exceeded screening levels, by station, were: five, station la; four, stations 2a
and lla; three, stations 3a, 5a, lOa, lib, and lie; two, stations 3b, 8c, 8e, 12a, 12b, and 12d;
one, stations 7a, 12c, 12e, and 15a; zero, stations 6a, 6b, 7b, 8a, 8b, 8d, 9a, and 9b. Two
other chemicals for which screening levels do not exist occurred at comparatively high
concentrations: parachlorometa cresol, stations la and lie; and phenol, station la.
The methylene chloride concentration at station 2a (Ciudad Jufrez sewage discharge canal) was
the highest in the study, while that at station 3b (Alamito Creek) was comparatively low (Table
10). Impacts were imperceptible in both cases, as no significant effects occurred in the sediment
eluate toxicity tests (Tables 16 and 17). Regarding effects on the Rio Grande/Rfo Bravo, inputs
associated with 2a may have been partially responsible for the elevated concentration at
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mainstem station 3. Although the intervening distance is considerable (385 km), the substantial
volume of inflow (1.7 cms or 61 cfs) makes this a possibility. Inputs associated with 3b
appeared negligible. The inflow volume was small (0.03 cms or 1.1 cfs), and the concentration
at mainstem station 4, located 0.6 km (0.4 miles) downstream, was below detection. Sewage
wastes emanating from Ciudad Judrez were the probable source of methylene chloride at station
2a. No potential sources are known for Alamito Creek, a remote, minimally-impacted stream.
The potential for impact and probable origin of toluene at station 2a were the same as for
methylene chloride. Resultant inputs again appeared to affect the Rio Grande/Rfo Bravo.
Although the concentration at the first downstream site, station 3, was below the screening level,
it represented one of only three instances where toluene was detected in the mainstem.
Regarding elevated arsenic levels at stations 3a (Rfo Conchos) and Sa (Terlingua Creek),
potential effects on the mainstem and possible origins were addressed in previous discussions.
The observed concentrations did not appear to be impacting the tributaries themselves, as no
significant effects occurred in the sediment eluate toxicity tests. In addition, fish community
integrity at station 3a was relatively high (fish from the site did, however, exhibit a slightly
elevated incidence of physical abnormalities).
Elevated levels of copper, mercury, selenium, silver, and phenol occurred at a single tributary
site, station la (El Paso Public Service Board Haskell R. Street wastewater treatment plant
effluent). Selenium, silver, and phenol were regarded as potential cofactors in the manifestation
of sediment eluate toxicity (along with parachlorometa cresol, discussed below). The discharge
was at least partially responsible for elevated levels of copper and mercury at station 2, the only
mainstem site where these two metals exceeded screening levels. Selenium, silver, and phenol
were not excessive at station 2. Additional information for these five chemicals is presented in
previous discussions. High levels of a variety of toxic chemicals in the El Paso Public Service
Board Haskell R. Street wastewater treatment plant sludge sample probably result from industrial
discharges to the collection system.
Zinc and parachlorometa cresol were the two other chemicals that were elevated at station la,
and the preceding discussion also applies here. Inputs to the mainstem were not apparent in data
from station 2. The factor that distinguished them from the previous five chemicals was that
they were also excessive at an additional station, 1 Ic (Arroyo el Coyote). At that site, the zinc
concentration was the second highest, and the parachlorometa cresol concentration the highest,
in the study. Impact by zinc appeared negligible, but parachlorometa cresol was considered a
possible cofactor in the manifestation of sediment eluate toxicity. Although the inflow volume
was small (0.05 cms or 1.8 cfs), contributions to the Rio Grande/Rfo Bravo were discernible,
as station 12 exhibited the highest zinc concentration of any mainstem site, and was one of only
two mainstem stations where parachlorometa cresol was detected.
Chromium exceeded aquatic life threshold values at 11 sites, and nickel at 12 sites, but no
impacts were evident. Sediment eluate toxicity was observed for one of these stations, 3a (Rfo
Conchos), but neither metal appeared to be a major factor. Effects of tributary inputs on the
Rio Grande/Rfo Bravo, potential sources, and other relevant information, were discussed under
"Mainstem".
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Chlordane was detected at five sites. A substantial concentration occurred at station la (El Paso
Public Service Board Haskell R. Street wastewater treatment plant effluent), although it did not
exceed the aquatic life threshold value. Threshold values were exceeded at the other four sites
(lla - Zacate Creek; lib - Chacon Creek; lie - Arroyo el Coyote; 15a - Anhelo Drain).
Stations lla and lib evidently were not adversely affected, as no significant effects occurred
in sediment eluate toxicity tests. The opposite was true for stations 1 Ic and ISa, and chlordane
appeared to be a primary factor. It may also have been partially responsible for the apparent
absence of fish from Anhelo Drain. Likely sources of chlordane were urban runoff from Laredo
(lla, lib), and a combination of urban runoff and domestic effluent from Nuevo Laredo (lie)
and Reynosa (15a). None of the tributaries appeared to affect the Rio Grande/Rio Bravo, as
chlordane was below detection at all mainstem sites.
DDE levels in excess of the national 85th percentile at stations lOa (Manadas Creek), lla
(Zacate Creek), and 12d (Arroyo Los Olmos) did not appear to have adverse impacts, as no
significant effects were observed in the sediment eluate toxicity tests. Contributions to the Rio
Grande/Rib Bravo were imperceptible, as no elevated DDE concentrations occurred anywhere
in the mainstem.
Dieldrin exceeded the national 85th percentile by a slight margin at station lla (Zacate Creek).
Implications for local impact and effects on the mainstem were the same as for DDE.
Two phthalate esters occurred at elevated concentrations: bis(2-ethylhexyl) phthalate at station
lie (Arroyo el Coyote), and di-n-butyl phthalate at station 12d (Arroyo Los Olmos). National
85th percentiles were exceeded by factors of 7.3X and 6.9X, respectively. Bis(2-ethylhexyl)
phthalate was considered a potential cofactor in the manifestation of sediment eluate toxicity at
station lie. No such effects were evident for di-n-butyl phthalate at station 12d. Inputs to the
Rio Grande/Rfo Bravo appeared negligible, as neither chemical exceeded screening levels in the
mainstem.
Toxic Chemicals in Fish Tissue
Fish tissue samples from the 18 major mainstem sites and six tributaries were analyzed for toxic
chemicals (Table 12). In all, 94 tissue samples were collected, including 45 fillet samples and
49 whole fish samples. Of the 140 toxic chemicals for which valid analytical results were
obtained, 29 occurred above detection limits, including 16 organic chemicals and 13 inorganic
chemicals (Table 13). Twelve toxic chemicals exceeded screening levels (Table 14). These,
together with the number of sites involved, were: cadmium (1); chromium (6); copper (20);
lead (2); mercury (17); selenium (23); zinc (14); chlordane (1); total DDT (5); dieldrin (2);
gamma-bhc (lindane) (1); and total PCB's (6).
Copper, selenium, and zinc were above detection limits in all U.S. samples, and p,p' DDE was
detected in most samples (Table 13). Aluminum, arsenic, cadmium, chromium, mercury, and
nickel were detected in more than half of the samples. Methylene chloride, lead, and thallium
were occasionally detected. Chlordane, p,p' DDD, arochlor 1248, arochlor 1254, cyanide, and
silver were infrequently detected. The remaining chemicals rarely occurred at detectable levels.
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Several organic chemicals for which no screening criteria are available were detected (Table 13).
Possible sources include field or laboratory contamination of samples, chlorinated municipal
effluent, and industrial effluent.
Methylene chloride was detected in 27 samples. It has been documented as a field contaminant
of water and sediment samples (USEPA, 1982), and has been identified as a possible laboratory
contaminant by TDK staff. It is commonly used as an organic solvent, and is a constituent of
chemical paint strippers. Methylene chloride was detected in samples from stations 6, 6a, 6b,
7, 8, 8d, 9, 9b, 10, 11, 13, 14, 15,17, and 18. It was not detected in water or sediment at any
of these sites. No municipal or industrial effluent influences are present at a number of these
sites, and it is likely that most of the methylene chloride detected in tissue was an artifact of
laboratory contamination.
Toluene was detected in three samples collected downstream from Laredo/Nuevo Laredo (station
12) and downstream from Hidalgo/Reynosa (station 16). At station 12, the concentration in
sediment exceeded the aquatic life threshold value. However, toluene was not detected in water
or sediment at station 16. Toluene is a solvent associated with industrial effluent, is a
component of petroleum products, and can be a field or laboratory contaminant (USDOE, 1993).
1,2-dichlorobenzene was detected in four samples. It can occur as a result of mixing of
chlorinated effluent and organic compounds, such as benzene, in the water column (USEPA,
1982; Joel Lusk, U.S. Fish and Wildlife Service, personal communication). This contaminant
was detected only at station 2 (downstream from El Paso/Ciudad Juarez), in whole body and
fillet samples of carp and channel catfish. Although 1,2-dichlorobenzene was not detected in
water or sediment from that site, it was present in water and sludge from station la, the El Paso
Public Service Board Haskell R. Street wastewater treatment plant outfall, located 13.8 km (8.6
miles) upstream. Thus, the discharge from that facility may contribute to body burdens at
station 2.
The remaining parameters were detected in only one or two samples, including chloroform
downstream from Hidalgo/Reynosa (station 16); trichlorofluoromethane downstream from
International Anzalduas Dam (station 14) and downstream from Brownsville/Matamoros (station
18); and 1,1,1-trichloroethane at the mouth of Santa Elena Canyon (station 5). These chemicals
are potentially associated with the combination of chlorinated municipal effluent and organic
compounds in water, and stations 16 and 18 are downstream of cities. None of them were
detected in water or sediment from the sites listed. Also, they are possible field or laboratory
contaminants, so their presence may not reflect actual concentrations in fish tissues (USEPA,
1982; USDOE, 1993).
Hexachlorobenzene was detected in one sample collected downstream from Laredo/Nuevo
Laredo (station 12). It is used as a pesticide, and also occurs as a breakdown product of and
impurity in other pesticides. Additionally, it may be generated as a byproduct during the
chlorination of wastewater (USEPA, 1992; Cain, 1993). It was not detected in water or
sediment at station 12.
The last of the organic chemicals for which no screening levels exist, bis(2-ethylhexyl) phthalate,
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was measured at 99 mg/kg (99 times the detection limit) in a sample from station 4 (downstream
from Presidio/Ojinaga). It generally is derived from industrial effluent, but may also be a field
or laboratory contaminant associated with plastic (USDOE, 1993; Verschueren, 1983). The fact
that it was not detected in water or sediment from station 4 suggests that sample contamination
may have been responsible for the occurrence.
Edible Fish Tissue
Data from fillet samples were evaluated for potential human health risks using USFDA action
or tolerance levels, USEPA screening levels, and TDK risk assessment values (Table 7). No
USFDA action or tolerance levels were exceeded. However, there were a number of instances
where USEPA fish tissue values were exceeded, and one instance where the TDK risk
assessment value for selenium was exceeded (Table 14).
Total DDT and total PCB concentrations exceeded USEPA fish tissue values in 11.1% and
13.3% of the fillet samples, respectively. Because the detection limit for PCB's (0.04 mg/kg)
is greater than the USEPA value (0.01 mg/kg), the number of samples shown to exceed the
screening level may be conservative. Mercury and dieldrin screening values were exceeded
twice, and chlordane and selenium screening levels once. Possible sources of these contaminants
include irrigation return flows/agricultural runoff (DDT, dieldrin, selenium), nonpoint sources
in urban areas (chlordane, PCB's), and nonpoint sources related to previous land use, including
mining, coal-fired power plants, and waste disposal sites (mercury, selenium, PCB's).
Elevated pesticide and PCB concentrations were noted only in blue catfish, channel catfish, and
carp. As these chemicals are lipophilic, they are more likely to bioaccumulate in fish with
higher lipid content, such as carp and catfish (Kanazawa, 1981; Irwin, 1988; Inmon et «/.,
1993). Metals, however, were elevated in largemouth bass and white bass at a limited number
of sites.
Dieldrin exceeded the USEPA fish tissue value upstream from Del Rio/Ciudad Acuna (station
7) and in San Felipe Creek (station 7b). Although not produced in or imported to the U.S. since
198S, dieldrin continues to enter aquatic systems in agricultural runoff (USEPA, 1992). Total
PCB's also exceeded the USEPA value at these two sites, as well as further downstream, below
Del Rio/Ciudad Acuna (station 8), and upstream and downstream from Eagle Pass/Piedras
Negras (stations 9 and 10). PCB's have been used extensively as lubricants, insulators, and
coolants, and occur in the environment throughout the U.S. (Eisler, 1986a; USEPA, 1992).
Neither chemical was detected in water or sediment from these sites.
Total DDT exceeded the USEPA fish tissue value upstream from the Rio Conchos confluence
(station 3), in the Rfo Conchos (station 3a), upstream from Del Rio/Ciudad Acuna (station 7),
and in San Felipe Creek (station 7b), then in the lower reaches of the mainstem, downstream
from International Anzalduas Dam (station 14), downstream from Hidalgo/Reynosa (station 16),
and upstream from Brownsville/Matamoros (station 17). DDT or its metabolites, DDE and
DDD, were detected in sediment samples from stations 2a and 3a. Large volumes of irrigation
return flow which enter the river via the Rfo Conchos and downstream from International
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Anzalduas Dam are the probable source of pesticide residues in fish tissue (Gamble et al., 1988;
Irwin, 1989; TNRCC, 1992a; USEPA, 1992).
Mercury exceeded the USEPA fish tissue value in the Rio Grande/Rib Bravo upstream and
downstream from Hildago/Reynosa (stations IS and 16). It was detected in water and sediment
from Anhelo Drain (station 15a), with the concentration in water exceeding human health
criteria. Mercury input from that tributary may contribute to tissue residues at stations 15 and
16.
Selenium exceeded the TDK risk assessment value only at station 6, near Langtry/San Ignacio.
The concentration in sediment also was substantial (2.23 mg/kg); although screening levels were
not exceeded, the value was greater than 1-2 mg/kg, the background level for aquatic sediments
identified by Eisler (1985) and Lemly (1985). Chlordane also exceeded the screening level
(USEPA fish tissue value) at one site, downstream from Laredo/Nuevo Laredo (station 12).
Chlordane was detected in sediment from several proximal tributaries (stations lla, lib, and
lie), and associated inputs probably are at least partially responsible for elevated Chlordane
levels in fish tissue at station 12.
San Felipe Creek (station 7b) and the Rfo Conchos (station 3a) were the only tributaries where
contaminant concentrations in fish tissue exceeded human health screening levels. Contaminant
levels in fish from the El Paso/Ciudad Juarez and Big Bend areas did not exceed human health
criteria, nor did residues in fish collected upstream from Laredo/Nuevo Laredo, upstream from
International Anzalduas Dam, and in the Brownsville/Matamoros area.
The mainstem upstream from Del Rio/Ciudad Acuna (station 7) had three contaminants in edible
fish tissue that exceeded human health screening levels (total DDT, total PCB's, dieldrin)
(Figure 2). Fish from stations 3, 3a, 6, 7b, 8, 9, 10, 12, 14, 15, and 16 contained one or two
contaminants that exceeded human health screening values. No exceedances occurred at the
remaining sites.
Whole Fish Tissue
Two types of screening were used to evaluate data from whole fish tissue samples.
Body Burdens.—This evaluation phase utilized screening levels derived by USFWS (national
85th percentiles and national geometric means), USEPA (national means), and TNRCC (state
85th percentiles). Information from supplemental sources also was used. All screening values
employed, and the sources from which they were adopted, are presented in Table 7.
Appropriate screening values were not available for aluminum, nickel, silver, thallium, or
cyanide.
Zinc exceeded the USFWS national 85th percentile in 14 instances (Table 14). Only whole carp
samples were involved. Zinc is a component of fish scales, and fish with large scales typically
contain substantial concentrations (Joel Lusk, USFWS, personal communication). Since zinc
35
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was elevated only in samples where large scales were analyzed, the observed concentrations
probably do not represent abnormally high levels.
Copper exceeded the USFWS national 85th percentile at most mainstem sites, and in all
tributaries except the Rfo Conchos (Table 14). Copper generally was above detection limits in
sediment, and in water was detected at nine mainstem sites and 11 tributaries. Copper is
associated with mining, plumbing, and electrical industries (Phillips and Russo, 1978; Moore
and Ramamoorthy, 1984; USEPA, 1985b). As many of the tributaries are dominated by
springflow, the copper levels in fish tissue may be a result of naturally elevated concentrations
in water and soil.
Selenium exceeded the USFWS national 85th percentile in two reaches of the river. The first
was from upstream of the Rfo Conchos confluence (station 3) to upstream of Del Rio/Ciudad
Acuna (station 7), including three tributaries, the Rfo Conchos, Pecos, and Devils rivers (stations
3a, 6a, 6b). Concentrations again were high at sites upstream from Laredo/Nuevo Laredo,
upstream from International Anzalduas Dam, and upstream from Hidalgo/Reynosa (stations 11,
13, 15). Levels were highest in the mainstem from station 3 to station 7, and in the Rio
Conchos.
Selenium concentrations in sediment exceeded 2 mg/kg at stations la, 5, 6, 10, lie, 12, and
12d. Selenium was detected at lower concentrations at stations la, 5, 5a, 5b, 6a, 11, 12a, 12b,
and 12c. In addition, concentrations in water exceeded the aquatic life chronic criterion at
stations 5a and 11. These occurrences generally coincided with areas where selenium was
elevated in fish tissue.
In rural areas, especially in the arid western U.S., the most likely source of selenium is
irrigation return flows or runoff from agricultural land (Phillips and Russo, 1978; Presser and
Barnes, 1985; CWRCB, 1988; TNRCC, 1992a). In urban areas, coal-fired power plants may
contribute selenium, via air deposition of fly ash, or return of cooling waters associated with fly
or bottom ash (Phillips and Russo, 1978; EPRI, 1986; Maier et
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Lead exceeded the USFWS national 85th percentile upstream from Eagle Pass/Piedras Negras
(station 9). Lead is a cumulative toxin, affecting growth, reproduction, development,
metabolism, and behavior in multiple species. It is associated with atmospheric deposition from
automobile exhausts and smelter stacks, discarded batteries and paints, and metal alloys (Phillips
and Russo, 1978; Moore and Ramamoorthy, 1984; Irwin, 1988). It was not detected in water
at the site, and was not elevated in sediment. Potential sources in the vicinity were not apparent.
Cadmium exceeded the USFWS national 85th percentile upstream from El Paso/Ciudad Ju&ez
(station 1). Cadmium is associated with lead and zinc deposits, smelters, fossil fuels, and
industrial wastes (Eaton, 1974; Brown and Lemay, 1977). It was not detected in water and was
not elevated in sediment from the site. No potential sources were evident.
