Hydrologic Conditions and Water Qual

in an Agricultural Area in Kleberg and 

Nueces Counties, Texas, 1996–98

Water-Resources Investigations Report 01–4101

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

U.S. Geological Survey

Hydrologic Conditions and Water Quality 

in an Agricultural Area in Kleberg and 

Nueces Counties, Texas, 1996–98

By Darwin J. Ockerman and Brian L. Petri

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01–4101

In cooperation with the Texas Agricultural Experiment Station, 

Caesar Kleberg Wildlife Research Institute, and 

Coastal Bend Bays and Estuary Program

Austin, Texas

2001
CONTENTS iii

CONTENTS

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

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

Purpose and Scope .................................................................................................................................................... 2

Description of Study Area ........................................................................................................................................ 2

Texas Water-Quality Standards ................................................................................................................................. 5

Acknowledgments .................................................................................................................................................... 6

Data-Collection Methods ..................................................................................................................................................... 6

Rainfall ..................................................................................................................................................................... 6

Runoff ....................................................................................................................................................................... 6

Water Quality ............................................................................................................................................................ 6

Fertilizer and Pesticide Application ......................................................................................................................... 7

Hydrologic Conditions ......................................................................................................................................................... 7

Rainfall ..................................................................................................................................................................... 7

Runoff ....................................................................................................................................................................... 9

Water Quality ....................................................................................................................................................................... 9

Rainfall ..................................................................................................................................................................... 11

Event-Mean Nutrient Concentrations ........................................................................................................... 11

Rainfall Deposition ....................................................................................................................................... 11

Runoff ....................................................................................................................................................................... 12

Event-Mean Concentrations .......................................................................................................................... 12

Nutrients and Major Inorganic Ions ................................................................................................... 12

Pesticides ........................................................................................................................................... 14

Suspended Sediment .......................................................................................................................... 14

Constituent Loads and Yields ....................................................................................................................... 15

Nutrients ............................................................................................................................................ 16

Pesticides ........................................................................................................................................... 18

Sediment ............................................................................................................................................ 19

Runoff-Weighted Average Concentrations .................................................................................................... 19

Nutrients ............................................................................................................................................ 20

Pesticides ........................................................................................................................................... 20

Summary .............................................................................................................................................................................. 21

Selected References ............................................................................................................................................................. 22

Appendix I–Rainfall Sample Analyses ................................................................................................................................ I–1

Appendix II–Runoff Event-Mean Concentrations ............................................................................................................... II–1

FIGURES

1–2. Maps showing:

1. Location of agricultural study area in Kleberg and Nueces Counties, Texas ............................................ 3

2. Location of data-collection sites in agricultural study area ....................................................................... 4

3. Diagram showing typical timing of agricultural activities in Nueces and Kleberg Counties, Texas ................... 5

4. Hydrograph showing rainfall, discharge, and subsample-collection timing at site 4 during storm 

event of September 22–24, 1998 .......................................................................................................................... 8

5–7. Graphs showing:

5. Annual rainfall at National Oceanic and Atmospheric Administration Kingsville weather 

station, 1969–98 ......................................................................................................................................... 8

6. Mean monthly rainfall at National Oceanic and Atmospheric Administration Kingsville

weather station, 1969–98 ........................................................................................................................... 10

7. Deviation of monthly study area rainfall from mean monthly rainfall at National Oceanic and 

Atmospheric Administration Kingsville weather station, 1969–98 .......................................................... 10
iv 

8. Hydrograph showing cumulative rainfall and runoff in study area, 1996–98 ...................................................... 11

9. Boxplots showing distribution of sulfate concentrations in runoff samples from sites 1–4 and 

site 5, 1996–98 ...................................................................................................................................................... 13

10. Graphs showing herbicide and insecticide detection frequency in 29 runoff samples, 1996–98 ......................... 15

11. Boxplots showing seasonal distribution of atrazine concentrations in runoff samples, 1996–98 ........................ 16

12–14. Graphs showing:

12. Relation between suspended sediment concentration (C) and discharge (Q) in runoff samples 

collected at site 5 ........................................................................................................................................ 17

13. Comparison of rainfall deposition and runoff yield of nitrogen during April 2–13, 1997, lower 

study area ................................................................................................................................................... 19

14. Relation between suspended sediment load and storm event runoff at site 5, 1996–98 ............................ 20

TABLES

1. Selected characteristics of data-collection sites .................................................................................................... 23

2. Applications of fertilizers and selected pesticides on 40,540-acre lower study area, 1996–98 ............................ 23

3. Rainfall, runoff volumes, and runoff coefficients for runoff events, 1996–98 ..................................................... 24

4. Summary statistics of selected nutrient concentrations for 19 rainfall samples, 1996–98 ................................... 25

5. Monthly and annual rainfall deposition of nitrogen and phosphorus, 1996–98 ................................................... 25

6. Summary statistics of selected major ion and nutrient concentrations in runoff samples, 1996–98, 

and selected regulatory water-quality criteria ....................................................................................................... 26

7. Pesticides applied on lower study area, 1996–98 ................................................................................................. 26

8. Summary statistics of selected pesticide concentrations in runoff samples, 1996–98, and selected 

regulatory water-quality criteria ............................................................................................................................ 27

9. Median concentrations of selected pesticides in runoff samples grouped by seasonal category .......................... 27

10. Monthly and annual loads of selected nutrients in runoff, 1996–98 ..................................................................... 28

11. Annual and average annual yields of selected nutrients in runoff, 1996–98 ........................................................ 31

12. Monthly and annual loads of selected pesticides in runoff, 1996–98 ................................................................... 32

13. Annual and average annual yields of selected pesticides in runoff, 1996–98 ...................................................... 33

14. Monthly and annual loads of sediment in runoff, study area, 1996–98 ................................................................ 34

15. Annual and average annual yields of sediment in runoff, study area, 1996–98 ................................................... 34

16. Study area runoff-weighted concentrations of selected nutrients, 1996–98 ......................................................... 35

17. Study area runoff-weighted concentrations of selected pesticides, 1996–98 ....................................................... 36
Abstract 1

Hydrologic Conditions and Water Quality 

in an Agricultural Area in Kleberg and 

Nueces Counties, Texas, 1996–98

By Darwin J. Ockerman and Brian L. Petri

Abstract

During 1996–98, rainfall and runoff were 

monitored on a 49,680-acre agricultural watershed 

in Kleberg and Nueces Counties in South Texas. 

