Water-Quality Assessment of the Trinity River Basin,

Texas Analysis of Available Information on Nutrients *

and Suspended Sediments, 1974-91

By PETER C. VAN METRE AND DAVID C. REUTTER

A Contribution of the

National Water-Quality Assessment

Program

U.S. GEOLOGICAL SURVEY WATER-RESOURCES 

INVESTIGATIONS REPORT 94-4086

Austin, Texas 

1995
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director

Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by 

the U.S. Government

For additional information write to:

District Chief 

U.S. Geological Survey 

8011 Cameron Road, Bldg. A 

Austin, Texas 78754-3898

Copies of this report can be purchased from:

U.S. Geological Survey 

Earth Science Information Center 

Open-File Reports Section 

Box 25286, MS 517 

Denver Federal Center 

Denver, Colorado 80225-0286

Information regarding the National Water-Quality Assessment (NAWQA) Program is available on 

the Internet via the World Wide Web. You may connect to the NAWQA Hone Page using the 

Universal Resource Locator (URL) at:

<URL:http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.html>
FOREWORD

The mission of the U.S. Geological Survey 

(USGS) is to assess the quantity and quality of the 

earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound 

decisions. Assessment of water-quality conditions and 

trends is an important part of this overall mission.

One of the greatest challenges faced by waterresources scientists is acquiring reliable information 

that will guide the use and protection of the Nation's 

water resources. That challenge is being addressed by 

Federal, State, interstate, and local water-resource 

agencies and by many academic institutions. These 

organizations are collecting water-quality data for a 

host of purposes that include: compliance with permits 

and water-supply standards; development of remediation plans for a specific contamination problem; operational decisions on industrial, wastewater, or watersupply facilities; and research on factors that affect 

water quality. An additional need for water-quality 

information is to provide a basis on which regional 

and national-level policy decisions can be based. Wise 

decisions must be based on sound information. As a 

society we need to know whether certain types of 

water-quality problems are isolated or ubiquitous, 

whether there are significant differences in conditions 

among regions, whether the conditions are changing 

over time, and why these conditions change from 

place to place and over time. The information can be 

used to help determine the efficacy of existing waterquality policies and to help analysts determine the 

need for and likely consequences of new policies.

To address these needs, the Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the 

National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation 

of the program. The NAWQA Program builds upon an 

existing base of water-quality studies of the USGS, as 

well as those of other Federal, State, and local agencies. The objectives of the NAWQA Program are to:

 Describe current water-quality conditions for a 

large part of the Nation's freshwater streams, 

rivers, and aquifers.

 Describe how water quality is changing over 

time.

 Improve understanding of the primary natural 

and human factors that affect water-quality 

conditions.

This information will help support the development 

and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local 

agencies to protect, use, and enhance water resources.

The goals of the NAWQA Program are being 

achieved through ongoing and proposed investigations 

of 60 of the Nation's most important river basins and 

aquifer systems, which are referred to as study units. 

These study units are distributed throughout the 

Nation and cover a diversity of hydrogeologic settings. More than two-thirds of the Nation's freshwater 

use occurs within the 60 study units and more than 

two-thirds of the people served by public water-supply 

systems live within their boundaries.

National synthesis of data analysis, based on 

aggregation of comparable information obtained from 

the study units, is a major component of the program. 

This effort focuses on selected water-quality topics 

using nationally consistent information. Comparative 

studies will explain differences and similarities in 

observed water-quality conditions among study areas 

and will identify changes and trends and their causes. 

The first topics addressed by the national synthesis are 

pesticides, nutrients, volatile organic compounds, and 

aquatic biology. Discussions on these and other waterquality topics will be published in periodic summaries 

of the quality of the Nation's ground and surface water 

as the information becomes available.

This report is an element of the comprehensive 

body of information developed as part of the NAWQA 

Program. The program depends heavily on the advice, 

cooperation, and information from many Federal, 

State, interstate, Tribal, and local agencies and the 

public. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch 

Chief Hydrologist

CONTENTS
CONTENTS

Abstract ............................................................... 1

Introduction............................................................................................................... 2

The National Water-Quality Assessment Program .......................................... 2

Purpose and scope ............................................................................................ 2

Nutrients in natural waters ............................................................................... 3

Acknowledgments............................................................................................ 4

Description of study unit........................................................................................... 4

Climate ............................................................................................................. 5

Geology............................................................................................................ 5

Population and land use ................................................................................... 5

Water use, diversions, wastewater treatment and returns ................................ 7

Streams and reservoirs ..................................................................................... 11

Aquifers............................................................................................................ 13

Assessment approach ................................................................................................ 13

Selection of constituents for analysis ............................................................... 15

Methods of analysis ......................................................................................... 15

Spatial distribution ............................................................................... 15

Relations to categorical variables ........................................................ 15

Relations to continuous variables ........................................................ 15

Loads and yields .................................................................................. 18

Available data ............................................................................................................ 18

U.S. Geological Survey .................................................................................... 18

Texas Water Commission ................................................................................. 20

Trinity River Authority .................................................................................... 20

City of Arlington .............................................................................................. 20

City of Fort Worth ............................................................................................ 22

Texas Water Development Board .................................................................... 22

National Atmospheric Deposition Program ..................................................... 22

Analysis of available data for nutrients .................................................................... 22

Atmospheric deposition ................................................................................... 22

Surface water ................................................................................................... 24

Spatial distribution ............................................................................... 24

Relations to point sources and reservoirs ............................................. 25

Relations to agricultural land use ......................................................... 27

Relations to streamflow ........................................................................ 31

Seasonal variations ............................................................................... 38

Temporal trends ................................................................................... 41

Loads and yields .................................................................................. 52

Relations to Livingston Reservoir .............................................. 62

Comparison of loads to atmospheric deposition ......................... 64

Ground water ................................................................................................... 65

Analysis of available data for suspended sediment .................................................. 68

Summary ................................................................................................................... 69

References ................................................................................................................. 70

CONTENTS IV
PLATE

[Plate is in pocket]

1. Map showing the National Water-Quality Assessment Program Trinity River 

Basin study unit

FIGURES

1-6. Maps showing:

1. Surficial geology of the Trinity River Basin ........................................ 6

2. Land use in the Trinity River Basin ..................................................... 8

3. Distribution of (A) total major crops, (B) wheat, (C) rice, and

(D) cotton production by county, 1989 ................................................ 9

4. Nitrogen and phosphorus fertilizer use by county ............................... 10

5. Major and minor aquifer outcrop areas in the Trinity River Basin ...... 14

6. Map showing locations and sampling frequency for U.S. Geological

Survey nutrient samples from streams, 1982-91 ................................. 19

7. Distribution of samples collected by the U.S. Geological Survey by

dectile of flow at four sites in the Trinity River Basin, 1982-91 .................. 20

8-14. Maps showing:

8. Locations and sampling frequency for the Texas Water Commission

nutrient samples from streams in the Trinity River Basin, 1982-91 ... 21

9. Locations of wells sampled by the Texas Water Development

Board in the Trinity River Basin, 1974-91 .......................................... 23

10. Spatial distribution of median and 90th percentile nitrogen

concentrations in Trinity River Basin streams, 1982-91 ..................... 25

11. Spatial distribution of median and 90th percentile nitrate-plusnitrate nitrogen concentrations in Trinity River Basin streams, 

1982-91................................................................................................. 26

12. Spatial distribution of median and 90th percentile ammonia plus 

organic nitrogen concentrations in Trinity River Basin streams, 

1982-91................................................................................................. 27

13. Spatial distribution of median and 90th percentile phosphorus

concentrations in Trinity River Basin streams, 1982-91 ..................... 28

14. Delineation of stream type in the Trinity River Basin ......................... 29

15-25. Boxplots showing:

15. Concentrations of nutrients in streams grouped by stream type

in the Trinity River Basin ..................................................................... 30

16. Relation between nutrient concentrations and percent of drainage

in agricultural land use in the Trinity River Basin ............................... 31

17. Relation between nitrogen concentrations and discharge

for streams in agricultural areas ........................................................... 32

18. Relation between ammonia plus organic nitrogen concentrations

and discharge for streams in agricultural areas .................................... 33

19. Relation between nitrite-plus-nitrate concentrations and

discharge for streams in agricultural areas ........................................... 34

CONTENTS V
20. Relation between phosphorus concentrations and discharge

for streams in agricultural areas ........................................................... 35

21. Relation between nitrogen concentrations and discharge for streams

in and downstream from Dallas-Fort Worth ........................................ 36

22. Relation between ammonia plus organic nitrogen concentrations 

and discharge for streams in and downstream from Dallas-Fort 

Worth.................................................................................................... 37

23. Relation between nitrite-plus-nitrate concentrations and discharge

for streams in and downstream from Dallas-Fort Worth ..................... 38

24. Relation between phosphorus concentrations and discharge for

streams in and downstream from Dallas-Fort Worth ........................... 39

25. Nitrogen and phosphorus concentrations and discharge for three

sites during a flow event on May 28-29, 1987 .................................... 42

26-29. Boxplots showing seasonal variations in flow-adjusted residuals of:

26. Nitrogen concentrations ....................................................................... 43

27. Ammonia plus organic nitrogen concentrations .................................. 44

28. Nitrite-plus-nitrate concentrations ....................................................... 45

29. Phosphorus concentrations ................................................................... 46

30-33. Boxplots showing temporal trends in flow-adjusted residuals of:

30. Nitrogen concentrations ....................................................................... 47

31. Ammonia plus organic nitrogen concentrations .................................. 48

32. Nitrite-plus-nitrate concentrations ....................................................... 49

33. Phosphorus concentrations ................................................................... 50

34. Boxplots showing change in percent of total nitrogen as ammonia plus 

organic nitrogen and nitrite plus nitrate for successive periods at two 

sites downstream from Dallas ....................................................................... 53

35-37. Diagrams showing:

35. Mean loads of nitrogen and phosphorus in the Dallas-Fort Worth

area and mainstem Trinity River, 1974 89 .......................................... 54

36. Mean loads of nitrite plus nitrate in the Dallas-Fort Worth area

and mainstem Trinity River, 1974-79, and 1984-89 ........................... 55

37. Mean loads of ammonia plus organic nitrogen in the Dallas-Fort

Worth area and mainstem Trinity River, 1974-79, and 1984-89 ........ 56

38-41. Plots showing average annual loads and yields for 1984-87 of:

38. Nitrogen ............................................................................................... 58

39. Phosphorus ........................................................................................... 59

40. Nitrite plus nitrate ................................................................................ 60

41. Ammonia plus organic nitrogen ........................................................... 61

42. Plots showing discharge and differences in nitrogen, phosphorus, and 

sediment loads for sites above and below Livingston Reservoir, 

1974-88 ........................................................................................................ 63

43. Boxplot showing nitrate concentrations for wells in seven major and

minor aquifers in the Trinity River Basin, 1974-91 ..................................... 65

44. Plots showing relation between nitrate concentration and depth of

well for seven aquifers, 1974-91 .................................................................. 67

CONTENTS VI
TABLES

1. Estimated withdrawals and consumptive water use for eight categories

in the Trinity River Basin in 1990 ................................................................... 11

2. Volumes of wastewater treated in 1990 by the seven largest wastewatertreatment plants in the Dallas-Fort Worth area, Texas ..................................... 12

3. Sources of data, time periods, number of analyses, and number of sites

used for analysis of nutrients in streams in the Trinity River Basin................. 16

4. Nutrient concentrations from atmospheric deposition recorded at 

National Trends Network stations in the vicinity of the study unit 

and the weighted-average concentration .......................................................... 24

5. Median concentrations for all stream sites and for sites assigned to three

groups based on stream type in the Trinity River Basin .................................. 30

6. Rank correlations between discharge and nutrient concentration in streams

in the Trinity River Basin, 1982-91 ................................................................. 40

7. Trends in nutrient concentrations at selected sampling sites in the Trinity

River Basin using the seasonal Kendall test .................................................... 51

8. Mean nutrient loads and mean daily discharge for nine sites in the Trinity

River Basin....................................................................................................... 57

9. Loads of ammonium and nitrate from atmospheric deposition for two

sites and the weighted average for the Trinity River Basin ............................. 64

10. Rank correlations for nitrate concentrations versus depth of well by

aquifer, 1974-91 .............................................................................................. 66

11. Summary of daily suspended-sediment data from selected

U.S. Geological Survey sites ........................................................................... 68

CONTENTS VII
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATED 

WATER-QUALITY UNITS

Multiply By To obtain

centimeter (cm) 0.3937

centimeter per year (cm/yr) 0.3937

meter (m) 3.281

cubic meter (m3) 35.31

cubic meters per second (m3/s) 35.31

million cubic meters per 810.7

year (MnrVyr)

kilometer (km) 0.6214

square kilometer (km2) 0.3861

kilogram per day (kg/d) 2.2046

kilogram per hectare per 0.8907

year [(kg/ha)/yr]

tonne 1.102

inch

inch per year

foot

cubic foot

cubic foot per second

acre foot per year

mile

square mile

pound per day

pound per acre per year

ton

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 a 

geodetic datum derived from a general adjustment of the first-order level nets of the United States 

and Canada, formerly called Sea Level Datum of 1929.

Chemical concentrations and water temperature are given in metric units. Chemical 

concentration is given in milligrams per liter (mg/L). Water temperature is given in degrees 

Celsius (°C), which can be converted to degrees Fahrenheit (°F) by the following equation:

°F=1.8(°C) + 32

CONTENTS VIII
Water-Quality Assessment of the Trinity River 

Basin, Texas Analysis of Available Information 

on Nutrients and Suspended Sediment, 1974-91

By Peter C. Van Metre and David C. Reutter

Abstract

The U.S. Geological Survey is conducting an 

assessment of water quality in the Trinity River 

Basin as part of the National Water-Quality 

Assessment Program. During the planning phase of 

this study, existing information on nutrients and 

suspended sediment was compiled and analyzed. A 

total of about 5,700 water-quality samples were 

analyzed from local, State, and Federal agencies. 

Of these, about 4,200 were from streams and about 

1,500 were from wells. Additionally, atmospheric 

deposition data for two locations were obtained 

and analyzed.

Spatial variations in nutrient concentrations in 

streams are related primarily to point sources and 

reservoirs. Median total nitrogen concentrations 

downstream from major point sources, 

downstream from reservoirs, and on tributaries 

were 6.0, 1.3, and 2.4 milligrams per liter, 

respectively. Median total phosphorus 

concentrations for the same three settings were 1.6, 

0.1, and 0.2 milligrams per liter, respectively. The 

largest concentrations occurred at low flow 

downstream from Dallas, Texas, when streamflow 

was dominated by treated wastewater from point 

sources. The smallest concentrations occurred 

immediately downstream from reservoirs, which 

act as sinks for nutrients. Nutrient concentrations 

in agricultural areas were positively correlated to 

percent of drainage in agricultural land use and to 

discharge, indicating washoff of nutrients from 

nonpoint sources during storms.

As with concentrations, nutrient loads were 

related to the presence of point sources and 

reservoirs. Loads increased substantially in the

Dallas-Fort Worth area with the addition of 

nutrients from point sources; loads decreased 

substantially as flow passed through Livingston 

Reservoir.

Concentrations of total nitrogen and total 

phosphorus did not change significantly, at the 95 

percent confidence level, from 1974 to 1991 at 

most sites. The exception was a decrease in 

phosphorus concentrations at two sites downstream 

from major wastewater-treatment plants in the 

Dallas area. Concentrations of organic nitrogen and 

ammonia declined and concentrations of nitrite 

plus nitrate increased at sites below major 

wastewater-treatment plants. These changes are 

indicative of improvements in wastewater 

treatment that converts organic nitrogen and 

ammonia to nitrite and finally nitrate. Because 

nitrogen conversion reactions consume oxygen, the 

occurrence of these reactions at the treatment 

plants instead of in the streams resulted in reduced 

loading of biochemical oxygen demand to the 

streams.

The only nutrient measured in ground water 

was nitrate. Nitrate concentrations varied by 

aquifer with the largest median concentrations in 

the Queen City and Nacatoch aquifers. There was a 

significant rank correlation between nitrate 

concentrations and depth of well for all seven 

aquifer groups sampled, with largest 

concentrations present in shallow wells. The large 

concentrations could result from nonpoint sources 

of nitrate associated with agricultural and urban 

land use; however, attempts to correlate nitrogen 

fertilizer application rates and agricultural land use 

to concentrations of nitrate in ground water were 

inconclusive.
Only limited suspended-sediment data were 

available. Four sites had daily sediment-discharge 

records for three or more water years (October 1 to 

September 30) between 1974 and 1985. An 

additional three sites had periodic measurements of 

suspended-sediment concentrations. There are 

differences in concentrations and yields among 

sites; however, the limited amount of data 

precludes developing statistical or cause-and-effect 

relations with environmental factors such as land 

use, soil, and geology. Data are sufficient, and the 

relation is pronounced enough, to indicate trapping 

of suspended sediment by Livingston Reservoir.

INTRODUCTION

The Trinity River drains about 48,000 km2 of 

north-central and east Texas (pi. 1). The 

headwaters are north and west of Dallas and Fort 

Worth, Texas, and the mouth of the river is at 

Trinity Bay, part of Galveston Bay, in the Gulf of 

Mexico. The population within the Trinity River 

Basin in 1990 was about 4.5 million, with about 

3.5 million living in the greater Dallas and Fort 

Worth area. The large population has caused 

stresses on water quality in the Trinity River, 

including 13 documented fish kills in the river 

from Dallas to Livingston Reservoir between 1970 

and 1985 (Davis, 1987).