Aluminum concentrations generally were above detection limits (Table 13), and ranged from 1.9
to 797 mg/kg. Aluminum levels in fish are strongly influenced by the consumption of soil by
bottom feeders, such as carp and catfish (Brumbaugh and Kane, 1985; Joel Lusk, USFWS,
personal communication). Predatory species, such as largemouth bass, smallmouth bass, and
white bass, never contained more than 3.2 mg/kg in this study. There was no apparent
relationship between aluminum concentrations in sediment and whole fish. The concentrations
that were observed in tissue did not appear to indicate environmental contamination.
Nickel concentrations ranged from 0.079 to 6.93 mg/kg, and were maximal (0.38 to 6.93
mg/kg) at stations 1, 2, 13, and 16. However, levels in other fish samples from the same sites
were substantially lower. Intermediate concentrations (0.142 to 0.249 mg/kg) occurred in fish
from stations 2, 3, 5, 6a, 6b, 7, 8, 8d, 9b, 11, 13, 14, and 15. Low concentrations (0.079 to
0.126 mg/kg) were found in fish from stations 1,4, 6, 6a, 7, 7b, 8, 8d, 13,14,15, 16, and 18.
Although nickel concentrations in sediment exceeded aquatic life threshold values at most sites
(Table 14), there was no apparent correlation with levels observed in fish tissue. Potential
sources of nickel include metal plating, alloys, and coal combustion (Phillips and Russo, 1978).
Silver was infrequently detected in whole fish (Table 13), with concentrations ranging from
0.035 to 0.088 mg/kg. A degree of clustering was evident, with detectable levels occurring
upstream and downstream from Eagle Pass/Piedras Negras, in the Rfo Escondido, and upstream
and downstream from Laredo/Nuevo Laredo (stations 9, 9b, 10, 11, 12). Additionally, both
samples collected downstream from Hidalgo/Reynosa (station 16) contained detectable silver
concentrations.
Although silver concentrations in water and sediment did not exceed screening levels at stations
9, 9b, 10, 11, or 16, the concentration in water at station 12 was high enough to potentially
exceed acute and chronic aquatic life criteria (Table 14). Industrial effluents associated with
metals, metal alloys, and electroplating are a potential source of silver (Phillips and Russo,
1978).
Thallium also was infrequently detected, with concentrations ranging from 0.038 to 0.053
mg/kg. It was found in fish from stations 7b, 8, 13, 15, and 16. Thallium is a byproduct of
iron, cadmium, and zinc processing, is associated with alloys, and has been used as a rodenticide
37
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and insecticide (Doull et a/., 1980). No water or sediment screening values for thallium were
exceeded at these sites.
Cyanide was detected in two samples, from stations 7b and 14. The measured concentrations
were 2.0 mg/kg, which is only slightly greater than the detection limit (1.0 mg/kg). Cyanide
in water exceeded the acute aquatic life criterion downstream from International Anzalduas Dam
(station 14), but elevated levels were not noted in water or sediment from station 7b.
Several organic chemicals exceeded screening criteria. Arochlor 1248 concentrations were
greater than the USFWS national geometric mean upstream from Eagle Pass/Piedras Negras
(station 9), and upstream from Laredo/Nuevo Laredo (station 11). PCB's were not detected in
water or sediment from these sites. The sources of PCB's in tissue were not apparent.
Chlordane exceeded the TNRCC state 85th percentile in one sample collected downstream from
Laredo/Nuevo Laredo (station 12). It was widely used in urban areas as a pesticide until sales
were banned in the United States in 1988. However, it is highly persistent in the environment,
and still sometimes occurs in reaches of rivers downstream from cities (USEPA, 1992). The
probable source of chlordane at station 12 is urban/industrial runoff from Laredo/Nuevo Laredo.
Gamma-bhc (lindane) exceeded the USEPA national mean upstream from the Rib Conchos
confluence (station 3). It was not detected in water or sediment from that site. Although usage
of this pesticide has been restricted in the U.S. since 1985, substantial agricultural activity in
the area is the probable source (Irwin, 1989; TNRCC, 1992a; USEPA, 1992).
To summarize the occurrence of toxic chemicals based on body burden screening, five
contaminants in fish tissue exceeded screening levels at station 9, four at station 11, and three
at station 13 (Figure 3). Eleven sites exhibited screening level exceedances for two contaminants
(1, 3, 4, 5, 6a, 65, 7, 9b, 10, 12, 16), and ten sites for one contaminant (2, 3a, 6, 7b, 8, 8d,
14, 15, 17, 18).
Predator Protection Limits.—Whole fish tissue data were subjected to a second type of
screening using predator protection limits (PPL's) (Table 7). These limits are maximum
concentrations recommended by USFWS, USEPA, and others for protection of predatory fish
and wildlife from the effects of ingesting contaminants in prey organisms.
For chromium, mercury, and selenium, PPL's are lower than 85th percentile values used in
body burden screening. Thus, some values for these contaminants which did not exceed 85th
percentiles did exceed PPL's. Also, the PPL for total PCB's is < 0.1 mg/kg; to be able to
estimate exceedances, a value of 0.1 mg/kg was employed. Therefore, the potential for effects
by PCB's on predatory species may be underestimated.
Selenium concentrations exceeded the PPL at every site except downstream from
Brownsville/Matamoros (station 18). Selenium can affect reproduction in fish and predatory
birds (Eisler, 1985; Lemly, 1985; Gillespie and Baumann, 1986; Ohlendorf et al., 1986;
Hoffman and Heinz, 1988; Ohlendorf, 1989). Although whole body concentrations of selenium
were greater than the PPL throughout the system, highest concentrations were from upstream
38
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of the Rfo Conchos confluence to Del Rio/Ciudad Acuna, and fish in that area may constitute
a more serious threat to predators. See previous discussions regarding potential sources of
selenium.
Similarly, mercury often exceeded the PPL. The only areas where concentrations were less than
the PPL included sites upstream from the Rfo Conchos confluence (stations 1-3), on the Pecos
and Devils rivers (stations 6a and 6b), downstream from Del Rio/Ciudad Acuna (station 8), and
downstream from Brownsville/Matamoros (station 18). See previous discussions regarding
potential sources, and occurrences in water and sediment at these sites.
Chromium occasionally exceeded the PPL. Concentrations in fish from the Pecos River, the Rfo
Escondido, upstream and downstream from Laredo/Nuevo Laredo, downstream from
International Anzalduas Dam, and downstream from Hidalgo/Reynosa were high enough to be
a potential risk to fish-eating predators. Chromium was detected in water from several
tributaries in these areas, including 6a, 9a, 12a, and 15a. Chromium levels in sediment from
stations 11, 14, and 16 exceeded aquatic life threshold values. General sources of chromium
include wastewater discharges from metal plating, chemical, power plant, and industrial facilities
(Eisler, 19865).
Total PCB's exceeded the PPL upstream from Eagle Pass/Piedras Negras and upstream from
Laredo/Nuevo Laredo (stations 9 and 11). PCB's were not detected in water or sediment at
these sites. There is extensive documentation regarding bioaccumulation of PCB's and
subsequent effects on organisms. PCB's are persistent, bioaccumulative, and carcinogenic, and
are known to cause reproductive failure in mammals (Parslow and Jefferies, 1973; Neidermyer
and Hicky, 1976; Addison and Brodie, 1977; Eisler, 1986a; USEPA, 1992).
The PPL for lead was exceeded only downstream from El Paso/Ciudad Juarez (station 2). Lead
was also elevated in sediment at that site, exceeding the aquatic life threshold value (Table 14).
It also occurred at a high concentration in sludge from station la, the El Paso Public Service
Board Haskell R. Street wastewater treatment plant outfall, located 13.8 km (8.6 miles)
upstream, implicating that facility as a probable contributor of lead to station 2. See previous
discussions regarding biological effects and potential sources.
To summarize indications of toxic chemical contamination from predator protection limit
screening, the following sites had three or four contaminants that exceeded PPL's in whole fish
tissue: upstream from Eagle Pass/Piedras Negras, the Rfo Escondido, upstream and downstream
from Laredo/Nuevo Laredo, downstream from International Anzalduas Dam, and downstream
from Hidalgo/Reynosa (stations 9, 9b, 11, 12, 14, 16) (Figure 4). Stations 2, 3a, 4, 5, 6, 6a,
7, 7b, 8d, 10, 13, and 15 had two contaminants that exceeded PPL's, while the remaining sites
had zero or one (1, 3, 6b, 8, 17, 18).
Toxicity Testing
Toxicity testing was performed on water and sediment eluate samples from all 45 sites. The
results are presented in Tables 14 and 15. Reference toxicant tests were conducted throughout
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the study, the results of which were within established acceptable limits based on previous tests
with the subject organisms (Terry Hollister, USEPA Laboratory-Houston, personal
communication).
Mainstem
In toxicity testing of water, significant adverse effects were seen in only one instance. This
involved the sample from station 1, where 100% mortality occurred in Ceriodaphnia dubia. As
that site was the upstream control station for the El Paso/Ciudad Juarez reach, this occurrence
was surprising. Five toxic chemicals were present at quantifiable levels, but none exceeded
screening levels. Thus, no causative agent was identified. Local fish and macrobenthic
communities were healthy, indicating that toxic impacts are not persistent. Supplemental data
for the site suggest that the occurrence may have been an aberration, as no significant effects
have been observed in periodic toxicity testing conducted since 1993 (Table 18).
Toxicity testing of sediment eluates also revealed only one instance of significant adverse effects.
This was in the sample from station 12, downstream from Laredo/Nuevo Laredo, where 100%
mortality occurred in fathead minnow embryo/larvae. Seventeen toxic chemicals were detected
in sediment, the highest number for any mainstem station. Concentrations of methylene
chloride, toluene, and nickel were in excess of aquatic life threshold values, and probably were
at least partially responsible for the observed effects. However, other chemicals, acting
synergistically or additively, may also have been involved. Fish and macrobenthic communities
at the site were moderately impaired; the observance of sediment eluate toxicity indicates that
toxic properties of local sediments were among the causative factors. Periodic toxicity testing
since 1991 has documented two other instances of significant adverse effects, involving one
water sample and one sediment eluate sample (Table 18).
A total of 114 toxicity determinations were made on mainstem samples (Ceriodaphnia dubia
survival in water and sediment, C. dubia reproduction in water and sediment, and fathead
minnow embryo/larval survival in water and sediment, at each of 19 sites). That significant
effects occurred in only two of 114 possible instances was an important finding regarding toxic
chemical impacts in the Rio Grande/Rib Bravo, indicating that such effects are rare during low
flow conditions.
Supplemental toxicity testing data, referenced for stations 1 and 12 above, exist for seven other
mainstem locations (Table 18). Most of the sites are located below major sister cities, to
monitor impacts in areas susceptible to toxic chemical contamination. In addition to occurrences
mentioned for station 12, significant adverse effects have been seen in water and/or sediment
eluate samples from stations 2,4, and 10. Thus, a potential for toxic effects exists at those three
sites, despite the fact that toxicity was not seen there during the present study.
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Tributaries
Toxicity was much more prevalent than for the mainstem, as samples from 14 of the 26 stations
produced significant adverse effects in at least one phase of the toxicity tests. The 14 affected
sites are discussed individually below.
Station la, the El Paso Public Service Board Haskell R. Street wastewater treatment plant
outfall, exhibited the most severe degree of toxicity in the study. One hundred percent mortality
occurred in Ceriodaphnia dubia and fathead minnow embryo/larvae in both the water and sludge
sample. Chemistry data for the water sample reflected the presence of 17 toxic chemicals, the
highest number in the study. Residual chlorine, chloroform, and arsenic were elevated, but only
the former exceeded an aquatic life screening level. The residual chlorine concentration was 63
times greater than the acute criterion, and undoubtedly was the main cause of water toxicity.
In sludge, copper, mercury, selenium, silver, and zinc concentrations exceeded national 85th
percentiles, and two other chemicals for which no screening levels exist occurred at
comparatively high concentrations (parachlorometa cresol, phenol). Effects by copper, mercury,
and zinc probably were negligible, as the observed concentrations were considerably less than
aquatic life threshold values. Selenium and silver do not have threshold values; however, the
margin by which they exceeded national 85th percentiles was slight for selenium (1.3X), but
extreme for silver (11.8X). The parachlorometa cresol concentration was the second highest
observed, while the detection of phenol was the only such occurrence in the study. Thus, among
potential causes of toxicity in the sludge sample, selenium may have played a minor role, while
silver, parachlorometa cresol, and phenol possibly had major involvement.
The water sample from station 2a, the Ciudad Juarez sewage discharge canal, induced 100%
mortality in C. dubia and fathead minnow embryo/larvae. Twelve toxicants were detected, six
of which exceeded screening levels (un-ionized ammonia, parachlorometa cresol, phenol,
phenolics recoverable, arsenic, mercury). Un-ionized ammonia, the only one that exceeded an
aquatic life screening level (at a concentration 2.6 times greater than the acute criterion),
evidently was the primary causative agent.
One hundred percent mortality occurred in C. dubia in sediment eluate from station 3a, the Rio
Conchos. Twelve toxicants were detected in sediment. Arsenic, chromium, and nickel exceeded
aquatic life threshold values and were potential causative agents. However, the arsenic
concentration stood out, as the amount by which it exceeded the threshold value was the greatest
in the study. The amounts by which chromium and nickel exceeded threshold values, on the
other hand, were comparable to amounts for sites where sediment toxicity was not observed.
Fish community integrity was relatively high at the site (although specimens exhibited a slightly
elevated incidence of physical abnormalities), indicating that any instream toxic stresses that
were being exerted were not appreciably affecting resident aquatic life.
In the water sample from station 6a, the Pecos River, C. dubia reproduction was significantly
reduced. Five toxicants were detected, but none exceeded screening levels. The causative agent
evidently was total dissolved solids, as the observed concentration approximated the range known
to induce stress in C. dubia (Terry Hollister, USEPA Region 6 Laboratory-Houston, personal
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communication). The local fish community was healthy, indicating that no appreciable instream
impacts were occurring.
Ceriodaphnia dubia reproduction was also significantly reduced in samples from the next three
sites. However, the effects probably were not ecologically important. The effect occurred in
water and sediment eluate from station fib, Devils River. Three toxicants were detected in water
and 11 in sediment, but none exceeded screening levels. Fish community integrity was high at
the site, providing evidence that no toxic impacts were occurring in the river.
For station 7a, Arroyo de las Vacas, only the water sample was involved. Of four toxicants that
were detected, only un-ionized ammonia appeared important. The measured concentration was
equivalent to the chronic aquatic life criterion, and probably was responsible for the observed
effect.
For station 7b, San Felipe Creek, the effect occurred only in the water sample. Two toxic
chemicals were detected; one of these, silver, exceeded aquatic life screening levels and was the
probable causative agent. The observed concentration was considerably greater than at any other
site. The local fish community was relatively healthy, indicating that any toxic effects that may
have been occurring in the creek were not severe.
In the water sample from station 9a, an unnamed tributary 3.6 km (2.2 miles) south of Eagle
Pass/Piedras Negras, C. dubia reproduction was significantly reduced, and 100% mortality
occurred in fathead minnow embryo/larvae. Seven toxic chemicals were detected, two of which
occurred at elevated levels. The arsenic concentration exceeded the human health criterion, but
was much lower than applicable aquatic life criteria. Un-ionized ammonia, on the other hand,
was over four times greater than the chronic aquatic life criterion, and most likely was
responsible for the observed effects.
In the water sample from station lOa, Manadas Creek, C. dubia survival and reproduction were
significantly reduced. Four toxic chemicals were detected, two of which occurred at
concentrations above screening levels. Antimony and thallium exceeded human health criteria,
with antimony also exceeding the national 85th percentile. However, concentrations of both
were well below aquatic life criteria. The probable causative agent was total dissolved solids,
as the observed concentration was within the range known to adversely affect C. dubia (Terry
Hollister, USEPA Laboratory-Houston, personal communication).
The water sample from station lla, Zacate Creek, produced 100% mortality in C. dubia. Eight
toxicants were present at detectable concentrations, two of which exceeded screening levels. The
selenium concentration exceeded the chronic aquatic life criterion, but only by a very slight
amount, and its impact probably was minimal. The diazinon concentration, however, was two
times greater than the acute screening value, and evidently was the main causative factor.
One hundred percent mortality occurred in C. dubia exposed to water from station 1 Ib, Chacon
Creek. Five toxic chemicals were detected, but none exceeded screening levels. Adverse effects
were attributable to total dissolved solids, which were at a level known to be highly stressful to
C. dubia (Terry Hollister, USEPA Laboratory-Houston, personal communication).
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For station lie, Arroyo el Coyote, C. dubia survival and reproduction were negatively affected
in the water sample. Ten toxic chemicals were detected, four of which exceeded screening
levels. Arsenic exceeded the human health criterion, but not the aquatic life criteria. Un-
ionized ammonia, selenium, and bis(2-ethylhexyl) phthalate exceeded chronic aquatic life criteria
by factors of 6.2X, 2. IX, and 3.3X, respectively. Thus, un-ionized ammonia appears to have
been the main cause of water toxicity, with selenium and bis(2-ethylhexyl) phthalate involved
in lesser roles. In the sediment eluate, 100% mortality occurred in both C. dubia and fathead
minnow embryo/larvae. Seventeen toxic chemicals were detected in sediment, with zinc,
chlordane, and bis(2-ethylhexyl) phthalate exceeding screening levels. Zinc exceeded the
national 85th percentile, but was far less than the aquatic life threshold value. Chlordane
exceeded the aquatic life threshold value by a factor of 1.9X. No aquatic life threshold value
exists for bis(2-ethylhexyl) phthalate, but the concentration was very high, exceeding the national
85th percentile by 7.3X and the next highest level in the study by 8X. One other chemical for
which no screening level exists, parachlorometa cresol, also was conspicuously elevated,
exceeding the next highest level in the study by 2.3X. Thus, indications are that chlordane,
bis(2-ethylhexyl) phthalate, and parachlorometa cresol were potentially important in the
manifestation of sediment eluate toxicity.
Significant adverse effects were recorded for the water sample from station 12d, Arroyo Los
Olmos, as 100% mortality occurred in both C. dubia and fathead minnow embryo/larvae. Seven
toxic chemicals were detected, with two exceeding screening levels. Cyanide exceeded the
chronic aquatic life criterion by a factor of 1.8X, and may have been marginally involved in the
observed effects. Diazinon exceeded the acute aquatic life criterion by 26.3X, and evidently had
a major causative role. Total dissolved solids probably contributed to the impact on C. dubia,
as the concentration was in the range known to adversely affect that organism (but below the
level known to stress fathead minnow embryo/larvae) (Terry Hollister, USEPA Laboratory-
Houston, personal communication). Local fish community structure reflected a moderate
probability that some form of instream impact was occurring.