Nineteen rainfall samples were analyzed for 

selected nutrients, and runoff samples from 29 

storms were analyzed for major ions, nutrients, and 

pesticides. Loads of nutrients in rainfall and loads 

of nutrients and pesticides in runoff were computed. For a 40,540-acre part of the watershed 

(lower study area), constituent loads entering the 

watershed in rainfall, in runoff from the upper 

study area, and from agricultural chemical applications to the lower study area were compared with 

runoff loads exiting the lower study area. 

Total rainfall for 1996–98 averaged 25.86 

inches per year, which is less than the long-term 

annual average rainfall of 29.80 inches for the area. 

Rainfall and runoff during 1996–98 were typical of 

historical patterns, with periods of below average 

rainfall and runoff interspersed with extreme 

events. Five individual storms accounted for about 

38 percent of the total rainfall and 94 percent of the 

total runoff.

During the 3-year study, the total nitrogen 

runoff yield from the lower study area was 1.3 

pounds per acre per year, compared with 49 pounds 

per acre per year applied as fertilizer and 3.1 

pounds per acre per year from rainfall. While 

almost all of the fertilizer and rainfall nitrogen was 

ammonia and nitrate, most of the nitrogen in runoff 

was particulate organic nitrogen, associated with 

crop residue. Total nitrogen exiting the lower study 

area in surface-water runoff was about 2.5 percent 

of the nitrogen inputs (fertilizer and rainfall nitrogen). Annual deposition of total nitrogen entering 

the lower study area in rainfall exceeded net yields 

of total nitrogen exiting the watershed in runoff 

because most of the rainfall does not contribute to 

runoff.

During the study, the total phosphorus runoff 

yield from the lower study area was 0.48 pound per 

acre per year compared with 4.2 pounds per acre 

per year applied as fertilizer and 0.03 pound per 

acre per year from rainfall.

Twenty-one pesticides were detected in 

runoff with varying degrees of frequency during 

the study. The herbicide atrazine was detected in 

all runoff samples. All of the most frequently 

detected pesticides (atrazine, trifluralin, simazine, 

pendimethalin, and diuron) exhibited higher concentrations during the pre-harvest period (March–

May) than during the post-harvest period (August–

October). 

During 1996–98, an average of 0.37 pound 

per acre per year of atrazine was applied to the 

lower study area. During the same period, 0.0027 

pound per acre per year of atrazine and its breakdown product deethylatrazine exited the lower 

study area in runoff (about 0.7 percent of the total 

atrazine applied to the cropland). During 1997, 

when heavy rainfall occurred during the months of 

April and May, the atrazine plus deethylatrazine 

exiting the lower study area was 1.8 percent of the 

applied atrazine.

The 1996–98 average sediment yield was 

610 pounds per acre per year. Sediment loads from 

the study area are associated with large storm 

events. Of the 45,300 tons of sediment transported 

from the study area during 1996–98 about 87 percent was transported during the three largest runoff 
2 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

events (April 1997, October 1997, and October 

1998).

Runoff-weighted average concentrations 

were computed for selected nutrients and pesticides. The 1996–98 runoff-weighted concentrations for total nitrogen and total phosphorus were 

1.3 and 0.50 milligrams per liter, respectively. The 

1996–98 runoff-weighted concentration for atrazine plus deethylatrazine was 2.7 micrograms per 

liter.

INTRODUCTION

Baffin Bay and Laguna Madre south of Corpus 

Christi, Texas (fig. 1), are shallow, poorly circulated 

estuaries that are potentially sensitive to point and nonpoint source contributions of water-quality constituents. 

These estuary systems experienced a persistent browntide algae bloom beginning in spring 1990 that threatened their biological productivity. Non-point source 

nutrient inputs are a possible concern, although the 

causes of the blooms are not well understood (Coastal 

Bend Bays and Estuaries Program, 1998). The brown 

tide bloom subsided in fall 1997, coincident with heavy 

rains in the area.

The U.S. Geological Survey (USGS) studied 

the hydrology and water quality of a 49,680-acre 

agricultural area adjacent to Baffin Bay during January 

1996–December 1998 to provide a better understanding 

of contributions of freshwater and runoff constituent 

loads from the agricultural area to Baffin Bay. The study 

was done in cooperation with the Texas Agricultural 

Experiment Station (TAES), Texas A&M UniversityCorpus Christi; Caesar Kleberg Wildlife Research 

Institute, Texas A&M University-Kingsville; and 

Coastal Bend Bays and Estuary Program. 

