ater-Quality Assessment of the Trinity River Basin, Texas—
in Two Coastal Prairie Streams Draining 
Agricultural Areas, 1994–95
GS) began nation-
ter-Quality Assessment 
AWQA are to 
lity of a large, repre-
ground-water 
ic understanding of the 
ing the quality of these 
rinity River Basin in 
 first 20 hydrologic 
y this program. The 
e Trinity River Basin 
d in September 1995. 
Study Area
The extreme southern part of the Trinity River Basin is 
known as the coastal prairie and has almost level relief, clay-rich 
soils, and a subtropical climate. The area receives an average of 
134 centimeters (53 inches) of rainfall per year. During calendar 
year 1994, 152 centimeters (60 inches) of rain fell at Anahuac 
and 178 centimeters (70 inches) fell in the Dayton area (National 
Oceanic and Atmospheric Administration, 1994). The coastal 
prairie is the only area within the river basin where there is 
extensive irrigation of farmlands. Historically, the coastal prairie 
has been a major producer of rice, cattle, oil, and natural gas. 
Because of the water requirements for growing rice and because 
the area is flood prone, a complex network of water-delivery 
canals and drainage canals has been built. These canals along 
with roadways have greatly altered the natural drainage of the 
Texas
EXPLANATION
Trinity River Basin
East Fork Double Ba
 watershed
West Prong Old Ri
 watershed
Stream sampling s
kilometers (70 miles) upstream from 
Trinity Bay, an inlet bay to Galveston 
Bay. 
With a focused objective of 
providing additional water-quality 
information in the intervening coastal 
prairie area and an overall objective of 
improving the understanding of the 
relations between farming practices 
and stream quality in the Trinity River 
Basin, a special study was conducted. 
This report provides a description of 
the occurrence and concentrations of 
nutrients in two streams in this inter-
vening area. An earlier report by 
Streams in the Trinity River Basin were assessed by sampling 
water, bed sediment, and tissue of biota and characterizing the 
aquatic communities and their habitat. Aquifers were assessed 
by sampling water from wells. 
The coastal prairie is a small part of the Trinity River Basin, 
but it is environmentally important because of its proximity to 
Galveston Bay and the extensive use of agricultural chemicals 
on many irrigated farms. Galveston Bay (fig. 1) was selected by 
Congress as an estuary of national significance and was included 
on a priority list for the National Estuary Program. The Trinity 
River is especially important because its watershed dominates 
the total Galveston Bay drainage area and because its flow con-
tributes substantial amounts of freshwater and water-quality 
constituents to the bay. Historically, measurements of the quan-
tity and quality of water entering Galveston Bay from the Trinity 
River Basin have been made using data from a station about 113 
U.S. Department of the Interior
U.S. Geological Survey
W
Nutrients 
Open-File Report 96–558
—Larry F. Land
Introduction
In 1991, the U.S. Geological Survey (US
wide implementation of the National Wa
(NAWQA) Program. Long-term goals of N
describe the status of and trends in the qua
sentative part of the Nation’s surface- and 
resources and to provide a sound, scientif
primary natural and human factors affect
resources (Leahy and others, 1990). The T
east-central Texas (fig. 1) was among the
areas, called study units, to be assessed b
first intensive data-collection phase for th
NAWQA began in March 1993 and ende
G
U
L
F
Figure 1.  Location of Trinity Ri
Brown (1996) describes the occur-
rence and concentrations of pesticides 
in these two streams. An overall anal-
ysis of nutrient data collected during 
1974–91 in the Trinity River Basin is 
given by Van Metre and Reutter 
(1995).
T
R
I
N
I
T
Y
R
I
V
E
R
West Prong
 Old River
East Fork
 Double Bayou
T
R
IN
IT
Y
 BAY
o
29 45’
30
o
95
o
o
94 30’
Anahuac
Dayton
LIBERTY
 COUNTY
CHAMBERS
 COUNTY
Study area
you
ver
ite
flat terrain. 
Two watersheds, East Fork Double Bayou and West Prong 
Old River (or simply East Fork and West Prong), were selected 
to characterize the quality of water draining agricultural areas of 
the coastal prairie. The location of these watersheds is shown in 
figure 1. The drainage areas and major crops are listed in table 1. 