In the water sample from station 15a, Anhelo Drain, C. dubia reproduction was significantly
reduced. Sixteen toxic chemicals were detected, with arsenic, mercury, and diethyl phthalate
exceeding screening levels. Of the three, only diethyl phthalate exceeded an aquatic life
screening level, with a concentration 2.7 times greater than the chronic criterion. In the
sediment eluate, fathead minnow embryo/larval survival was significantly reduced. Fourteen
toxic chemicals were detected in sediment, but only chlordane exceeded screening levels, with
a concentration 3.7 times greater than the aquatic life threshold value. Fish apparently were
absent from the drain, as an attempt to collect samples for tissue analysis produced no
specimens. This was reflective of severe instream stress, but whether toxic or conventional
pollutants were primarily responsible was not clear.
Macrobenthic Community Assessment
Macrobenthic bioassessments were performed at the 18 major mainstem stations (Table 19).
One hundred and ninety-nine taxa were collected, a high total that reflected physiographic
complexity and varying zoogeographical influences along the longitudinal gradient.
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Prior to interpreting the data, sample collecting techniques and data evaluation methods
(described in the "Biological Techniques" section) were reviewed to evaluate their relative
abilities to characterize macrobenthic integrity.
Evaluation of Collecting Techniques
Because snag sampling had not previously been used in TNRCC studies, a comparison to Surber
sampling was performed. This involved the employment of both collecting techniques at two
stations. At station 8, macrobenthic integrity was higher in the Surber sample, as indicated by
greater species richness, diversity, equitability, and EPT index, plus a more optimally-sized
standing crop. This was not reflected by TNRCC Mean Point Scores (MPS's), which were
identical. It was, however, by the Ohio Invertebrate Community Index (ICI); although both ICI
values were within the range associated with an intermediate aquatic life use rating, that for the
Surber sample was 36% greater than that for the snag sample.
At station 12, a similar relationship was evident, for the same reasons. However, in this case
both rating techniques clearly indicated better integrity in the Surber sample. The MPS for the
Surber sample was 23% greater than that for the snag sample, the latter being reduced by one
subcategory (from high to intermediate). The ICI reflected an intermediate use rating for both
samples; however, the value for the Surber sample was 57% greater than that for the snag
sample.
Thus, the two sampling methods resulted in slightly different indications of macrobenthic
integrity. Surber sampling produced 0-23% greater MPS's, and 36-57% greater Id's, than did
snag sampling. Two natural factors relating to physical habitat probably are instrumental in this
relationship. First, rocky-bottomed substrates from which Surbers were taken were more
physically complex than snags, which had relatively smooth surfaces. This afforded a greater
variety of microhabitats, which would be expected to support a more diverse macrobenthic
assemblage. Second, current velocity associated with Surber sample habitats typically was
considerably greater, as snags generally occurred in slackwaters along the shorelines. Maximal
macrobenthic diversity in streams typically is associated with relatively swift current velocity
(Hynes, 1970).
However, a possibility also exists that snag communities were more affected by certain
environmental stresses, such as toxicants, than were riffle communities, in relation to
aforementioned hydrological variability. Kerans et al. (1992) showed that effects of
environmental stresses on certain invertebrate metrics are manifested to a greater degree in
slackwater habitats than in riffles. Potential stresses may be ameliorated in riffles by the effects
of high reaeration, continuous flushing, and low degree of accumulation of (potentially
contaminated) fine particulates on the substrate, whereas the reverse appears true for
slackwaters.
The evaluation revealed an inherent tendency for Surber sampling to sometimes produce more
favorable indications of macrobenthic integrity than snag sampling. However, supplemental
information suggests that environmental stresses may compound innate differences. Therefore,
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in interpreting the data, indications of reduced macrobenthic integrity from snag sampling were
not automatically discounted. Rather, the possibility that environmental stresses might be
involved was closely scrutinized.
Comparison of Data Evaluation Methods
In a TNRCC study of 81 minimally-impacted ecoregion stream sites, the MPS indicated
exceptional or high ratings at a 15% greater frequency than did the ICI (Bayer et a/., 1992).
Thus, intrinsic differences in the way the techniques function appear to account for this amount
of divergence.
In the present study, the MPS produced an aquatic life use rating one subcategory higher than
did the ICI in nine instances. Ratings were consistent in the other 11 cases. Therefore, the
MPS resulted in a higher aquatic life use rating 45% of the time.
Thus, the frequency of divergence for the Rio Grande/Rfo Bravo exceeded what would be
expected in minimally-impacted streams by 30%. This may reflect differing sensitivities of the
two techniques in situations where toxicants or certain other types of environmental stresses
occur, with the ICI producing harsher indications. There is evidence from other TNRCC studies
that the MPS is very sensitive to the effects of organic enrichment, but less so to toxic stresses
(Davis, 1991), which precipitated employment of both techniques in the present study.
Comparison of the two rating methods showed that the MPS can be expected to sometimes give
a more favorable impression of macrobenthic integrity than the ICI, with the gap widening
where certain types of environmental impact occur. Accordingly, in cases where the ICI was
reduced relative to the MPS, the potential for inherent variation, and the possibility of stress-
induced divergence, were given equal consideration in interpreting the data.
Macrobenthic Integrity
A high aquatic life use is in effect for 17 of the 18 macrobenthic stations. The only deviation
is for station 2, where a limited aquatic life use is applicable (TNRCC, 1991).
An important finding was that no limited aquatic life use ratings were indicated by either rating
method. Such a rating usually reflects severe instream impact.
The 18 sampling stations were placed into four categories based on indications of macrobenthic
integrity. Category 1 included seven sites, for which both rating methods indicated attainment
of a high or exceptional aquatic life use (stations 4, 6, 7, 9, 10, 11, 14). Determinations for
all seven were based on Surber samples. Ranges and means for rating method values and
principal metrics included: MPS, 3.17-3.67, 3.45; ICI, 36-46, 40.3; species richness, 33-65,
49.6; standing crop, 443-22,637,5,794 individuals/m2; diversity, 3.88-4.51,4.16; equitability,
0.71-0.80, 0.74; EPT index, 10-20, 15; prevalence of dominant functional feeding group,
23.31-42.97, 31.60%.
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All but the lowermost of these seven sites were between the Rio Conchos confluence and
International Falcon Reservoir, and five were clustered in the reach between Langtry/San Ignacio
and Laredo/Nuevo Laredo. Thus, zoogeographical factors appeared to be partially responsible
for superior macrobenthic integrity, i.e., overlap of influences within a transitional zone between
three biotic provinces (see "Study Area" section, and discussion by Davis, 1980).
Macrobenthic integrity at station 6 stood out from all other sites. It was the only site
categorized as exceptional by both rating methods, and the MPS and ICI values were the highest
in the study. This was fostered by the remoteness of the area, together with general
environmental and zoogeographical considerations discussed for category 1 sites.
Levels of integrity at category 1 sites reflected optimal environmental conditions, lack of effects
by toxic chemicals or other detrimental factors, and full attainment of the designated high aquatic
life use. Supplemental information supported this assessment, as no significant effects occurred
in water or sediment eluate toxicity tests (Tables 16 and 17), and few toxic chemicals exceeded
screening levels (Table 13). Such conditions were not surprising for the three sites that were
upstream control stations (station 7, Del Rio/Ciudad Acuna reach; station 9, Eagle Pass/Piedras
Negras reach; station 11, Laredo/Nuevo Laredo reach), or for station 6 which was in a remote
reach. For the other three, however, which were downstream sites in reaches where potential
contaminant sources exist (station 4, Presidio/Ojinaga; station 10, Eagle Pass/Piedras Negras;
station 14, International Anzalduas Dam), the findings were particularly noteworthy in indicating
that effects of pollutants introduced from those areas were negligible.
Category 2 consisted of six sites which partially attained a high aquatic life use (stations 1, 2,
5, 8, 13, 16). These were rated high by the MPS but intermediate by the ICI. Ranges and
means for rating method values and principal metrics included: MPS, 2.50-3.17, 2.76; ICI,
22-30,26.6; species richness, 15-51,36.3; standing crop, 495-35,642,10,687 individuals/m2;
diversity, 2.78-3.79, 3.37; equitability, 0.58-0.71, 0.66; EPT index, 5-10, 6.9; prevalence
of dominant functional feeding group, 29.62-77.15, 48.00%.
Stations 1 and 13 were upstream control sites for the El Paso/Ciudad Juarez reach and the
International Anzalduas Dam reach, respectively. As such, no appreciable instream impacts
were anticipated. Respective MPS values of 3.00 and 3.17 were in the mid- to upper portion
of the range associated with a high aquatic life use, while ICI values of 30 were only slightly
below the minimum associated with a high use. These were the highest values observed in the
study for snag samples.
The inherent tendency for snag samples to slightly underrate macrobenthic integrity could
account for the amount by which ICI values fell short of the high range at these two sites. In
addition, physical macrohabitat characteristics were not particularly suitable for macrobenthos
at either site, and indications of slightly depressed macrobenthic integrity could have been a
product of physical habitat limitations. Station 1 had a monotonous, sandy substrate, and was
located in a channelized reach subject to episodic scour and refill. The harsh nature of the
aquatic environment at this site has been described by Davis (1980). Station 13 also had a
homogenous, predominantly sand substrate, plus various other limitations as discussed for
stations 15, 17, and 18 below. In addition, instream flow at both sites, which is dependent on
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upstream reservoir releases, is highly variable and sometimes changes abruptly, resulting in wide
physicochemical fluctuations.
The indicated level of macrobenthic integrity at station 5 was slightly lower than at stations 1
and 13, even though Surber sampling was employed. The MPS was in the lower portion of the
high use subcategtory range, and the ICI was in the middle of the intermediate use subcategory
range. In those respects, it resembled characteristics for stations 2, 8, and 16. However, it was
lumped with stations 1 and 13 for the following reasons. It was located in a remote reach far
removed from any wastewater discharges or other likely sources of contamination. Physical
habitat characteristics in the immediate vicinity were very poor, as has been described by Davis
(1980), primarily with respect to extreme substrate embeddedness and lack of microhabitat
diversity. This condition evidently is induced by inflow from Terlingua Creek, which enters
immediately upstream from the collecting area, during spates. That the physical nature of the
substrate was the primary factor limiting macrobenthic integrity was substantiated by additional
field observations. Subsequent to sampling activities in the area, communication with the
fisheries team revealed that conditions were substantially different a kilometer or so downstream,
where fish collecting was performed. No embeddedness was evident, and large invertebrates
such as Megaloptera, which were absent from the macrobenthic sample, were collected in
abundance during seining.
Based on physical habitat, sampling technique, macrobenthic integrity, and geographical
considerations, it was concluded that the potential that toxic chemical-induced environmental
stress was occurring at stations 1, 5, and 13 was very slight. Associated data substantiated this
conclusion, as few toxic chemicals exceeded screening levels (Table IS) and no adverse effects
were seen in sediment eluate toxicity tests. In addition, no significant effects occurred in
toxicity testing of water from stations 5 and 13 (Tables 16 and 17). Although Ceriodaphnia
dubia survival was significantly reduced in water from station 1, no potential causative agents
were identified, and the healthy condition of local fish and macrobenthic communities indicated
that instream toxicity is not persistent.
Indications of macrobenthic integrity at the remaining sites in category 2 were summarized in
the second paragraph preceding. All three were downstream sites in reaches where potential
contaminant sources exist (station 2, El Paso/Ciudad Juarez; station 8, Del Rio/Ciudad Acuna;
station 16, Hidalgo/Reynosa).
At station 2, low flow is dominated by the £1 Paso Public Service Board Haskell R. Street
wastewater treatment plant effluent. In addition, physical habitat is poor, due to the effects of
stream channelization, lack of substrate complexity, and other factors (as described by Davis,
1980). Based on these considerations, a limited aquatic life use has been designated for the
segment (TNRCC, 1991). In an effort to eliminate physical habitat effects, Surber samples were
collected from localized, gravel-bottomed riffles about 2 km (1.2 miles) downstream,
immediately below Riverside Diversion Dam. The results showed that although the designated
limited aquatic life use was attained, macrobenthic integrity was considerably reduced compared
to levels that would be expected in unimpacted situations. Furthermore, absolute MPS and ICI
values were somewhat lower than at the upstream control site, despite the fact that station 1 data
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was based on snag sampling. A degree of organic enrichment was indicated by the prevalence
of miners (organisms that feed on deposited fine particulate organic matter), primarily
oligochaetes which are highly tolerant of such conditions. Whereas organic enrichment may
have contributed to depressed macrobenthic integrity, a possiblity that effects by toxic chemicals
may also have been involved was not ruled out.
Station 8 was subjected to Surber and snag sampling. Regarding the Surber sample, the physical
habitat was not ideal, but consisted of runs about 0.3 m (1 foot) deep over coarse gravel partially
embedded in sand and silt. Also, a degree of organic enrichment was indicated by the
prevalence of miners, mostly tolerant species of oligochaetes. Comparison to data from the
control site (station 7) reflected a moderate reduction in macrobenthic integrity, which was
partially attributable to less-suitable physical habitat. For the snag sample, the ICI value was
the second lowest observed in the study, and was appreciably less than that for the Surber, as
was reflected by reductions in individual metrics such as species richness and EPT index.
Whereas physical habitat limitations, organic enrichment, and inherent variability among
sampling techniques and rating methods may have contributed to indications of reduced
macrobenthic integrity, a possibility also existed that slight effects by toxic chemicals may have
been occurring.
Indications of macrobenthic integrity at station 16 were influenced by the same limiting factors
discussed for station 13, as physical habitat characteristics and the employment of snag sampling
were common to both sites. Other limitations are addressed in the discussion for stations IS,
17, and 18 below. Another possible detriment was slight organic enrichment, which was
reflected by the large standing crop and predominance by miners. Cumulatively, these factors
undoubtedly contributed to indications of reduced macrobenthic integrity. In fact, there were
strong indications that unsuitable physical habitat conditions were the primary determinant, as
macrobenthic integrity actually was better than at the upstream control site (station 15).
However, in light of the fact that this was the downstream station for the Hidalgo/Reynosa
reach, where potential contaminant sources exist, together with the level of macrobenthic
integrity that was observed, a possibility also existed that slight effects by toxic chemicals may
have been occurring.
With ample consideration ascribed to physical habitat characteristics, sampling techniques,
macrobenthic integrity, geographical location, chemical data, and toxicity testing results, it was
concluded that the potential that toxic chemical-induced environmental stress was occurring was
slight for stations 8 and 16, but moderate for station 2. At stations 8 and 16, few toxic
chemicals exceeded screening levels, and no significant effects occurred in water or sediment
eluate toxicity tests. At station 2, no significant effects were seen in the toxicity tests, but a
number of toxic chemicals occurred at elevated concentrations, most notably in sediment (Table
IS). For all three sites, however, observed levels of macrobenthic integrity indicated that if
toxic stresses were being manifested, the effects were relatively minor.
Category 3 consisted of four sites where an intermediate use was indicated by both rating
methods (stations 3, IS, 17, 18). Thus, the designated high aquatic life use level was not
attained. However, the degree by which they failed to do so was relatively slight (with the
exception of the MPS value for station 3). The main difference between these sites and those
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in category 2 was that MPS values were slightly lower and fell into the upper end of the
intermediate use range (while ICI values were comparable, in most cases actually being slightly
higher). Ranges and means for rating method values and principal metrics included: MPS,
1.50-2.33,2.12; ICI, 27-30,28.3; species richness, 19-34,28.3; standing crop, 4,869-42,517,
16,078 individuals/m2; diversity, 1.50-3.43, 2.78; equitability, 0.11-0.69, 0.51; EPT index,
5-8, 6.5; prevalence of dominant functional feeding group, 54.36-95.73, 69.10%.
Station 3 was in a remote area far removed from any obvious contaminant sources, and for that
reason was established as the upstream control site for the Presidio/Ojinaga reach. The Surber
sample from the site exhibited anomalously low diversity, equitability, and MPS, as well as next-
to-lowest species richness in the study. In addition, functional feeding group composition was
more unbalanced than at any other site. The main contributing factor was the presence of
dominant numbers of filter-feeding, facultative blackfly larvae (Simulium nr bivittatwn).
Although values for certain metrics resulted in a depressed MPS, the ICI was more normal, in
the upper half of the range associated with an intermediate aquatic life use. Also, sensitive
species were fairly well represented, as reflected by the EPT index value. Thus, the degree by
which macrobenthic integrity was suppressed was not as severe as was implied by the MPS.
Numerous elements contribute to poor macrobenthic habitat in the station 3 area, as discussed
by Davis (1980). Among these are: elevated dissolved solids resulting from evapotranspiration
and irrigation return flows in the reach from El Paso/Ciudad Juarez to Presidio/Ojinaga;
excessive turbidity and sedimentation; predominance of fine sediment and high degree of
embeddedness of stones on the substrate; wide physicochemical fluctuations promoted by highly
variable flow; and seasonal intermittency of flow. Collectively, these factors result in reduced
physical habitat complexity/suitability, and generally stressful environmental conditions. Optimal
macrobenthic integrity would not be expected under such conditions.
Of the other sites in category 3, two were upstream control sites (station 15, Hidalgo/Reynosa
reach; station 17, Brownsville/Matamoros reach), and the other was a downstream site (station
18, Brownsville/Matamoros reach). All three were subjected to snag sampling, and resultant
indications of macrobenthic integrity were very consistent. Habitat characteristics also were
relatively similar. Low gradient and stream channel morphometry result in physical
homogeneity, i.e., the river typically is wide and deep, and lacks riffles and runs. The substrate
is monotonous, comprised mainly of sand and silt. Instream flow is highly variable (see Study
Area description), resulting in wide physicochemical fluctuations. Cumulatively, these factors
act to create relatively poor macrobenthic habitat.
Based on physical habitat, sampling technique, macrobenthic integrity, and geographical
considerations, it was concluded that the potential that toxic chemicals were exerting instream
stress at stations 3, 15, 17, and 18 was slight. Further evidence supporting this conclusion
included the relative insignificance of toxic chemical concentrations (Table 15), and the lack of
significant effects in water and sediment eluate toxicity tests (Tables 16 and 17).
Category 4 contained a single site, station 12, which did not fit previous patterns. Both
sampling techniques were employed, and respective indications of macrobenthic integrity were
considerably different. For the Surber sample, the MPS indicated a high use, but the ICI, the
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lowest observed for Surber samples, was in the lower half of the intermediate use range. For
the snag sample, both rating methods reflected an intermediate use; however, the MPS was the
lowest observed for snag samples, and the ICI was the lowest recorded for any sample by a
substantial margin. As three of the four ratings were in the intermediate range, it was concluded
that the designated high aquatic life use was not attained. Ranges and means for rating method
values and principal metrics included: MPS, 2.17-2.67,2.42; ICI, 14-22,18; species richness,
27-41, 34; standing crop, 14,801-18,300, 16,551 individuals/m2; diversity, 3.28-3.89, 3.59;
equitability, 0.69-0.73, 0.71; EFT index, 2-5, 3.5; prevalence of dominant functional feeding
group, 63.15-70.99, 67.07%.