Purpose and Scope

This report describes the quantity and quality 

of rainfall and runoff in an agricultural watershed in 

Kleberg and Nueces Counties and presents constituent 

loads and yields associated with the runoff. Data were 

collected at five streamflow-gaging stations and one 

weather station in an agricultural watershed in the 

Baffin Bay drainage area. Nineteen rainfall event 

samples were collected at the weather station and 

analyzed for nutrient concentrations. Twenty-nine 

event-composite runoff samples were collected at the 

five streamflow stations and analyzed for major ions, 

nutrients, and pesticides. Event-mean concentrations 

and constituent loads and yields were determined from 

these data. Loads and yields of selected constituents in 

rainfall and runoff were compared with applications of 

fertilizers and pesticides for that part of the study area 

where the application data were available. 

Description of Study Area

Alice and Kingsville (1997 populations of 20,301 

and 26,646, respectively) (Texas State Data Center, 

1998) are the primary urban centers in the region 

(fig. 1). Land use in the Baffin Bay watershed is about 

33 percent cropland and about 64 percent rangeland. 

The study area (fig. 2) within the Baffin Bay watershed 

consists of 49,680 acres of agricultural land located in 

Nueces and Kleberg Counties. The study area is primarily cropland except for 3,392 acres of rangeland and 

about 500 acres of canals, ditches, and roads in the 

lower study area. 

The climate of the area is classified as subtropical, with warm tropical air from the Gulf of Mexico contributing to mild winter temperatures and hot, humid 

summer weather (Baird and others, 1996). Prevailing 

winds are southeasterly throughout the year.

Streams in the study area are ephemeral and have 

runoff only after heavy rains. Most of the runoff from 

the study area enters Petronila Creek, Alazan Bay, and 

Baffin Bay (from Tunas Creek). Some of the runoff 

from the lower study area enters rangeland and ephemeral wetlands, and by overland flow, moves toward 

Baffin Bay through Cayo del Grullo (fig. 2).

Besides climate and rainfall, the type and nature 

of the soils and, to a lesser degree, the agricultural practices are crucial in the rainfall-runoff process. VictoriaClaireville Association clays are the dominant soil in 

the watershed (R. Schmidt, Natural Resource Conservation Commission, Kingsville, Texas, oral commun., 

1996). These soils are dark, calcareous, and crumbly. 

Because Victoria soils dry and crack almost every summer, their subsoil can take in and store much water in a 

short time during heavy rains in the fall. However, after 

the soils are saturated, they take in water very slowly, 

resulting in substantial runoff during heavy rains (Soil 

Conservation Service, 1992). Slopes in the study area 

are nearly flat, ranging from 0 to 3 percent.

The primary crops in the study area are cotton and 

grain sorghum, which are rotated annually. The following table shows the acreage of each crop planted in the 

lower study area during 1996–98. Crop acreage was not 
I

NTRODUCTION3

Sand and mud flats

Water body

Baffin Bay watershed

Agricultural study area (fig. 2)

Land subject to inundation

EXPLANATION

CORPUS CHRIST

 BAY

GULF OF MEXICO LAGUNA MADRE

BAFFIN BAY

LAKE CORPUS CHRISTI

NUECES

RIVER

ALAZAN BAY

Alice

Kingsville

Corpus Christi

NUECES BAY

0 5 10 15 20 MILES

97o 98 30' o 98o 99 30' o

28o30'

28o

27o30'

WEBB

LA SALLE

JIM WELLS

LIVE OAK

DUVAL

MCMULLEN

JIM HOGG

ZAPATABROOKS KENEDY

KLEBERG

NUECES

SAN PATRICIO

Figure 1. Location of agricultural study area in Kleberg and Nueces Counties, Texas.
4 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

Figure 2. Location of data-collection sites in agricultural study area.

S

W

In

A

EXP

R

St g r c

San

Fernando

Creek

Carreta

Creek

Fernando San

Gertrudis

Creek Creek

Pinto Creek Canal

Lower Madero Canal

Petronila

Creek

Creek

Petronila

Chiltipin

Creek

Creek

Tunas

Upper Madero

Canal

Santa

Cayo del

Grullo 

KINGSVILLE 

NAVAL AIR

STATION

97o37'30" 97o30' 97o52'30" 97o45'

27o37'30"

27o30'

0 1 2 3 4 5 MILES

NUECES COUNTY

KLEBERG COUNTY

BISHOP

KINGSVILLE

US 77

FM 3354

FM 70

US 877

US 77

US 77

FM 1118

W N A T E

NOAA

TAES

Site 1

Site 2

Site 3

Site 4

2

Site 5
INTRODUCTION 5

available for the upper study area; however, farming 

practices and crop ratios are similar to those in the lower 

study area.

Crop acreage within the lower 40,540-acre study area, 

1996–98

[About 3,932 acres are rangeland; about 500 acres are 

canals, ditches, and roads]

Crop production begins during mid-July to lateAugust with shredding of crop residue from the previous crop and deep tilling (fig. 3) to plow out stubble and 

prepare the ground to absorb fall and winter rains. The 

soil is retilled during September and October to destroy 

winter weeds and to prepare the seedbed for the succeeding crop. Pre-plant fertilizers and broadcast preemergence herbicides are usually applied during the low 

rainfall months of November and December. During 

January to mid-February, additional tilling is done. 

Planting begins in early- to late-March, depending on 

the available soil moisture, soil temperature, and type of 

crop. After the crops have emerged from the soil, production practices throughout the growing season consist 

of pesticide application and row cultivation. From midto late-April to early-July, few field operations are 

required for grain sorghum. However, for cotton, this is 

a period of heavy activity involving insect control. 