The U.S. Geological Survey (USGS) is 

conducting an assessment of water quality in the 

Trinity River Basin as part of the National WaterQuality Assessment (NAWQA) Program. During 

the planning phase of this study, existing 

information on nutrients and suspended sediment 

was compiled and analyzed. This report presents 

the results of that analysis.

The National Water-Quality Assessment 

Program

The goals of the NAWQA program (Hirsch, 

Alley, and Wilber, 1988) are to:

1. Provide a nationally consistent description 

of current water-quality conditions for a

large part of the Nation's surface- and 

ground-water resources,

2. Define long-term trends (or lack of trends) 

in water quality, and

3. Identify, describe, and explain, as possible, 

the major factors that affect observed 

water-quality conditions and trends.

The NAWQA program is being executed 

through 60 (proposed) separate investigations of 

river basins and aquifer systems of the Nation, 

referred to as study units. Each study-unit 

investigation will include assessments of surfacewater and ground-water quality. Study units will 

undergo cycles of 3 years of intensive study, 

followed by 6 years of limited monitoring, with the 

cycle repeated decadally (Leahy, Rosenshein, and 

Knopman, 1990). The planning phase for the first 

cycle began in 1991 for 20 of the 60 study units 

and the Trinity River Basin is one of the 20.

In addition to the study-unit investigations, 

teams of scientists are conducting nationalsynthesis investigations to develop a regional and 

national scale understanding of water quality. The 

first national synthesis topics being investigated 

are pesticides and nutrients. These investigations 

mostly will rely on data collected by the study-unit 

investigations. This approach will provide results 

useful both in understanding and managing the 

water resources of the study unit and in answering 

regional and national questions about water 

quality.

Purpose and Scope

This report describes the occurrence of 

nutrients and suspended sediments in streams and 

nutrients in ground water in the Trinity River 

Basin, and relates that occurrence to environmental 

factors. This evaluation precedes the intensive 

sampling phase of the NAWQA study and is, in 

part, intended to help design that sampling effort.

The scope of this report includes the Trinity 

River Basin and small areas on each side of the 

Trinity River Basin near its mouth (pi. 1). Streams
and aquifer systems were addressed; reservoirs 

were not. This effort was limited by the availability 

of water-quality and environmental data. Only 

existing water-quality data collected between 

October 1, 1973, and September 30,1991 (water 

years 1974-91), were used. Additional limitations 

on the inclusion of data for particular analyses are 

described in a later section, "Assessment 

Approach." In an effort to expand the availability 

of data, water-quality data from agencies other 

than the USGS were also used.

Nutrients in Natural Waters

Living organisms require at least 30 to 40 

elements for their growth and development. The 

most important of these elements are carbon, 

hydrogen, oxygen, phosphorus, potassium, 

nitrogen, sulfur, calcium, iron, magnesium, boron, 

zinc, chlorine, molybdenum, cobalt, iodine, and 

fluorine (Smith, 1980). Essential nutrients that are 

in the shortest supply tend to control plant 

production rates and are said to be "limiting." 

Because phosphorus and nitrogen are commonly 

the limiting nutrients in aquatic systems, this 

report focuses on those elements. In sufficiently 

large concentrations, phosphorus and nitrogen 

can adversely affect water quality through

(1) eutrophication (abundant accumulation of 

nutrients causing excessive plant growth),

(2) toxicity to aquatic life, and (3) toxicity to 

warm-blooded animals that drink the water.

Chemical and biological processes that transfer 

nitrogen to and from the lithosphere, atmosphere, 

hydrosphere, and biosphere represent the nitrogen 

cycle. Nitrogen makes up 79 percent of the 

atmosphere as molecular nitrogen, N2 . Processes 

by which nitrogen gas (N2) is changed in oxidation 

state and converted to chemical compounds 

containing nitrogen are referred to as "nitrogen 

fixation." Ammonia is the product of biological 

fixation; nitrate is the product of high-energy 

fixation by lightning. Biological fixation 

contributes roughly 90 percent of the fixed 

nitrogen contributed to the earth each year. 

Biological fixation is accomplished by blue-green 

algae, symbiotic bacteria living in association with 

plants, and free-living aerobic bacteria (Smith,

1980). In addition to fixation, nitrogen is also made 

available through the breakdown of organic matter 

containing nitrogen, a process referred to as 

"respiration."

Nitrogen in reduced forms is converted by 

bacteria into nitrite (NO2) and nitrate (NO3). This 

process is commonly termed "nitrification." 

Nitrification can be described by the following 

reactions:

NH/ + 3/2O2 -> NO2- + 2H+ + H2O ,

1/2O2 -> NO3- .

(1)

(2)

Phosphorus and nitrogen mainly are provided 

to plants in aquatic systems by phosphate (PO4) 

and nitrate (NO3) or ammonium (NH4). The 

average proportions of the major elements in algal 

biomass are described by the Redfield formula 

(Morel, 1983):

Protoplasm = C106H263O110N16P1 . (3)

A more complete stoichiometric description of 

photosynthesis and respiration in natural waters is 

provided by the following reactions:

106CO2 + 16NO3- + H2PO4- 

Protoplasm + 138O2 ,

122H2O + 17H+ =

106CO2 + 16NH4+ + H2PO4- + 106H2O = 

Protoplasm + 106O2 + 15H+ .

(4)

(5)

Reactions moving from left to right in equations 4 

and 5 represent photosynthesis; reactions moving 

from right to left represent respiration.

As indicated by equations 1, 2, 4, and 5, 

nitrification and respiration exert an oxygen 

demand on natural waters. This oxygen demand is 

sometimes reported as the nitrogen biochemical 

oxygen demand (NBOD) and can be 

environmentally significant, for example, in rivers 

receiving effluents containing ammonia and 

organic nitrogen. Rickert and others (1976) found 

that nitrification was the dominant control on 

dissolved oxygen (DO) in some reaches of the 

Willamette River in Oregon during the summers of
1973 and 1974. The nitrification was caused by 

ammonia loading from a pulp mill.

Nitrate in anoxic environments can be reduced 

by bacteria to nitrous oxides or nitrogen gas. This 

process is commonly referred to as 

"denitrification" and is described by the following 

reaction:

4NO3-+ 5"CH2O" + 4H+ -> 2N2 + 5CO2 + 7H2O , (6)

where "CH2O" is a symbol for organic matter, 

which is the carbon source needed for this reaction 

to proceed.

Phosphorus is a common element in igneous 

rock and is also fairly abundant in sediments, but 

concentrations present in solution in natural water 

generally are no more than a few tenths of a 

milligram per liter (Hem, 1985). The most 

common phosphate species present at pHs found in 

natural waters are H2PO4" and HPO42". Sources of 

phosphorus in the Trinity River Basin include the 

breakdown and erosion of phosphorus-bearing 

minerals in soil, decaying vegetation, phosphate 

fertilizers, sewage effluent, and metabolic wastes 

from animals. The transport of phosphate in 

fertilizers to streams may be partly restricted 

because phosphates are not very mobile in soils 

and sediments. Soil erosion, however, could 

contribute suspended phosphate to streams.

Acknowledgments

We gratefully acknowledge Steve Twidwell, 

Texas Water Commission; Roger Quincy, Texas 

Water Development Board; Stair Birch, City of 

Arlington; Richard Talley, City of Fort Worth; and 

Robert McCarthy, Dallas Water Utilities, for 

providing water-quality data collected by their 

agencies in the Trinity River Basin. We also wish 

to thank Sam Brush with the North-Central Texas 

Council of Governments for providing data on 

wastewater-treatment plants and for his insights 

and information on historical changes in treatment 

practices in the basin.

DESCRIPTION OF STUDY UNIT

The Trinity River Basin NAWQA study unit is 

located in the south-central United States, in northcentral and east Texas (pi. 1). It extends on a 

southeast diagonal for about 570 km, from 

immediately south of the Oklahoma-Texas State 

boundary to Trinity Bay, a part of Galveston Bay, 

in the Gulf of Mexico. The study-unit boundary is 

the surface-water drainage divide of the Trinity 

River except in the area near the coast where it 

includes parts of Chambers and Liberty Counties. 

The study unit includes about 48,000 km2 or about 

6.5 percent of the total area of the State of Texas, 

with parts or all of 38 Texas counties within its 

limits.

Phosphorus is present in animal waste and is 

therefore present in sewage. During the 1950's and 

1960's, the increased use of phosphate in 

household detergents tended to increase the output 

of phosphate by sewage-treatment plants. Public 

awareness of problems caused by phosphorus, 

particularly eutrophication of lakes, has led to 

various measures to limit the use of phosphate in 

detergents (Hem, 1985). To discourage excessive 

growth of aquatic plants in flowing water, the U.S. 

Environmental Protection Agency (1986) 

recommended that total phosphorus concentrations 

should not exceed 0.1 mg/L as P. Total phosphorus 

is a measure of the organic and inorganic forms of 

dissolved and suspended phosphorus.

The Trinity River Basin study unit can be 

described as a modified sedimentary landform 

reflecting a depositional geologic history of 

successive climate and sea level changes altered by 

the subsequent uplift or subsidence of areas of the 

study unit. The topography varies according to the 

nature of the rocks exposed and the stage reached 

in the erosion cycle. The study unit is dissected by 

alternate bands of rolling, treeless prairies, smooth 

to slightly rolling prairies, rolling timbered hills, 

and a relatively flat coastal plain. The study unit 

slopes gradually from about 300 m above sea level 

at the headwaters in the northwest to sea level in 

the southeast. Land-surface elevation decreases at 

about 1.3 m/km over the length of the study unit.
Climate

The climate of the study unit is best described 

as modified marine, subtropical humid, with warm 

summers and a predominant onshore flow of 

tropical maritime air from the Gulf of Mexico. This 

onshore flow is modified by a west to east increase 

in moisture and by intermittent seasonal intrusions 

of continental air. This variation in climate is 

attributed to changes in land elevation and the 

proximity tp the Gulf of Mexico and the southern 

Great Plains. Most of the study unit has a winter 

surplus and a summer deficit of precipitation. The 

most northwestern section of the study unit 

experiences little or no water surplus in any season, 

and the lower tip of the basin experiences no water 

deficit in any season.

Average annual precipitation ranges from 

about 70 cm in the northwestern part of the study 

unit to about 130 cm in the southeastern part. The 

percent of precipitation that runs off also increases 

from northwest to southeast. Average annual 

temperature ranges from about 18 °C in the 

northwest to 20 °C in the southeast.

Geology

The surface geology (fig. 1) is composed of 

deposits ranging in age from Pennsylvanian to 

Quaternary (Peckham and others, 1963). 

Sediments of the Pennsylvanian and Permian 

Periods of the Paleozoic Era crop out in the 

northwestern part of the study unit and dip to the 

northwest. These sediments underlie about 6 

percent of the study unit and consist of marine and 

nearshore sand, shale, and limestone. Cretaceous 

formations underlie about 48 percent of the study 

unit. The formations crop out in the northern and 

middle parts of the study unit (fig. 1) and dip to the 

southeast toward the Gulf of Mexico. Cretaceous 

deposits are composed of nearshore sand, marineshale, and limestone. Formations of the Tertiary 

and Quaternary Periods crop out in the middle and 

southern parts of the study unit and dip to the 

southeast (fig. 1). Tertiary formations underlie 

about 30 percent of the study unit; Quaternary 

formations underlie the remaining 16 percent of the 

study unit. Tertiary and Quaternary formations

contain a mix of marine and continental deposits. 

The marine deposits consist of clay, shale, and 

marl, with minor amounts of sand. Continental and 

nearshore deposits consist primarily of sand, with 

lesser amounts of clay, shale, and lignite.

Population and Land Use

Texas is the seventh fastest growing state in the 

United States, and the third largest in total 

population behind California (30 million), and 

New York (18 million) with a 1990 population of 

almost 17 million (A.H. Belo Corp., 1991). The 

study unit contains two of the four most populous 

counties in the State, Dallas and Tarrant Counties, 

with a combined 1990 population of about 3 

million, or about 19 percent of the State's total 

population. These two counties account for about 

66 percent of the total population of 4.5 million in 

the study unit. Between 1980-90, Denton County 

had the largest percent increase of all counties in 

the State (91 percent), followed by Collin County 

(86 percent), and Rockwall County (76 percent). 

During the same period, the total population in 

Texas increased by about 19 percent; however, the 

population of the study unit increased by about 26 

percent, indicating that the study unit continues to 

be one of the major growth areas in the State. 

Population density of the State is about 26 persons 

per square kilometer, but the density of population 

within the study unit is about 100 persons per 

square kilometer. Dallas and Tarrant Counties both 

have population densities of over 386 persons per 

square kilometer.

Land-use and land-cover data are available as 

Geographic Information and Retrieval System 

(GIRAS) files for the entire United States (U.S. 

Geological Survey, 1990). Land-use and landcover information was interpreted from topographic maps and high altitude aerial photos. The 

final boundaries were compiled at a scale of 

1:250,000 and digitized at that scale. Land use is 

mapped to the detail described as level 2 by 

Anderson and others (1976). One limitation of the 

level-2 classification scheme when attempting to 

relate land use to water quality is that cropland and 

pasture are grouped. The level-2 category 

"cropland and pasture" accounts for about 99
34'

32'

30°

98- ~r 96*

EXPLANATION

fffffj QUATERNARY

p^ TERTIARY

[ | CRETACEOUS

mHU PERMIAN

IB! PENNSYLVANIAN

> »! - ^-r f- - -*rU T^^m

94- ~T

Modified from Godfrey. McKee, and Oakes. 1973. and 

Kier, Gamer, and Brown, 1977.

Base from U.S. Geological Survey, 1:250,000 topographic quadrangles, 1972-88. 

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees.

SCALE 1:3.168,000

0

1 

0

1 

50

50

1

1 

100

100

I

KILOMETERS

Figure 1. Surficial geology of the Trinity River Basin.
percent of all agricultural lands in the study unit. 

This classification precludes comparisons between 

pasture and the more intensively farmed and 

fertilized cropland and comparisons among crops. 

The distribution of land use within the study unit is 

shown on figure 2.

Historically, agriculture has been economically 

important in the study unit. The percentage of 

county area planted in a particular crop or crops 

in 1989 is shown on figure 3. The total major crop 

category (fig. 3A), includes corn, cotton, peanuts, 

sorghum, soybeans, rice, and wheat. Wheat, rice, 

and cotton are shown separately (figs. 3B, C, D) 

because of the significant differences in the 

geographic distribution and management practices 

of these crops. The major cotton producing area 

within the study unit is in the Blackland Prairie. 

Nitrogen and phosphorus fertilizer use in the study 

unit is generally distributed in patterns similar to 

agricultural land use (fig. 4).

Water Use, Diversions, Wastewater 

Treatment and Returns

Surface water is the main source of water 

supply for the study unit. The removal of water 

from the rivers and streams within the Trinity 

River Basin is reported in two different ways, withdrawals and consumptive use. Consumptive use 

is water which does not return to the system in its 

original form. Water-use data are provided by the 

Texas Water Development Board and aggregated 

as reported here by the USGS (Dee Lurry, U.S. 

Geological Survey, written commun., 1992). 

Total withdrawals for 1990 were estimated to be 

3,800 Mm3 and, of that amount, an estimated 

520 Mm3 was consumptive use (table 1). An 

estimated 96 percent of total withdrawals were 

from surface water with the remaining 4 percent 

coming from ground water.

The largest consumptive use in 1990 was for 

domestic water supply, the majority of which 

occurs in Dallas and Tarrant Counties due to their 

large populations (table 1). The largest category of 

withdrawals in 1990 was power generation with an 

estimated total of 2,500 Mm3 (table 1); however, 

only 1.2 percent of that was estimated to be

consumptive use. Farming and ranching activities 

are found in large areas of the basin. Irrigation 

withdrawals occur predominately in the riceproducing coastal area of the basin. Most of this 

water is supplied by surface water with some 

ground water pumped mainly from the Gulf Coast 

aquifer (Texas Department of Water Resources, 

1984). Withdrawals for irrigation totaled 180 Mm3 

in 1990 with an estimated 84 Mm3 for consumptive 

use. Livestock water use in the Trinity River Basin 

totaled 28 Mm3 in 1990.

Transfers of water from the adjoining basins 

and from reservoirs below the Dallas-Fort Worth 

area are required to meet the needs of the DallasFort Worth metropolitan area. In 1989,111 Mm3 of 

water was transferred into the Trinity River Basin 

from the Sabine, Neches, and Brazos River Basins 

(Texas Water Development Board, 1990). In 1989, 

354 Mm3 of water was exported from the Trinity 

River Basin which was equal to 5.4 percent of the 

mean annual discharge measured at site 08066500, 

the Trinity River at Romayor, Texas (pi. 1). The 

majority of the transfers were to the Houston area 

from Livingston Reservoir and the mainstem of the 

Trinity River downstream from Livingston 

Reservoir. Other exports were to adjacent and 

nearby basins (Texas Water Development Board, 

1990).

In 1972, the Federal Water Pollution Act 

established higher standards for wastewater 

treatment. Included were lower levels for the 

biochemical oxygen demand (BOD) and ammonia 

in wastewater effluent. The BOD is the measured 

biochemical depletion of oxygen in a sample under 

controlled conditions (usually 5 days at 20 °C). It is 

considered a useful way of expressing streampollution loads. The treatment of wastewater is 

commonly described in terms of three main 

processes primary, secondary, and tertiary 

treatment. In the major wastewater-treatment 

plants (WWTPs) in the Dallas-Fort Worth area 

primary treatment consists of using screens, 

aerated grit basins, and settlement ponds to remove 

solids. The secondary treatment process treats the 

primary treated water with activated sludge. 