Rocky-bottomed riffles were common in the area, and overall physical habitat characteristics
were favorable. Nevertheless, sensitive species were poorly represented, as was reflected by
low EFT index values (that from the Surber being the lowest observed for Surber samples, while
that from snags was the lowest seen for any sample). The scope of reduction was broad, as the
Surber EPT index value represented a 71 % decrease from that at the control site (station 11).
A degree of organic enrichment was indicated by the predominance of miners, mainly tolerant
species of oligochaetes and chironomids, and by the relatively large standing crops. This
probably was partially responsible for reduced macrobenthic integrity.
The site was the downstream station for the Laredo/Nuevo Laredo reach, where the potential
for contaminant introduction is high. Based on this consideration, together with the prevalence
of favorable physical habitat, the observed level of macrobenthic integrity, and the paucity of
sensitive species, it was concluded that the potential that toxic chemical-induced environmental
stress was occurring at the site was high. This presumption is supported by the fact that
significant effects occurred in the sediment eluate toxicity test (Table 17). Nonetheless, the level
of macrobenthic integrity observed indicated that if toxic stresses were being manifested, the
effects were not severe.
In conclusion, the 18 major mainstem stations were grouped according to the potential that
macrobenthic communities were being affected by toxic chemicals, as follows.
indicated potential station(s)
none 4, 6, 7, 9, 10, 11, 14
very slight 1,5, 13
slight 3, 8, 15, 16, 17, 18
moderate 2
high 12
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Fish Community Assessment
A total of 53 species of fish were collected from 18 sites on the mainstem Rio Grande/Rfo Bravo
and seven tributaries sampled during this study. The initial inspection of the fish community
data revealed several major faunal breaks that could be defined by differences in fish species
occurrence. One division occurred surrounding the Rio Conchos, with another at International
Amistad Reservoir. The final one was observed surrounding International Falcon Reservoir.
These trends appear more related to changes in stream hydrology than other factors.
Collections from stations 1 and 2, upstream and downstream of El Paso/Ciudad Juarez,
contained similar species to those found in the Texas portion of the upper Rio Grande/Rfo Bravo
by previous researchers (Hubbs et al., 1977; Bestgen and Platania, 1988). Species considered
common in the upper river by Hubbs et al. (1977) were gizzard shad (Dorosoma cepedianum),
red shiner (CyprineUa lutrensis), common carp, river carpsucker (Carpiodes carpio), channel
catfish, western mosquitofish (Gambusia affinis), and green sunfish (Lepomis cyanellus).
Subsequently, Bestgen and Platania (1988) indicated that those species were still common and
added bullhead minnow (Pimephales vigilax) and longear sunfish (Lepomis megalotis). Collected
during the study, but not reported by Hubbs et al. (1977) or Bestgen and Platania (1988), was
an introduced species, walleye (Stizostedion vitreum), from station 1. Hubbs et al. (1977)
characterized Rio Grande/Rfo Bravo fauna upstream of the Rfo Conchos as widely distributed
and salt tolerant.
The influence of the Rfo Conchos (3a) on species assemblages in the Rio Grande/Rfo Bravo
became apparent at station 3, upstream of the confluence, and continued downstream to
International Amistad Reservoir. Hubbs et al. (1977) discussed the Rfo Conchos influence and
the influx of species that occur in the Rio Grande/Rfo Bravo beginning in the reach that contains
the confluence of the two rivers. The faunal differences upstream and downstream of the Rfo
Conchos also appear to be longstanding (Hubbs et al., 1977).
In this study, International Amistad Reservoir presented a distinct boundary between a slightly
turbid upstream reach influenced by stream modifications, irrigation return flows, runoff, and
the Rfo Conchos (stations 3, 3a, 4, 5, and 6) and a downstream reach influenced by clear
inflows of water from the reservoir and to a lesser degree, by springflow emanating from
tributaries on both sides of the border. Modification of this downstream habitat through changes
in flow patterns and reduced turbidity (Table 10) has apparently created a longitudinal gap in
the occurrence of some fish species. Several members of the fish community were observed
upstream of the reservoir and further downriver, around Eagle Pass/Piedras Negras and
Laredo/Nuevo Laredo (stations 9, 10,11, and 12), but were absent immediately downstream of
International Amistad Reservoir at stations 7 and 8. Tamaulipas shiner (Notropis braytoni) was
collected at stations 3, 3a, 4, 5, and 6 upstream of the reservoir and station 11 downstream,
being absent from intervening sites. Other species demonstrating a similar pattern were Rio
Grande shiner (Notropis jemezanus), collected at stations 5, 6, 9, 9b, and 11; speckled chub
(Macrhybopsis aestivalis), collected at stations 3, 3a, 4, 5, 6, 6a, 11, and 12; and blue sucker
(Cycleptus elongates), collected at stations 3a, 4, 5, and 12. Longnose dace (Rhinichthys
cataractae) was observed at stations 3a, 4,5, and 6, but not downstream. River carpsucker and
blue catfish were present upstream of International Amistad and downstream of International
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Falcon Reservoirs, but were absent from the entire middle reach. Platania (1991) observed
similar patterns of occurrence for Tamaulipas shiner, Rio Grande shiner, speckled chub, and
longnose dace. Those four species, plus river carpsucker and blue catfish were all collected in
the vicinity of stations 7 and 8 prior to the impoundment of International Amistad Reservoir
(Trevino-Robinson, 1959). Coupled with the absence of certain species downstream of Amistad,
was the presence of "stream" or "creek" oriented fishes (Smith and Miller, 1986) in the
mainstem at stations 7, 8 and 9. The occurrence of roundnose minnow (Dionda episcopa) and
Rio Grande darter (Etheostoma grahami) in a big river habitat illustrates the influence of
Amistad and associated spring-fed tributaries on the Rio Grande/Rio Bravo in that reach. Other
species largely unique to the middle reach and tributaries were Texas shiner (Notropis amabilis),
gray redhorse (Maxostoma congestion), and smallmouth bass (Micropterus dolomieui).
International Falcon Reservoir provides a break between the river's freshwater middle reach and
a lowermost reach that becomes increasingly brackish toward the river's mouth. Estuarine and
marine species were present at all sites downstream of the reservoir, but assumed the greatest
proportions of the fish community at the two lowermost stations, 17 and 18, which are at river
kilometer 1S5.8 and 78.3, respectively. Species representing this estuarine and marine fauna
were American eel (Anguilla rostrata), Atlantic needlefish (Strongylura manna), gulf killifish
(Fundulus grandis), sheepshead minnow (Cyprinodon variegatus), Amazon molly (Poecilia
formosa), sailfin molly (Poecilia latipinna), mountain mullet (Agonostomus monticola), striped
mullet (Mugil cephalus), and bigmouth sleeper (Gobiomorus dormitor). Based upon the
collection from Arroyo Los Olmos (12d), whose confluence is more than 330 km (205 miles)
upstream from the Rio Grande/Rib Bravo mouth, brackish water species move far upriver during
periods of reduced flow probably following changing salinity gradients. Estuarine or marine
species accounted for approximately 96 percent of the total collection for Arroyo Los Olmos.
That site had very high conductivities. A collection from the next downstream station on the
mainstem was not dominated by brackish water species, possibly because flows were elevated
at the time of sampling. Absent from our collections throughout the reach downstream of
International Falcon Reservoir were several minnow species—Tamaulipas shiner, Rio Grande
shiner, and speckled chub—which were historically observed in this area (Trevino-Robinson,
1959), suggesting a shift in the fish community. Edwards and Contreras-Balderas (1991)
observed the elimination of freshwater restricted species downstream of Brownsville/Matamoros
and replacement by estuarine and marine forms. They also noted an increase in the proportion
of estuarine species between International Anzalduas Dam and Brownsville/Matamoros.
Changes in both segments were attributed primarily to decreasing stream flows and concurrent
increases in salinity. They also speculated that increases in chemical pollution were affecting
indigenous species in conceit with changes in stream flow. Species such as largemouth bass and
white bass also become common downstream of International Falcon Reservoir, undoubtedly
because of the influence of the reservoir fisheries.
In summary, hydrologic modifications have largely shaped the present Rio Grande/Rio Bravo
fish community. Upstream of the Rio Conchos, where flow is undependable, species
assemblages are small and adapted to highly variable conditions. The Rfo Conchos provides a
reliable supply of water and habitat for species that characterize the river downstream to
International Amistad Reservoir. The reservoir reduces turbidity and alters water quality and
along with spring system influences, causes a shift in the fish community towards a clear water,
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small stream assemblage. Further downstream around Eagle Pass/Piedras Negras and Laredo,
some characteristic community members observed upstream of Amistad are again present.
International Falcon Reservoir and subsequent diversions serve to reduce flows downstream,
with estuarine and marine species invading further upstream. Any evaluation of fish
communities for the purposes of ascertaining pollutant impacts must be conducted within the
context of these altered fauna! patterns.
Fish Community Measures - EJ Pftso/Cjudad Judrez to International Falcon Reservoir (Stations
1-12.
Species Richness, Composition, and Similarity. — Species number (Figure 5) ranged from
nine to 19 species at Rio Grande/Rfo Bravo sites upstream of International Falcon Reservoir and
on the Rio Conchos. The median value was 12. The Rio Conchos (3a) sample contained
noticeably more species (18) than surrounding sites. Hubbs et al. (1977) noted this same trend.
Sites with fewer species than the median (stations 1-3) were all upstream of the Rfo Conchos
with one exception, station 7, downstream of International Amistad Reservoir, which had the
lowest number of species. The collection from station 9, upstream of Eagle Pass/Piedras
Negras, had the most species of any site between El Paso/Ciudad Jua*rez and International Falcon
Reservoir. The most notable variation in species richness between sites bracketing the major
sister cities came at stations 9 and 10, with a difference of seven species. Species present
upstream but not downstream were roundnose minnow, Rio Grande shiner, Mexican tetra
(Astyanax mexicanus), flathead catfish (Pylodictis olivaris), smallmouth bass, largemouth bass,
and Rio Grande darter. Several of those species-the two minnows and darter-are among the
more habitat sensitive species in the mainstem, suggesting the potential for impacts based upon
community composition. Texas shiner was collected at station 10, but not station 9. These
changes were not well reflected in the similarity index (Table 21) given the fact that the two sites
also had many species in common. It should be noted, however, that station 9 shared higher
similarity to an intervening tributary, the Rio Escondido (9b), than to station 10.
The only other drop in species number between upstream and downstream sites came at station
2 relative to station 1 and amounted to a decrease of one, although the community composition
was slightly different. Flathead catfish, walleye, and bluegill (Lepomis macrochirus) were
collected at station 1, but not station 2, whereas white bass and largemouth bass were taken
downstream but not upstream. From an impact analysis standpoint, little can be ascribed to the
community differences between those sites, and the index of similarity value for those collections
was the highest in the study when comparing upstream-downstream sites in the mainstem (Table
21).
Increases in species richness were observed at downstream stations relative to upstream when
comparing stations 7 and 8, four species; 11 and 12, three species; and 3 and 4, two species.
Collected at station 7 but not 8 were Texas shiner, smallmouth bass, and Rio Grande cichlid
(Cichlasoma cyanoguttatum), whereas spotted gar (Lepisosteus oculatus), blacktail shiner
(Cyprinella venusta), roundnose minnow, gray redhorse, channel catfish, green sunfish, and
largemouth bass were taken downstream but were absent upstream. Similarity between the sites
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was relatively low (Table 21). These community changes probably reflect the fish community's
state of flux given the aforementioned influence of International Amistad Reservoir.
Though the increase in species richness was not large at station 12 relative to 11, the similarity
between the two stations was low (Table 21). Western mosquitofish, Tamaulipas shiner, Rio
Grande shiner, and fiathead catfish were all collected upstream, but were absent downstream,
whereas the reverse was true for blue sucker, gray redhorse, Mexican tetra, white bass,
warmouth (Lepomis gulosus), longear sunfish, and largemouth bass. Physical habitat was
substantially more diverse at station 12 and consisted of more backwaters, snags, and instream
vegetation, which would tend to favor additional centrarchid species. A specific habitat favoring
blue sucker, boulder fields with swift velocities, was also observed downstream. The low
similarity and elimination of the two shiner species, however, suggest potential impacts from
intervening inputs of wastewater from Laredo/Nuevo Laredo.
The difference in species richness at stations 3 and 4 was minor and index of similarity between
the two sites was high (Table 21). Present upstream but not downstream were fathead minnow
(Pimephales promelas) and Mexican tetra, whereas gizzard shad, blue sucker, longnose dace,
and longear sunfish were collected downstream but were absent upstream. Two of these species,
longnose dace and blue sucker, were not observed upstream of the Rio Conchos and their
absence from station 3 may reflect the lack of reliable flows upstream of the confluence and the
influence of irrigation return flows. The river was turbid (110 jtu) at station 3 and was of high
conductivity (2,640 pmhos/cm). That situation also precluded effective electrofishing and may
have biased the sample slightly, providing a somewhat tenuous comparison between upstream
and downstream sites. However, species richness and composition comparisons for stations 3
and 4 do not suggest substantial impact from intervening pollutant inputs.
Species richness was the same at stations 5 and 6 with community similarity being moderate.
Present at station 5 but not 6 were blue sucker, blue catfish, fiathead catfish, and blue tilapia
(Jllapia aurea). Those found downstream but absent upstream were smallmouth buffalo
(Ictiobus bubalus), white bass, largemouth bass, and freshwater drum (Aplodinotus gntnniens).
Physical habitat, particularly substrate, at station 5 was different from that at station 6. Many
cobble riffles were present, with some rubble and small boulders at station 5, whereas station
6 had smaller substrate particle sizes, mainly gravel and sand with some cobble. Those
conditions may have favored blue sucker at station 5 over 6, whereas the proximity of
International Amistad Reservoir downstream probably explains the presence of white bass and
largemouth bass at station 6.
Index of Biotic Integrity,—Index of Biotic Integrity (IBI) scores for sites upstream of
International Falcon Reservoir and in the Rfo Conchos ranged from 13 to 24 out of a possible
30 points (Table 22). The median value for those sites was 18. Stations with a score in the
bottom two quartiles were 3a, S, 6, 7, 8, 12. Station 12, downstream from Laredo/Nuevo
Laredo, posted the lowest score in this reach (IBI=13). Species richness was rated high, though
the site was downrated slightly for a reduced number of minnow species. Other areas in which
it was downrated were for dominance by a single species, red shiner, which made up 60% of
the sample; a relatively low catch rate; an elevated number of introduced individuals, primarily
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common carp; and the highest incidence of deformities, lesions, and tumors of any sample
collected in the study. We noted deformities, lesions, and tumors in 10.9% of the electrofishing
catch at this site and 2.75% of the total catch.
Stations 7 and 8 also had scores that were less than the median (14 and 17, respectively). As
noted earlier, the collection from station 7 contained the fewest species in the study and only one
minnow species, Texas shiner. The catch rate was the lowest in the river upstream of
International Falcon Reservoir and the second lowest in the overall study. In addition, the
number of introduced individuals-common carp and smallmouth bass-was elevated. Station 8
had moderate species richness and two minnow species, roundnose minnow and blacktail shiner,
the latter being an introduction. Catch rate was again relatively low and the number of
introduced individuals was elevated and consisted of common carp and blacktail shiner. As
already noted, those sites, particularly station 7, demonstrated modified fauna! patterns related
to the presence of International Amistad Reservoir. Though a potential for contaminant impact
is present at station 8 through point sources upstream, it is largely impossible to differentiate
from the aforementioned influences.
Stations 5 and 6 were downrated for incidence of deformities, lesions, and tumors and relatively
low catch rates. Station 5 was also downrated for a high incidence of introduced individuals,
primarily common carp. The station 6 sample was dominated by more than 65% red shiner,
which caused it to receive a less than optimal rating. Both of these sites contained many
characteristic Rio Grande/Rio Bravo species, however, including speckled chub, Tamaulipas
shiner, Rio Grande shiner, and longnose dace. Given the presence of those species, the
remoteness of the sites, and a low potential for pollutant inputs, the probability of contaminant
impact appears very slight.
The final site with a score less than the median was the Rfo Conchos (3a). This site had high
species richness relative to surrounding sites on the Rio Grande/Rfo Bravo, though the collection
was downrated for the percentage of introduced fishes, primarily represented by common carp.
Several sensitive minnow species were observed, including speckled chub, Tamaulipas shiner,
and longnose dace. However, this site also had the third highest incidence of deformities,
lesions, and tumors in the study. When only electrofishing samples were considered, 4.5 % of
the catch had some physical anomaly.
In evaluating differences between paired samples bracketing the major sister cities, two (stations
3 and 4 and stations 7 and 8) had higher EBI scores downstream rather than upstream. Sampling
difficulties noted earlier may have influenced the score at station 3 as well as return flow impacts
on water quality. In addition, the influx of species from the Rfo Conchos (3a) may have reduced
dominance by a single species at station 4, the sole metric that improved over station 3. The
situation at stations 7 and 8 has already been discussed.
Three paired sites had lower downstream values. Station 12 was nine points lower than station
11, station 10 was four points lower than station 9, and station 6 was two points lower than
station 5. No difference was noted between stations 1 and 2. Though station 12 had more
species than 11, two characteristic minnow species-Rio Grande shiner and Tamaulipas shiner-
were absent downstream. As noted earlier station 12 also had a greater dominance by one
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species, a lower catch rate, and a higher incidence of disease. The substantial decrease in IBI
scores between stations 11 and 12, the low similarity index value, and the overall depressed
score for station 12 indicates a high probability of pollutant impact. The drop in IBI scores
between stations 9 and 10 is also supported by the aforementioned decrease in species richness,
and suggests a moderate probability of pollutant impact. Given the small variation between the
scores at stations 5 and 6 and the lack of pollutant inputs, the potential for impact is very slight.
In summary, based upon a variety of fish community measures station 12 has a high probability
of pollutant impact; station 10, a moderate probability; and station 3a, a slight probability. In
the latter case, the presence of fin deformities was the primary indicator, though Bestgen and
Platania (1988) noted impacts in the Rio Grande/Rfo Bravo at their sites downstream of the Rio
Conchos. Downstream of the Rio Conchos, they observed decreases in fish density and noted
thick deposits of anoxic silt. Station 3 demonstrated very slight impacts that may relate to
irrigation return flows. Stations 5 and 6 demonstrated some potential for contaminant impact,
but as noted, that probability appears very slight. Finally, the situation at stations 7 and 8
preclude any definitive evaluation. Stations 1, 2, 4, 9 and 11 demonstrated no potential for
contaminant impact based upon the fish community evaluation.