In 1994, the two watersheds had more than one-half of their area 
in cropland, pasture, and rangeland. However, the area is in a 
transition because of a decline in the agricultural economy. The 
result has been a decline in cropland and an increase in aban-
doned farms, pasture, rangeland, industry, and home sites. 
East Fork is on the east side of the Trinity River, drains 
directly into Trinity Bay, has clay soils, and is the larger water-
shed. Rice is the dominant crop. Cattle are raised in the pasture 
areas, especially in the headwaters area. West Prong drains into 
the Trinity River from the west and has soils ranging from clay 
G
A
L
VES
T
O
N
 BAY
O
F
M
EXI
C
O
0 5 10 15 20 KILOMETERS
ver Basin, coastal prairie study area, and stream sampling sites.
ammonia plus organic nitrogen [known as total Kjeldahl nitro-
gen (TKN)] in both streams were less than 1.0 mg/L (milligram 
per liter) for 75 percent of the samples from both streams. 
Nitrate concentrations as nitrogen were less than 0.15 mg/L for 
more than one-half the samples from both streams. Concentra-
tions of total nitrogen equal to or less than 1.0 mg/L occurred in 
50 percent of the samples from East Fork and 75 percent of the 
samples from West Prong. The maximum concentration of total 
nitrogen of all samples was 4.8 mg/L from East Fork. Total 
nitrogen is generally composed of more than 80 percent TKN. 
Concentrations of total phosphorus were less than 0.1 mg/L 
for about 80 percent of the samples from East Fork and 75 per-
cent of the samples from West Prong. The maximum total phos-
phorus concentration of all samples was 0.35 mg/L from West 
Prong. At the median concentration, dissolved phosphorus 
makes up about one-half the total phosphorus from both streams. 
On the basis of these samples, concentrations of nitrogen 
compounds were greater in East Fork where rice is the dominant 
loam to clay. The West Prong watershed has a higher percentage 
of land in crop production and a greater variety of crops (mostly 
rice, sorghum, and soybeans) than the East Fork watershed. 
Other agricultural activities in the West Prong watershed include 
growing hay, turf, and nursery plants and cattle ranching. Rice is 
planted between April and mid-June, depending on the weather 
and variety. Sorghum is planted as early as the first part of 
March, and soybeans are planted in the May-to-June timeframe. 
Fertilizer generally is applied during April to July but varies 
with the crop, planting schedule, and weather. Most of the appli-
cations are in May and June when plants are in the stage of 
vigorous growth.
Detailed crop acreage was obtained by use of aerial photo-
graphs of the watersheds taken during the sampling period and 
by assistance from Consolidated Farm Service Agency (CFSA) 
officials in Liberty and Chambers Counties. The aerial photo-
graphs were originally printed at a scale of 1:24,000, and the 
watershed boundaries and crop information from the CFSA were 
overlain on the photograph (Zey, 1995). The three crops identi-
fied through the CFSA account for most of the cropland in the 
watersheds. Figure 2 shows the aerial photographs overlain with 
the watershed boundaries, streams, canals, and crops.
Sampling
Water samples were collected at a downstream site in each of 
the two watersheds from March 1994 through February 1995. 
Sampling frequency varied from weekly during part of the grow-
ing season (May and June) to once every 2 months during the 
winter. A total of about 20 samples from each site was available 
for analysis. Additional samples were collected to ensure consis-
tent, reproducible results. 
Water samples were analyzed at the USGS National Water-
Quality Laboratory. In addition to nutrient analyses, laboratory 
analyses included major inorganic ions, pesticides, and sus-
pended sediment. Field measurements included stream stage and 
discharge, water temperature, pH, dissolved oxygen, and spe-
cific conductance. Stream stage was measured three times per 
week at each site from April through September. These stage 
data were used to estimate discharge with a linear-regression 
equation that was calculated from paired stage and discharge 
measurements. 