Aerated basins are used to develop a large 

community of microorganisms which consume the 

organic contaminants in the wastewater. Clarifiers
94?

34?

32>

3D1

EXPLANATION

 URBAN

^| AGRICULTURE

H RANGE

f FOREST

 WATER

Modified from U.S. Geological Survey, 1990

Digital base from U.S. Geological Survey

Alters equal-area projection based on standard parallels 29.5 and 45.5 degrees

SCALE 1:3,168.000

50

____I______

100 MILES

I l 

.50 100 KILOMETERS

Figure 2. Land use in the Trinity River Basin.
PERCENTAGE OF COUNTY

AREA PLANTED IN MAJOR CROPS. 1

PERCENTAGE OF COUNTY 

AREA PLANTED IN WHEAT. 1989

(C)

EXPLANATION

PERCENTAGE OF COUNTY 

AREA PLANTED IN RICE. 1989

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

SCALE 1:6.336,000 

0 50 100 MILES

0 50 100 KILOMETERS

Figure 3. Distribution of (A) total major crops, (B) wheat, (C) rice, and (D) cotton production by county, 

1989.
NITROGEN APPLIED PHOSPHOROUS APPLIED

EXPLANATION

NITROGEN IN KILOGRAMS PER HECTARE (BY COUNTY) 

1990 DATA

NO DATA

LESS THAN 5 

5-9.9 

10 -19.9 

GREATER THAN 20

EXPLANATION

PHOSPHOROUS IN KILOGRAMS PER HECTARE (BY COUNTY) 

1990 DATA

NO DATA

LESS THAN 2.5

2.5 - 4.9

5-7.5

GREATER THAN 7.5

SCALE 1:6.336,000

50 100 MILES

I

I I I

0 50 100 KILOMETERS

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

Figure 4. Nitrogen and phosphorus fertilizer use by county.

10
Table 1. Estimated withdrawals and consumptive water use for eight categories in the Trinity River Basin 

in 1990

[Water-use data provided by the Texas Water Development Board and aggregated by the U.S. Geological Survey (Dee Lurry, U.S. 

Geological Survey, written cornmun., 1992).]

Category

Commercial

Domestic

Industrial

Power

Mining 

Livestock

Irrigation 

Losses 

Total

Withdrawals and deliveries 

from public supply 

(in million cubic 

meters per year)

52

880

100

2,500

17

28

180 

12

3,800

Estimated consumptive use

Million cubic meters 

per year

26

300

43

29

17

28

84 

12

520

Percent of supply

5.0

34

43

1.2

100 

100

47 

100

'14

1 Average percent of total supply as consumptive use.

are then used to separate most of the 

microorganism population from the wastewater. 

Tertiary processes included filtration of the 

wastewater through sand filters or, in a few cases, 

carbon adsorption basins. The final product is 

disinfected with chlorine or sulfur dioxide, 

dechlorinated, then released into the natural waters.

Since 1972, regional WWTPs were 

constructed within the Dallas-Fort Worth area 

(pi. 1) to eliminate many of the community-owned 

WWTPs that provided only the primary treatment 

of wastewater (table 2). In 1970 the regional 

WWTPs treated and discharged about 390 Mm3 

(equal to 283 million gallons per day). The volume 

treated in 1990 by the seven largest WWTPs was 

approximately 760 Mm3 (equal to 550 million 

gallons per day) and represents over 95 percent of 

the wastewater generated in the Dallas-Fort Worth 

metropolitan area (Brush and Promise, 1990). 

These facilities have both secondary and tertiary 

treatment processes. Bypasses from sewagecollection systems and sewage-treatment plants 

have historically contributed raw sewage 

containing nutrients to the Trinity River system.

Total annual bypass volumes from major WWTPs 

ranged from 760,000 m3 to 33 Mm3 from 1978 to 

1985 (Davis, 1987).

Several studies have documented 

improvements in water quality since the 1970's in 

the Trinity River below the Dallas-Fort Worth area. 

The improvements mainly were attributed to 

reductions in the loading of oxygen-demanding 

materials in sewage as a result of improved sewage 

treatment (Davis, 1987; Schertz, 1990).

Streams and Reservoirs

The western part of the basin is drained by the 

Clear Fork Trinity River and the West Fork Trinity 

River, which join in Fort Worth (pi. 1). The northcentral part of the basin is drained by the Elm Fork 

Trinity River which joins the West Fork Trinity 

River in Dallas to form the Trinity River. The 

northeast part of the basin is drained by the East 

Fork Trinity River which joins the Trinity River 

about 32 km southeast of Dallas. Runoff increases 

from an average of 5.3 cm/yr in the west to 15 

cm/yr in the east corresponding with the increase in

11
Table 2. Volumes of wastewater treated in 1990 by the seven largest wastewater-treatment plants in the Dallas-Fort 

Worth area, Texas

Average effluent discharges from the seven largest wastewater-treatment plants

and total discharges form all plants

(in million cubic meters per year)

period

1970-74

1975-79

1980-84

1985-89

1990-92

Fort 

Worth 

Village 

Creek

44

69

118

153

187

Dallas 

Central

162

187

206

206

199

Dallas 

Southside

5.4

8.3

32

56

100

Trinity River 

Authority 

Central

38

70

102

122

154

City of 

Garland 

Duck 

Creek

22

22

26

29

31

North Texas 

Municipal 

Water District 

Mesquite

7.4

7.3

11

14

16

Trinity River 

Authority 

Ten Mile 

Creek

7.0

8.9

11

16

22

Total

302

402

559

672

789

average annual precipitation from 71 cm to more 

than 100 cm.

The middle part of the Trinity River Basin is 

drained mostly by two tributaries, Cedar Creek 

from the east and Richland Creek from the west. 

Runoff from Richland Creek averaged 17 cm/yr 

from 1939 to 1989. Runoff from Cedar Creek 

averaged 21 cm/yr from 1963 to 1987.

Downstream from the mouth of Richland 

Creek the width of the basin narrows to about 70 

km. Average runoff from Kickapoo Creek in the 

southeastern part of the basin was 25 cm/yr from 

1965 to 1990 and average annual precipitation 

there was about 130 cm. Not only does runoff 

increase with increasing precipitation but the 

percent of precipitation running off also increases, 

from about 8 percent of precipitation in the 

northwest to about 20 percent in the southeast. 

There are several factors that could cause this. 

Rainfall exceeds soil infiltration capacities and 

evapotranspiration rates more frequently in wet 

areas than in dry areas, which results in increased 

runoff. Greater base flow from ground-water 

discharge to streams would also be expected in 

wetter areas where ground-water recharge is 

greater and ground-water levels tend to be closer to 

land surface.

The Trinity River discharges to Trinity Bay, a 

part of Galveston Bay, in the Gulf of Mexico. Total

flow out of the Trinity River Basin can be 

approximated from flow measured at site 

08066500, the Trinity River at Romayor, Texas. 

The drainage area at the Romayor gage is about 

44,000 km2 or about 96 percent of the Trinity River 

Basin. Average discharge at the Romayor gage 

from 1924 to 1991 was 208 m3/s or 6,560 Mm3/yr. 

Average runoff was 15 cm/yr, about 16 percent of 

the average precipitation in the drainage. The 

remainder of the precipitation either evaporates, is 

diverted out of the basin, or enters storage either in 

reservoirs or aquifers. The largest flows in the 

Trinity River generally occur during winter and 

spring and the smallest flows occur during summer 

and early fall.

Human activities have caused extensive 

changes in the stream network and streamflow in 

the basin. Agricultural and urban development 

have modified the landscape over most of the 

basin converting grasslands and forests to 

ranches, farms, and cities. Numerous reservoirs 

have been built to retain runoff on all major 

tributaries and the mainstem of the Trinity River, 

and diversions move water within the basin and to 

and from adjacent river basins. Some of that water 

is used by the cities in the Dallas-Fort Worth 

metropolitan area, treated, and then returned to the 

Trinity River, thus adding solutes to the water and 

new pathways for water flow.

12
Human development has caused changes in the 

variability of streamflow in some streams in the 

study unit (Ulery and others, 1993). Two processes 

are responsible for the changes, discharge of 

effluents from WWTPs and construction of 

reservoirs. Under natural flow conditions 

streamflow is predominantly from two sources, 

baseflow from ground-water discharge to the 

stream and runoff from precipitation. The 

distribution of flows at a site is dependent on the 

amount of precipitation and the local physical 

characteristics, for example, soil infiltration rates. 

The effect of discharge of effluents has been to 

increase baseflow downstream from the DallasFort Worth area. The effect of reservoirs has been 

to supplement baseflow and to reduce flood peaks 

in some reaches.

In addition to the reservoirs shown on plate 1, 

there are about 1,000 smaller reservoirs in the 

basin. Most of these are floodwater-retarding 

structures, constructed by the Soil Conservation 

Service (U.S. Department of Agriculture, 1979), 

with capacities generally between 0.5 and 1 Mm3 . 

Total conservation capacity in the basin, including 

the capacities of the floodwater-retarding 

structures, is estimated to be about 9,300 Mm3. 

That is about 1.4 times greater than the mean 

annual flow at the Romayor gage.

Aquifers

The aquifers within the State have been 

classified as major and minor water-bearing 

formations based on their varying abilities for 

supplying ground water (Texas Board of Water 

Engineers, 1958). Based on that classification, 

there are three major aquifers in the Trinity River 

Basin, the Trinity Group, Carrizo-Wilcox, and Gulf 

Coast (fig. 5). Minor aquifers in the basin include 

the Woodbine, Nacatoch, Queen City, and Sparta. 

In addition, there are many other water-bearing 

formations which yield small or moderate 

quantities of water and are important locally. More 

detailed descriptions of ground water in the Trinity 

River Basin are found in Peckham and others 

(1963), Muller and Price (1979), Nordstrom 

(1982), and Texas Department of Water Resources 

(1984). Several reports have been written for the

U.S. Geological Survey Gulf Coast Regional 

Aquifer-Systems Analysis study that includes the 

lower half of the Trinity River Basin. Hosman and 

Weiss (1991) described in detail the hydrogeologic 

framework for the Texas coastal uplands aquifer 

system. Pettijohn, Weiss, and Williamson (1988) 

presented maps of dissolved-solids concentrations 

in the gulf coast aquifer systems. Ryder (1988) 

described the hydrogeology and predevelopment 

flow in the Texas Gulf Coast aquifer systems.

Aquifers in the Trinity River study unit 

generally dip to the southeast. Recharge to the 

aquifers is primarily in the outcrop areas and 

discharge is to wells, adjacent beds, and the 

downdip saline zone. Major cones of depression 

have formed in the Trinity Group aquifer where 

declines of 30 to 76 m for extensive areas were 

reported during the period 1976-89 (Baker and 

others, 1990). Declines have also occurred in the 

Woodbine aquifer. Wells tapping zones under 

artesian conditions over a large area experienced 

declines of 7 m or more from 1976 to 1989 and 

declines in Grayson and Fannin Counties were as 

much as 45 m (Baker and others, 1990). Groundwater withdrawals have resulted in local and 

regional cones of depression in the Gulf Coast 

aquifer centering in the Houston area. As a result 

of this, upward vertical gradients have been 

reversed in those areas. Some water is also derived 

from the reduction in storage associated with the 

compaction of clays (Muller and Price, 1979).

ASSESSMENT APPROACH

The approach to this assessment followed four 

steps. They were:

1. Select constituents for analysis on the 

basis of the NAWQA study design, 

biochemical considerations, and data 

availability.

2. Select types and methods of analysis.

3. Compile and screen available data to 

develop data sets for analyses.

4. Analyze and interpret data.

13
98-

34-

32-

30°

96' ~T

EXPLANATION

MAJOR AQUIFERS 

[] TRINITY GROUP AQUIFER 

3 CARRIZO-WILCOX AQUIFER

GULF COAST AQUIFER

MINOR AQUIFERS

| | WOODBINE AQUIFER 

HH NACATOCH AQUIFER 

| | QUEEN CITY AQUIFER 

SPARTA AQUIFER

Modified from Peckham and others, 1963

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

SCALE 1:3,168.000 

50

I 

50

100 MILES

100 KILOMETERS

Figure 5. Major and minor aquifer outcrop areas in the Trinity River Basin.

14
Selection of Constituents for Analysis

Constituents evaluated included total nitrogen, 

ammonia plus organic nitrogen (commonly 

referred to as total Kjeldahl nitrogen (TKN)), 

nitrite plus nitrate (NO2+NO3), and total 

phosphorus. Additionally, trends were evaluated 

for the component parts of TKN and NO2+NO3: 

organic nitrogen, ammonia, nitrite, and nitrate. 

Concentrations are reported in milligrams per liter 

as N for the various nitrogen species and as P for 

total phosphorus. TKN and NO2+NO3 were chosen 

for analyses because the sum of TKN and 

NO2+NO3 approximately equals total nitrogen in 

natural waters and the relative proportions of each 

reflects differences in chemical behavior related to 

photosynthesis and oxygen demand.

Methods of Analysis

Spatial Distribution

The spatial distribution of constituents is 

presented using maps showing median and 90th 

percentile values in surface water. The time period 

used was water years 1982 to 1991 (October 1 to 

September 30), and data from other agencies were 

included. This time period was chosen as a 

compromise between using a long time period to 

maximize the likelihood of data and using a short 

time period to minimize the likelihood of possible 

temporal trends in the data. The sites used were 

those that had 12 or more samples collected during 

the time period. Sites with fewer than 12 samples 

were not used in order to avoid the potential for 

short-term variability and unknown variability due 

to flow conditions arising at some sites. Time 

period, number of analyses by constituent, and the 

number of sites for each type of analysis are listed 

in table 3.

Maps showing constituent concentrations were 

produced using different line symbols for reaches 

of the streams that vary based on differences in 

concentrations. This approach was chosen so that 

the reader can visualize spatial patterns in streams 

associated with features such as reservoirs and 

point sources. Stream reaches were broken at

confluences of major tributaries, at reservoirs, and 

at major point sources. Generally, stream reaches 

are represented on maps using data from the 

sampling site in the reach with the largest number 

of samples. Actual concentrations at locations not 

sampled are not known.

Relations to Categorical Variables

The relation between concentrations in streams 

and one categorical variable, stream type, was 

evaluated. Three stream types were identified 

(1) reach downstream from a major point source 

or sources (below point source), (2) reach 

downstream from reservoirs with no major point 

sources between the reservoir and the site (below 

reservoir), and (3) tributary without major 

reservoirs or major point sources (tributary). Each 

sampling site was assigned to one of these three 

stream types. Sites were grouped by stream type 

and concentrations for groups were compared 

graphically and statistically. The data set used was 

stream samples from the USGS and other agencies 

from 1982 to 1991. Graphical comparisons are 

made using side-by-side modified boxplots. The 

modified boxplots show the 25th, 50th, and 75th 

percentiles as a rectangle or box and have whiskers 

on the lower and upper ends of the box extending 

to the 10th and 90th percentiles. Outliers and 

extreme values are not shown. Statistical 

comparisons between data grouped by stream type 

were made using the Kruskal-Wallis test. The test 

was used to determine the probability that each 

group of samples came from the same population.

Only one categorical variable was evaluated 

relative to nutrients in ground water. Nitrate 

concentrations in ground water were grouped by 

aquifer and compared using the same methods 

described above, side-by-side boxplots and the 

Kruskal-Wallis test.

Relations to Continuous Variables

Relations between concentrations in streams 

and three continuous variables were investigated: 

the percent of drainage in agricultural land use, 

streamflow, and time. These analyses required

15
Table 3. Sources of data, time periods, number of analyses, and number of sites used for analysis of nutrients in streams in the Trinity River Basin 

[USGS, U.S. Geological Survey; TWC, Texas Water Commission; TRA, Trinity River Authority; ARL, city of Arlington; FW, city of Fort Worth; ~, indicates parameter not analyzed]

Type of 

analysis

Spatial 

distribution

Relations to

point source 

and reservoirs

Relations to

agricultural land use

Relations to

stream

discharge

Seasonal

variation

Temporal 

trends

Loads

Minimum

Sources Time number

of data period samples

per site

USGS 

TWC

TRA ARL 

FW 1982-91 12

USGS

TWC 

TRAARL

FW 1982-91 1

USGS 1982-91 6

USGS 1982-91 19

USGS 1974-91 60USGS 1974-91 60

USGS 1974-90 114

Number of analyses and number of sites (in parentheses)

Nitrogen

1,043 

(21)

1,106

(34)

322 

(14)

760

(11)

1,190

(8)

1,190 

(8)

1,095

(9)

Phosphorus

2,197 

(41)

2,762

(66)

385 

(14)

807

(11)

1,534

(8)

1,534 

(8)

1,120

(9)

Ammonia 

plus 

organic 

nitrogen

1,881 

(34)

2,267

(68)

385 

(14)

830

(11)

1,250

(8)

1,250 

(8)

1,200

(9)

Nitrite ~ Organic piUS AmmoniS nHmnon

nitrate

1,208 

(23)

2,428

(74)

384 

(14)

827

(11)

1,229

(8)

1,229 1,552 1,390 

(8) (8) (8)

1,125

(9)

Nitrite Nitrate

... ...

...

..

...

1,526 1,465 

(8) (8)

o\
digitized drainage areas, data for the related 

continuous variables including streamflow, and 

some knowledge of sampling and laboratory 

methods over time. Additionally, to evaluate 

relations to time, a minimum of 60 samples 

distributed over a minimum of 10 years was used 

(table 3).

Relations between concentrations and percent 

of drainage in agricultural land use were 

investigated by digitizing contributing drainage 

areas to each sampling site, then digitally 

overlaying those areas with GIRAS land-use data. 