Fish Community Measures - International Falcon Reservoir to Brownsville/Matamoros (Stations
12d. 13-18^
Species Richness, Composition, and Similarity.—Downstream of International Falcon
Reservoir, species richness ranged from 11 to 21, with the lowest numbers at stations 16 and
12d (11 species) and station 18 (12 species). Species richness was highest (21 species) at
stations 13 and 14, the first stations downstream of International Falcon Reservoir, having been
augmented by estuarine fishes, introduced species, and gamefish. Richness was noticeably lower
at stations 12d, 16, and 18. Comparing sites bracketing the major sister cities, a difference of
seven species was noted between stations 15 and 16 and six species between 17 and 18. In the
former instance, none of the species present upstream and absent downstream are particularly
sensitive to pollution impacts, though the magnitude of difference in species number suggests
an effect. Species present at station 15 but not 16 were longnose gar (Lepisosteus osseus),
gizzard shad, bullhead minnow, smallmouth buffalo, western mosquitofish, white bass, Rio
Grande cichlid, blue tilapia, and striped mullet. Present downstream but not upstream were blue
catfish and gulf killifish. Similarity (Table 21) was moderate.
The situation at stations 17 and 18 was somewhat similar, with a clear pattern not evident in
considering species present at 17 but not 18. Those present at the upstream site but absent
downstream were spotted gar, gizzard shad, threadfin shad (Dorosoma petenense), red shiner,
Mexican tetra, blue catfish, western mosquitofish, white bass, redear sunfish (Lepomis
microlophus), and mountain mullet. Those present at station 18 but absent at 17 were
sheepshead minnow, Amazon and sailfin mollies, and Rio Grande cichlid. The downstream
species are more estuarine in nature, suggesting a generally higher salinity in the reach.
However, several of the species that dropped out between the two sites are tolerant of brackish
conditions and the assemblage at station 18 contains freshwater elements, including longear
sunfish and largemouth bass. Similarity was the lowest in the study (Table 21).
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Though stations 13 and 14 had the same number of species, several differences were apparent.
Upstream but not downstream were Atlantic needlefish, sailfin molly, redbreast sunfish (Lepomis
auritus), green sunfish, freshwater drum, Rio Grande cichlid, and threadfin shad. Species
observed at station 14 but not 13 were American eel, river carpsucker, blue catfish, western
mosquitofish, mountain mullet, striped mullet, and bigmouth sleeper. No particular pattern in
terms of species sensitivity is apparent from those differences. However, the number of
estuarine/marine species appears slightly greater downstream of International Anzalduas Dam
(station 14), which presents a migrational barrier at times. Similarity (Table 21) between the
two sites was moderate.
Station 12d had what appeared to be a highly modified community given its location several
hundred kilometers inland and the presence of 96% euryhaline species. Conductivity was
elevated (Table 10) which undoubtedly accounts for the brackish nature of the community. The
collection was dominated by Amazon molly.
Index of Biotic Integrity.—All of the upstream sites had higher IBI scores than their
downstream counterparts (Table 23). The largest difference was at station 16, which had a six
point lower IBI than station IS. Station 14 was four points lower than station 13, primarily
because of the second highest incidence of deformities, lesions, and tumors in the study. Station
18's IBI score dropped three points from the score at 17. Station 12d's score was comparable
to the downstream stations.
Station 16 posted a slightly depressed IBI from station 15, because of the already mentioned drop
in species richness and a greater domination of the sample by a single species, Mexican tetra.
As noted earlier, these sites had lower similarity. In fact, the fish community at station IS was
more similar to station 17 than to station 16. That factor, coupled with the lowered IBI score
and the sharp drop in species richness compared to surrounding sites, suggests probable
contaminant impact at station 16.
The lower IBI score at station 14 relative to 13 was the result of an increased number of
individuals with deformities, lesions, and tumors. The percentage was the second highest in the
study. Similarity was relatively high for the two sites and species richness at station 14 was the
second highest in the study. Consequently, the only indicator pointing to potential impact is the
percentage of diseased individuals. Based only upon that finding, the potential for pollutant
impact appears slight.
The decrease in IBI score at station 18 relative to station 17 is difficult to evaluate in terms of
probable pollution impacts. The site was downrated from station 17 for lower than optimal
species richness and the elevated percentage of estuarine and marine species. The latter metric
suggests that the community changes observed at station 18 are "real", but partitioning the
influences of decreased flows from contaminant impacts would require additional study.
Station 12d was downrated because of the aforementioned dominance by euryhaline species,
which exceeded even that of the lowermost site, station 18. The site was also downrated for
dominance by a single species. Seine catch rates were high compared to mainstem sites, but that
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may relate more to the lack of flow and small size of the stream, a circumstance making
collecting more efficient. Given the modified nature of the community, the potential for impact
appears moderate.
In summary, of the sites downstream of International Falcon Reservoir, 12d and 16 appear to
have a moderate potential for impact, with station 14 having a slight probability. The only
indicator pointing to station 14, however, is the presence of fishes with deformities, lesions, or
tumors. Station 18 is difficult to interpret given the aforementioned factors. Stations 13, 15,
and 17 demonstrate no impact attributable to contaminants, based upon this evaluation.
Fish Community Measures - Middle Reach Tributaries (Stations 6a. 6br 7b. 8d. 9b)
Species Richness, Composition, and Similarity.—Species richness in the middle reach
tributaries ranged from 12 to 23 (Figure 5), with the fewest number in the Pecos River (6a) and
San Felipe Creek (7b). In the latter case, seining sites were sparse in the reach sampled, which
may have influenced the collection. The Rio Escondido (9b) and Rio San Rodrigo (8d) had the
greatest number of species with 23 and 18, respectively. The Devils River (6b) collection
contained IS species. Similarity (Table 21) among the tributaries was generally low and in the
range of 0.333 to 0.667, with the widely varying habitats probably responsible. A major
exception was a value of 0.829 between the Rio Escondido and San Rodrigo. The Pecos River
was most dissimilar to the other tributaries with 75% of its fauna comprised of cyprinid species
and no centrarchids being represented. Several native minnow species were observed, including
proserpine shiner (Cyprinella proserpina), speckled chub, Texas shiner, and Tamaulipas shiner.
Centrarchid species were numerous in the Rio Escondido and Rio San Rodrigo with six and
seven species, respectively. The collections from those two streams included several
characteristic minnow species such as roundnose minnow and Texas shiner in both streams and
Rio Grande shiner in the Rio Escondido. Four centrarchid species were in the Devils River
collection along with several unique minnow species, including proserpine shiner and sand shiner
(Notropis stramineus), the latter only being taken from this site. San Felipe Creek had only two
minnow species—proserpine and blacktail shiner—and two centrarchid species. Blacktail shiner,
an introduced species, was quite abundant throughout the middle reaches of the river. Gray
redhorse, a catostomid, was also common at all of these tributary sites. Rio Grande darter was
found in the Pecos River, Rio Escondido, and Rio San Rodrigo.
Index ofBiotic Integrity.—I&l scores ranged from 18 to 23 (Table 24), with the lowest score
at San Felipe Creek (7b). That site was downrated for lower than optimal species richness, few
minnow species (one of which is introduced), lower than optimal catch rates, and a high
percentage of introduced species, primarily carp and blue tilapia. All of the tributaries were
downrated for a high percentage of introduced species. The IBI score for San Felipe Creek,
together with low species richness and few numbers of minnow species, would appear to indicate
a slight potential for impact. Also notable is the lack of Rio Grande darter, which Trevino-
Robinson (1959) collected at several stations on San Felipe Creek. Platania (1991), however,
collected additional minnow species and Rio Grande darter further upstream in Hinds Spring,
a tributary to San Felipe Creek. He also observed roundnose minnow and Texas shiner
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immediately downstream of the spring pool in Del Rio. Consequently, any impacts would
appear to be in the lowermost reach of the creek.
The Pecos River (6a) had the second lowest JEBI based upon slightly depressed species richness,
low catch rate, and high number of introduced species, primarily blacktail shiner. The Pecos
did have the highest number of minnow species of all sites in the study, several of which are
relatively sensitive to habitat degradation. The low catch rate may have in part related to
elevated conductivity (4,330 pmhos) making electrofishing difficult. Elevated conductivity
undoubtedly also selects against certain species that might otherwise populate the area. The
source of conductivity is partially natural, from brine springs, and also partially attributable to
man's activities. However, the potential for impact appears very slight.
Based upon this evaluation of the middle river tributaries, San Felipe Creek (7b) has slight
potential for contaminant impact; the Pecos River (6a), very slight potential; and the remainder
of the tributaries (6b, 8d, and 9b), no potential.
A summary of all of the fish community data yielded the following ratings for potential impacts:
indicated potential station(s)
inconclusive 7, 8, 18
none 1, 2, 4, 6b, 8d, 9, 9b, 11, 13, 15, 17
very slight 3, 5, 6, 6a
slight 3a, 7b, 14
moderate 10, 12d, 16
high 12
Integration of Data
The final aspect of the evaluation was to integrate available information to identify sites and
chemicals of potential concern. The objective was prioritization for purposes of water quality
management and future monitoring.
Sites of Potential Concern
Sites were grouped according to potential for toxic chemical impact, based on cumulative
information. Mainstem and tributary sites were treated separately, since the scope of evaluation
was different for the two.
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Mainstem.—Sites were ranked for 17 individual components belonging to seven categories,
as described below.
Water chemistry: (1) number of toxic chemicals detected; (2) number of toxic chemicals
that exceeded screening levels; (3) mean factor by which screening levels were
exceeded.
Sediment chemistry: (4), (5), and (6), same as for water chemistry.
Tissue chemistry: (7), (8), and (9), same as for water chemistry.
Toxicity testing of water: (10) Ceriodaphnia dubia mortality, percent greater than in the
control; (11) Ceriodaphnia dubia reproduction, percent less than in the control; (12)
fathead minnow embryo/larval mortality, percent greater than in the control.
Toxicity testing of sediment eluates: (13), (14), and (IS), same as for toxicity testing of
water.
Macrobenthic community evaluation: (16) indicated potential for effects by toxic
chemicals.
Fish community evaluation: (17) indicated potential for effects by toxic chemicals.
Station 5b, (Rio Grande/Rfo Bravo downstream from mouth of Lozier Canyon), was excluded
because no tissue data, macrobenthic data, or fish community data were generated for that site.
For a parameter that exceeded multiple screening levels, the exceedance factor utilized was that
associated with the most stringent screening level. Ranks based on evaluations of macrobenthic
communities and fish communities were multiplied by a factor of three to give all categories
equal weight. Rankings for individual components were added for each site, and the rank sums
were divided by the appropriate number of components. The latter step was necessary because
fish community evaluations were inconclusive for stations 7, 8, and 18, and could not be used
in the ranking. Thus, the divisor was 18 for stations 7, 8, and 18, and 21 for the other 15
stations. A final ranking was derived from the respective quotients (Table 25).
The ranking was used along with supplemental information to group the mainstem sites
according to potential for toxic chemical impact. Accessory information included
USEPA/TNRCC TOXNET data (Table 18), and other historical data relative to toxic chemicals
(see "Historical Information" section). The term "impact" as applied here refers to adverse
effects on aquatic life, or human health hazards associated with regular consumption of water
and/or fish.
Groupings were based on eight characteristics. Sites for which at least six characteristics
reflected a potential for toxic chemical impact were placed in the first group. The first group,
for which a high potential for toxic chemical impact was indicated, included stations 2 and 12.
Both were downstream from areas where appreciable amounts of wastewater enter the river (El
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Paso/Ciudad Juarez, and Laredo/Nuevo Laredo, respectively). These sites were characterized
by: significant effects in water and/or sediment eluate toxicity tests, and/or in multiple instances
in the USEPA/TNRCC TOXNET program (Table 18); four or more toxic chemicals that
exceeded screening levels in water and/or sediment; exceedance of human health criteria in
water and/or tissue by one or more toxic chemicals; moderate to high potential for instream
impact as indicated by macrobenthic and fish communities; a high ranking, from 1.0 to 3.0;
and indications of toxic chemical effects from historical data sources. Station 12 exhibited all
of these characteristics. Station 2 did not exhibit any human health criteria exceedances, nor did
the local fish community reflect any impact. Indications are that toxic effects at that site may
be selective for invertebrates, as suggested by macrobenthic community structure and the fact
that TOXNET effects have involved Ceriodaphnia dubia, but not fathead minnow embryo/larvae
(Table 18).
Characteristics of the remaining sites were too diverse for specific limits to be established; the
following general characteristics were used for grouping purposes: significant effects in water
and/or sediment eluate toxicity tests, in the present study or the TOXNET program; four toxic
chemicals that exceeded screening levels in water and/or sediment; exceedance of human health
criteria in water and/or tissue by one or more toxic chemicals; moderate potential for instream
impact as indicated by macrobenthic or fish communities; a ranking from 2.0 to 9.0; and
indications of toxic chemical effects from historical data sources. Sites exhibiting three or more
of these characteristics were placed in the second group; those exhibiting two or less were placed
in the third group.
The second group, for which a slight to moderate potential for toxic chemical impact was
indicated, included stations 3, 10, 14, and 16. The latter three were downstream from areas
where a substantial potential for toxic chemical input exists (Eagle Pass/Piedras Negras,
International Anzalduas Dam, and Hidalgo/Reynosa, respectively), while station 3 was the
upstream control site for the Presidio/Ojinaga reach. Characteristics placing these sites into the
second group consisted of the following. Station 3 had four toxic chemicals that exceeded
screening levels in water and/or sediment, one chemical that exceeded human health criteria in
tissue, and a relatively high ranking (5.0). Station 10 had one toxic chemical that exceeded
human health criteria in tissue, a moderate potential for toxic chemical impact as indicated by
the fish community, and two instances of significant toxic effects from the TOXNET program.
Station 14 had four toxic chemicals that exceeded screening levels in water and/or sediment, one
toxic chemical that exceeded human health criteria in tissue, a relatively high ranking (6.5), and
implications of toxic effects from historical information. Station 16 had two toxic chemicals that
exceeded human health criteria in tissue, a moderate potential for toxic chemical impact as
indicated by the fish community, and a relatively high ranking (4.0).
The third group, for which little or no potential for toxic chemical impact was indicated,
included stations 1, 4, 5, 6, 7, 8, 9, 11, 13, 15, 17, and 18. Not surprisingly, the group was
dominated by upstream control sites (1, 7, 9, 11, 13, 15, 17) and remote sites (5, 6). Three,
however, were downstream sites (4, 8, 18), and associated characteristics indicate that effects
of toxic chemical inputs from the Presidio/Ojinaga, Del Rio/Ciudad Acuna, and
Brownsville/Matamoros areas are minimal.
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Three of the sites in the third group (4,7,8) exhibited two of the aforementioned characteristics,
five (1,5, 6,9, IS) exhibited one of the characteristics, and four (11, 13, 17, 18) exhibited none
of the characteristics. Stations 7 and 8 displayed exceedances of human health criteria in tissue,
8 had a slightly elevated ranking (8.0), and 7 had a high ranking (2.0). The latter mainly
resulted from elevated levels of a variety of toxic chemicals in fish tissue. This may be related
to hypolimnetic releases from International Amistad Reservoir, as there are no known extrinsic
inputs of contaminants between the dam and station 7. The only adverse indications for station
4 were from historical data, involving one instance of significant effects in the TOXNET
program (Table 18), and documentation of elevated pesticide levels during the 1970's and early
1980's (TNRCC, 1992a). There were no indications of toxic chemical-related problems at
station 4 during the present study.
Among sites having one negative characteristic, stations 6, 9, and 15 exhibited exceedances of
human health criteria in fish tissue, station 5 ranked relatively high (6.5), due mainly to
exceedances of screening levels by metals in fish tissue, and station 1 displayed significant
effects in toxicity testing of water. The latter occurrence may have been an aberration. No
causative agent was apparent in the accompanying chemical data, and no other finding gave any
indication of toxic chemical impact. Furthermore, there have been no significant effects in five
samples collected for the TOXNET program (Table 18).
Tributaries.—Sites were ranked for each of 12 components belonging to four categories
(water chemistry, sediment chemistry, toxicity testing of water, and toxicity testing of sediment
eluates, as described for mainstem sites). Toxic chemical data for fish tissue, and bioassessment
data for macrobenthic and fish communities, were not employed in the ranking because they
were not generated for all tributary sites. For a parameter that exceeded multiple screening
levels, the exceedance factor utilized was that associated with the most stringent screening level.
Rankings for the 12 components were added for each site, and a final ranking was derived from
the rank sums (Table 26).
The ranking was used along with supplemental information to group the tributaries according
to potential for toxic chemical impact. Accessory information included exceedances of human
health criteria in tissue (available for six sites), and observations on the condition of fish
communities (available for eight sites). The term "impact" as applied here refers to adverse
effects on aquatic life within the tributaries themselves, or human health hazards associated with
regular consumption of water and/or fish from these systems. Potential effects tributary inflows
exert on the Rio Grande/Rio Bravo are considered separately.
The first group, for which a high potential for toxic chemical impact was indicated, included
stations la, 2a, lOa, lla, lie, and 15a. These were characterized by: significant effects in
water and/or sediment eluate toxicity tests, for which probable toxic chemical causative agents
were identified; four or more toxic chemicals that exceeded screening levels in water and/or
sediment; exceedance of human health criteria in water and/or tissue by one or more toxic
chemicals, by factors > 5X; high potential for instream impact as indicated by fish community
attributes (if the fish community was assessed); and a high ranking, from 1.0 to 7.0. Stations
la, 2a, and 15 a exhibited all of these characteristics, and stations lOa, lla, and lie, all but
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one. Divergences were: lOa, total dissolved solids as the probable cause of toxicity testing
effects; Ha, no exceedance of human health criteria; and lie, exceedance of human health
criteria by a factor < SX.
The second group, for which a slight to moderate potential for toxic chemical impact was
implied, consisted of stations 3a, 7b, 9a, and 12d. Traits of this group were: significant effects
in water and/or sediment eluate toxicity tests, for which probable toxic chemical causative agents
were identified; two or three toxic chemicals that exceeded screening levels in water and/or
sediment; exceedance of human health criteria in water and/or tissue by one or more toxic
chemicals, by a factor of 2 to 3X; slight or moderate potential for instream impact as reflected
by the condition of the fish community (if the fish community was assessed); and a moderate
ranking, from 8.0 to 14.0. Stations 3a and 9a exhibited all of these traits, station 12d all but
one, and station 7b all but two. Deviations were: 12d, a ranking higher than 8.0; and 7b,
fewer than two toxic chemicals that exceeded screening levels in water and/or sediment, and
exceedance of human health criteria by a factor > 3X.