Grain sorghum usually is harvested in early- to midJuly. Cotton harvest usually begins with application of 

harvest aids (defoliants) around mid-July and ends 

around mid-August (Dr. Bobby Eddleman, Texas Agricultural Experiment Station, Texas A&M UniversityCorpus Christi, written commun., 1998) 

Texas Water-Quality Standards

The Texas Natural Resource Conservation 

Commission (TNRCC) has designated water-quality 

standards and appropriate uses (such as aquatic life, 

contact or non-contact recreation, drinking water) 

for specific stream, estuary, and bay segments (Texas 

Natural Resource Conservation Commission, 1999). To 

support the designated use of the water-body segments, 

standards for common water-quality indicators such as 

dissolved oxygen, temperature, pH, dissolved minerals, 

and fecal bacteria have been established for some 

stream and bay segments. The TNRCC has not developed segment-specific standards for any of the canals 

or creeks monitored during this study. However, some 

segment-specific standards have been established for 

nearby Petronila Creek and Baffin Bay, which eventually receive runoff from the study area. The water-body 

segment that includes Baffin Bay is designated for contact recreation, high aquatic life, and oyster waters 

Year Cotton Grain sorghum Total

1996 21,814 14,354 36,168

1997 19,100 17,028 36,128

1998 16,770 19,358 36,128

Shredding previous crop

and deep tilling

Applying fertilizers and 

herbicides

Tilling in preparation

for planting

Planting

Applying pesticides and

cultivating

Controlling insects

(cotton)

Harvesting

Activity Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Figure 3. Typical timing of agricultural activities in Nueces and Kleberg Counties, Texas.
6 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

(State of Texas, 1997). Similarly, the tidal segment of 

Petronila Creek is designated for contact recreation.

Acknowledgments

Special thanks are extended to Dr. Bobby 

Eddleman and Clinton Livingston with the Texas Agricultural Experiment Station, Texas A&M UniversityCorpus Christi, for their substantial support to this 

project. Dr. Eddleman provided valuable project oversight and technical assistance, while Clinton Livingston 

assisted with site and equipment maintenance and sample collection. 

DATA-COLLECTION METHODS

The study area is defined by the location of five 

streamflow-gaging and water-quality sampling stations 

with raingages (fig. 2) that were installed in January 

1996 and operated through December 1998. A weather 

station with rainfall sample collector (operated by 

TAES) also is located in the study area. A long-term, 

National Oceanic and Atmospheric Administration 

(NOAA) operated raingage is located near Kingsville. 

Table 1 (at end of report) lists selected characteristics of 

the data-collection sites.

The watershed of the upper study area is defined 

by the drainage area for sites 1 and 2 (fig. 2). Site 1 is 

located on Upper Madero Canal just inside Kleberg 

County. Site 2 is located on Chiltipin Creek just inside 

Nueces County. The combined drainage area of the 

upper study area is 9,140 acres, although the specific 

drainage areas gaged by each station are indeterminate, 

primarily because of the interconnecting canal between 

the two gaging stations. 

Sites 3, 4, and 5 are located at the outlets of the 

study area (fig. 2). Site 3 is located on Pinto Creek Canal 

and site 4 on Lower Madero Canal. Runoff exiting the 

study area through sites 3 and 4 makes its way toward 

Cayo del Grullo (an arm of Baffin Bay) via overland 

flow through rangeland and ephemeral wetlands. Site 5 

is located on Tunas Creek 3 miles (mi) upstream from 

the confluence with Petronila Creek and Alazan Bay 

(another arm of Baffin Bay). The combined drainage of 

the lower study area is 40,540 acres. The individual 

drainage areas gaged by each of the lower stations are 

not well defined (again, because of interconnecting 

canals between gaging stations). However, most of the 

upper and lower study area ultimately drains to site 5. 

During the study, about 93 percent of all runoff leaving 

the study area exited through site 5.

Rainfall

Five tipping bucket raingages at sites 1–5 and one 

at the TAES weather station recorded hourly rainfall for 

the study area. The raingages at sites 1–5 have recorded 

data since January 1996, and the TAES raingage has 

recorded data since 1988. The average rainfall volumes 

on the study area were determined from a Theissenweighted average (Wanielista, 1990, p. 51) of the six 

raingage measurements. 

Runoff 

Gage height was continuously recorded at all sites 

using a gas-bubbler and pressure transducer system. A 

relation between gage height and discharge was developed by making independent discharge measurements 

during runoff events. At site 5, the relation between 

gage height and discharge was variable and not well 

defined because of tidal influence. Therefore, an acoustic velocity meter (AVM) also was installed to measure 

stream velocity along with gage height. Independent 

stream discharge measurements at site 5 were used to 

develop a relation between gage height, stream velocity, 

and stream discharge (Patino and Ockerman, 1997).

Water Quality

Rainfall samples were collected at the TAES 

weather station by an automatic rainfall sample collector. The collector is equipped with an empty polyethylene bucket that is covered prior to rainfall. A moisture 

sensor activates a mechanism to uncover the collection 

bucket when rainfall begins and cover the bucket when 

rainfall ends to prevent evaporation and contamination 

of the sample. About 0.2 inch (in.) of rain is required to 

provide sufficient sample volume for analysis. Rainfall 

samples were collected as single-event composite samples during rainfall events and therefore represent rainfall event-mean concentrations (EMCs). The samples 

were retrieved as soon as possible after a rainfall event, 

chilled, and shipped overnight to the USGS National 

Water Quality Laboratory (NWQL) in Denver, Colorado, for analysis. 

Rainfall samples were analyzed for the following 

forms of nitrogen: ammonia, nitrite plus nitrate, dissolved organic, and total organic. Total nitrogen concentrations were computed for each sample as the sum of 

ammonia, nitrite plus nitrate, and total organic nitrogen. 

The samples also were analyzed for total phosphorus, 

dissolved phosphorus, and dissolved orthophosphate 
HYDROLOGIC CONDITIONS 7

phosphorus. Field measurements of pH and alkalinity 

also were made for selected samples. 