Agricultural lands were then summed within each 

drainage area, divided by total area, and multiplied 

times 100 to calculate percent. The relation 

between concentrations and percent agricultural 

land use is displayed graphically using scatter plots 

of concentrations versus percent agricultural land 

use. Spearman's rank correlations were computed 

between the ranks of concentrations and percent 

agricultural land use to test whether concentrations 

generally increased or decreased with increasing 

agricultural land use. If such a relation exists, the 

two variables are said to possess a monotonic 

correlation (Helsel and Hirsch, 1992).

Relations to streamflow were evaluated 

graphically by plotting concentrations versus 

discharge. LOWESS, or LOcally WEighted 

Scatterplot Smoothing, curves are shown on these 

plots to emphasize the shape of the relationship 

between the variables. LOWESS involves fitting at 

least 2n weighted least-square equations and, 

because of the weighting, is a robust method to 

compute a moving average (Helsel and Hirsch, 

1992). Plots of concentrations versus discharge 

were grouped by stream type, as described above, 

to show variations in the relation to discharge for 

different hydrologic conditions. Relations to 

streamflow were evaluated statistically by 

computing rank correlations between 

concentrations and streamflow and evaluating the 

significance of the relation on the basis of p-values.

Seasonal variations of concentrations in 

streamflow were evaluated by plotting flowadjusted residuals of concentrations by day of the 

year (Julian date) with LOWESS curves. Flow 

adjustment was accomplished by taking the

difference between concentrations and the 

LOWESS curve of concentration versus discharge. 

The flow adjustment was made to remove some of 

the variance in concentrations caused by variations 

in streamflow. Seasonal variations and relations to 

time were evaluated for the 1974 to 1991 water 

years. The longer time period was used to include 

the occurrence of major changes in wastewater 

treatment in the 1970's (Brush and Promise, 1990).

Relations to time, or trends, were evaluated at 

selected stream sites using the seasonal Kendall 

test on the residuals of concentrations versus 

streamflow. The analyses were done using 

residuals to remove the variance in concentrations 

caused by variations in streamflow so that any 

"trend" signal present could be more easily 

detected. LOWESS curves were used to compute 

residuals because they describe the relation 

between concentrations and streamflow without 

assuming linearity or normality of residuals. The 

seasonal Kendall test accounts for seasonal ity by 

computing the Mann-Kendall test on each of the 

specified number of seasons separately and then 

combining the results (Helsel and Hirsch, 1992). 

The null hypothesis, that there was no relation 

between residuals and time, was evaluated at a 

confidence level of 95 percent. If a p-value of 0.05 

or less was computed by the seasonal Kendall test, 

the null hypothesis was rejected and it was 

concluded that there was a trend.

Changes over time in the proportions of total 

nitrogen as TKN and as NO2+NO3 were also 

evaluated. Total nitrogen is approximately equal to 

the sum of TKN and NO2+NO3 ; therefore, as one 

constituent increases as a proportion of total 

nitrogen, the other will decrease. These changes 

are illustrated using side-by-side boxplots of 

proportion of total nitrogen as TKN and as nitriteplus-nitrate at two sites for successive time periods 

(1974-76,1977-81,1982-86, and 1987-91).

Relations between nutrient concentrations and 

three continuous variables were evaluated in 

ground water. The variables were depth of well, 

nitrogen fertilizer application rate, and percent 

agricultural land use. The relation between nitrate 

concentrations and depth of well, by aquifer, was 

plotted with LOWESS curves and evaluated using

17
rank correlations. The relation between nitrate 

concentrations and nitrogen fertilizer application 

rate, by county, was evaluated using rank 

correlations for wells less than 30 m deep. Wells 

were assigned fertilizer application rates based on 

what county they were located in. The relation 

between nitrate concentrations and percent 

agricultural land use was evaluated for wells lessthan 30 m deep and wells less than 60 m deep. 

Percent agricultural land use was determined for 

each of these wells by digitally overlaying a 1-km 

radius circle around the well with the GIRAS landuse data and calculating the percent of area within 

that circle that was agricultural land use. The 

relation between nitrate concentrations and percent 

agricultural land use was plotted with LOWESS 

curves and evaluated using rank correlations.

were obtained in digital format; however, in a few 

cases, paper copies were obtained and entered into 

a data base by USGS personnel. Although a large 

number of analytical results were obtained, this 

was not intended to be an inventory of all existing 

water-quality data for the basin. Sources of data, 

time periods, number of samples, and number of 

sites used for each type of nutrient analysis in 

streams are summarized in table 3.

Data from the U.S. Environmental Protection 

Agency's Storage and Retrieval System (STORET) 

data base were helpful in determining what 

agencies had collected water-quality data in the 

basin. The agencies were contacted directly for 

their data. Available data, by agency, is briefly 

described below.

Loads and Yields

Load is the mass of a constituent passing a 

location during a given time period, for example, 

kilograms of nitrogen per day passing a streamflow 

gaging station. Load is the product of 

concentration and discharge. Yield is the load per 

unit area and is calculated by dividing the load by 

the drainage area. Loads and yields of nitrogen, 

phosphorus, TKN, and NO2+NO3 were calculated 

for nine USGS streamflow gaging stations. 

Calculation of loads was accomplished using a 

program developed by the USGS (Tim Cohn, 

written commun., 1992) called the Minimum 

Variance Unbiased Estimator (MVUE). The 

MVUE estimates load by performing a regression 

analysis on the measured concentrations using 

linear and nonlinear functions of discharge and 

time. The MVUE accounts for both temporal trend 

and seasonality and includes estimates of the 

standard error of prediction. This program was 

used to estimate monthly and annual loads for each 

calendar year.

AVAILABLE DATA

Historical water-quality data were requested 

from State, local, and other Federal agencies that 

operate in the Trinity River Basin. Preferably, data

U.S. Geological Survey

The USGS collected a total of 2,324 nutrient 

samples from 42 stream-sampling locations in the 

Trinity River Basin from October 1,1973, to 

September 30,1991 (water years 1974-91). Each 

of those 2,324 samples had at least one analysis of 

the eight nutrient constituents used in the analysis 

of temporal trends. Seasonal variations, temporal 

trends, and loads were evaluated for the period 

1974-91. Other analyses were for the period 1982- 

91. USGS stream-sampling locations for the period 

1982-91 are shown on figure 6. Sampling at some 

sites began in the 1960's and continues to the 

present (1994).

Data from selected USGS sites were used to 

evaluate relations to time and to calculate loads. To 

accurately calculate the load of a constituent, the 

data set used must include samples collected at 

higher flows, when much of the load occurs. The 

distribution of samples over the discharge 

hydrograph is also relevant to the evaluation of 

relations to time. The distribution of samples as a 

function of flow for four sites used in the 

evaluation of relations to time and calculation of 

loads is shown on figure 7. Dectiles of flow were 

determined using daily discharge data. Samples 

generally were evenly distributed over the range of 

flow at these sites and high flows were well 

represented.

18
AMMONIA PLUS ORGANIC NITROGEN (TKN) NITRITE PLUS NITRATE

TOTAL NITROGEN TOTAL PHOSPHORUS

-M \N 

.-< \ *

-v

ATV^

V^U.'

:,'V 33-frx \ ^tf-UVft 

98^X0 v-t^ 1$ l 7

» x flf '

96 0 

-- 33

EXPLANATION

 1 TO 11 SAMPLES

O 12 OR MORE SAMPLES

SCALE 1:6.336.000

100 MILES

__I

I I I

0 50 100 KILOMETERS

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

Figure 6. Locations and sampling frequency for U.S. Geological Survey nutrient samples from streams, 

1982-91.

19
40

30

20

10

08049500

West Fork Trinity River at Grand Prairie

40

30

20

10

08057410 

_ Trinity River below Dallas

40

30

20

10

08062500

Trinity River near Rosser

0 10 20 30 40 50 60 70 80 90 100 

DECTILE OF FLOW

25

20

15

10

08066500

Trinity River at Romayor

0 10 20 30 40 50 60 70 80 90 100 

DECTILE OF FLOW

Figure 7. Distribution of samples collected by the U.S. Geological Survey by dectile of flow at four sites in 

the Trinity River Basin, 1982-91.

Texas Water Commission

The Texas Water Commission (TWC) 

provided samples from streams at more than 200 

locations within the basin (fig. 8). The nutrients 

sampled at most sites included nitrite, nitrate, 

ammonia, TKN, total phosphate, orthophosphate, 

and total phosphorus. A total of 1,144 analyses, 

collected during the 1982-91 water years, were 

used for this analysis. Sampling results compared 

well at sites where sampling by the USGS and the 

TWC coincided. The TWC data were used for 

analyses of spatial distribution of nutrients and 

relations to point sources and reservoirs.

Trinity River Authority

A total of 418 analyses from 8 sites sampled by 

the Trinity River Authority (TRA) were included

in the database obtained from the TWC and were 

used in this analysis (the TWC database contains 

data from several agencies in Texas). Nutrients 

commonly sampled included ammonia, nitrate, and 

TKN. These were used in the analyses of spatial 

distribution and relations to point sources and 

reservoirs.

City of Arlington

The city of Arlington provided data from 16 

sampling locations within the basin; 5 in Lake 

Arlington and 11 on tributaries to Lake Arlington. 

The nutrients sampled included ammonia, nitrite, 

nitrate, TKN, total phosphate, and orthophosphate. 

Forty-two analyses from one site sampled by the 

city of Arlington were used in the analysis of 

spatial distribution and relations to point sources

20
AMMONIA PLUS ORGANIC NITROGEN (TKN) NITRITE PLUS NITRATE

96 0 

-- 33

TOTAL PHOSPHORUS

EXPLANATION

 1 TO 11 SAMPLES

O 12 OR MORE SAMPLES

SCALE 1:6,336.000

50 100 MILES

 

0 50 100 KILOMETERS

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

Figure 8. Locations and sampling frequency for the Texas Water Commission nutrient samples from 

streams in the Trinity River Basin, 1982-91.

21
and reservoirs. Samples from Lake Arlington were 

not within the scope of this report.

City of Fort Worth

The city of Fort Worth provided 182 analyses 

from 7 sampling locations in the greater Fort Worth 

area that were used in this analysis. The nutrients 

sampled included ammonia, nitrite, nitrate, TKN, 

total phosphate, and orthophosphate. These 

samples were used in the analysis of spatial 

distribution and relations to point sources and 

reservoirs.

Texas Water Development Board

The Texas Water Development Board is the 

only agency that provided ground-water quality 

data for nutrients to this study and the only nutrient 

measured was nitrate. Ground-water data used in 

this analysis were from seven major and minor 

aquifers in the basin, had an associated well depth, 

and were from water years 1974-91. A total of 

1,482 nitrate analyses from 1,041 wells were 

available (fig. 9).

Sampling objectives for specific wells or 

analyses were generally not known; however, both 

routine monitoring and drinking water compliance 

sampling are performed by the State (Robert 

Blodgett, Texas Water Commission, oral commun., 

1993). Additionally, the type of well was not 

known, and it may have been a domestic-supply 

well, monitoring well, or public-supply well. 

Sampling objectives, type of well, and well 

completion can affect the interpretation of results. 

If, for example, only monitoring wells near 

hazardous waste sites are included in the data set 

an obvious bias would result. On the basis of well 

locations and depths, and conversations with TWC 

(Blodgett, oral commun., 1993), it was assumed 

that these wells represent ambient water quality 

and were mostly domestic- and public-supply wells 

in the study unit.

National Atmospheric Deposition Program

Atmospheric-deposition data were obtained 

from the National Trends Network (NTN) 

Atmospheric Deposition Program. The data 

collected from two NTN stations, TX56 and TX38, 

were used in this report. Station TX56 is located in 

the northwestern part of the Trinity River Basin at 

the LB J National Grasslands in Wise County, 

which is approximately 225 km from the 

geographic midpoint of the study unit (pi. 1). 

Station TX38 is located east of Palestine, Texas, 

approximately 80 km outside of the study unit and 

approximately 150 km from the geographic 

midpoint of the study unit. Concentrations and 

loads, estimated as the product of concentrations 

and precipitation, were available for ammonia and 

nitrate. Orthophosphate was also measured, but all 

samples were below the detection limit.

ANALYSIS OF AVAILABLE DATA FOR 

NUTRIENTS

Atmospheric Deposition

Precipitation is known to be an effective agent 

in cleansing the atmosphere of contaminants. This 

can result in precipitation having low pHs and 

containing large amounts of nutrients and toxic 

metals (Novotny and Chesters, 1981).

Concentrations of ammonium and nitrate in 

atmospheric deposition at the two NTN stations 

and the estimated average concentrations for the 

Trinity River Basin are listed in table 4. The 

estimated average concentrations were calculated 

as the average of the two NTN stations weighted 

by their distance to the geographic midpoint of the 

basin. Median concentrations of ammonium and 

nitrate from atmospheric deposition were both 0.2 

mg/L as N. In comparison, the median 

concentrations of ammonia and nitrate from 

tributary stream-sampling sites in the Trinity River 

Basin were 0.1 and 0.3 mg/L, respectively. The 

comparatively large nutrient concentrations in

22
96° 96° 94*

34"

32°

30° -

EXPLANATION

MAJOR AQUIFERS

[[[III TRINITY GROUP AQUIFER 

F^ CARRIZO-WILCOX AQUIFER 

GULF COAST AQUIFER

MINOR AQUIFERS

WOODBINE AQUIFER 

NACATOCH AQUIFER 

QUEEN CITY AQUIFER 

SPARTA AQUIFER 

 WELL LOCATION

Modified from Peckham and others, 1963

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

SCALE 1:3.168.000

50

____I______

50 100 KILOMETERS

100 MILES

__I

Figure 9. Locations of wells sampled by the Texas Water Development Board in the Trinity River Basin, 

1974-91.

23
Table 4. Nutrient concentrations from atmospheric deposition recorded at National Trends Network stations in the 

vicinity of the study unit and the weighted-average concentration1

[NH4, ammonium; NO3, nitrate; Ppt., precipitation; mg/L, milligrams per liter; cm, centimeters; N, nitrogen; , no data]

National Trends Network stations

Water 

year

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

Median

NH4 

(mg/L 

asN)

0.19

.20

.16

.08

.07

.14

.14

.26

.24

.18

.17

TX38

N03 

(mg/L 

asN)

0.19

.16

.20

.16

.13

.13

.18

.19

.16

.18

.17

Ppt 

(cm)

105.72

154.05

87.12

110.52

141.11

108.22

76.07

135.74

105.40

162.74

109.4

NH4 

(mg/L 

asN)

...

--

0.22

.27

.17

.19

.10

.32

.32

.23

.23

TX56

NOg

(mg/L 

asN)

...

__.

0.23

.24

.17

.19

.16

.27

.21

.21

.21

Ppt 

(cm)

...

...

64.07

90.44

103.19

101.25

80.38

100.58

111.76

96.73

98.6

Basin weightedaverage 

concentration

NH4 

(mg/L 

asN)

...

...

0.19

.15

.10

.17

.14

.31

.29

.23

.18

N03 

(mg/L 

asN)

...

...

0.21

.18

.14

.15

.17

.21

.17

.18

.18

1 To calculate the weighted average for the Trinity River Basin, each monitoring site was spatially weighted by the inverse of the squared 

distance of the site to the geographic midpoint of the basin.

atmospheric deposition suggest that it could be a 

significant source of nutrients to streams.

Surface Water

Spatial Distribution

Median and 90th percentile concentrations of 

nitrogen, NO2+NO3,TKN, and phosphorus for 

stream reaches with sampling sites with 12 or more 

analyses during water years 1982-91 are shown on 

figures 10 through 13. Not all sites fitting this 

criteria were used to construct these figures. If two 

or more sites were nearby in a similar reach of a 

stream, the site with more samples generally was 

used to represent the reach.

The most obvious spatial patterns indicated on 

figures 10 through 13 are large concentrations in 

the West Fork Trinity River, East Fork Trinity 

River, and Trinity River below the Dallas-Fort 

Worth area. The large concentrations are caused 

primarily by point-source discharges of nutrients in 

sewage effluent from the major WWTPs. Other 

spatial patterns are indicated by small 

concentrations below reservoirs. The small 

concentrations downstream from reservoirs are 

caused by the trapping of sediments and associated 

nutrients in the reservoirs and by uptake of 

nutrients in the reservoirs by plants. With few 

exceptions, relatively small concentrations of 

nutrients also occur on tributaries upstream from 

reservoirs and point sources.

24
MEDIAN 90TH PERCENTILE

EXPLANATION

NITROGEN CONCENTRATIONS

_ Greater than 5.0 

milligrams per liter

_ 2.0 to 5.0

milligrams per liter

Less than 2.0 

milligrams per liter 

Insufficient samples to 

calculate percentHes

A SAMPLING SITE

SCALE 1:4,000.000

so i

I_________

100 KILOMETERS

Figure 10. Median and 90th percentile nitrogen concentrations in Trinity River Basin streams, 1982-91.

It is evident from the figures showing spatial 

distribution that some parts of the study unit are 

lacking in nutrient data. There were few sites on 

tributaries in the middle and lower parts of the 

basin and there were no sites in the Cedar Creek 

drainage with 12 or more samples. There also were 

no large-scale synoptic sampling efforts over the 

basin designed to sample many streams during one

season and flow condition, hence the use of 

statistical summaries.