Table 1.  Watershed drainage areas and major crops 
[km
2
, square kilometers] 
Watershed
Drainage
area
(km
2
)
Crops
(percentage of watershed area)
Rice Sorghum Soybeans
East Fork 112 10.5 0 0
West Prong 75 5.6 5.9 20
Nutrients in Two Coastal Prairie Streams
Nutrients in streams include several species of nitrogen and 
phosphorus. Their concentrations are affected by many environ-
mental and human factors, such as season, rainfall, runoff, 
instream processes, soil types, land use, and proximity to 
sources. 
Concentrations
The distribution of concentrations for total (dissolved plus 
suspended sediment) ammonia plus organic nitrogen, nitrate, 
and total nitrogen (nitrogen compounds) and for dissolved and 
total phosphorus are shown in figure 3. Concentrations of total 
94 33’15" 94 30’
o o
29 45’
9 41’45"
o
o
9 48’15"
o
I-10
A.  East Fork Double Bayou
30
95 94 56’15"
30 03’45"
o
o
oo
US 9
0
TX 146
B.  West Prong Old River
0 1 2 3 4 KILOMETERS
EXPLANATION
MAJOR CROPS
Rice
Sorghum
Soybeans
East Fork Double Bayou and
 We s t  P ro n g  O l d  R i ve r
Other streams or canals
Watershed boundary
Stream sampling site
Figure 2.  Aerial photographs overlain with watershed 
boundaries, streams, canals, and major crops, March 1994–
February 1995.
crop, and concentrations of phosphorus compounds were greater 
in West Prong where there is more of a variety of crops. For 
nitrate, the maximum concentration of all the samples was 4.8 
mg/L, which is less than one-half of the U.S. Environmental 
Protection Agency (1996) Maximum Contaminant Level for 
drinking water. To avoid excessive algae and other aquatic plant 
growth, the U.S. Environmental Protection Agency (1986) 
recommends that total phosphorus be less than 0.1 mg/L in 
streams except where they enter lakes and reservoirs; there, the 
concentrations should be less than 0.05 mg/L. About 20 percent 
5
4
N
, 
 L
I
T
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R
,
5
4N
,
 LIT
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R
,
NITROGEN
TOTAL AMMONIA
PLUS 
ORGANIC NITROGEN
TOTAL
NITROGEN
of the samples contained more than 0.1 mg/L of total phospho-
rus, but about 80 percent contained more than 0.05 mg/L of total 
phosphorus.
Relation to Seasons
The seasonality of nutrient concentrations is shown by graph-
ing the concentrations of total nitrogen and total phosphorus 
over time for the two sites (fig. 4). The data plots show that con-
centrations of total nitrogen and total phosphorus have seasonal 
highs during May and June. The seasonal pattern of total nitro-
gen shows an increase from an average of 0.8 mg/L during 
March, April, and July–February to 1.5 mg/L during May and 
June. For the same time periods and sampling sites, the average 
concentrations of total phosphorus increased from 0.07 to 0.12 
mg/L. The seasonally high concentrations followed a number of 
small runoff events and one large runoff event in early May. 
Runoff events in August and September did not seem to cause a 
subsequent change in nutrient concentrations. A small runoff 
event in October appears to have caused an increase in phospho-
rus concentrations at both sites, but the sampling was too infre-
quent to identify a pattern. These seasonally high patterns of 
nutrient concentrations in the two streams generally coincide 
with the timing of fertilizer applications.
Loads
Whereas constituent concentrations define the levels of the 
nutrients in the stream, constituent loads define the total amount 
of nutrients being transported by the streams for a given period. 
The annual loads (March 1994–February 1995) of total nitrogen 
and total phosphorus for the two streams were estimated using 
the "unbiased stratified ratio estimator" method (Thomann and 
Mueller, 1987). This method uses the concentrations and dis-
charges measured during sampling along with the mean dis-
charge for the period to determine the load. It also allows for the 
data to be divided into two or more seasons to account for vari-
able flow or concentrations. In this case, May and June were 
selected to represent one season when concentrations are typi-
cally higher, and the remaining months represent a second sea-
son when concentrations are typically lower. 