The third group, for which little or no potential for toxic chemical impact was indicated, was
comprised of stations 3b, 5a, 6a, 6b, 7a, 8a, 8b, 8c, 8d, 8e, 9b, lib, 12a, 125, 12c, and 12e.
These displayed the following attributes: no significant effects in water or sediment eluate
toxicity tests, significant effects attributable to total dissolved solids, or significant effects
regarded as ecologically unimportant; two or fewer toxic chemicals that exceeded screening
levels in water and/or sediment; no exceedance of human health criteria, or exceedance only
by selenium in water by a factor £ 1.4X; very slight or no potential for instream impact as
indicated by the fish community (if the fish community was assessed); and a low ranking, from
15.0 to 26.0. Stations 6a, 6b, 7a, 8a, 8b, 8c, 8d, 9b, and 12e exhibited all of these
characteristics, stations 3b, 5a, 12b, and 12c all but one, and stations 8e, lib, and 12a all but
two. Divergences were: 3b, Sa, and 12b, more than two toxic chemicals that exceeded
screening levels in water and/or sediment; 12c, a ranking higher than 15.0; and 8e, lib, and
12a, more than two toxic chemicals that exceeded screening levels in water and/or sediment, and
a ranking higher than 15.0.
During low-flow conditions such as were prevalent during the study, the potential for adverse
effects on the Rio Grande/Rio Bravo is related to the above grouping and the volume of inflow.
Among tributaries in the first group, the potential appears to be high for la and 2a (1.3-1.7 cms
or 45-61 cfs), moderate for lla and 15a (0.17-0.45 cms or 5.9-16 cfs), and slight for lOa and
lie (< 0.06 cms or 2 cfs). The potential associated with tributaries in the second group is
slight to moderate for 3a and 7b (4-15 cms or 141-530 cfs), and very slight for 9a and 12d (<
0.03 cms or 1 cfs). Tributaries in the third group would not be expected to affect the mainstem,
regardless of inflow volume.
A possibility exists that tributaries in the first and second group may exert greater relative effects
during high flow events, due to scouring of contaminated bottom sediments. However, this has
not been documented.
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Toxic Chemicals of Potential Concern
The 30 toxic chemicals that exceeded screening levels are considered to be of potential concern
in the Rio Giande/Rfo Bravo system. These chemicals were assigned an approximate level of
importance based on their occurrence.
A high priority group includes 19 chemicals that exceeded screening levels in the mainstem.
residual chlorine lead chlordane
methylene chloride mercury p,p' DDE
toluene nickel dieldrin
arsenic selenium gamma-bhc (lindane)
cadmium silver total PCB's
chromium zinc cyanide
copper
A medium priority group includes four chemicals that exceeded screening levels at multiple
tributary sites.
un-ionized ammonia phenol diazinon
parachlorometa cresol
A low priority group includes seven chemicals that exceeded screening levels at only one
tributary site.
phenolics recoverable thallium diethyl phthalate
chloroform bis(2-ethylhexyl) phthalate di-n-butyl phthalate
antimony
Identification of these 30 chemicals is very consistent with earlier indications for the system.
In its review of historical water quality data for the Rio Grande/Rfo Bravo basin, TNRCC
(1992a) identified toxic chemicals of potential concern. All but five of those (endrin,
hexachlorobenzene, toxaphene, 2,4,5-T, total PAH's) are included in the preceding list. 2,4,5-
T, historically elevated in the lower Pecos River, was not analyzed in the present study, but the
other four were. Neither endrin nor toxaphene was detected in any matrix. Hexachlorobenzene
was detected in tissue at one site (station 12), but did not exceed screening levels. Of fifteen
PAH's that were analyzed, naphthalene was the only one that occurred above detection limits,
in water from stations la and 15a, but again, screening levels were not exceeded. Thus, the
subject chemicals were not shown to be problemmatic during the present study, and for that
reason were not included in the preceding list.
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RECOMMENDATIONS
Follow-up monitoring should be conducted at sites of potential concern as identified in the
present study. The objectives would be to reexamine or better define the degree of impact, to
assess temporal variation, and, in some cases, to try to further identify sources of toxic
chemicals. Surveillance should be conducted at sites where a slight-to-moderate or high potential
for toxic chemical impact was shown to exist (mainstem stations 2, 3, 10, 12, 14, 16; tributary
stations la, 2a, 3a, 7b, 9a, lOa, lla, He, 12d, 15a). Except for cases addressed below, the
scope of evaluation should be the same as that employed in the present study (see "Types of
Analyses" under "STUDY DESCRIPTION").
Expanded monitoring is recommended for the two mainstem locations where a high potential for
toxic chemical impact was indicated (station 2, downstream from El Paso/Ciudad Juarez; station
12, downstream from Laredo/Nuevo Laredo). In reevaluating the station 2 area, three stations
should be sampled, including la, 2, and a new mainstem site located a short distance upstream
from la. The purpose of sampling the new site would be to try to distinguish the effects of
upstream urban influences from those of the El Paso Public Service Board Haskell R. Street
wastewater treatment plant discharge. The scope of evaluation at the new site should include
toxic chemical analyses and toxicity testing of water and sediment. Bioassessments and analysis
of contaminants in fish tissue are not recommended for the new site because physical habitat in
the vicinity is extremely poor (channelized, concrete-lined streambed). Addition of another
mainstem site downstream from station 2 would serve no purpose, because all flow in the river
typically is diverted a short distance downstream, at Riverside Diversion Dam.
In reevaluating the station 12 area, sampling should be performed at station 12, at a new site
downstream from station 12, and in local tributaries sampled during the present study (lOa, lla,
lib, lie). A number of additional inflows exist in the vicinity (Buzan, 1990), and as many of
these as possible should also be sampled in an effort to identify toxic chemical inputs. The new
mainstem site should be established 10-15 km (6.2-9.3 miles) downstream from station 12, to
examine longitudinal variation and to further define the extent of impact of toxic chemicals
emanating from Laredo/Nuevo Laredo.
Another recommendation is that the Rfo Conchos (3a) and San Felipe Creek (7b), the only
tributaries of potential concern that support significant aquatic life habitat, should be subjected
to intensive surveillance. Multiple sites along the gradient of each stream should be evaluated
similarly to the way mainstem stations were evaluated in the present study. Point source
discharges should also be sampled, and evaluated as were tributaries during the present study.
Finally, surveillance to specifically evaluate toxic chemical concentrations in fish tissue (whole
body and edible portions) is recommended for six locations. Four are sites where the potential
for overall toxic chemical impact was shown to be low in the present study, but which did
exhibit elevated numbers of toxic chemicals that exceeded screening levels, or anomalously high
concentrations of certain contaminants, in fish tissue (stations 6, 7, 9, 11). These should be
reevaluated to further characterize possible risks to fish communities, predatory species that feed
on fish, and human health. Additionally, contaminant levels in fish tissue should be evaluated
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at a minimum of one site in International Amistad Reservoir and one site in International Falcon
Reservoir, neither of which was sampled during the present study. Both support significant sport
fisheries, and baseline data is needed to characterize existing tissue contaminant levels. The
most important locations, if single sites were utilized, would be in the extreme upper end of each
reservoir, where the potential for contamination is greatest in association with riverine inflow.
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75
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APPENDIX A
Tables
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Table 1
List of Sampling Stations
Map Station
Code Description
Mainstem stations
1 Rio Grande at Courcheane Bridge (Hwy. 273) in El Paso, 2.7 km upstream from
American Dam, at river km 2,020.8
2 Rio Grande at Zaragosa International Bridge in El Paso, at river km 1,992.8
3 Rio Grande 5.0 km upstream from Rio Conchos confluence near Presidio, at
river km 1,552.2
4 Rio Grande below Rio Conchos confluence, 14.4 km downstream from
Presidio/Ojinaga International Bridge, at river km 1,528.5
5 Rio Grande at mouth of Santa Elena Canyon in Big Bend National Park, at
river km 1,424.7
5b* Rio Grande immediately downstream from mouth of Lozier Canyon, 44 km
southeast of Dryden, at river km 1,062.7
6 Rio Grande at Foster Ranch near Langtry, at river km 1,058.2
7 Rio Grande 0.4 km upstream from Del Rio/Ciudad Acuna International Bridge,
at river km 903.2
8 Rio Grande 6.4 km downstream from Del Rio/Ciudad Acuna International
Bridge, at river km 896.8
9 Rio Grande 1.0 km upstream from Eagle Pass/Piedras Negras International
Bridge, at river km 799.8
10 Rio Grande 14 km downstream from Eagle Pass/Piedras Negras International
Bridge, near irrigation canal lateral 50, at river km 785.8
11 Rio Grande at Laredo water treatment plant, 5.1 km upstream from old
Laredo/Nuevo Laredo International Bridge (U.S. 81), at river km 585.9
12 Rio Grande at pipeline crossing, 13.2 km downstream from old Laredo/Nuevo
Laredo International Bridge (U.S. 81), at river km 567.6
13 Rio Grande at Los Ebanos, 54.7 km upstream from Anzalduas Dam, at river km
328.8
14 Rio Grande 0.8 km downstream from Anzalduas Dam, at river km 273.3
15 Rio Grande at Hidalgo/Reynosa International Bridge (U.S. 281), at river km
256.7
16 Rio Grande below Anhelo Drain south of Las Milpas, at river km 244.1
17 Rio Grande 6.3 km downstream from San Benito pumping plant and 15.3 km
southwest of San Benito, at river km 155.8
18 Rio Grande 0.3 km downstream from El Jardin pumping plant and 11.2 km
downstream from Brownsville/Matamoros International Bridge (U.S. 77), at
river km 78.3
79
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Table l (continued)
List of Sampling Stations
Map Station
Code Description
Stations
la El Paso Public Service Board Haskell street WWTP outfall, in El Paso
County, Texas
2a Ciudad Juarez sewage discharge canal (Dren de Interceptacion) immediately
above Alamo grade control structure, 2.0 km northeast of Colonia Esperanza,
in the Mexican state of Chihuahua
3a Rio Conchos 0.2 km upstream from mouth and 4.8 km northwest of Ojinaga, in
the Mexican state of Chihuahua
3b Alamito Creek at FM 170, 0.5 km upstream from mouth, and 9.7 km southeast
of Presidio, in Presidio County, Texas
5a Terlingua Creek 0.2 km upstream from mouth and 13.7 km south of Ter lingua,
in Brewster County, Texas
6a Pecos River at Shumla Bend, 12.1 km east of Langtry, in Val Verde County,
Texas
6b Devils River at Pafford Crossing, 18.5 km east of Comstock, in Val Verde
County, Texas
7 a Arroyo de las Vacas 0.2 km upstream from mouth in Ciudad Acuna, in the
Mexican state of Coahuila
7b San Felipe Creek at Silos Farm road bridge, 1.8 km upstream from the mouth
and 3.2 km south-southeast of Del Rio, in Val Verde County, Texas
8a Pinto Creek at Moody Ranch, 2.6 km upstream from mouth, in Kinney County,
Texas
8b Rio San Diego at highway crossing, 2.4 km upstream from mouth and 6.0 km
west of Jimenez, in the Mexican state of Coahuila
8c Las Moras Creek at U.S. 277 north of Quemado, 1.8 km upstream from mouth,
in Maverick County, Texas
8d Rio San Rodrigo 1.6 km upstream from mouth at El Moral, in the Mexican
state of Coahuila
8e Maverick Canal return flow to Rio Grande, immediately downstream from
Maverick Power Plant, 14.5 km north-northwest of Eagle Pass, in Maverick
County, Texas
9a Unnamed tributary 0.1 km upstream from mouth and 3.6 km south of Eagle
Pass/Piedras Negras International Bridge, in the Mexican state of Coahuila
9b Rio Escondido 0.1 km upstream from mouth and 5.9 km east of Villa de
Fuente, in the Mexican state of Coahuila
lOa Manadas Creek 0.8 km upstream from mouth and 1.2 km downstream from FM
1472, near northern city limit of Laredo, in Webb County, Texas
80
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Table 1 (continued)
List of Sampling Stations
Map Station
Code Description
lla Zacate Creek 0.1 km upstream from mouth in Laredo, in Webb County, Texas
lib Chacon Creek 0.1 km upstream from mouth in Laredo, in Webb County, Texas
lie Arroyo el Coyote 0.1 km upstream from mouth and 7.2 km south-southeast of
Nuevo Laredo, in the Mexican state of Tamaulipas
12a Rio Salado at flow gage located 10 km southeast of Las Tortillas, in the
Mexican state of Tamaulipas
12b Rio Alamo at flow gage located 8 km upstream from mouth and 1 km north of
Ciudad Mier, in the Mexican state of Tamaulipas
12 c Rio San Juan at flow gage located 5 km upstream from mouth in Camargo, in
the Mexican state of Tamaulipas
12d Arroyo Los Olmos 2.1 km upstream from mouth at U.S. 83 south of Rio Grande
City, in Starr County, Texas
12e Puertecitos Drain 3.8 km upstream from mouth and 12.3 km west-northwest of
Ciudad Diaz Ordaz, in the Mexican state of Tamaulipas
15a Anhelo Drain 0.1 km upstream from mouth and 3.2 km east of Reynosa, in the
Mexican state of Tamaulipas
* - supplemental mainstem station established to provide a baseline for
assessing future effects of Lozier Canyon inflow; parametric coverage
similar to that for tributaries
81
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Table 2
Toxic Chemicals Targeted for Analysis
in Water, Sediment, and Fish Tissue8
Phenols and Creaols
parachlorometa cresol
pentachlorophenol
phenol (CaHsOH) »ingle compound
phenolica recoverable
2-chlorophenol
2-nitrophenol
2,4-dichlorophenol
2,4-dimethylphenol
2,4-dinitrophenol
2,4,6-trichlorophenol
4-nitropheno1
4,6-dinitro-ortho-cre8ol
Ethers
bia(chloromethy1) ether*
bis(2-chloroethyoxy) methane
bis(2-chloroethyl) ether
bia(2-chloroisopropyl) ether
2-chloroethyl vinyl ether*
4-bromophenyl phenyl ether
4-chlorophenyl phenyl ether
Halooenated Aliphatics
bromodichloromethane
bromoform
carbon tetrachloride
chloroethane
chloroform
dibromochloromethane
dichlorodifluoromethane
hexachlorobutadiene
hexachlorocyclopentadiene
hexachloroethane
methyl bromide
methyl chloride
methylene chloride
tetrachloroethylene
trichloroethylene
trichlorofluoromethane
vinyl chloride
1,1-dichloroethane
1,1-dichloroethylene
1,1,1-trichloroethane
1,1,2-trichloroethane
1,1,2,2-tetrachloroethane
1,2-dichloroethane
1,2-dichloropropane
1,2-trana-dichloroethylene
1,3-trana-dichloropropene
1,3-cia-dichloropropene
Polvcvclic Aromatic Hydrocarbona
acenaphthene
acenaphthylene
anthracene/phenanthrene
benzo(A) anthracene 1,2-benzanthracene
benzo(B) fluoroanthene
benzo(GHI) perylene 1,12-benzoperylene
benzo(K) £luoranthene
benzo-A-pyrene
chryaene
fluoranthene
fluorene
indeno(1,2,3-CO) pyrene
naphthalene
pyrene
1,2,5,6-dibenzanthracene
Monocvelie Aromatics
benzene
chlorobenzene
ethylbenzene
hexachlorobenzene
nitrobenzene
Btyrene"
toluene
xylene*
1,2-dichlorobenzene
1,2,4-trichlorobenzene
1,3-dichlorobenzene
1,4-dichlorobenzene
2,4-dinitrotoluene
2,6-dinitrotoluene
NitroBamii
id Other N Compounds
acrylonitrile
benzidine
n-nitroaodi-N-propylamine
n-nitrosodimethylamine
n-nitrosodiphenylamine
1,2-diphenylhydrazine
3,3-dichlorobenzidine
Metals
aluminum4
antimony
arsenic
beryllium
cadmium
chromium
copper
82
-------
Table 2 (continued)
Toxic Chemicals Targeted for Analysis
in Water, Sediment, and Fish Tissue8
Metals (continued)
lead
mercury
nickel
selenium
silver
thallium
zinc
Pesticides
acrolein*
aldicarbc
aldrin
alpha benzene hexachloride
atrazinec
beta benzene hexachloride
carbarylb
carbofuran"
chlordane .
chlorfenvinphosc
chlorothalonilc
chlorpyrifosb
chlorsulfuron'*
p,p' ODD
p,p' DOE
p,p' DDT
delta benzene hexachloride
demetonb*
diazinon*
dibromochloropropane (dbcp)c
dicamba"
2,4-dichlorophenoxyacetic
acid (2,4-D)b
dicofol (kelthane)"
dicrotophosc*
dieldrin
dinosebc
endosulfan alpha
endosulfan beta
endosulfan sulfate
endrin
endrin aldehyde
fenthion (baytex)e*
gamma-bhc (1indane)
guthion
heptachlor
heptachlor epoxide
isophorone
malathionb
metsulfuron'*
methomyl*
methoxychlorb
metolachlore
mirexb
parathionb
picloram0
prometon6*
simazine*
tetraethylpyrophosphate (tepp)c*
toxaphene
2,4,5-TP (silvex)"
PCS'a and Related Compounds
arochlor 1016
arochlor 1221
arochlor 1232
arochlor 1242
arochlor 1248
arochlor 1254
arochlor 1260
2-chloronaphthalene
Phthalate Esters
bis(2-ethylhexyl) phthalate
di-n-butyl phthalate
di-n-octyl phthalate
diethyl phthalate
dimethyl phthalate
n-butyl benzyl phthalate
General Inorganics
cyanide
- parameters without superscripts are designated as priority pollutants by the
Code of Federal Regulations Part 423 Appendix A; parameters with
superscripts are non-priority pollutants whose inclusion is accounted for
in footnotes b, c, and d; two additional toxicants, ammonia and residual
chlorine, also were analyzed in water, but are included in the conventional
parameter category
83
-------
Table 2 (continued)
Toxic Chemicals Targeted for Analysis
in Water, Sediment, and Fish Tissue8
b - parameter* for which numerical criteria have been established by the State
of Texas
c - parameters which were recommended for inclusion by USEPA Region 6
d - parameters which Lewis et al. (1991) showed to have a potential for
affecting the Rio Grande
* - parameters which the laboratory did not have the capability to analyze
84
-------
Table 3
Sample Specifications
Parameters
Sample Volume/
Type of Container
Preservation
Holding
Time
WATER
•TSS, TDS, chloride,
sulfate
one 1 qt. cubitainer*
ice
7 days
•total hardness,
turbidity
one 1 qt. cubitainer*
ice
24 hrs.