Relations to Point Sources and Reservoirs

The spatial distribution of concentrations 

suggested three types of streams in the study unit

25
MEDIAN 90TH PERCENTILE

EXPLANATION

NITRITE PLUS NITRATE 

CONCENTRATIONS

_ Greater than 3.0 

milligrams per liter

_ 1.0 to 3.0

milligrams per liter

_, Less than 1.0 

milligrams per liter

Insufficient samples to 

calculate percentiles

A SAMPLING SITE

SCALE 1:4,000,000

SO 100 MLES

I 

100 KILOMETERS

Figure 11. Median and 90th percentile nitrite-plus-nitrate nitrogen concentrations in Trinity River Basin 

streams, 1982-91.

with differing concentrations of nutrients (fig. 14). 

Each sampling site was assigned to one of three 

types: (1) reach downstream from a major point 

source or sources (below point source), (2) reach 

downstream from reservoirs with no point 

sources between the reservoir and the site 

(below reservoir), and (3) tributary without 

major reservoirs and point sources (tributary). 

As expected, concentrations are larger below the

major point sources than for tributaries and below 

reservoirs (fig. 15, table 5). The differences in 

concentrations between the three stream types are 

statistically significant for all four constituents 

using the Kruskal-Wallis test, with p-values in each 

case less than 0.0001.

Differences in concentrations by stream type 

indicate the effects of two major environmental

26
MEDIAN 90TH PERCENTILE

EXPLANATION

AMMONIA PLUS ORGANIC 

NITROGEN CONCENTRATIONS

_ Greater than 3.0 

milligrams per liter

_ 1.0 to 3.0

milligrams per liter

Less than 1.0 

milligrams per liter

Insufficient samples to 

calculate percentiles

A SAMPLING SITE

SCALE 1:4.000,000

100 KILOMETERS

Figure 12. Median and 90th percentile ammonia plus organic nitrogen concentrations in Trinity River 

Basin streams, 1982-91.

factors in the Trinity River Basin, reservoirs and 

point sources. Reservoirs generally reduce nutrient 

concentrations, and discharge of sewage effluents 

from major WWTPs increases concentrations.

Relations to Agricultural Land Use

An important nonpoint source of nutrients 

to streams nationally is agricultural use of

fertilizers. Contributing drainage areas were 

digitized and overlaid with GIRAS land-use data 

for 14 tributary sites sampled by the USGS. The 

percent of drainage area in agricultural land use 

was then calculated for comparison with nutrient 

concentrations. Only tributary sites were used 

because the effects of reservoirs and major point 

sources on downstream water quality overshadow 

the effects of nonpoint sources (land use) in the 

upstream drainage.

27
MEDIAN 90TH PERCENTILE

EXPLANATION

PHOSPHORUS CONCENTRATIONS

_ Greater than 3.0 

milligrams per liter

_ 0.5 to 3.0

milligrams per liter

Less than 0.5 

milligrams per liter

Insufficient samples to 

calculate percentiles

A SAMPLING SITE

"v

" "Mis,

SCALE 1:4,000.000

100 KILOMETERS

Figure 13. Median and 90th percentile phosphorus concentrations in Trinity River Basin streams, 1982-

91. -

There is a positive relation between percent 

agricultural land use and some constituent 

concentrations for tributary sites; however, there 

is a large amount of variability in concentrations 

(fig. 16). The lines shown on figure 16 are 

LOWESS curves. Statistically significant rank 

correlations, at a 95 percent confidence level, were 

computed between percent of drainage area

classified as agricultural land use and nitrogen and 

NO2+NO3. Correlations to phosphorus and TKN 

were not statistically significant. The positive 

correlations are an indication that more nitrogen 

and NO2+NO3 are present in tributaries draining 

agricultural lands than in tributaries draining 

nonagricultural lands. Most of the nonagricultural 

land in the tributary drainages was either range or

28
94>

34-

30- -

EXPLANATION

 BELOW POINT SOURCE

-- BELOW RESERVOIR

TRIBUTARY 

A SAMPLING SITE

Digital base from U.S. Geological Survey

Albers equal-area projection based on standard parallels 29.5 and 45.5 degrees

SCALE 1:3.168.000 

0 50

I T 

50 100 KILOMETERS

100 MILES

__I

Figure 14. Delineation of stream type in the Trinity River Basin.

29
1O.U

. 12.5

? 

b. 10.0

0 7.5 

O

g 5.0 

Z 

2.5

0.0

-

*

*

* *

, }

1 B

.^ ».*, ^-v^ -^ ^%f*

Cr A° <<Jr

52

z

UJ

DC

Z

 §>

E

P

S

t

Z

0.

UJ

E

z

1O.U

12.5

10.0

7.5

5.0

2.5

.

-

"

-

-

*

*

* 

*

* |

I T 

r^E-, 1 |

0.0 - 

<r

E 

eo zl|

ip

O 

rr 

O

o.u 

6.0 

4.0 

2.0 

0.0

I ! :

: ] j :

T '

i i i

I \^ \^

\S ^ xA. 

«M 0^ A -<X* >P A*x^ tyys

4?

STREAM TYPE

,

f=

to

§i

O.

8

I

D.

IU.U

7.5

5.0

2.5

nn

* 

f

1

:L i

*

 * 

i

i

 1

| 1

"'"n i *

STREAM TYPE

Figure 15. Concentrations of nutrients in streams grouped by stream type in the Trinity River Basin. (All 

concentrations of nitrogen expressed as N.)

Table 5. Median concentrations for all stream sites and for sites assigned to three groups based on stream type in 

the Trinity River Basin

[Number of samples shown in parentheses. TKN, Total Kjeldahl nitrogen; NO2, nitrite; NO3, nitrate]

Constituent

Nitrogen

TKN

NO2+NO3

Phosphorus

Median concentration in milligrams per liter and number of samples (in parentheses)

All sites

2.9 

(1,106)

1.2 

(2,267)

.9 

(1,497)

.3 

(2,762)

Point source

6.0 

(564)

1.5 

(981)

3.7 

(620)

1.6 

(1,244)

Reservoir

1.3 

(229)

.8 

(725)

.4 

(480)

.1 

(794)

Tributary

2.4 

(313)

1.1 

(561)

.8 

(397)

.2 

(724)

30
15

10

20 40 60 80 

PERCENT AGRICULTURE

100

gg.

II

20 40 60 80 100 

PERCENT AGRICULTURE

ooc

Z UJ

II 4

:-u

20 40 60 80 

PERCENT AGRICULTURE

100

OC UJ 3

CO DC

If

Q-H

20 40 60 80 100 

PERCENT AGRICULTURE

EXPLANATION

 LOWESS SMOOTH LINE

Figure 16. Relation between nutrient concentrations and percent of drainage in agricultural land use in the 

Trinity River Basin.

forest; none of the drainages had large areas of 

urban land use.

There are two known limitations with the landuse data used to determine percent agricultural land 

use for this analysis. The first is that the data were 

from the 1970's and land use changes have 

occurred in some parts of the study unit since then. 

The second is that the Anderson level-2 

classification for agricultural land use combines 

cropland and pasture (Anderson, 1976). There are 

large differences in the use and timing of fertilizer 

application between different crops and between

crops and pasture. Those differences cannot be 

evaluated using the available land-use data.

Relations to Streamflow

Two different physical phenomena can cause 

differences in concentrations in relation to 

Streamflow at a site. One phenomenon is dilution. 

A solute may be delivered to the stream at a 

reasonably constant rate (due to a point source or 

ground-water discharge to the stream). Runoff will 

dilute the concentration of the solute as discharge

31
08051500 Clear Creek near Sanger

0.1 1 10 100 

08052780 Hickory Creek at Denton

0.01 0.1 1 10 100 

08064100 Chambers Creek near Rice

1,000

08058900 East Fork Trinity River at McKinney

1,000 0.0001 0.001 0.01 0.1 1 10 

08052700 Littte Elm Creek near Aubrey

100

0.1 10 100 0.001 0.01 0.1 1 10 100 1,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 17. Relation between nitrogen concentrations and discharge for streams in agricultural areas.

increases. The result is a decrease in concentration 

with increasing flow. The other phenomenon is 

washoff. A solute, sediment, or a constituent 

attached to sediment can be delivered to the stream 

primarily from overland flow from paved areas or 

cultivated fields, or from streambank erosion. In 

this case, concentrations tend to increase with

increasing flow. Both of these phenomena occur in 

the Trinity River Basin.

Twelve sites sampled by the USGS, 

representing all three stream types described 

above, were used to evaluate the relations between 

nutrient concentrations and streamflow. Rank

32
lit 

o 

o

o

z

i

o

CO

12

08051500 Clear Creek near Sanger

0.01 0.1 1 10 100 1,000 

08052780 Hickory Creek at Denton

12

08058900 East Fork Trinity River at McKinney

0.01 0.1 1 10 100 

08064100 Chambers Creek near Rice

1.000

12

0.0001 0.001 0.01 0.1 1 10 

08052700 Little Elm Creek near Aubrey

100

\ ,

0.001 0.01 0.1 10 100 0.001 0.01 0.1 10 100 1,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 18. Relation between ammonia plus organic nitrogen concentrations and discharge for streams in 

agricultural areas.

correlations and p-values indicate the amount of 

monotonic correlation between the constituent 

concentration and discharge, and the significance 

of the relation (table 6). Increasing concentrations 

with increasing flow (washoff) occur at tributary 

sites in agricultural or mixed agricultural and range 

areas (figs. 17-20). This relation indicates natural

and human nonpoint sources of nutrients are 

present in the tributary drainages.

Decreasing concentrations with increasing 

flow (dilution) occur at sites below the major point 

sources of wastewater in the Dallas-Fort Worth 

area (figs. 21-24). Concentrations at high flows in

33
o

3

2

I

z

CO

08051500 Clear Creek near Sanger

0.1 1 10 100 

08052780 Hickory Creek at Denton

1,000

8

08058900 East Fork Trinity River at McKinney

0.01 0.1 1 10 100 

08064100 Chambers Creek near Rice

6

1,000 0.0001 0.001 0.01 0.1 1 10 100 

08052700 Little Elm Fork Creek near Aubrey

0.001 0.01 0.1 10 100 1,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 19. Relation between nitrite-plus-nitrate concentrations and discharge for streams in agricultural 

areas.

the tributary drainages are larger than 

concentrations at high flows at sites below major 

point sources (figs. 17-24). Plots on figures 21 

through 24 are "stacked" in downstream order 

from upstream from the first major WWTP to 

downstream from Livingston Reservoir. The 

upper-left graph is the most upstream site and

the lower-right graph is the most downstream 

(figs. 21-24). Increased concentrations and the 

change in relation to discharge caused by the 

addition of wastewater effluent are indicated by 

comparing site 08048543, West Fork Trinity River 

at Beach Street in Fort Worth (the most upstream 

site), with site 08049500, West Fork Trinity River

34
cc

08051500 Clear Creek near Sanger

0.01 0.1 1 10 100

08064100 Chambers Creek near Rice

0

0.001 0.01 0.1 10

0.1 1 10 100 1,000 

08052780 Hickory Creek at Denton 06058900 East Fork Trinity River at McKinney

1,000 0.0001 0.001 0.01 0.1 1 10 

08052700 Little Elm Creek near Aubrey

100

100 0.001 0.01 0.1 10 100 1,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 20. Relation between phosphorus concentrations and discharge for streams in agricultural areas.

at Grand Prairie. Livingston Reservoir is between 

site 08065350, Trinity River nearCrockett, and site 

08066500, Trinity River near Romayor. 

Constituent concentrations decline significantly 

between these two sites, and the relation to 

discharge changes.

Concentrations at sites 08048543 and 

08066500 varied little with changes in discharge. 

Site 08048543 has about 93 percent of its drainage 

area regulated by Benbrook Lake on the Clear Fork 

Trinity River and Lake Worth on the West Fork 

Trinity River. Site 08066500 has about 96 percent

35
15

10

08048543 West Fork Trinity River at Beach St

0.1 10 100 1,000

15

10

08049500 West Fork Trinity River at Grand Prairie

15

10

1 10 100 

08057410 Trinity River below Dallas

10 100

1,000

1,000

08062500 Trinity River near Rosser

10 100

08065350 Trinity River near Crockett

10 100

08066500 Trinity River at Romayor

1,000

1,000

10 100 1,000 10,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 21 . Relation between nitrogen concentrations and discharge for streams in and downstream from 

Dallas-Fort Worth.

of its drainage area regulated by Livingston 

Reservoir. Mixing of inflows in the reservoirs 

results in relatively little change in concentrations 

with changes in discharge downstream from the 

reservoirs. Some washoff from the adjacent 

urbanized areas of Fort Worth may contribute 

nutrients to site 08048543; however, it does not

appear to be significant compared to the effects of 

the reservoirs.

The largest nutrient concentrations at sites 

subject to washoff often occur during the initial 

rise in streamflow. Larger concentrations during

36
tr LU

Q. 

CO

12

08048543 West Fork Trinity River at Beach St

12

0.1 1 10 100 1.000 

08049500 West Fork Trinity River at Grand Prairie

12

1 10 100 

08057410 Trinity River below Dallas

10 100

1.000

1,000

08062500 Trinity River near Rosser

10 100

08065350 Trinity River near Crockett

. /

10 100

06066500 Trinity River at Romayor

10 100 1,000

1.000

1,000

10,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 22. Relation between ammonia plus organic nitrogen concentrations and discharge for streams in 

and downstream from Dallas-Fort Worth.

the initial rise in streamflow are indicated by plots 

of discharge and concentrations for multiple 

samples for a single flow event at three tributary 

sites (fig. 25). The initial runoff from fields or 

developed areas can carry more sediment,

nutrients, and other constituents that have 

accumulated during the preceding dry period. As 

rainfall continues less readily transported material 

is encountered and concentrations in washoff 

decrease.

37
15

10

08048543 West Forte Trinity River at Beach St. 08062500 Trinity River near Rosser

15

10

0.1 1 10 100 1.000 

08049500 West Fork Trinity River at Grand Prairie

10 100

08065350 Trinity River near Crockett

1,000

15

10

1 10 100

08057410 Trinity River below Dallas

1,000 10 100

08066500 Trinity River at Romayor

1.000

10 100 1.000 10 100 1,000 10.000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 23. Relation between nitrite-plus-nitrate concentrations and discharge for streams in and 

downstream from Dallas-Fort Worth.

Seasonal Variations

Nutrient concentrations can vary seasonally 

because of seasonal changes in growth and decay 

of vegetation, changes in temperature and 

precipitation, seasonal applications of fertilizers 

containing nutrients, and other environmental

factors. Seasonal variations in concentrations were 

evaluated at eight sites that each had 60 or more 

analyses collected from 1974 to 1991. Evaluations 

were made by plotting flow-adjusted residuals of 

concentrations by day of the year (Julian day) with 

LOWESS curves (figs. 26-29). Data from a longer 

time period was used for evaluation of seasonal

38
cc

UJ

13 

CC 

UJ 

0.

10

8

6

4

2

08048543 West Fork Trinity River at Beach SL 08062500 Trinity River near Rosser

0.1 1 10 100 1,000 

08049500 West Fork Trinity River at Grand Prairie

1 10 100 

08057410 Trinity River below Dallas

1,000

10 100

08065350 Trinity River near Crocket!

10 100

08066500 Trinity River at Romayor

1,000

1,000

100 1,000 10,000 10 100 1,000 10,000

DISCHARGE, IN CUBIC METERS PER SECOND

EXPLANATION

 LOWESS SMOOTH LINE

Figure 24. Relation between phosphorus concentrations and discharge for streams in and downstream 

from Dallas-Fort Worth.

variations, temporal trends, and loads to include 

major improvements in sewage treatment that 

occurred in the 1970's and early 1980's (Brush and 

Promise, 1990).

While there is considerable variability, the 

largest concentrations of nutrients generally occur

during fall and winter and the smallest 

concentrations during spring and summer (figs. 

26-29). This could reflect less utilization of 

nitrogen during fall and winter by plants. 

Exceptions for NO2+NO3 are at sites 08049500 

and 08057410, where this pattern is reversed. At 

these two sites seasonal patterns of TKN and

39
Table 6. Rank correlations between discharge and nutrient concentrations in streams in the 

Trinity River Basin, 1982-91

[USGS, U.S. Geological Survey; TKN, Total Kjeldahl nitrogen; NOz, nitrite; NO3 nitrate]

USGS site Constituent

Number 

of 

samples

Rank 

correlation 

(rho)

p-value

Significant 

at 95 percent 

level 1

TVibutary sites

08051500

Clear Creek

near Sanger

08064100

Chambers Creek

near Rice

08052780

Hickory Creek

at Denton

08052700

Little Elm Creek

near Aubrey

08058900

East Fork

Trinity River at 

McKinney

Nitrogen

TKN

NO2+NO3

Phosphorus

Nitrogen

TKN

NO2+NO3

Phosphorus

Nitrogen

TKN

NO2+NO3

Phosphorus

Nitrogen

TKN

NO2+NO3

Phosphorus

Nitrogen

TKN

NO2+NO3 

Phosphorus

64

73

72

73

30

44

44

44

55

56

56

56

64

74

74

74

19

19

19 

19

0.69

.69

.65

.80

.45

.28

.72

.39

.32

.33

.21

.58

.06

.29

.24

.22

.69

.62

.46 

.65

0.00

.00

.00

.00

.01

.06

.00

.01

.02

.01

.13

.00

.65

.01

.04

.06

.00

.00

.05 

.00

yes

yes

yes

yes

yes

no

yes

yes

yes

yes

no

yes

no

yes

yes

no

yes

yes

yes 

yes

t

t

t

t

t

t

t

t

T

t

t

t

t

t

t 

t

Sites below major point sources

08049500

West Fork Trinity

River at Grand

Prairie

08057410

Trinity River

below Dallas

08062500

Trinity River

near Rosser

Nitrogen

TKN

NO2+NO3

Phosphorus

Nitrogen

TKN

NO2+NO3

Phosphorus

Nitrogen

TKN

N02+NO3

Phosphorus

118

119

119

107

105

105

106

106

104

104

104

93

-.53

-.58

-.40

-.43

-.60

-.65

-.30

-.50

-.71

-.42

-.62

-.75

.00

.00

.00

.00

.00

.00

.00

.00

.00

.00

.00

.00

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

yes

4

4

4

4

4

4

4

4

4

4

4

4

40
Table 6. Rank correlations between discharge and nutrient concentrations in streams in the 

Trinity River Basin, 1982-91 Continued

USGS site Constituent

Number 

of 

samples

Rank 

correlation 

(rho)

p-value

Significant 

at 95 percent 

level 1

Sites below major point sources Continued

08065350

Trinity River

near Crockett

Nitrogen

TKN

NO2+NO3

Phosphorus

62

63

63

62

-0.81

-.16

-.83

-.67

0.00

.21

.00

.00

yes

no

yes

yes

1

1

1

Sites below major reservoirs

08048543

West Fork Trinity

River at Beach 

Street at Fort Worth

08066500

Trinity River

near Romayor

Nitrogen

KN

NO2+NO3 

Phosphorus

Nitrogen

TKN

NO2+NO3

Phosphorus

88

96

96 

95

51

77

74

78

-.16

-.24

-.22 

-.07

.20

-.07

.49

.07

.13

.02

.03 

.51

.16

.56

.00

.55

no

yes

yes 

no

no

no

yes

no

4

1

1

1 = no change with discharge; T = positive relation to discharge; 4< = negative relation to discharge.