Estimates of fertilizer applications were made for the three 
major crops grown in the coastal prairie area. These estimates 
were made by multiplying the area planted for a given crop in 
each watershed by application rates used by the Texas Agricul-
tural Extension Service as typical applications for estimating 
crop-production costs (Texas Agricultural Extension Service, 
1991). No estimates were made for fertilizers applied by home-
owners and farmers growing crops such as hay, fruits, nuts, and 
turf because these amounts are believed to be insignificant in 
comparison to the application of fertilizers to the three major 
crops.
As shown in table 2, loads of total nitrogen and total phos-
phorus for East Fork were about 20 and 12 percent of the 
fertilizer applications to the major crop in the watershed, which 
is rice. For West Prong, loads of total nitrogen and total phos-
phorus were about 13 and 9 percent of fertilizer applications to 
the major crops (rice, sorghum, and soybeans) in this watershed. 
The percentage of fertilizers lost to the stream from the fields 
0
1
2
3
CO
NCE
NT
RA
T
I
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L
L
I
G
R
AM
S PE
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 AS
 N
I
T
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EN
2143 4 5 6 7 8 9 10 11 12 13
TOTAL NITROGEN
0
0.4
0.1
0.2
0.3
CO
NCE
NT
RA
T
I
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N
, 
I
N
 M
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A
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S PER
 L
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R
,
 AS PH
O
SPH
O
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U
S
2143 4 5 6 7 8 9 10 11 12 13
TOTAL PHOSPHORUS
0
12
2
4
6
8
10
E
S
T
I
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A
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D DIS
CHA
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,
 
IN CUB
IC ME
T
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P
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 S
E
CO
ND
2142 3 4 5 6 7 8 9 10 11 12 13
STREAMFLOW
EXPLANATION
East Fork
West Prong
East Fork
West Prong
MAR
MAY
JULY SEPT NOV JAN FEB
0
1
2
3
CO
NCE
NT
RA
T
I
O
IN
 MILLIG
R
A
M
S
 P
E
R
AS
 N
I
T
R
O
G
EN
0
0.4
0.1
0.2
0.3
CO
NCE
NT
RA
T
I
O
N
,
IN
 MILLI
G
R
A
M
S
 P
E
R
 LIT
E
R
,
AS
 PH
O
S
P
H
O
R
U
S
EXPLANATION
EAST FORK
Detection limit
Minimum concentration
25th percentile
Median
75th percentile
Maximum concentration
WEST PRONG
PHOSPHORUS
NITRATE
DISSOLVED
PHOSPHORUS
TOTAL 
PHOSPHORUS
DOUBLE BAYOU
OLD RIVER
Figure 3.  Distribution of nutrient concentrations, March 1994–
February 1995.
Figure 4.  Seasonal pattern of nutrient concentrations and 
streamflow, March 1994–February 1995.
90–174, 10
National Oceanic
Ocean Servi
Asheville, N.
paged].
was less than 0.15 mg/L in more than one-half the samples 
from both streams. Total nitrogen generally comprised more 
than 80 percent total ammonia plus organic nitrogen.
 Total phosphorus concentrations were less than 0.1 mg/L 
for about 80 percent of the samples from East Fork and for 
about 75 percent of the samples from West Prong. At the 
median concentration from both streams, dissolved 
phosphorus made up about one-half of the total phosphorus.
 Total nitrogen and total phosphorus exhibited a seasonality 
with higher concentrations occurring in May and June. 
However, the average concentrations from the two streams 
during these 2 months were less than two times the 
concentrations during other times of the year. These 
seasonally high patterns of nutrients in the two streams 
generally coincided with the timing of fertilizer 
applications.
 Loads of total nitrogen in the two streams ranged from 13 to 
20 percent of the fertilizer applied to major crops in the 
associated watersheds. Loads of total phosphorus ranged 
from 9 to 12 percent of the fertilizer applied to major crops. 
The percentage of fertilizers lost to the streams from the 
fields growing these major crops would be somewhat less 
than these amounts because there are other sources of 
nutrients in streams. 
growing these major crops would be somewhat less than the 
amounts shown in table 2 because there are other sources of 
nutrients in streams such as fertilizer applied to other crops, 
landscape plants, and lawns and also manure from livestock.