•ammonia, TOC,
phenol
one 1 qt. Mason jar
w/ teflon lid liner*
cone. H2SO4 to
pH <2; ice
28 days
•dissolved metals
one 1 qt.
plastic bottle"
filter; HNOj
to pH <2; ice
28 days
•volatile organics
two 40-mL glass
screw top vials w/
teflon lid liners0
ice
7 days
•pesticides
two 1 qt. Mason jars
w/ teflon lid liners'*
•ice
7 days
•other organics
one 1 qt. Mason jar
w/ teflon lid liner*
ice
7 days
•cyanide
one 1 qt. cubitainer*
*NaOH to pH
<12; ice
14 days
•toxicity testing
two 1 gal. cubitainers*
ice
24 hrs.
SEDIMENT
•organics
one 1 qt. Mason jar
w/ teflon lid liner"1
ice
7 days
•metals
one 1 qt. Mason jar w/
teflon lid liner
ice
28 days
•TOC, acid volatile
sulfide, particle
size distribution
one 1 qt. Mason jar w/
teflon lid liner*
ice
7 days
85
-------
Table 3 (continued)
Sample Specifications
Parameters Sample Volume/ Preservation Holding
Type of Container Time
•toxicity testing one 1 gal. cubitainer* ice 7 days
1- containers pretreated by rinsing with site water
k - containers pretreated by rinsing with 10% metals-grade HNOj followed by type
2 deionized water
e - containers pretreated by manufacturer
4 - containers pretreated by rinsing with methylene chloride, then baked at 40
°C overnight to remove traces of solvent
* - initially, 100 mg sodium thiosulfate added if residual chlorine present
' - initially, 0.6 g ascorbic acid added if residual chlorine present
86
-------
Table 4
Methods Utilized by Texas Department of
Health Environmental Chemistry Laboratory
Anah/te
Matrix
Pieputtkn/DigeKioii/
Analytical
Method
Method
DctcnpCMD
GRAIN SIZE ANALYSIS
INORGANICS
ifijf volatile fvl
chloride
cyanide
tipid
* acrccocQ
sample filtered
ample filtered
sulfale
total dissolved solids
total hardness
total oig nic cafbon
total upended wfidt
witer
umple ground
with chlorofonn
nmpke aeteeaed
umple filtered
umple filtered
•ample filtered
simple screened, dried
Method 9030A
EPA 350.1
EPA 325.2
EPA 335.2
EPA 335.2
EPA 335.2
AOAC15 ed. 964.12
EPA 420.1
EPA 420.1
EPA 420.1
EPA 9036
EPA 160.1
EPA 130.2
EPA 415.1
EPA 415.1
EPA 160.2
EPA 180.1
ootoranrtnc,
autoouted pheaate
ferricy anidc AAO
total, ipectropbotometfic
total, ipfctrophotomctnc
total, ipectropbotometnc
4-AAPwrthdutilUtioo
ipoctrophouiBiBtnc.
4-AAPwi
-------
Table 4 (continued)
Methods Utilized by Texas Department of
Health Environmental Chemistry Laboratory
Analyte Matrix PnpMttna/Difeatiao/
Extraction Method
anemc water SM3113B
iriiomt EPA SW Method 3050
time modification of EPA
SW Method 200 J
beryOwm water
iriijamt EPA SW Method 3050
time •edification of EPA
SW Method 200.3
ecdauuni wvtar
irilimmf EPA SW Method 3050
time modification of EPA
SW Method 200.3
dno*. w«er
eednctt EPA SW Method 3050
time BodifietfionorEPA
SW Mettwd 200 J
copper wMer
inilimrnt EPA SW Mediod 3050
time modifiutiaaorEPA
SW Method 200 J
tad wMer
KOmeot EPA SW Method 3050
time Bodtficttna of EPA
SWMe*od200J
mercury wMer EPA 245.1
irtomt EPA SW Method 3050
laeae modiBririon of EPA
SW Method 200.3
nickel water
•odiBeat EPA SW Method 3050
time modification of EPA
SW Method 200 J
sekaium water SM3113B
•eoneol EPA SW Method 3050
Analytical
VldlUKl
EPA 206.2
EPA 206.2
EPA 206 J
EPA 200.7
EPA 200.7
EPA 200.7
EPA 213 .2
EPA 213.2
EPA 213.2
EPA 200.7
EPA 200.7
EPA 218.2
EPA 200.7
EPA 200.7
EPA 220.2
EPA 239.2
EPA 239.2
EPA 239.2
EPA 245.1
EPA 245 J
EPA 245.6
EPA 200.7
EPA 200.7
EPA 249.2
EPA 270.2
- 1 .limiinn^ihltiilrnr
Method
DncripttOD
OFAA
OFAA
•MMfj««bl
njuiHib
ICP
ICP
ICP
OFAA
OFAA
OFAA
ICP
ICP
OFAA
ICP
ICP
OFAA
OFAA
OFAA
OFAA
manual cold vapor
maoaal cold vapor
manual cold vapor
ICP
ICP
OFAA
OFAA
flu ii ••^••^•ir
nMoraBBnc
88
-------
Table 4 (continued)
Methods Utilized by Texas Department of
Health Environmental Chemistry Laboratory
AnaJyte
tOver
th.nw— .
OK
VOLATILE ORGANICS
SEMIVOLATILE OROANICS
INSECTICIDES
Matrix Prcpantion/DicefUan/
Extraction Method
time EPA SW Method 3050 2,
water
tedinent EPA SW Method 3050
time modification of EPA
SW Method 200 3
water
•edaaent EPA SW Method 3050
time modification of EPA
SW Method 200 J
water
aednent EPA SW Method 3050
time modificition of EPA
SW Method 200 J
water EPA 5030, purfe A trap
mMtmffttt EPA 5030, mrthamrt extraction,
purge A trap
tiaaue EPA Region VQ Lab tonication
water EPA 3520, conrinnniit liquid/liquid
aedaaent EPA 3540, Soxhkt extraction
tiaaue EPA 3540, Sinrhlft extraction;
EPA 3640, OPC ckanup
water EPA 3510, fepanlory funnel
aedaoeot EPA 3540. Soxakt extraction; EPA
Analytical
Method
EPA 200.7
EPA 272.2
EPA 272.2
EPA 279.2
EPA 279 3.
EPA 279.2
EPA 200.7
EPA 200.7
EPA 200.7
EPA 8260
EPA 8260
EPA 8260
EPA 8270
EPA 8270
EPA 8270
EPA 8080
EPA 8080
Method
Deacription
ICP
OFAA
OFAA
OFAA
OFAA
OFAA
ICP
ICP
ICP
OC/MS
OC/MS
OC/MS
OC/MS
OC/MS
OC/MS
OC-ECD
OC-ECD
3620, ckanup florail fractionation
USFDA PAM Method 211, blender
extraction; EPA 3640, OPC cleanup;
EPA 3620, ckanup floruit fractionation
EPA 8080
OC-BCD
HERBICIDES
EPA 3510, Kpantory funnel. EPA 8150
diazomelhane eMerificalion
EPA 8150, shaker, aepanlory EPA 8150
funnel, diazomethane eateriTicalion
OC-ECD
OC-ECD
CARBAMATES
•Kt"'mn. direct injection
EPA 531
HPLC poat cohnm
89
-------
-------
Table 4 (continued)
Methods Utilized by Comisi6n Nacional del Agua
-------
-------
ANALYTICAL iHgiTHiirnrn-Bgj SAMPLE Hil4»ARftTICN, DESCRIPTION OF
EQCDjPMENT, AID DETECTION LIMITS
ANALYTICAL MEXB3DOUX3ES
PHYSICAL-CHEMICAL PARAMETERS IN HATER SAMPLES:
The samples were filtered in the field to 0.45 microns, except for those to be
analyzed for Total Organic Carbon, Total Suspended Solids, and Turbidity.
The techniques employed are those recommended by Mexican Official Norms, listed
in the National Water Commission's (CNA) Manual of Techniques. These procedures
are similar to the analytical methods described in the APHA-AWWA-WEF's •'Standard
Methods for the Examination of Water and Wastewater" 18th edition (17th for
Sulf ates) . Below are described the methods employed in this study, as well as
the limits of detection.
HYDROGEN POTENTIAL (pH) :
Field determination, electrometric method vising a CheckMate 90 Analyzer,
Corning. L.D.= 0.5.
ELECTRICAL CONDDCnVTIY:
In the first phase, the reported values were field determinations using the
electrometric method. In the 2nd, 3rd and 4th phases, the CheckMate 90 Analyzer
was used with method NOM-AA-— 93-1984 / Method 2510 B Standard Methods, L.D.= 1
ndcromhos/cm.
DISSOLVED OXW3QJ:
Field determination, electrometric method using the CheckMate 90 Analyzer,
Corning L.D. =0.3 mg/1.
CHLORIDES (CL):
Method NOM-AA-73-1981 / Method 4500-C1 B Standard Methods by Argentometry,
titration with AgNO3 0.0141 N, using Potassium Chromate as an indicator, L.D. =2
mg/1.
TOTAL DISSOLVED Hnr.Tns (TDS) :
Method NOM-AA-20-1980 / Method 2540 C by gravimetry, drying at 178-182 degrees
Centigrade. L.D.= 1 mg/1.
TOTAL SOSPENDED Hnff.TTlS (TSS) :
Method NCM-AA-34-l981/Method 2540 D Standard Methods by gravimetry, determining
the amount of material retained by a 0.45 micron filter, and drying at 103-105
degrees Centrigrade, L.D.= 1 mg/1.
-------
-2-
TGORL
In the 3rd phase, Method 2340 B Standard Methods was employed, calculated after
the separate determinations for Calcium and Magnesium by atomic absorption
expressed in mg/1 of CaCO3. L.D.=3 mg/1. In the first stage, there being no
sample, a calculation was made using lineal regression analysis of electrical
conductivity - hardness and total dissolved solids - hardness. In stages 2 and
4, method NOM-AA-72-1981 / Method 2340 C Standard Methods, titration with EDTA,
L.D.-1 mg/1 as CaO03.
SOLfKCES (SD4):
Method NOM-AA-74-1981 / Method 4500-SO4 E, Standard Methods 17th edition, by
turbidity, precipitation using Barium Chloride. In the first stage, an HF
Instruments DRT 100 turbidity meter was used, L.D.= 5 mg/1. In stage 2, a
Bausch and Lomb Spectronic 20 spectrophotometer was used, L.D. =1 mg/1. In
stage 3, a Milton Roy Spectronic 21 D spectrophotometer was used, L.D.=1 mg/1.
In stage 4, a Coleman Jr. II Perkin Elmer spectrophotometer was used, L.D. 1
mg/1.
Nephelometric method, Method 2130 B Standard Methods. In stage 1, an HF
Instruments DRT 100 turbidity meter was used. In stage 2, a Digital Monitec TA1
nephelometer was used. In stages 3 and 4, a Cole Farmer 8391-35 turbidity meter
was used. L.D. = 0.05 UIN (?) .
AMOOACAL NTraOGH* (N-*«3) :
In stages 1,3 and 4, the titrimetric method with prior distillation was used.
Method 4500-NH3 E Standard Methods. A Macro Kjeldhal Lab Conco distiller was
employed using a Boric acid solution as an indicator as an absorbent of the
distillate and titrating with H2SO4 0.02N. L.D.=0.ll mg/1. In stage 2, Method
4500-NH3 Standard Methods colorimetric Nesslerization with distillation, using a
Spectronic 20 spectrophotometer was used. L.D.= 0.02 mg/1.
TOTAL ALKALDtTIY:
Method NOM-AA-36_1980 , titration with HC1 0.02 N using Orange Methyl as an
indicator and expressing the results in mg/1 of CaC03, L.D.= 3mg/l.
CYANIDES (Oi):
For stage 2, Method 4500-CN E Standard Methods was used. Colorimetric with
reaction to Barbituric-Pyridine acid. A B&L Spectronic 20 spectrophotometer was
used. L.D.= 0.02 mg/1. For stage 4, Method 4500-CN F Standard Methods ion
cyanide selective was used. A Corning 250 Analyzer was used. L.D. =0.001 mg/1.
-------
-3-
FHHICtSl
Method 5530 C Standard Methods, extraction with chloroform with
4-Aminoantipyrine. For stage 2, a B&L Spectronic 20 spectrophotometer was vised;
for stage 4, a Coleman Jr. II Perkin Elmer spectrophotometer was used. L.D.
=0.001 mg/1.
ANALKSIS FGR MERES IN WATH* SAMFIES
The samples were filtered to 0.45 microns in the field, treated with ultrapure
nitric acid to a pH of < 2, and refrigerated until the analysis was
accomplished. The analyses were done using atomic absorption.
In stages 1 and 3, a Varian Spectra-20 spectrophotometer was used; in stages 2
and 4, a Perkin reimsr 5000 spectrophotometer was used. Both of these are double
light source with background correction, using single element bulbs with hollow
cathodes. The calibration curves and standards are prepared daily using 5
standards plus the blank, which are acid treated the same as the samples. For
the parameters Od, Cu, Fe (in stages 1 and 3), Ag, Ni, Pb, Zn, Al (in stages 2
and 4) and Cr, the f lame-by-direct-aspiration of the sample system was used; and
for the analysis of As, Se and Hg in stages 1 and 3, the hydrate generator
system and cold vapor Varian VGA -76 was used; and, in stages 2 and 4, the
hydrate generator system and cold vapor Perkin Elmer was used. As and Se were
sequentially analyzed in the same manner following the hydrate generation
system. The sample was prepared in HC1 6 M, placing in the recipient of the
reducer: 0.6% NaBH4; 0.5% NaOH: 10% KI, HC1 10 M was put into the acid canal.
Mercury was analyzed in a separate sample using the cold vapor technique, adding
nitric acid (5% v/v) and chlorhydric acid (5% v/v), placing in the recipient of
the reducer 25% w/v SnC12 en 20% v/v HC1, and HC1 5 M was put into the acid
canal.
ANALYSIS FOR MORES IN SEODfHlT SAMPIES
In stages 1 and 3, the sample preparation included drying and screening. An
extraction with diluted HC1 using 20g of sample and 125 ml of HC1 0.5 N, 16 hour
reaction time in an Bderbach mechanical agitator, and later filtration to 0.45
microns was accomplished. The analyses were accomplished in a similar manner
with the same equipment as the water samples reporting the results in mg/kg of
dry material. This corresponds to the extractable or non-residual metals, in
which are included the metallic particles that are deposited on the sediment
particles, as well as metal-organic material compounds and metals found the form
of insoluble salts.
For the analysis for metals in sediments for stages 2 and 4, 3 g of dry,
screened material were treated with 5 ml of suprapure nitric acid, digested by
microwave, and the extract was diluted with 50 ml of distilled water, and then
quantification was accomplished by atomic absorption.
The limits of detection in micro-grams/1 of solution in the metals' analysis by
atomic absorption were as follows:
-------
Bar Stages 1 and 3:
AgAsCdCrCuFeHgNiPbSeZn
L.D. 3 1 3 10 3 8 0.2 10 13 1 3
S.D 1 0.5 1 5 1 3 0.1 5 7 0.5 2
For Stages 2 and 4:
L.D. 30 1 2 40 40 10 0.5 50 80 0.5 10
S.D. 9 0.5 1 10 5 5 0.1 10 10 0.1 3
-------
TABLE 5. Modified Index of Biotic Integrity rating criteria for sites on
the Rio Grande and tributaries.
Metric (Sites 1-12, 3A, 6A, 6B, 7B, 8D, 9b)
1.
2.
3.
4.
5.
6.
Total number of species
Number of minnow species
% of individuals in most abundant species
Total number of individuals*
a. Individuals per hour electrofishing
b. Individuals per seine haul
% diseased individuals
% of individuals as introduced species
Metric (Sites 13-18,120)
1.
2.
3.
4.
5.
6.
Total number of species
% of individuals as estuarine/marine species
% of individuals in most abundant species
Total number of individuals*
a. Individuals per hour electrofishing
b. Individuals per seine haul
% diseased individuals
% of individuals as introduced species
5
>14
>5
<40
>224
>67
<0.5
<6
5
>14
0-18
<40
>224
>67
<0.5
<6
Ratings
3
8-14
3-5
40-55
112-224
34-67
0.5-1
6-12
3
8-14
>18-49
40-55
112-224
34-67
0.5-1
6-12
1
<8
<3
>55
<112
<34
>1
>12
1
<8
>49
>55
<112
<34
>1
>12
90
-------
-------
TABLE 6 - Status (N=native, l=introduced) and preferred habitat (F=freshwater, E=estuarine
or marine) of fish species collected in the Rio Grande and tributaries.