Automatic water samplers at sites 1–5 collected 

runoff samples during storm events. When the 

streamflow-gaging instrumentation detected runoff, 

automatic samplers were activated to collect runoff 

samples. Discrete aliquots (subsamples) were collected 

by the automatic samplers at a frequency proportional 

to stream discharge. As the flow at a particular site 

increased, the sampler collected subsamples more frequently; thus, the samples were automatically flowweighted during the collection process. At the end of 

the event, the subsamples from each site were combined 

into a single discharge-weighted composite sample (one 

sample from each site). Analysis of the composite samples yielded EMCs, which represent the dischargeweighted average concentrations during the runoff 

event. Figure 4 shows a rainfall-discharge hydrograph 

of a runoff event at site 4 during Sept. 22–24, 1998, and 

also shows the subsample-collection timing during the 

event. 

During several of the longer duration runoff 

events, multiple composite samples were collected at 

some sites. For example, after 24 hours of sampling, the 

subsamples already collected would be composited, 

processed, and shipped to the laboratory for analysis 

while automatic sampling continued at the site. In this 

way, subsamples collected during the early phase of a 

runoff event would not be degraded if the runoff event 

lasted several days1. When multiple composite samples 

were collected and analyzed at a particular site during 

an event, EMCs for the entire event were calculated by 

discharge-weight averaging of the concentrations from 

the separate composites. Runoff samples were analyzed 

by the NWQL for major inorganic ions, nutrients, and 

dissolved pesticides. 

Total suspended solids concentrations were determined by analysis of the samples collected with automatic samplers. In addition, five suspended sediment 

samples were collected manually at site 5 using the 

equal-discharge-increment (EDI) method (Wells and 

others, 1990). The EDI method involves collection of a 

sample at a specific time and discharge that is integrated 

horizontally and vertically through the channel cross 

section. Because sediment concentrations vary with 

location in the channel cross section, the EDI method is 

more accurate for determining sediment concentrations 

than using automatic samplers, which collect samples 

from a single fixed point in the channel.

Sand-break analysis was performed on several of 

the EDI sediment samples to determine the sand-clay 

composition of the sediment. Sand-break analysis gives 

the percentage of sediment, by weight, that is finer than 

0.062 millimeter.

Fertilizer and Pesticide Application

Detailed fertilizer and pesticide application data 

for the 40,540-acre lower study area (excluding area 

gaged by sites 1 and 2) were provided by the agricultural 

producer and TAES. Runoff data collected at sites 1 and 

2 provide information on runoff and constituents entering the lower study area. Runoff data for sites 3, 4, and 

5 provide information on runoff exiting the entire study 

area. The chemical application data can be used, along 

with runoff data, to compare loads and yields of nitrogen, phosphorus, and pesticides entering (through rainfall and chemical applications) and exiting (through 

surface runoff) the lower study area. Annual applications of nitrogen and phosphorus fertilizers and selected 

pesticides are listed in table 2 (at end of report).

HYDROLOGIC CONDITIONS

Rainfall

Long-term rainfall data from the NOAA weather 

station at Kingsville Naval Air Station (fig. 2) were 

compared with study area rainfall data during 1996–98. 

The NOAA station annual rainfall for 1969–98 is shown 

in figure 5. The 30-year mean annual rainfall is 29.80 in. 

with a minimum of 17.14 in. during 1988 and a maximum of 46.01 in. during 1981.

During January 1996–December 1998, the study 

area received 77.59 in., as determined by a Theissen 

average (Wanielista, 1990) based on the six study area 

raingages. Annual distribution of the rainfall is listed in 

the table below. 

Annual and mean annual rainfall for study area 

(1996–98) and mean annual rainfall for NOAA station, 

1969–98 

1 During the October 1997 event, 5 days of sampling at site 5 

resulted in 104 subsamples, representing three composite samples 

that were analyzed and aggregated by discharge to determine the 

EMCs.

Study area rainfall

(inches)

1969–98 mean

annual NOAA

station rainfall

(inches) 1996 1997 1998

1996–98

mean annual

12.86 38.08 26.65 25.86 29.80
8 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

0

2400 1200

September 22 September 23 September 24

4

3

2

1

CUMULATIVE RAINFALL, IN INCHES

14

12

10

8

6

4

2

0

DISCHARGE, IN CUBIC FEET PER SECOND

1200 2400 1200

Cumulative rainfall

Discharge

Subsample collection

1969–98 mean annual rainfall

 29.80 inches

50

40

30

20

10

0

RAINFALL, IN INCHES

1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997

Figure 5. Annual rainfall at National Oceanic and Atmospheric Administration Kingsville weather station, 1969–98.

Figure 4. Hydrograph showing rainfall, discharge, and subsample-collection timing at site 4 during storm event of 

September 22–24, 1998.
WATER QUALITY 9

The study area rainfall of 12.86 in. during 1996 

was less than any individual annual rainfall during 

1969–98 at the NOAA station (fig. 5). The 1997 study 

area rainfall exceeded the NOAA station mean annual 

rainfall because of heavy April and October rains. 

However, the study area received only 0.84 in. of rain 

during June–August (8.75 in. less than the NOAA mean 

monthly rainfall for June–August 1969–98, shown in 

fig. 6). In 1998, study area rainfall was less than the 

NOAA mean annual rainfall, despite heavy September 

and October rains.

The NOAA station mean monthly rainfall (1969–

98) indicates a bimodal rainfall pattern, with May–June 

and August–October generally receiving the greatest 

rainfall (fig. 6). Heavy rainfall during August–October 

often is associated with tropical disturbances originating from the Gulf of Mexico.