NO2+NO3 are opposite, with larger concentrations 

of TKN corresponding to smaller concentrations of 

NO2+NO3 . Both sites are in the Dallas area and are 

dominated by wastewater effluents. TKN in 

wastewater is converted to NO2 and NO3 in the 

presence of dissolved oxygen. Dissolved oxygen 

concentrations at these sites also vary seasonally 

with larger concentrations during winter than 

summer (Brush and Promise, 1990, p. 206-207). 

More dissolved oxygen could lead to more rapid 

conversion of TKN to NO2+NO3 during winter and 

result in the observed seasonal patterns of TKN 

and NO2+NO3 .

Phosphorus concentrations are larger at most 

sites during the fall and smaller during the spring 

(fig. 29). This pattern could indicate seasonal 

variations in the growth and decay of vegetation 

with smaller concentrations in spring resulting 

from more utilization of phosphorus by plants.

Temporal Trends

Trend analysis was performed on the same 

eight sites where seasonal variations were 

evaluated. Trends were evaluated for the period 

1974-91 using the seasonal Kendall test on 

flow-adjusted residuals. The period varied slightly 

depending on available data (table 7). Trend results 

are presented in three ways: (1) graphically by 

plotting flow-adjusted residuals of nitrogen, TKN, 

NO2+NO3 , and phosphorus versus time with 

LOWESS curves (figs. 30-33); (2) by symbols on 

table 7 indicating either no trend, increasing trend, 

or decreasing trend, at the 95 percent confidence 

level; and (3) numerically by multiplying the 

Kendall slope estimated for the trend (for 

statistically significant trends) times 15 years to 

show the expected change in median 

concentrations in milligrams per liter from about 

1975 to about 1990 (table 7). In some cases, 

conclusions of trend analysis vary compared with 

other studies (Schertz, 1990) because of

41
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ISCHARGE

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NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

I 3

£

+.0-

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DISCHARGE, IN CUBIC METERS PER SECOND

PHOSPHORUS CONCENTRATION, IN MILLIGRAMS PER LITE

-k

t

-k

§

o ro ^ g> o 

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DISCHARGE, IN CUBIC METERS PER SECOND
o

8*

sr

93 cos

08050500 Elm Fork Trinity River near Sanger

-1

_2 ' i i i i i i i i i i i i 

08048543 West Fork Trinity River at Beach St.

,,,, .._...,,,,,... ; . ...

 * ' '

08049500 West Fork Trinity River at Grand Prairie

-2

* .* . s *""*"** 

4 rr

08057410 Trinity River below Dallas

-2

.4 ii i I .i" i i ' i i i.i i i 

J FMAMJ JASOND

08062000 East Fork Trinity River near Crandall

-2

J__I

08062500 Trinity River near Rosser

-2

08065350 Trinity River near Crockett

-2

 '* *" . **../'.. .»..'.!

* ! * " s * * » 

.4 i i i i i i i i I I i I I 

08066500 Trinity River at Romayor

.2 i i i i i i i i i i

JFMAMJJASOND

EXPLANATION

LOWESS SMOOTH LINE

Figure 26. Seasonal variations in flow-adjusted residuals of nitrogen concentrations.

43
08050500 Elm Fork Trinity River near Sanger

-2 1 1 1 1 1 1 1 1 1

08048543 West Fork Trinity River at Beach St.

8 

Z 1

cc

2 0

UQ:

Z UJ ^ 5

g£ '1

0,0- 

3OT s!| -2

<S

II

22 4

£ Z 

O

1

o

COuj n

cc ° 

o 

P

3 -2 

B

5

1 1 1 1 1 1 1 1 1 1 1 1

""** *..* "

08049500 West Fork Trinity River at Grand Prairie

i i i i i i i ( i i i .1 i

~ * ..." ** ~

 ".* *«" ' "' * ; '*.* *. ! .* 

_ * * t * _

t i i i i i i i i t i i

08057410 Trinity River below Dallas

-2

 , ? !» ** *

i t 'i i i i i i i i J i 

JFMAMJJASOND

08062000 East Fork Trinity River near Crandall 

*|l I I

-2

1 1 1 1 1

08062500 Trinity River near Rosser 

4| ! I I I I I I I I

-2

.4 i i i i i» i i i I i i i 

08065350 Trinity River near Crocket!

-2

08066500 Trinity River at Romayor 

^ ~l I I I I I 1 1 1 I T

.**.' ' V-* '. "* " ' - ''':"''.'

J FMAMJ JASOND

EXPLANATION

LOWESS SMOOTH LINE

Figure 27. Seasonal variations in flow-adjusted residuals of ammonia plus organic nitrogen concentrations.

44
z m 

coii 

Roc

f\ Ul

Si Q

COui 

oc

Q 

| 

|

i

08050500 Elm Fork Trinity River near Sanger

-1

_2 ' i i i i i i i i i i i i 

08048543 West Fork Trinity River at Beach St.

-2 i i i i i

08049500 West Fork Trinity River at Grand Prairie

-

08057410 Trinity River below Dallas

-2

J FMAMJ JASOND

08062000 East Fork Trinity River near Crandall

-2

r * i

i i i i i i i i

08062500 Trinity River near Rosser

-2

' '.».:''.: ' t '''*..'*

08065350 Trinity River near Crocket!

-2

.4 i 'i i i i i f i i i i i 

08066500 Trinity River at Romayor

.2 i i i i i i i i i i i i i 

J FMAMJ JASOND

EXPLANATION

LOWESS SMOOTH LINE

Figure 28. Seasonal variations in flow-adjusted residuals of nitrite-plus-nitrate concentrations.

45
08050500 East Fork Trinity River near Sanger 08062000 East Fork Trinity River near CrandaU

ICO 0-2

. 

25

1

0

-1

-2

2

1 

0

-1

4

2

0

-4

4

2

0

-2

-/I

. I I I ( ) I I I I I I I.

_ % _

:t~ ' -...':

i i i i i i i i i i i i

08048543 West Fork Trinity River at Beach St.

-

'* '*""*' " * ;

-

08049500 West Fork Trinity River at Grand Prairie

.. *" . 

'. .' '*'*: " .' 

; ; .: ^^T^n^4 r*r

_ * * % * _

i i i i i i i * i i i i i

08057410 Trinity River below Dallas

i i i I i i . i i i i i i

. : .....[ ' ; - '{. . ;': ' £_._.

i i. i i i i i i i i i i

2

0

-2

-4

4

2

0

-2

4

2

0

-2

-4

2

1

0

-1 

 >

.1 i i i i* i i i i.i i i

~ * % ~

^ * 

 " * * * * * * 

 .

i i i i i i i i i i i i

08062500 Trinity River near Rosser

i i i i i i r i i i i i

* \ ' . t ..

. . .".*"" . * « ;H-.V: -:V :«::A;:V.;.OTV^-;

- 1 '

08065350 Trinity River near Crockett

i i i i i i i i i i i i

.** " * . "*"*." * ... >. * ..*. ii "i 'i J

. . . * *

i i i i i i i i i i i i

08066500 Trinity River at Romayor

i i i i i i i i i i i i

" - . » -

'* "" "^ UJ " ' ' '" x<r" ix :

i i i i i i i i i i i i

J FMAMJ JASOND J FMAMJ JASOND

EXPLANATION

LOWESS SMOOTH UNE

Figure 29. Seasonal variations in flow-adjusted residuals of phosphorus concentrations.

46
UJ 

O

L $£8ccfc 

bo.

Z>;j

9d ffi 2

KZ

o

UJ

§ §<

I

10

08050500 Elm Fork Trinity River near Sanger 08062000 East Fork Trinity River near Crandall

-5

-10

10

08048543 West Fork Trinity River at Beach St

08049500 West Fork Trinity River at Grand Prairie

en

-10

10

08057410 Trinity River below Dallas

-5

-10

08062500 Trinity River near Rosser

'** " TTTTJVt'*"-"'

"

08065350 Trinity River near Crockett

08066500 Trinity River at Romayor

v . ". .

'^J^^T^

1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995

EXPLANATION

- LOWESS SMOOTH LINE

Figure 30. Temporal trends in flow-adjusted residuals of nitrogen concentrations.

47
O 13

CO &.

Pco

zo

10

08050500 Elm Fork Trinity River near Sanger 08062000 East Fork Trinity River near Crandall

-5

-10

10

08048543 West Fork Trinity River at Beach St.

-5

08049500 West Fork Trinity River at Grand Prairie

10 i i i i i

-10

10

08057410 Trinity River below Dallas

-5

-10

08062500 Trinity River near Rosser

08065350 Trinity River near Crockett

08066500 Trinity River at Romayor

1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995

EXPLANATION

 LOWESS SMOOTH LINE

Figure 31. Temporal trends in flow-adjusted residuals of ammonia plus organic nitrogen concentrations.

48
o o

UJ i.?£COD 3 = 

^S

ti

CCrf

ten

Z(3

&3

08050500 Elm Fork Trinity River near Sanger

08048543 West Fork Trinity River at Beach St.

08049500 West Fork Trinity River at Grand Prairie

08057410 Trinity River below Dallas

08062000 East Fork Trinity River near Crandall

08062500 Trinity River near Rosser

08065350 Trinity River near Crockett

08066500 Trinity River at Romayor

;».t..w.*.vwVvV' -

1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995

EXPLANATION

 LOWESS SMOOTH LINE

Figure 32. Temporal trends in flow-adjusted residuals of nitrite-plus-nitrate concentrations.

49
08050500 Elm Fork Trinity River near Sanger 08062000 East Fork Trinity River near Crandall

08048543 West Fork Trinity River at Beach St. 08062500 Trinity River near Rosser

z ° 

o

!c

EC ..

z 4

Ul

z

8 °

O i -4

I

£cc

03 Ul

103 a

0.5 -8

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-

-

.

08049500 West Fork Trinity River at Grand Prairie

.

0

-*".* * -

 % *

 :% .-!" i .

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"*«..*/^y."^ ^"»-^-*;'.'. " ft '«'^ ..'

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08065350 Trinity River near Crockett

8 r-T

08057410 Trinity River below Dallas 08066500 Trinity River at Romayor

-8 

1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 1995

EXPLANATION

LOWESS SMOOTH LINE

Figure 33. Temporal trends in flow-adjusted residuals of phosphorus concentrations.

50
Table 7. Trends in nutrient concentrations at selected sampling sites in the Trinity River Basin using the seasonal Kendall test

[Number shown is estimated change in concentration over 15 years, in milligrams per liter; USGS, U.S. Geological Survey; TKN, Total Kjeldahl nitrogen; NO2, nit

 , no change in time, T, increase with time, at the 95 percent confidence level]

USGS Perlod(s) 

i»_ / «.-\ Nitrogen 

sne \year)

West Fork

Trinity River

at Beach Street

at Fort Worth 1977-91 4

08048543 -0.42

West Fork

Trinity River

at Grand 1975-91 

Prairie

08049500

Trinity River

below Dallas 1974-91 

08057410

East Fork

Trinity River 1971-81 

near Crandall 1986-91

08062000

Trinity River

near Rosser 1975-91 

08062500

Trinity River

near

Crockett 1974-91 

08065350

Trinity River

at Romayor 1974-91 

08066500

Elm Fork

Trinity River

near Sanger 1974-85 

08050500

.. Ammonia 

nitrogen

4 4

-0.39 -0.16

4

-1.8

4 4

-1.0 -3.3

4

-3.0

4 4

-.50 -2.2

4

-.091

 

 

TKN

4

-0.51

4

-3.3

4

-4.3

4

-3.8

4

-3.7

4

-.71

Nitrite Nitrate

T

0.13

4 t

-0.32 2.2

t

3.3

T t

.14 1.5

t

3.0

T

.56

T

.003

 

N
Small increasing trends in nitrite and 

NO2+NO3 were indicated at 08066500, Trinity 

River at Romayor (table 7). The increasing trends 

could result from increases in nitrate and 

NO2+NO3 concentrations in the Trinity River 

above Livingston Reservoir being passed through 

the reservoir. The magnitude of change of these 

trends is small, estimated to be about 0.1 mg/L 

over 15 years for NO2+NO3 . No trends were 

indicated at this site for nitrogen or phosphorus.

Decreasing trends in all constituents except 

nitrite and nitrate were indicated at 08048543, the 

West Fork Trinity River at Beach Street in Fort 

Worth. There was a small increasing trend in 

nitrate concentration. There are no large WWTPs 

upstream from the site and the causes of these 

trends are not known.

Similar patterns of trend were observed at each 

of the other five sites; decreases in ammonia and 

TKN and increases in nitrate and NO2+NO3 . All 

five sites are downstream from major WWTPs in 

the Dallas-Fort Worth area. Changes in treatment 

practices since the mid-1970's have resulted in 

reduced BOD concentrations in effluents, primarily 

by converting ammonia and organic nitrogen to 

nitrate. Those changes have not significantly 

changed total nitrogen concentrations (table 7) but 

have significantly reduced TKN and increased 

NO2+NO3 concentrations. At 08057410, the 

Trinity River below Dallas, median TKN 

decreased about 4.3 mg/L over 15 years while 

median NO2+NO3 increased about 3.6 mg/L 

(table 7).

The conversion of TKN to NO2+NO3 over 

time is indicated by boxplots showing the percent 

of total nitrogen as TKN and as NO2+NO3 for 

successive periods at the Trinity River below 

Dallas and the Trinity River near Rosser (fig. 34). 

In the mid 1970's more than 75 percent of total 

nitrogen at these sites was TKN and less than 25 

percent was NO2+NO3 . By the late 1980's these 

proportions had reversed.

As indicated on table 7, there is a gap from 

1982 to 1985 in the data for site 08062000. Helsel 

and Hirsch (1992; p. 349) recommend that if the 

gap in a sample record is more than about one-third

the entire period of data collection a step trend 

procedure is probably best. Because the gap in 

record for site 08062000 was 4 years out of a 

period of 18 years, less than one-third of the 

record, a step trend procedure was not used. 

Pairwise slopes computed to estimate the Seasonal 

Kendall trend slope are calculated "across" the 

gap.

Statistically significant decreases in 

phosphorus occurred at the two sites downstream 

from major WWTPs in the Dallas area (08049500 

and 08057410). The causes of the decreases in 

phosphorus concentrations are not known but 

could have resulted from reductions in the use of 

phosphate in detergents. There currently (1994) is 

not a phosphate ban in the Dallas-Fort Worth area 

(Samuel Brush, North-Central Texas Council of 

Governments, oral commun., 1993).

Loads and Yields

Loads of nitrogen, TKN, NO2+NO3 , and 

phosphorus were calculated for nine sites in the 

study unit (table 8). Simplified diagrams of 

selected streams in the Trinity River Basin are used 

to illustrate the spatial changes that occur with 

nutrient loads (figs. 35-37). Streams were 

segmented at the locations of WWTP effluent 

discharge and at the midpoint between sites used to 

calculate loads. The thickness of stream segments 

on figures 35 to 37 was varied proportionally to 

mean loads for sites represented by the segments; 

the larger the load, the wider the line segment on 

these figures. Mean loads of nitrogen and 

phosphorus are presented graphically for calendar 

years 1974-89 because of the general lack of 

temporal trends in concentrations. Mean loads of 

TKN and NO2+NO3 are presented graphically for 

two time periods, 1974-79 and 1984-89, because 

of temporal trends in concentrations of these 

constituents during the 1970's and 1980's. There 

were significant decreases in TKN and increases in 

NO2+NO3 loads in streamflow downstream from 

the Dallas-Fort Worth metropolitan area from the 

late 1970's to the late 1980's (figs. 36, 37).