Scientific name
Lepisosteus oculatus
Lepisosteus osseus
Anguilla rostrata
Oorosoma cepedianum
Oorosoma petenense
Cyprinella lutrensis
Cyprinella proserpina
Cyprinella venusta
Cyprinus carpio
Dlonda episcopa
Macrhybopsis aestivalis
Notropis amabilis
Notropis braytonl
Notropis femezanus
Notropis stramineus
Plmephales promelas
Plmephales vigllax
Rhinichmyes eataractae
Carpiodes carpio
Cycleptus elongates
Ictiobus bubalus
Moxostoma austrinum
Moxostoma congestion
Astyanax mexicanus
Ictalurus furcatus
Ictalurus punctatus
Ictalurus lupus
Pylodictis olivaris
Strongylura marina
Cyprinodon variegatus
Fundulus grandis
Fundulus zebrinus
Gambusia affinis
Poecilia formosa
Poecilia latipinna
Menidia beryllina
Morone chrysops
Lepomis auritus
Lepomis cyanellus
Lepomis gulosus
Lepomis macrochirus
Lepomis megalotis
Lepomis microlophus
Micropterus dolomieu
Micropterus salmoides
Etheostoma grahami
Stizostedion vitreum
Aplodinotus grunniens
Cichlasoma cyanoguttatum
Tilapia aurea
Agonostomus monticola
Mugil cephalus
Gobiomorus dormitor
Common name
Spotted gar
Longnose gar
American eel
Gizzard shad
Threadfin shad
Red shiner
Proserpine shiner
Blacktail shiner
Common carp
Roundnose minnow
Speckled chub
Texas shiner
Tamaulipas shiner
Rio Grande shiner
Sand shiner
Fathead minnow
Bullhead minnow
Longnose dace
River carpsucker
Blue sucker
Smallmouth buffalo
Mexican redhorse
Gray redhorse
Mexican tetra
Blue catfish
Channel catfish
Headwater catfish
Rathead catfish
Atlantic needlefish
Sheepshead minnow
Gulf killifish
Plains killHish
Mosquitofish
Amazon molly
Sailfin molly
Inland silverside
White bass
Redbreast sunfish
Green sunfish
Warmouth
Bluegill sunfish
Longear sunfish
Redear sunfish
Smallmouth bass
Largemouth bass
Rio Grande darter
Walleye
Freshwater drum
Rio Grande cichlid
Blue tilapia
Mountain mullet
Striped mullet
Bigmouth sleeper
Status
N
N
N
N
N
N
N
1
1
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
1
N
N
N
N
1
1
N
N
N
N
1
1
N
N
1
N
N
1
N
N
N
Preferred
habitat
F
F
E
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
E
E
E
F
F
E
E
E
F
F
F
F
F
F
F
F
F
F
F
F
F
F
E
E
E
91
-------
Table 7
Screening Level Concentrations
VO
ro
Parameter
CONVENTIONAL
ammonia (unionixed)
reiidual chlorine
PHENOLS AND CRBSOLS
parachtoromela crctol
phenol (CjHjOH) >«|te compound
pbenolici recoverable
HALOQENATED ALIPHATICS
bfomodkhlotcmethanc*
chloroform*
mclhykne chloride*
leuachloroelhylene*
IrichlorofluoronKlhane
t.t.l-trichtoioethane
Hater
Human Health
Ntlionil
>5th
PeicenUk '
0.(/L)
NV
NV
25
13
24
10
27
61
4
NA
20
Connimptkn
ofFiih
and Water "
0.1/L)
NV
NV
NV
21, 000 l4
NV
2.7 l4
100"
47 u
597"
NA
200"
CooMunption
of Pub
Only14
(M'L)
NV
NV
NV
4,600,000 u
NV
220 l4
12,130"
16,000 l4
1.832 "
NA
1,030,000*
Sediment
Aqutfic Life
Acute
Vabie
WU
SS
,,14
30»
10.200 4
NV
11,000*
21,900 J
11,000*
5,210'
NA
11,000'
Chronic
Value
(MI/L)
SS
II14
NV
2.560 4
NV
NV
1,240 J
NV
MO7
NA
NV
National
ISih
Pcrcenlile '
(m|ftt)
NM
NM
NV
NV
NM
NA
NV
NV
NA
NA
NA
Other
NM
NM
NV
NV
NM
NA
NV
SS
NA
NA
NA
Tiaaue
Whole Body
National
15th
Percentile1
(mt/kt)
NM
NM
NA
NA
NM
NA
NV
NV
NA
NV
NV
Other
NM
NM
NA
NA
NM
NA
NV
NV
NA
NV
NV
Edible
Tiiiue11
(ovAt)
NM
NM
NA
NA
NM
NA
NV
NV
NA
NV
NV
POLYCYCLIC AROMATIC
HYDROCARBONS
naphthalene
MONOCYCLIC AROMATICS
benzene*
chlorobcnzene
tlhytbcnzcne
hexachlorobeiucnc*
toluene
xylene
1.2-dichlorobenzene
1,4 -dichlotobenzene
METALS
iluroinum
antimony
arsenic*
10
100
NA
10
NA
31
NV
10
10
NV
54
10
36'
5"
NA
3.100 M
NA
6.100 "
10.000 "
2.700 l4
400'4
NV
14 l4
JO"
2.600 M
312"
NA
29.000 '4
NA
200.000 M
NV
17.000 M
2.600 l4
NV
4.300 "
1.4'4
2,300 »
J.30010
NA
32.000 "
NA
n.soo «
10.000 2l
250"
250"
991"
9.000IJ
360"
620*
NV
NA
NV
NA
NV
5.000 n
50"
SO"
»7'4
1.600"
190"
NA
NA
NV
NA
NA
NV
NV
NV
NV
NV
NM
14
NA
NA
NV
NA
NA
SS
NV
S3
SS
NV
NM
SS
NA
NA
NA
NA
NV
NV
NA
NV
NA
NV
NM
0.2
NA
NA
NA
NA
NV
NV
NA
NV
NA
NV
NM
0.27 "; 0.5 M
NA
NA
NA
NA
0.07
NV
NA
NV
NA
NV
NM
NV
-------
Table 7 (continued)
Screening Level Concentrations
10
w
Parameter
beryllium
cadmium
chromium, total
bivalent
hexavalenl
copper
lead
mercury
nickel
selenium
silver
Ihillium
zinc
PESTICIDES
chlordane*
chlorpyrifos
p,p' DDD*
p,p' DDE*
p.p' DDT*
diazinoo
dicldrin*
gamma-bhc (lindane)*
Hater
Human Heallh
National
85lh
Percentile '
0-g/U
NA
6
20
NV
NV
20
20
1.3
20
10
10
NV
80
NA
NA
NA
NA
NA
NV
NA
0.1
Consumption
of Fish
and Water "
(M/L)
NA
10"
50"
33,300 M
50"
1.30014
5"
0.0122 "
610"
10"
50"
1.7'4
5,000"
NA
NA
NA
NA
NA
NV
NA
4"
Consumption
of Fish
Only24
0-g/U
NA
NV
NV
673,000"
NV
NV
25"
0.0122 "
4.600 l4
NV
NV
6.3 14
NV
NA
NA
NA
NA
NA
NV
NA
16"
Sediment
Aquatic Life
Acute
Value
04; I.O27-*2
O.n 24; I.O21'12
NA
0.01 J4; 0.1 »
0.0027 "
Edible
Tissue 2>
(mg/kg)
NA
10
NV
NM
NM
NV
NV
0.6; I.O2*
NV
50; 2.0 M
NV
NV
NV
0.08 M;
NA
0.3 "
5.0 "•»
0.3 "
5.0 ».»•
0.3 "
5.0 "•»
NA
0.007
0.3 2»
0.08
PCB's AND RBLATBD
COMPOUNDS
arochlor 1248*
arochlor 1254*
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA NA NV 0.06"; <0.1 JO-J2 0.01 w
2.0 M-»
NA NA 9.6 0.21Jl; <0.1 JO-M 0.01 M
-------
Table 7 (continued)
Screening Level Concentrations
Hater
Sediment
Tissue
vo
Parameter
Human Health
Aquatic Life
Whole Body
National
85lh
Pcrcealile '
f>g/L)
Consumption
of Fish
and Water "
O-I/L)
Consumption
of Fish
Only14
(rt/L)
Acute
Value
(M'U
Chronic
Value
0-g/U
National
85th
Percealife1
(mf/kg>
Other
(mg/kg)
National
85th
Perccntile1
(mg/kg)
Other
(mg/kg)
Edible
Tissue11
(mt/kg)
PHTHALATE ESTERS
bis(2-clhylhcxyl) phlhalale*
dielhyl phlhalalc
dip butyl phlhalate
GENERAL INORGANICS
cyanide
5
20
NA
23.000 M
NA
59 14
120.000'«
NA
940"
940"
NA
3"
J"
NA
1.900 M/kg
NA
0.068 mgiL 0.7 mg/L M 220 m|/L M 0.046 m|/L " 0.011 mg/L "
It
NV
NA
S3
NV
NV
NA
NA
NV
NV
NA
NA
NV
NV
NA
NA
NV
from Orcenspun ind Tiylor, 1979
from Norberg-King el a/., 1989 (represents 41 hr. LCSO value for Ceriodaphnla dubla)
from Arthur el at.. 1983 (represent! lowest recorded adverse effccl level, which reiulled in lowered emergence and elevated drift of abeam inaccU)
froraUSEPA, 1980 a
from USBPA, 1980 b
from USEPA. 1980 c
horn USEPA. 1980 d
from USEPA. 1980 e
from USEPA, 1980 f
from USEPA. 1980 f
from USEPA, 1980 h
from USEPA, 1980 i
from USEPA, 1980 j
national criterion (USEPA, 1986)
from USEPA, 1980k
from USEPA, 19801
from USEPA, 1980m
drinkinf water maximum contaminant level (TNRCC, 1993)
Male criterion (TNRCC. 1991)
from USEPA, 1980 n
minimum concentration reported to kill Tub (McKee and Wolf, 1963)
concentration reported to adversely affect bluegill sunfiih within a ten hour period (McKee and Wolf, 1963)
applicable to all mairutem alalioru, and tributary tlalioni 3a, 3b, Sa. 6a, 6b, 7b, 81, 8b, 8c. Id. (e, %, lOa. I la, lib. 12a, I2b. I2c, I2d, I2e
applicable to Iribulariei dominated by treated or untreated domestic sewage cfnucnl, which are not regarded at potential drinking water auppliei (la, 2a, 7a, 9a, lie, I5a)
eslabliihed ai a national criterion, but later withdrawn in National Toxics Rule (USBPA, 1986)
from a risk assessment by TDH (1992)
value is for total DDT (sum of ODD -f DDE + DDT)
10
II
II
I)
14
15
If
17
II
19
10
21
n
n
24
25
16
n
-------
Table 7 (continued)
Screening Level Concentrations
21 • from Guidance for Aliening Chemical Contaminant Data for Use in Pish Advisories (USBPA, 1993)
n • action or tolerance kvel (USFDA. 1993)
M - value u ror lotil PCBi (mm of all arochlon)
11 • geometric mean from (be USFWS National Contaminant BJomonitoring Program (Sdunht a at., 1990)
" - U.S. FUh and Wildlife Service predator protection limit
" • tSih percenlile value from the USFWS National Contaminant Biomoniloring Program (Schraitt and Brumbaugh, 1990)
" . 85th perccntile value from the TNRCC Surface Water Quality Monitoring program (TNRCC. 1994)
M • mean concenlfation from National Study of Chemical Reiiduea in Fiih (USBPA, 1992)
14 • value ii foe total cMordane (turn of Irani + cii isomcri)
NV - no value exists
NA • not applicable because parameter wat not delected in Ihii matrix
NM • parameter waa not measured n this matrix
SS - criterion is site-specific; tee Table I for water, Table 9 for acdiment
* - parameter identified as a carcinogen; human health criteria baaed on risk factor of 10 '*
VO
in
-------
Table 7 (continued)
Mexico Water Quality Standards
Criteria in Water for Specific Toxic Materials
Aquatic Life Protection
M6xico Fresh Water
Parameter Aquatic Life Criteria*
Aldrin 3
Aluminum 50
Arsenic 200
Cadmium @exp(0.785[ln(hardness)]-3.490)
Chlordane 2
Chromium (Hex) 10
Copper @exp(0.8545pn(hardness)]-l .465)
Cyanide** 5
DDT 1
Dieldrin 2
Endosulfan 0.2
Endrin 0.02
Heptachlor 0.5
Hexachlorocyclohexane 2.5
(Lindane)
Lead @exp(1.273[ln(hardness)]-4.105)
Mercury 0.01
Nickel @exp(0.846[ln(hardness)] +1.1645)
Total PCB's 0.01
Parathion 0.04
Pentachlorophenol 0.5
Selenium 8
Silver, as free ion @exp(1.72[ln(hardness)]-6.52)
Toxaphene 0.0002
2,4,5 Trichlorophenol 10
Zinc @exp(0.8473pn(hardness)] +10.36)
* All Values Listed or Calculated in Micrograms per Liter - Hardness Concentrations are
Input as Milligrams per Liter
** Amenable to Chlorination
-------
Table 8
Site-Specific Screening Level Concentrations for Water"
Parameter
CONVENTIONAL (mj/L)
unmoai* (unionized) m'n
METALS to/L) b
udmiura *•*'*
chromium, trivikat '•'•'
copper I*.'
lud '•)•'
nickel *•'••
tine °.P-«
Station
1
0.096/0018
NA
NA
NA
477/11.6
NA
NA
la
0.302/0.041
NA
NA
33.4/21.1
173/6.7
2,332/259
193/174
2
NA
146/3.1
NA
65.2/38.8
NA
NA
351/318
2a
0.151/0.029
NA
NA
NA
433/16.9
4,300/47*
355/322
3
0.127/0.025
NA
NA
NA
715/27.9
5,999/667
496/449
3a
NA
NA
NA
46.4/28.5
NA
3,131/34*
NA
3b
NA
NA
NA
13.0/9.0
NA
NA
82. 3/74.5
4
NA
NA
NA
65.1/39.1
NA
4,290/477
355/321
5
NA
NA
NA
NA
475/11.5
4,573/508
378/342
Sa
NA
NA
NA
NA
NA
NA
NA
5b
0.123/0.024
NA
NA
NA
NA
NA
NA
6
0.146/0.028
NA
NA
62.2/37.1
NA
NA
336/304
vo
01
-------
Table 8 (continued)
Site-Specific Screening Level Concentrations for Water*
Parameter
CONVENTIONAL (mj/L)
ammonia (unionized) m>n
METALS On/U b
cadmium c-d'*
chromium, Irivalcat *•'••
copper I-11-1
lead '•>•*
nickel *•'••
me °'P'§
Station
6a
0.099/0.016
NA
10,404/1,240
NA
NA
NA
NA
6b
0.131/0.025
NA
NA
34.1/21.5
NA
NA
NA
7
NA
NA
NA
452/27.8
NA
NA
253/229
7a
0.092/0.010
NA
NA
62.3/37.2
NA
NA
NA
7b
NA
NA
NA
NA
NA
NA
NA
8
0.111/0 023
NA
NA
NA
NA
NA
NA
8a
0.157/0.030
NA
NA
NA
NA
NA
NA
8b
NA
NA
NA
NA
NA
NA
NA
8c
NA
NA
NA
NA
NA
NA
NA
8d
NA
NA
NA
NA
NA
NA
NA
Be
NA
NA
NA
33.9/21.4
NA
NA
NA
9
NA
NA
NA
NA
NA
NA
NA
to
vl
-------
Table 8 (continued)
Site-Specific Screening Level Concentrations for Water*
Parameter
CONVENTIONALS (m|/L)
unmoai* (unionized) m-°
METALS 04/L) b
ctdmium c'd'§
chromium, trivtlent *•'••
copper I-0-1
L-*d 'J'*
nickel *•'•'
zinc °'P'«
Station
9a
0.135/0.017
NA
6,(I6/(12
NA
NA
NA
NA
9b
NA
137/3.0
NA
6I.«/37.0
NA
NA
NA
10
NA
NA
NA
NA
NA
NA
NA
10a
NA
(07/10.3
NA
272/142
NA
NA
NA
11
NA
107/2.5
NA
50.5/30.7
NA
NA
NA
lla
0.202/0.037
71.6/1.9
NA
36.0/22.6
NA
NA
206/1(7
lib
NA
1,017/12.2
NA
330/169
NA
NA
NA
lie
0.14(/0.014
NA
NA
NA
NA
NA
NA
12
0.224/0.041
NA
NA
NA
NA
NA
NA
12a
NA
613/1.6
14,2(0/1,702
NA
NA
NA
NA
12b
0.2(3/0.041
NA
NA
NA
NA
NA
NA
12c
0.302/0.041
155/3.3
NA
6I.9/40.(
NA
4,4667496
NA
10
00
-------
Table 8 (continued)
Site-Specific Screening Level Concentrations for Water"
Parameter
12d 12e 13 14
CONVENT10NALS (m|/L)
ammonia (unionued) m-a 0.016/0.020 0.114/0.015 0.152/0.029 NA
METALS 0-i/L) b
cadmium c-d>1 NA NA NA NA
chromium, trivalent '•'•' NA NA NA NA
copper I-*'* 109/61.7 NA 47.1/28.9 46.5/28.6
lead 'J'§ NA NA NA NA
nickel k.'.» NA NA NA NA
zinc °'P'' 557/504 534/484 NA NA
NA - not applicable became parameter wai below die detection lira* at Ihil the
* - value* in labk rcpreaent criteria for protection of freihwaler aquatic life, preaented at acute v
m - calculated according to equation! deicribed by USBPA (1984)
0 - vahiei for Ihia parameter represent national criteria (USBPA, 1986)
yf) ' - vahiea for Ihil parameter reprewol Mate criteria (TNRCC, 1991)
\Q - metalt criteria calculated uiinf bardneit conccntrationi from Table (, According to following <
c . e(1.128>ln(hardae»)M.6774) i . e(l.273|ln(hardne»)]-1.460)
d . e(0.7852(ln(hardneii)l-3.490) j . c(1.273[m(hardneu)J-4.705)
e . e(0.8!90(ln(hMdneii))+3688) k . e(0.8460(ln(hardneu))+3.36I2)
f . e(0.8190|ln(haraneM)] + 1.56l) I . e(0.8460(ln(hardneii))-«-l.l645)
I . e(0.9422[bi(hardneM))-1.3844) o . e(0.8473[ta(h»rdne.i))+0«604)
b . e(0.»$45lto(b«doe.»)| 1.386) p . e(0.8473(ln(hardneM)) +0.7614)
Station
15 15a 16 17
NA 0.102/0.013 0.143/0.028 NA
NA NA NA NA
NA 7,533/898 NA NA
NA 104/59.2 48.6/29.7 63.2/37.7
NA NA NA NA
NA NA NA NA
NA 534/484 270/244 342/309
atae/chronic value
equation!'.
IB
NA
NA
NA
77.4/45.3
NA
NA
NA
-------
Table 9
Site-Specific Screening Level Concentrations for Sediment*
Parameter
la
Station
2a
3a
3b
5a
5b
HALOOBNATBD AL1PHATICS Oi(/kf)
mdhykae chloride •
110
NA
139
315
230
NA
550
NA
NA
NA
NA
NA
MONOCYCUC AROMATICS 0"|/k«)
. b
1.2-dkhlorabcazcoec
1,4-dichlorobenzene °
550
NA
NA
39,350
11,018
11,018
695
NA
NA
1.575
NA
NA
1.150
NA
NA
NA
NA
NA
2.750
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
O
O
METALS |/k()
cMordwe1
chtorpyrifot ••
p.p' ODD m
p.p' DDE a
dicldrtn •••
1.12
1.71
1.38
7.41
7.26
0.04
1.10
41.8
130
122
98.4
535
519
3.15
78.7
2.991
2.29
2.15
1.74
9.45
9.17
0.06
1.39
52.8
5.20
4.88
3.94
21.4
20.8
0.13
3.15
120
3.80
3.57
2.88
15.6
15.2
0.09
2.30
87.4
2.39
2.25
1.81
9.86
9.57
0.06
1.45
55.1
9.08
8.53
6.88
37.4
36.3
0.22
5.50
209
4.46
4.19
3.38
18.4
17.8
0.11
2.70
103
5.36
5.04
4.06
22.1
21.5
0.13
3.25
124
4.54
4.26
3.44
18.7
18.2
0.11
2.75
105
3.96
3.72
3.00
16.7
15.8
0.10
2.40
91.2
13.6
12.8
10.3
56.1
54.5
0.33
8.25
314
NA
NA
NA
NA
NA
78.7
507
NA
NA
NA
NA
NA
NA
NA
NA
NA
20.3
NA
4,410
NA
NA
NA
NA
NA
NA
NA
NA
NA
2.030
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
PHTHALATB ESTERS |