A graph of the deviation of monthly study area 

rainfall during 1996–98 from the NOAA station mean 

monthly rainfall during 1969–98 shows that the study 

area rainfall was below the NOAA mean monthly during 24 months (fig. 7). Even during 1997, when the 

study area received above-average rainfall, rainfall was 

below average during 6 months. This pattern of periods 

of below-average rainfall punctuated by extreme rainfall events is typical of historical patterns (Ward, 1997). 

Runoff

Runoff conditions during the study period 

generally corresponded to the rainfall pattern, with runoff events interspersed between long periods of no runoff. During 1996–98, 77.59 in. of rain fell on the study 

area resulting in 12.05 in. (fig. 8) or 50,300 acre-feet 

(acre-ft) of runoff. The runoff coefficient (ratio of runoff 

volume to rainfall volume) for the entire study period is 

0.16. Annual coefficients were less than 0.01 in 1996, 

0.21 in 1997, and 0.15 in 1998. 

During the study, 18 separate runoff events 

(runoff at one or more of the five sites) were recorded. 

Table 3 (at end of report) lists the dates of the runoff 

events at each of the five monitoring sites; study area 

rainfall for each event (Theissen average); runoff volume at each site; total runoff exiting the study area; and 

runoff coefficients for individual events. Runoff coefficients for individual events ranged from less than 0.001 

(Aug. 23–25, 1996, and Aug. 31–Sept. 1, 1996) to 0.488 

(Oct. 18–31, 1998).

In the study area, runoff largely depends on rainfall, rainfall intensity, and antecedent soil moisture conditions. The combination of climate, soil type, and 

agricultural practices typically results in runoff only 

after rainfall of 2 in. or more, depending on antecedent 

conditions. Daily rainfalls of 2.71 and 1.96 in. (Sept. 21, 

1997, and Aug. 14, 1998, respectively) resulted in no 

runoff because of dry antecedent soil conditions. In contrast, rainfall of 0.37 in. (Feb. 21–22, 1998) produced 

runoff (at only one site) during wet antecedent soil conditions (runoff had occurred 7 days previously). Most of 

the total volume of runoff during the study period 

resulted from relatively extreme storm events (greater 

than 5.0 in. of rainfall). The table below shows the percentage of runoff that resulted from specific amounts of 

event rainfall.

Distribution of 18 runoff events by rainfall amount, 

1996–98 

[>, greater than]

During the study, the five largest storm events 

contributed about 38 percent of the total rainfall for the 

study area and 94 percent of the total runoff (table 3). 

During 1996, the study area received only about 13 in. 

of rain; furthermore, no daily rainfall was greater than 

1.5 in., resulting in very little runoff. During 1997, rainfall of 5.50 in. on Apr. 2–13, 2.54 in. on May 15–19, and 

11.16 in. on Oct. 10–24, accounted for about 97 percent 

of the annual runoff. During 1998, rainfall of 4.62 in. on 

Sept. 17–Oct. 4 and 5.45 in. on Oct. 18–31, accounted 

for about 90 percent of the annual runoff. During 1996–

98, there were no runoff events during June, historically 

one of the months with the greatest rainfall. 

Most of the runoff from the study area exited 

through site 5. The total runoff from the study area during 1996–98 was 50,300 acre-ft (table 3). The runoff 

exiting the study area through site 5 (46,900 acre-ft) was 

93 percent of the total study area runoff.

WATER QUALITY

Rainfall and runoff quality were investigated by 

statistical analysis of EMCs; computation of constituent 

Event

rainfall

(inches):

0

 to

0.99

1.00

to

1.99

2.00

to

2.99

3.00

to

3.99

4.00

to

4.99

>5.00

Number of 

events

3 9 20 1 3

Runoff (acrefeet)140 2,040 3,510 0 3,980 40,600

Percent of 

total study 

area runoff

.28 4.1 7.0 0 7.9 81
10 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

10

8

6

4

2

0

-2

DEVIATION FROM NOAA MEAN MONTHLY RAINFALL, IN INCHES

-4

1996 1997

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

1998

5

4

3

2

1

0

RAINFALL, IN INCHES

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

Figure 6. Mean monthly rainfall at National Oceanic and Atmospheric Administration Kingsville weather station, 

1969–98.

Figure 7. Deviation of monthly study area rainfall from mean monthly rainfall at National Oceanic and Atmospheric 

Administration Kingsville weather station, 1969–98.
WATER QUALITY 11

deposition, loads, and yields; and determination of 

average study area constituent concentrations. Also, 

amounts of fertilizer and pesticide applications were 

compared with amounts of chemicals present in rainfall 

and runoff.

Rainfall

During 1996–98, 19 rainfall samples were 

collected at the TAES weather station and analyzed 

(appendix 1); 13 rainfall events that produced runoff 

were sampled and 6 other selected rainfall events that 

did not produce runoff were sampled. During the study 

period, 75.74 in. of rainfall were recorded at the TAES 

station. The 19 samples represent 28.84 in. of rainfall, 

or about 38 percent of the total rainfall (recorded at the 

TAES station) during the study period.

Event-Mean Nutrient Concentrations

Descriptive statistics computed from EMCs of 

selected nutrients in the rainfall samples are shown in 

table 4 (at end of report). Most of the nitrogen is in the 

form of dissolved ammonia (NH4) and dissolved nitrate 

(NO3), which were detected in all samples. The median 

ammonia and nitrate concentrations were 0.15 and 

0.10 milligram per liter (mg/L), respectively. Dissolved 

nitrite (NO2) and dissolved and total forms of organic 

nitrogen were detected in about one-half of the samples. 