Average loads and average yields for 1984-87 

for nine sites are shown versus distance upstream

52
1.00

0.75

g 0.50

§ 0.25

ffi

0.00

T 1

T

1.00

3 z 0.75 

F ^

M

feg 0.50

II

W < 0.25

0.00

1

T

fe

1.00

0.75 -

0.50 -

0.25 -

0.00

1.00

g £ 0.75

z z

o ^ 0.50

^1

^ I 0.25

0.00

1

T

YEARS YEARS

Figure 34. Change in percent of total nitrogen as ammonia plus organic nitrogen and nitrite plus nitrate for 

succesive periods at two sites downstream from Dallas.

from the mouth of the Trinity River on figures 38 

through 41. Yield is the load at a site divided by the 

drainage area at the site. In natural streams, loads 

generally increase with increasing drainage area 

because of the positive relation between drainage 

area and discharge. Yields in natural streams 

remain relatively constant as drainage area 

increases. Small declines in yield with increasing 

drainage area can be caused by uptake of nutrients 

by plants, denitrification, and accumulation of 

sediments and their associated nutrients in channel 

deposits. In the Trinity River Basin, however, 

reservoirs and major point sources significantly 

change these relations.

The two most upstream sites shown on figures 

38 to 41 are 08051500, Clear Creek near Sanger, 

and 08052700, Little Elm Creek near Aubrey

(pi. 1). Both sites are tributaries above major point 

sources and reservoirs. Clear Creek drainage is 

largely pasture and range, and Little Elm Creek 

drainage is dominated by agriculture. Loads at 

these sites are small compared to downstream sites 

because their drainage areas are much smaller, 

however, yields of nitrogen from these drainages 

are similar in magnitude to yields for sites 

downstream from major WWTPs in Dallas (fig. 

38). The 4-year average yield of nitrogen for Little 

Elm Creek was 6.7 (kg/ha)/yr compared with 2.8 

(kg/ha)/yr at Clear Creek. More intensive fanning 

in the Little Elm Creek drainage could cause the 

larger yield compared with Clear Creek; however, 

much of the difference can be explained by greater 

runoff in Little Elm Creek.

53
LEWISVILLE LAKE

BENBROOK LAKE

LAKE RAY HUBBARO

O CEDAR CREEK 

RESERVOIR NITROGEN

a

RICHLAND-CHAMBERS 

RESERVOIR

LEWISVILLE LAKE

LAKE WORTH Q^ ^

BENBROOK LAKE

O

LAKE RAY HUBBARD

A Q

CEDAR CREEK 

RESERVOIR

RICHLAND-CHAMBERS 

RESERVOIR

PHOSPHORUS

EXPLANATION

SEGMENTS HAVING NO LOAD DATA 

A USGS STATION USED IN LOAD CALCULATIONS 

 »- WASTEWATER TREATMENT PLANT EFFLUENT INPUT 

Q RESERVOIR

WIDTH OF STREAM SEGMENT:

1/4 INCH IS APPROXIMATELY 40,000 KILOGRAMS PER DAY OF TOTAL NITROGEN

1/4 INCH IS APPROXIMATELY 10.000 KILOGRAMS PER DAY OF TOTAL PHOSPHORUS 

NOT TO SCALE

LIVINGSTON RESERVOIR

LIVINGSTON RESERVOIR

Figure 35. Mean loads of nitrogen and phosphorus in the Dallas-Fort Worth area and mainstem Trinity 

River, 1974-89.

54
LEWISVILLE LAKE o

LAKE WORTH Q^^^ I ^

O^

BENBROOK LAKE

LAKE RAY HUBBARD

CEDAR CREEK 

RESERVOIR

O

RICHLAND-CHAMBERS 

RESERVOIR

^KE WORTH

BENBROOK LAKE

LEWISVILLE LAKE

LAKE RAY HUBBARD

O

CEDAR CREEK 

RESERVOIR

1984-1989

RICHLAND-CHAMBERS 

RESERVOIR

1974-1979

LIVINGSTON RESERVOIR

EXPLANATION

SEGMENTS HAVING NO LOAD DATA 

A USGS STATION USED IN LOAD CALCULATIONS 

 » WASTEWATER TREATMENT PLANT EFFLUENT INPUT 

O RESERVOIR

WIDTH OF STREAM SEGMENT:

1/4 INCH IS APPROXIMATELY 20.000 KILOGRAMS PER DAY OF NITRITE PLUS NITRATE

NOT TO SCALE

LIVINGSTON RESERVOIR

Figure 36. Mean loads of nitrite plus nitrate in the Dallas-Fort Worth area and mainstem Trinity River, 

1974-79 and 1984-89.

55
LEWISVILLE LAKE

LAKE WORTH

RICHLAND-CHAMBERS 

RESERVOIR

LEWISVILLE LAKE

LAKE RAY HUBBARD

LAKE WORTH

BENBROOK LAKE

CEDAR CREEK 

RESERVOIR

1984-1989

EXPLANATION

SEGMENTS HAVING NO LOAD DATA 

A USGS STATION USED IN LOAD CALCULATIONS 

-> WASTEWATER TREATMENT PLANT EFFLUENT INPUT 

Q RESERVOIR

WIDTH OF STREAM SEGMENT:

1/4 INCH IS APPROXIMATELY 20,000 KILOGRAMS PER DAY OF 

AMMONIA PLUS ORGANIC NITROGEN (TKN)

NOT TO SCALE

O

RICHLAND-CHAMBERS 

RESERVOIR

1974-1979

LIVINGSTON RESERVOIR

LIVINGSTON RESERVOIR

Figure 37. Mean loads of ammonia plus organic nitrogen in the Dallas-Fort Worth area and mainstem 

Trinity River, 1974-79 and 1984-89.

56
Table 8. Mean nutrient loads and mean daily discharge for nine sites in the Trinity River Basin

[USGS, U.S. Geological Survey; TKN, Total Kjeldahl nitrogen; NO2, nitrite; NO3 nitrate; kg/d, kilograms per day; m3/s, cubic meters per seco

USGS 

site 

number

08048543

08049500

08053000

08057410

08061750

08062500

08062700

08065350

08066500

TKN 

(kg/d)

a 800

5,900

...

19,900

...

24,500

24,600

20,600

15,700

1974 to 1979

NO2 + NO3 

(kg/d)

400

1,600

...

3,200

...

3,700

9,000

15,800

6,300

1984 to 1989

Mean dally 

discharge 

(m3/s)

a 8

16

...

52

...

76

90

157

237

TKN 

(kg/d)

800

2,100

1,400

8,900

2,700

11,800

12,500

16,500

16,400

NO2+NO3 

(kg/d)

500

3,800

700

15,600

1,400

25,100

...

24,200

9,700

Mean daily 

discharge 

(m3/s)

13

22

17

64

16

91

105

158

207

Total 

nitrogen 

(kg/d)

b 1,700

5,900

C 2,700

23,700

C 3,700

32,600

...

36,900

24,000

a 1977 to 1979. 

b 1977 to 1989. 

c 1981 to 1989.
100,000

10,000

1,000

100

 D

= 5

Z 0.

8

CC 

UJ

E 2

z

' D

800 600 400 200

DISTANCE TO MOUTH OF TRINITY RIVER, IN KILOMETERS

EXPLANATION

 ELM FORK TRINITY RIVER AND TRIBUTARIES

 WEST FORK TRINITY RIVER

........ TRINITY RIVER

n STATION

 RESERVOIR

A MAJOR WASTEWATER TREATMENT PLANT DISCHARGE

Figure 38. Plots showing average annual loads and yields for 1984-87 of nitrogen.

58
100,000

10,000

M»

1

Z 1,000 I

100

10

oc

OCX

Ooc

X UJ °-

2.0

1 '5

1.0

0.5

800 600 400 200 

DISTANCE TO TRINITY RIVER MOUTH, IN KILOMETERS

	EXPLANATION

__ ELM FORK TRINITY RIVER AND TRIBUTARIES

. WEST FORK TRINITY RIVER

....... TRINITY RIVER

D STATION

 RESERVOIR

A MAJOR WASTEWATER TREATMENT PLANT DISCHARGE

Figure 39. Plots showing average annual loads and yields for 1984-87 of phosphorus.

59
100,000

38

10,000

1,000

100

pco

t UJ zi 

co a:

-3 UJ

=! 1 *

800 600 400 200 

DISTANCE TO MOUTH OF TRINITY RIVER, IN KILOMETERS

EXPLANATION

ELM FORK TRINITY RIVER AND TRIBUTARY

WEST FORK TRINITY RIVER

TRINITY RIVER

STATION

RESERVOIR

MAJOR WASTEWATER TREATMENT PLANT DISCHARGE

Figure 40. Plots showing average annual loads and yields for 1984-87 of nitrite plus nitrate.

60
100.000

oc o 10,000 

to:

Z UJ

isl£

1,000

100

 &<

So: 

£S

00

z m

off. DC W

2.0

1.5

2 ^

0

800 600 400 200 

DISTANCE TO MOUTH OF TRINITY RIVER, IN KILOMETERS

	EXPLANATION

 ELM FORK TRINITY RIVER

 WEST FORK TRINITY RIVER

....... TRINITY RIVER

D STATION

 RESERVOIR

A MAJOR WASTEWATER TREATMENT PLANT DISCHARGE

Figure 41. Plots showing average annual loads and yields for 1984-87 of ammonia plus organic nitrogen.

61
Flow from sites 08051500 and 08052700 

passes through Lake Lewisville before reaching 

site 08053000, Elm Fork Trinity River near 

Lewisville (pi. 1). Load increases between these 

sites, however, the two tributary sites represent 

only 22 percent of the drainage area to site 

08053000. The yield of nitrogen decreases from 

2.8 and 6.7 (kg/ha)/yr at the tributary sites to 2.1 

(kg/ha)/yr at site 08053000, downstream from 

Lake Lewisville. This decrease is caused by 

trapping and uptake of nutrients in the reservoir. 

No loads were calculated for sites on the West Fork 

or Clear Fork Trinity River upstream from site 

08048543. Ninety-three percent of the drainage 

area to site 08048543 is captured by reservoirs and 

the 4-year average yield of nitrogen at the site was 

0.7 (kg/ha)/yr, the smallest nitrogen yield 

calculated for any of these nine sites.

Similar downstream patterns of loads and 

yields occur for NO2+NO3 , TKN, and 

phosphorous. Loads and yields are greatly 

increased by the addition of wastewater and are 

decreased by reservoirs. The magnitude of the 

increase in load caused by the WWTPs can be 

demonstrated by comparing loads for sites 

upstream and downstream from WWTP discharge. 

Site 08048543, the West Fork Trinity River at 

Beach Street, and site 08053000, the Elm Fork 

Trinity River near Lewisville, are both upstream 

from the major WWTPs. The combined drainage 

area of these two sites is 11,300 km2 . Site 

08057410, the Trinity River below Dallas, is 

downstream from the confluence of the West Fork 

Trinity and Elm Fork Trinity Rivers and has a 

drainage area of 16,300 km2 . Discharges from 

three major WWTPs enter the river between the 

two upstream sites and site 08057410. For the 

period 1984-87, the combined mean loads for the 

two upstream gages were 3,800 kg/d of nitrogen 

and 310 kg/d of phosphorus, and the combined 

mean daily discharge was approximately 30 m3/s. 

During this time the downstream site had mean 

loads of 25,000 kg/d of nitrogen and 8,400 kg/d of 

phosphorus and a mean daily discharge of 64 m3/s. 

This is a 560 percent increase in nitrogen load and 

a 2,600 percent increase in phosphorus load, but 

only a 110 percent increase in discharge.

Loads continue to increase downstream from 

site 08057410; however, because drainage area is 

also increasing, the annual yield remains relatively 

constant between sites 08057410 and 08062500 

and declines downstream at site 08065350. This 

decline could result from dilution of flow 

dominated by wastewater effluent at the more 

upstream sites by inflow from tributaries, some of 

which have reservoirs that could reduce nutrient 

loads.

Relations to Livingston Reservoir

Livingston Reservoir is a significant sink for 

nutrients traveling down the Trinity River (figs. 

35-41). To examine the magnitude of this process, 

annual nutrient loads and yields for site 08065350, 

the Trinity River near Crockett, and site 08066500, 

the Trinity River at Romayor, were compared 

(fig. 42). The site near Crockett is located about 

110 km upstream from Livingston Reservoir and 

the site at Romayor is located about 50 km 

downstream from the reservoir. From 1974 to 

1989, the Trinity River near Crockett had mean 

monthly loads of 36,900 kg/d of nitrogen and 

10,200 kg/d of phosphorus, whereas the Trinity 

River at Romayor had mean monthly loads of 

24,000 kg/d of nitrogen and 3,600 kg/d of 

phosphorus. Although there was approximately a 

30 percent increase in discharge between the two 

sites, there was a 35 percent decrease in nitrogen 

loads and a 65 percent decrease in phosphorus 

loads. Uptake of nutrients by plants, and settling of 

sediments and associated nutrients in Livingston 

Reservoir are two processes that contribute to the 

decrease in nutrient loads.

There was some indication of a decrease in 

phosphorus trapping in Livingston Reservoir with 

time (fig. 42). If in fact there was a decrease, as 

suggested by figure 42, then either loads into the 

reservoir were decreasing or loads out were 

increasing. There was no trend in phosphorus 

concentrations (table 7) or loads, based on a rank 

correlation between annual load and time, at the 

Trinity River at Romayor. Therefore, there is no 

evidence that loads out of the reservoir are 

increasing. There was a decrease in the phosphorus 

load at the Trinity River near Crockett during the 

time period. The rank correlation between annual

62
z

o

ui 400

CO

jjOC

OQ. 300

<2

It 200

52

o

m 100

3

- 0

6,000

CO

OQ 4,000 X jOE

-J 2 2,000 I s

0

15,000

z'co

o o 10,000

io

TOTAL Nil METRI IN 

pi

w

4,000

^O 3,000

CO £

at 2,000

z 8

ug

gj--' 1,000 

o to z~

o 

 i i i r 1 i i i

 08065350 (ABOVE RESERVOIR)

Q 08066500 (BELOW RESERVOIR)

_

-

-

,

-

- y

i 1

 i

- i

= 1

 B (

i

n

B I

E

i

i

- F

- t

= E

- B 1

i

i

z F

- ^

- !

= p

- B~

i i i i

(

-

1

P

^

E

B 

1

_

[

= '.

- - -

E BZ :

- 1

- K -

= E ^ ;

- B~ BH .

1 i ' i

NINETY-FIVE PERCENT CALCULATED DIFFERENCE CONFIDENCE INTERVAL ^^^^' CIJ ""-rcncn^c

I | 11 v

o o

I <

\

) 1

i , i . i . i

--^.

\

» 4

'*

i

}

,

-

< >

-

 0 , -

i.i.

' i ' i ' i ' i

-

'. i » ^ '^^l 1

i

i > < '^

1

*

i . i .

-

<

-

» 

<

!

0 NO 

' ^ DATA

1.1.

1 i ' i ' i i

-

-

- 1

t 

I

'H.u

1 i 

i

1 * >

i

1972 1974 1976 1978 1980 1982

i

.

1 \ ' i

-

< 1 < > O

, . t .

1984 1986 1988 19<

Figure 42. Discharge and differences in nitrogen, phosphorus, and sediment loads for sites above and 

below Livingston Reservoir, 1974-88. (Load differences were calculated by substracting the loads below 

Livingston Reservoir from the loads above Livingston Reservoir.)

63
Table 9. Loads of ammonium and nitrate from atmospheric deposition for two sites and the weighted average for the 

Trinity River Basin1

[NH4, ammonium; NO3> nitrate; kg/ha, kilogram per hectare; N, nitrogen; , no data]

National Trends Network stations Basin weighted-average

Water year

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

Median

TX38

NH4 

(kg/ha as N)

2.01

3.06

1.41

.92

.96

1.51

1.04

3.57

2.62

3.01

1.75

N03 

(kg/ha as N)

2.01

2.51

1.73

1.76

1.84

1.44

1.33

2.53

1.64

2.91

1.81

TX56

NH4 

(kg/ha as N)

1.26

2.15

1.51

1.75

.68

2.91

3.15

1.%

1.85

N03 

(kg/ha as N)

1.51

2.15

1.71

1.88

1.32

2.72

2.37

2.00

1.93

concentration

NH4 

(kg/ha as N)

1.36

1.11

1.12

1.58

.93

3.39

2.77

3.47

1.47

N03 

(kg/ha as N)

1.67

1.88

1.80

1.57

1.33

2.58

1.85

2.65

1.83

1 The weighted average for the Trinity River Basin was calculated by weighting site loads by the inverse of the squared 

distance of the site to the geographic midpoint of the basin.

phosphorus load and time was -0.58 with ap-value 

of 0.02. Additionally, time was a significant 

variable in the regression estimate of load at 

Crockett using the MVUE and the coefficient was 

negative. There was a small negative trend in 

phosphorus concentrations at this site but it was 

not statistically significant (table 7). The decrease 

in trapping by Livingston Reservoir is, therefore, 

attributed to a decrease in phosphorus load to 

Livingston Reservoir since the mid-1970's.

Comparison of Loads to Atmospheric Deposition

The precipitation that falls onto the study 

unit contains appreciable concentrations of 

ammonium and nitrate (table 4). Nutrient 

concentrations of this magnitude, times 

precipitation, result in a relatively large source of 

nitrogen to the Trinity River Basin. The mean

annual loads for ammonium and nitrate reported at 

the two NTN stations (TX38 and TX56) of interest 

to the study unit are listed in table 9. Also listed are 

the spatially weighted averages of the loads 

estimated for the Trinity River Basin.

The atmospheric loads recorded at the NTN 

station at LBJ National Grasslands (TX56) were 

compared with the yields in streamflow calculated 

for site 08051500, Clear Creek near Sanger, 

located approximately 50 km from TX56 (pi. 1). 

For the water years 1984-88, the mean annual load 

of nitrate plus ammonium from atmospheric 

deposition was 3.5 kg/ha. In comparison, the mean 

annual yield at Clear Creek near Sanger for the 

same period was only 2.7 kg/ha of total nitrogen. 