Summary
Water-quality samples were collected from East Fork Double 
Bayou and West Prong Old River in the coastal prairie area 
immediately upstream from Trinity Bay from March 1994 to 
February 1995. Rice is grown in the East Fork Double Bayou 
watershed; and rice, sorghum, and soybeans are grown in the 
West Prong Old River watershed. Results of sampling include 
the following:
 Total nitrogen concentrations were equal to or less than 1.0 
mg/L in 50 percent of the samples from East Fork and in 75 
percent of the samples from West Prong. Nitrate as nitrogen 
Table 2.  Estimated fertilizer applications to major crops and 
estimated loads of nutrients in two coastal prairie streams, March 
1994–February 1995
[kg/yr, kilograms per year] 
Stream/nutrient
Estimated
fertilizer
application to
major crops
(kg/yr)
Estimated
nutrient
load—all
sources
(kg/yr)
Percentage
of fertilizer
applications
leaving
watersheds
by streams
East Fork
Total nitrogen 210,000 42,000 20
Total phosphorus 28,000 3,500 12
West Prong
Total nitrogen 120,000 16,000 13
Total phosphorus 28,000 2,600 9
October 1996
Texas Agricultural Extension Service, 1991, Results of 1991 
agricultural demonstrations, Liberty County: College 
Station, Tex., Texas A&M University, 98 p.
Thomann, R.V., and Mueller, J.A., 1987, Principles of surface 
water quality modeling and control: New York, Harper 
Collins Publishers, 644 p.
U.S. Environmental Protection Agency, 1986, Quality criteria 
for water 1986: Washington, D.C., Report 440/5–86–001, 
453 p.
          1996, Drinking water regulations and health advisories: 
Washington, D.C., 16 p.
Van Metre, P.C., and Reutter, D.C., 1995, Water-quality 
assessment of the Trinity River Basin, Texas—Analysis of 
available information on nutrients and suspended 
sediments, 1974–91: U. S. Geological Survey Water-
Resources Investigations Report 94–4086, 71 p.
Zey, E.B.,1995, National Water-Quality Assessment Program—
A GIS application for the Texas coastal prairie survey: 
College Station, Tex., Texas A&M University, Rangeland 
Ecology and Management, Professional Paper, 42 p.
Information on technical reports and hydrologic data related 
to the NAWQA Program can be obtained from:
Project Chief—Trinity River Basin 
NAWQA Study
U.S. Geological Survey
8011 Cameron Road
Austin, Texas 78754–3898
e-mail: lfland@usgs.gov
In 1991, the U.S. Geological Survey, U.S. 
Department of the Interior, began a National Water-
Quality Assessment (NAWQA) Program. The long-
term goals of the NAWQA Program are to describe 
the status of and trends in the quality of a large 
representative part of the Nation’s surface- and 
ground-water resources and to identify the major 
factors that affect the quality of these resources. In addressing these 
goals, NAWQA will produce water-quality information that is useful 
to policymakers and managers at Federal, State, and local levels.
Studies of 60 hydrologic systems that include parts of most major 
river basins and aquifer systems are the building blocks of the 
national assessment. The 60 study units range in size from less than 
1,000 to more than 60,000 square miles and represent 60 to 70 percent 
of the Nation’s water use and population served by public water 
supplies. Twenty investigations began in 1991, 15 investigations 
began in 1994, and 20 are scheduled to begin in 1997.
U.
S.
D
E
P
A
R
T
M
N OF
H
I
G
LC S
U
V
Y
E
T
T
E
N
T
E
R
O
R
I
E
O
A
R
I
O
L
G
E
ation plan for the National Water-Quality 
t Program: U.S. Geological Survey Open-Report 
 p.
 and Atmospheric Administration, National 
ce, 1994, Climatological data—Texas: 
C., National Climatic Data Center [variously 
References
Brown, M.F., 1996, Water-quality assessment of the Trinity 
River Basin, Texas—Pesticides in a coastal prairie 
agricultural area, 1994–95: U.S. Geological Survey Open-
File Report 96–124, 6 p.
Leahy, P.P, Rosenshein, J.S., and Knopman, D.S., 1990, 
Implement
Assessmen