Nitrite concentrations were less than the laboratory 

minimum reporting level (0.001 mg/L) in eight samples; the median concentration was 0.001 mg/L. The 

median total organic nitrogen concentration was 

0.02 mg/L.

Total phosphorus was detected in all of the rainfall samples; the median concentration was 0.005 mg/L. 

Most of the phosphorus was dissolved phosphorus 

(median concentration 0.004 mg/L). Orthophosphate 

phosphorus was detected above the minimum reporting 

level (0.001 mg/L) in nine samples; the maximum concentration was 0.02 mg/L.

Rainfall Deposition

The deposition of rainfall constituents (in pounds 

per acre) can be defined as the product of the event constituent concentration and the rainfall volume. Daily 

rainfall deposition on the lower study area was computed for total nitrogen, dissolved ammonia, dissolved 

80

70

60

50

40

30

20

10

0

RAINFALL AND RUNOFF, IN INCHES

1996 1997 1998

Cumulative rainfall

Cumulative runoff

1996–98 study area rainfall: 77.59 inches

1996–98 study area runoff: 12.05 inches

Figure 8. Hydrograph showing cumulative rainfall and runoff in study area, 1996–98.
12 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

nitrate, and total phosphorus. For rainfall events during 

which rainfall samples were collected and analyzed, 

thus providing EMCs, daily deposition was computed 

as:

D = EMC x R x Cf, (1)

where 

D = daily deposition, in pounds per acre;

EMC = nutrient concentration in rainfall, in milligrams per liter;

R = study area daily rainfall, in inches; and 

Cf = conversion factor of 0.2266.

For unsampled events, in which rainfall constituent concentration data were not available, daily deposition of total nitrogen, ammonia, and nitrate was 

computed by regression equations that relate daily rainfall and daily nutrient deposition. The regression equations were developed from a larger database that 

includes data from this study along with rainfall nitrogen data collected from another location in the region 

(Ockerman and Livingston, 1999). Daily rainfall deposition of total phosphorus for unsampled events was 

computed using the median total phosphorus concentration in the rainfall samples (table 4). The daily rainfall 

depositions of nitrogen and phosphorus were aggregated to produce monthly and annual rainfall depositions of nitrogen and phosphorus to the study area as 

shown in table 5 (at end of report). 

Applying the deposition rates (table 5), 465,000 

pounds (lb) of total nitrogen were delivered to the 

49,680-acre study area during 1996–98. Ammonia and 

nitrate accounted for 88 percent of the total nitrogen 

rainfall deposition. Compared to total nitrogen, total 

phosphorus deposition from rainfall was small, 4,870 lb 

delivered to the study area during 1996–98. 

Runoff 

Event-Mean Concentrations

During August 1996–October 1998, 29 runoff 

event samples were collected at sites 1–5 and analyzed 

to characterize the quality of runoff from the study 

area. Runoff EMCs, by site and event, are listed in 

appendix 2.

Nutrients and Major Inorganic Ions

The first step in analyzing nutrient and major 

inorganic ion concentrations in runoff was to test for 

statistically significant differences in concentrations 

among sampling sites. The Kruskall-Wallis test (Helsel 

and Hirsch, 1992, p. 159), a nonparametric test, 

compares the medians of groups differentiated by one 

explanatory variable (in this case, sampling site) to 

determine whether all groups have the same median 

(null hypothesis), or whether at least one median is different (alternative hypothesis). The p-values2 of each 

test are shown in the table below. A statistical difference 

in concentrations among sites was deemed to exist when 

p-values were less than 0.05.

Results of Kruskal-Wallis test for statistical differences 

in nutrient and major inorganic ion concentrations in 

runoff samples among sites 1–5 

The results of the Kruskall-Wallis tests indicate a 

significant difference in all major inorganic ion concentrations and ammonia nitrogen concentrations among 

sites but no significant difference in other nutrient concentrations. Because the major inorganic ion EMCs at 

site 5 were greater than major inorganic ion EMCs at all 

other sites (appendix 2), the Mann-Whitney test (Helsel 

and Hirsch, 1992, p. 118) was used for further comparison of the major inorganic ion data from site 5 with the 

data from sites 1–4. The Mann-Whitney test, also a nonparametric test similar to the Kruskal-Wallis test, is 

intended to compare only two datasets. Results of this 

test indicated major inorganic ion concentrations were 

2 The p-value is the “attained level of significance” (the 

significance level attained by the data), which is the probability of 

obtaining the computed test statistic, or one even less likely, when 

the null hypothesis is true (Helsel and Hirsch, 1992, p. 108).

Constituent p-value

Is there a

difference

 in concentrations

among sites?

Nitrogen, total 0.22 No

Ammonia nitrogen, dissolved (as N) .02 Yes

Nitrate + nitrite nitrogen, dissolved (as N) .75 No

Ammonia + organic nitrogen, total (as N) .28 No

Phosphorus, total .72 No

Calcium, dissolved .02 Yes

Magnesium, dissolved .003 Yes

Potassium, dissolved .01 Yes

Chloride, dissolved .001 Yes

Sulfate, dissolved .001 Yes
WATER QUALITY 13

significantly greater at site 5 compared with sites 1–4, 

as shown in the table below.

Results of Mann-Whitney test for statistical differences 

in major inorganic ion concentrations in runoff samples 

between sampling sites 1–4 and site 5 

A likely cause of greater major inorganic ion 

concentrations at site 5, compared to those at sites 1–4, 

is the proximity of site 5 to Baffin Bay. Sites 1–4 are 

located on canals that carry only freshwater runoff. 