In addition, the results of a 1988 survey by the 

Texas Agricultural Extension Service estimate that 

18 kg/ha of total nitrogen is applied annually as

64
I

III

I

z

CO

<

z

UJ

1,000

100 -

10 -

0.1

-

_

/

t

*

* *

* 

*

*

^

*

*

*

*

I

T J*

*

*

-

^ £ £ / <f ^ ^

£ /// & £/ £/ A? P/

^^ e (> ^ ^^

cT

AQUIFERS

Figure 43. Nitrate concentrations for wells in seven major and minor aquifers in the Trinity River Basin, 

1974-91.

fertilizer to Denton County, where Clear Creek 

near Sanger is located (Texas Agricultural 

Extension Service, written commun., 1992). These 

numbers suggest that only about 10 to 15 percent 

of the atmospheric and fertilizer load of nitrogen 

runs off. Other possible fates of the nitrogen 

include transport to ground water, uptake by plants, 

ammonium adsorption by soils, and denitrification 

(Novotny and Chesters, 1981).

Ground Water

A total of 1,482 measurements of nitrate from 

seven aquifers were available for this analysis. The 

median nitrate concentration of these 

measurements is 0.1 mg/L as N. The largest 

median concentrations by aquifer are in the Queen

City and Nacatoch aquifers (fig. 43). The KruskalWallis test of nitrate concentrations by aquifer had 

a p-value of 0.00001, indicating there are 

statistically significant differences in median 

concentrations by aquifer. A multiple-stage test 

using the Kruskal-Wallis test was performed to 

determine which aquifers were different (Helsel 

and Hirsch, 1992, p. 200). Nitrate concentrations 

fell into three groups by aquifer using this test and 

a 95 percent confidence interval. The largest 

median concentrations were in the Nacatoch and 

Queen City aquifers. Intermediate concentrations 

were in the Woodbine aquifer. The smallest 

concentrations were in the Trinity Group, CarrizoWilcox, Gulf Coast, and Sparta aquifers. The 

causes of these differences are not known; 

however, the Nacatoch and Queen City aquifers 

had relatively few samples and most of these

65
Table 10. Rank correlations for nitrate concentrations versus depth of well1 by 

aquifer, 1974-91

Aquifer

Trinity Group

Woodbine

Nacatoch

Carrizo-Wilcox

Number 

of 

samples

649

347

21

130

Rank 

correlation, 

rho

-0.20

-.11

-.45

-.34

p-value

0.00

.04

.04

.001

Significant at 

95 percent 

level

yes

yes

yes

yes

Queen City

Sparta

Gulf Coast

All wells

32

37

45

1,261

-.39

-.51

-.36

-.20

.03

.001

.02

.000

yes

yes

yes

yes

1 Only wells less than 500 meters deep were used.

samples were from relatively shallow wells. These 

relations can be summarized by:

NCTC = QNCT > WDBN > TRIN = CZWX = GULF = SPRT,

where,

NCTC = Nacatoch 

QNCT = Queen City 

WDBN= Woodbine 

TRIN = Trinity 

CZWX= Carrizo-Wilcox 

GULF = Gulf Coast 

SPRT = Sparta

There is a significant rank correlation between 

nitrate concentration and depth of well for all seven 

aquifers with larger concentrations in shallow 

wells (fig. 44, table 10). The largest concentrations 

mostly occur in wells that are less than about 100 

m deep. Less than 1 percent of samples exceeded 

the Environmental Protection Agency's maximum 

contaminant level (MCL) for nitrate of 10 mg/L; 

however, 5 of the 28 samples (18 percent) from 

wells less than 15 m deep exceeded the MCL. Ten 

of the 84 samples (12 percent) from wells less than 

30 m deep exceeded the MCL. Larger 

concentrations in shallow wells could result from

nonpoint sources of nitrate in water recharged 

through agricultural or residential areas.

The relation between nitrate concentrations in 

wells and agricultural land use was evaluated for 

wells less than 60 m deep. Land use was 

determined for wells by digitally overlaying 1-km 

radius circles around wells with the GIRAS landuse data. The percent of area within the circles 

classified as agricultural land use was than 

computed and assigned to the wells. There was not 

a significant rank correlation between percent 

agricultural land use and nitrate concentration for 

wells 0 to 60 m deep or for wells 0 to 30 m deep. 

For wells 0 to 60 m deep the correlation coefficient 

was -0.03, the p-value was 0.61, and the sample 

size was 250.

There are a number of limitations and 

assumptions to the analysis of relations to 

agricultural land use. These include: (1) the 

assumption that recharge to the wells occurs within 

1 km of the well; (2) the assumption that water 

sampled from the well was recharged during a time 

period of similar land use to what is mapped by the 

GIRAS data (mid-1970's); (3) the limitation that 

other land uses such as urban are not accounted for; 

and (4) the limitation that a variety of land uses are

66
1,000

100

10

0.1

TRINITY

EXPLANATION

LOWESS SMOOTH LINE

1,000

100

10

0.1

1,000

100

10

0.1

WOODBINE

QUEEN CITY

CARRIZO-WILCOX

NACATOCH

1,000

100

10

0.1

100

SPARTA

200 300 100

GULF COAST

200 300

DEPTH, IN METERS

Figure 44. Relation between nitrate concentration and depth of well for seven aquifers, 1974-91.

67
Table 11. Summary of daily suspended-sediment data from selected U.S. Geological Survey sites

[USGS, U.S. Geological Survey; mg/L, milligrams per liter; kg/ha, kilograms per hectare]

USGS

site Site

number

08044000 Big Sandy Creek

Daily

suspendedsediment

concentrations

(mg/L)

Median Maximum Minimum

68 1,150 3

Dally

suspendedsediment

loads

(tonnes)

Mean

70

Daily

suspendedsediment

yields

(kg/ha)

Mean

1

Period of

record

used

10-73 to 7-76

near Bridgeport

08051500 Clear Creek 

near Sanger

36 2,920 317 10-73 to 8-76

08065000 Trinity River 105 1,920 \5 

near Oakwood

08065800 Bedias Creek near 35 915 10 

Madisonville

2,586

68

10-75 to 6-80 

10-83 to 7-85

included in the agricultural class in the GIRAS 

data.

ANALYSIS OF AVAILABLE DATA FOR 

SUSPENDED SEDIMENT

The relation between nitrate concentrations 

and nitrogen fertilizer application data by county 

(Texas Agricultural Extension Service, written 

commun., 1992) was also evaluated. Wells were 

assigned nitrogen fertilizer application rates based 

on what county they were located in. Nitrate 

concentrations did not have statistically significant 

rank correlations to fertilizer application rates 

using all wells and using wells less than 30 m deep. 

For wells less than 30 m deep, the correlation 

coefficient was less than 0.01. Therefore, no 

relation to fertilizer application rates by county can 

be demonstrated.

These evaluations indicate that nitrate 

concentrations are larger in shallow zones of all 

seven aquifer groups. The zone of larger 

concentrations extends to a depth of about 100 m 

below land surface. Significant relations between 

nitrate concentrations and nitrogen fertilizer 

application rates and between nitrate 

concentrations and agricultural land use were not 

found using existing data.

Suspended-sediment concentrations were 

monitored by the USGS at seven sites in the Trinity 

River Basin. Daily suspended-sediment 

concentrations were measured and loads were 

calculated at four USGS sites for periods from 3 to 

6 years between water years 1974 and 1985. Three 

of these sites are on tributaries and one is on the 

mainstem of the Trinity River. In addition, three 

USGS sites on the Trinity River were sampled 

periodically, usually six times per year. Periodic 

data for two of these sites were used to calculate 

loads using the MVUE.

Minimum, median, and maximum suspendedsediment concentrations, loads, and yields were 

calculated for the daily suspended-sediment sites 

(table 11). The largest median concentration 

occurred at the Trinity River near Oakwood and 

the smallest occurred at Bedias Creek near 

Madisonville and at Clear Creek near Sanger. 

These differences could be related to land use, soil, 

geology, and other physical factors; however, no 

conclusions regarding relations to environmental 

factors were warranted because of the limited

68
number of sites and because the periods of record 

differ between sites.

Livingston Reservoir is located between two 

sites, 08065350 and 08066500, for which periodic 

suspended-sediment data are available. The annual 

load of suspended sediment was smaller for the site 

below Livingston Reservoir (08066500) than for 

the site above the reservoir (08065350), even 

though mean discharge was approximately 30 

percent greater at the downstream site during the 

study period (fig. 42). The differences in loads 

observed between these two sites can be attributed 

to trapping of sediment in Livingston Reservoir. 

Although no temporal trends are indicated by the 

annual load differences (fig. 42), there is a relation 

between annual load differences and discharge at 

these sites. The largest differences in the annual 

suspended-sediment loads take place during years 

of greatest discharge.

SUMMARY

A total of about 5,700 water-quality samples 

from the Trinity River Basin were analyzed. Of 

these, about 4,200 were from streams and about 

1,500 were from wells. Additionally, atmospheric 

deposition data for two locations were obtained 

and analyzed.

Spatial variations in nutrient concentrations are 

related primarily to point sources and reservoirs. 

The smallest concentrations occurred immediately 

downstream from reservoirs, which act as sinks for 

nutrients. Nutrient concentrations in agricultural 

areas were positively correlated to percent of 

drainage in agricultural land use and to discharge, 

indicating washoff of nutrients from nonpoint 

sources during storms. Nutrient concentrations 

downstream from point sources were inversely 

related to discharge, indicating dilution at higher 

flows.

Total nitrogen and phosphorus concentrations 

did not change significantly, at the 95 percent 

confidence level, from 1974 to 1991 at most sites. 

The exception was a decrease in phosphorus 

concentrations at two sites in and downstream from

major wastewater-treatment plants in the Dallas 

area. Concentrations of organic nitrogen and 

ammonia declined and concentrations of nitrite 

plus nitrate increased at sites below major 

wastewater-treatment plants. These changes are 

indicative of improvements in wastewater 

treatment that convert organic nitrogen and 

ammonia to nitrite and finally nitrate. Because 

nitrogen conversion reactions consume oxygen, 

this conversion at the treatment plants instead of in 

the streams resulted in reduced loading of 

biochemical oxygen demand to the streams.

As with concentrations, nutrient loads and 

yields were related to the occurrence of point 

sources and reservoirs. Loads calculated at sites 

above and below three major wastewater-treatment 

plants in the Dallas area showed a 560 percent 

increase in nitrogen load and a 2,600 percent 

increase in phosphorus load, but only a 110 percent 

increase in discharge below the plants. Nitrogen 

and phosphorus loads were 35 percent and 65 

percent smaller, respectively, at the Trinity River at 

Romayor, downstream from Livingston Reservoir, 

than at the Trinity River near Crockett, upstream 

from Livingston Reservoir, even though flow was 

30 percent greater at the downstream site.

The only nutrient measured in ground water 

was nitrate. Nitrate concentrations varied by 

aquifer with the largest median concentrations in 

the Queen City and Nacatoch aquifers. There was a 

significant rank correlation between nitrate 

concentrations and depth of well for all seven 

aquifer groups sampled with larger concentrations 

in shallow wells. The larger concentrations could 

result from nonpoint sources of nitrate in water 

recharged in agricultural and urban areas; however, 

concentrations of nitrate in ground water did not 

correlate to nitrogen fertilizer application rates or 

agricultural land use.

Only limited suspended-sediment data were 

available. Four sites had daily sediment-discharge 

records for three or more years between water 

years 1974-85. An additional three sites had 

periodic suspended-sediment concentrations 

measured. There are differences in concentrations 

and yields among sites; however, the limited 

amount of data precludes developing statistical or

69
cause-effect relations with environmental factors 

such as land use, soils, and geology. Data are 

sufficient, and the relation is pronounced enough, 

to indicate trapping of suspended sediment by 

Livingston Reservoir.

REFERENCES

A.H. Belo Corp., 1991, 1992-93, Texas Almanac 

(56th ed.): The Dallas Morning News, 

656 p.

Anderson, J.R., Hardy, E.E., Roach, J.T., and 

Witmer, R.E., 1976. A land use and land 

cover classification system for use with 

remote sensor data: U.S. Geological Survey 

Professional Paper 964, 28 p.

Baker, Bernard, Duffin, Gail, Flores, Robert, and 

Lynch, Tad, 1990, Evaluation of water 

resources in part of north-central Texas: 

Texas Water Development Board Report 

318, 67 p.

Brush, Samuel, and Promise, John, 1990, Influence 

of wastewater discharges on Trinity River 

water quality, in Jensen, Ric, ed., How 

healthy is the upper Trinity River Proceedings of the Symposium on the biological and 

water quality perspectives of the Trinity 

River, October 2, 1990: Fort Worth, Texas, 

Water Resources Institute, p. 197-213.

Davis, J.R., 1987, Analysis offish kills and associated water-quality conditions in the Trinity 

River, Texas I. Review of historical data, 

1970-1985: Texas Water Commission LP 

87-02, 95 p.

Godfrey, C.L., McKee, G.S., and Oakes, Henry 

(comps.), 1973, General soil map of Texas, 

pt. 2, Soils, formation, classification and 

use: College Station, Texas A&M University Systems, Texas Agricultural Experimental Station, scale 1:1,500,000.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical 

methods in water resources: New York, 

Elsevier Scientific Publishing Co., Studies 

in Environmental Science 49, 522 p.

Hem, J.D., 1985, Study and interpretation of the 

chemical characteristics of natural water:

U.S. Geological Survey Water-Supply Paper 

2254,263 p.

Hirsch, R.M., Alley, W.M., and Wilber, W.G.,

1988, Concepts for a National Water Quality 

Assessment Program: U.S. Geological Survey Circular 1021,42 p.

Hosman, R.H., and Weiss, J.S., 1991, Geohydrologic units of the Mississippi embayment 

and Texas coastal uplands aquifer systems, 

south-central United States: U.S. Geological Survey Professional Paper 1416-B, 19 p.

Kier, R.S., Garner, L.E., and Brown, L.F., Jr., 

1977, Land Resources of Texas: Austin, 

The University of Texas at Austin, Bureau 

of Economic Geology Special Report, 4 

sheets, scale 1:500,000.

Leahy, P.P., Rosenshein, J.S., and Knopman, D.S., 

1990, Implementation plan for the National 

Water-Quality Assessment Program: U.S. 

Geological Survey Open-File Report 90- 

174,10 p.

Morel, P.M., 1983, Principles of aquatic chemistry: 

New York, John Wiley and Sons, Inc., 

446 p.

Muller, D.A. and Price, R.D., 1979, Ground-water 

availability in Texas, estimates and projections through 2030: Texas Department of 

Water Resources Report 238, 77 p.

Nordstrom, P.L., 1982, Occurrence, availability, 

and chemical quality of ground water in the 

Cretaceous aquifers of north-central Texas: 

Texas Water Development Board Report 

269, p. 3-7.

Novotny, Vladimir, and Chesters, Gordon, 1981, 

Handbook of nonpoint pollution, sources 

and management: New York, Van Nostrand 

and Reinhold Co., 555 p.

Peckham, R.C., Souders, V.L., Dillard, J.W., and 

Baker, B.B., 1963, Reconnaissance investigation of the ground-water resources of the 

Trinity River Basin, Texas: Texas Water 

Commission Bulletin 6309, 100 p.

Pettijohn, R.A., Weiss, J.S., and Williamson, A.K., 

1988, Distribution of dissolved-solids concentrations and temperature in ground water 

of the gulf coast aquifer systems, south-central United States: U.S. Geological Survey

70
Water-Resources Investigations Report 88- 

4082,5 sheets, scale 1:3,500,000.

Rickert, D.A., Mines, W.G., and McKinzie, S.W., 

1976, Methodology for river-quality assessment with application to the Willamette 

River Basin, Oregon: U.S. Geological Survey Circular 715-M, 55 p.

Ryder, P.D., 1988, Hydrogeology and predevelopment flow in the Texas gulf coast aquifer 

systems: U.S. Geological Survey WaterResources Investigations Report 87-4248, 

109 p.

Schertz, T.L., 1990, Trends in water-quality data in 

Texas: U.S. Geological Survey WaterResources Investigations Report 89-4178, 

177 p.

Smith, R.L., 1980, Ecology and field biology (3d 

ed.): New York, Harper and Row, p. 34-72.

Texas Board of Water Engineers, 1958, Texas 

water resources planning at the end of the 

year 1958: Texas Board of Water Engineers 

Progress Report to the Fifty-Sixth Legislature, p. 33.

Texas Department of Water Resources, 1984,

Water for Texas, A comprehensive plan for

the future: Texas Department of Water 

Resources Report GP-4-1, v. 2, pt. Ill, 

sec. 8, p. 3.

Texas Water Development Board, 1990, Water for 

Texas, today and tomorrow: Texas Water 

Development Board GP-5-1, p. 3-32 to 3- 

92.

Ulery, R.L., Van Metre, P.C., and Crossfield, A.S., 

1993, Trinity River Basin, Texas: Water 

Resources Bulletin, v. 29, no. 4, p. 685-711.

U.S. Department of Agriculture Soil Conservation 

Service, 1979, Final environmental impact 

statement, Trinity River watershed: 

U.S. Department of Agricultural (ADM) 79- 

l-(F)-TX,94p.

U.S. Environmental Protection Agency, 1986, 

Quality criteria for water: U.S. Environmental Protection Agency EPA-440/5-86- 

001,453 p.

U.S. Geological Survey, 1990, Land use and land 

cover digital data from 1:250,000- and 

l:100,000-scale maps: U.S. Geodata Users 

Guide 4, 33 p.

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