However, the channel at site 5 is close enough to Baffin 

Bay that the channel occasionally is inundated by bay 

water (with greater major inorganic ion concentrations; 

Hem, 1985, p. 7)3. Deposits of major ions in bed sediment of Tunas Creek could be washed out during runoff 

events and contribute to the greater concentrations in the 

runoff. 

Discharges of oil-field brines also have occurred 

previously in Tunas Creek4 and nearby Petronilla Creek 

(the confluence of Tunas and Petronilla Creeks is about 

3 mi downstream of site 5). Brine discharges (with chloride concentrations exceeding 50,000 mg/L, Armstrong 

and Ward, 1998) over the years could have saturated 

channel sediments and might continue to be flushed out 

during runoff events5

.

Concentrations of major inorganic ions at site 5 

are not considered representative of cropland runoff. 

The difference between distribution of sulfate concentrations at sites 1–4 and site 5 are shown by boxplots in 

figure 9. 

Constituent p-value

Are concentrations

greater at site 5

than at sites 1–4?

Calcium, dissolved 0.004 Yes

Magnesium, dissolved .0001 Yes

Potassium, dissolved .022 Yes

Chloride, dissolved .0002 Yes

Sulfate, dissolved .0008 Yes

3 During August 1998, salt deposits on the channel bed and 

banks were observed along the reach of Tunas Creek where site 5 is 

located, likely caused by evaporation of bay water, which had inundated the creek during high tides.

4 Until 1987, 24 permitted brine discharge points into Tunas 

Creek combined for a permitted discharge of 1.47 acre-ft per day 

(Armstrong and Ward, 1998).

5 Fifty years of continual brine discharges to Petronilla Creek 

ended in 1987. Instream salinity values near discharge sites did not 

recover quickly, suggesting brine constituents are recalcitrant and 

reside in sediment after discharge (Texas Natural Resource Conservation Commission, 1996).

(20)

(7)

1,000

100

10

1

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

SITES 1–4 SITE 5

(20)

Largest data value within 1.5 times 

the IQR above the box

75th percentile

Median (50th

 percentile)

25th percentile

Interquartile

 range (IQR)

Smallest data value within 1.5

times the IQR below the box

Data value 1.5 to 3.0 times 

the IQR outside the box

EXPLANATION

Number of samples

Figure 9. Distribution of sulfate concentrations in 

runoff samples from sites 1–4 and site 5, 1996–98.
14 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

The difference in ammonia concentrations among 

sites also was further examined. Concentrations at the 

upper study area sites 1 and 2 were significantly less 

than concentrations at the lower study area sites 3, 4, 

and 5 (Mann-Whitney test, p-value 0.0007). However, 

the cause of the difference was not apparent. 

The next step in analyzing major ion and 

nutrient concentrations was to perform statistical 

tests to identify seasonal trends in concentrations. 

Major ion (except for site 5) and nutrient concentrations 

from runoff samples were grouped into two seasonal 

categories—pre-harvest and post-harvest. The preharvest season includes February–May, and the postharvest season includes August–November. All 

runoff events during the study period were in these 

two seasonal categories—13 runoff samples were collected during the pre-harvest season and 16 runoff 

samples were collected during the post-harvest season. 

The Mann-Whitney test was used to determine whether 

one season tends to produce greater concentrations 

than the other season. Test results of median concentrations did not indicate statistically significant seasonal 

differences for any of the major ion or nutrient concentrations, although the p-value for ammonia, 0.07, indicates a possible seasonal difference. 

Results of Wilcoxon rank-sum test for statistical 

seasonal differences in nutrient and major inorganic ion 

concentrations in runoff samples 

The final step in analyzing concentrations was to 

produce summary statistics of the data to characterize 

cropland runoff-event concentrations. Data from all 

sites, except major inorganic ion concentrations from 

site 5, were combined, and summary statistics were generated (table 6 at end of report). Table 6 also lists, for 

comparative purposes, TNRCC Texas Surface Water 

Quality Standards (TSWQS) for aquatic life protection 

or human health protection for nitrite plus nitrate nitrogen and total phosphorus.

Pesticides

The runoff samples were analyzed for a suite 

of 88 pesticides, some of which are not used in the 

study area (appendix 2). The pesticides that were 

applied on the lower study area during 1996–98 are 

listed in table 7 (at end of report). The table also indicates which pesticides were analyzed in the runoff 

samples. 

One measure of pesticide occurrence is the frequency of detection among all samples. The bar graphs 

in figure 10 show the percentage of samples with herbicide and insecticide detections. The runoff samples had 

detections of 21 pesticides (15 herbicides and 6 insecticides). The herbicide atrazine and its breakdown product deethylatrazine were detected in all samples. Other 

pesticides detected in at least 25 percent of the samples 

were the herbicides trifluralin, simazine, pendimethalin, 

diuron, and metolachlor and the insecticide malathion. 

A statistical summary of concentrations of pesticides 

detected in the runoff samples and selected waterquality standards are listed in table 8 (at end of report).

Certain pesticide concentrations exhibited 

significant seasonal differences, probably because of 

the timing of pesticide application. The boxplots in 

figure 11 show the distribution of atrazine concentrations in runoff samples grouped by the two seasonal categories. The median pre-harvest, median post-harvest, 

and median of all samples for selected pesticides are 

listed in table 9 (at end of report). 

Suspended Sediment

Five suspended sediment samples were collected 

during two runoff events at site 5 as shown in the table 

below. Sand-break analysis was performed on three of 

the five sediment samples. Sediment composition, by 

weight, ranged from 98.4 to 99.7 percent silt and clay. 

Suspended sediment concentrations in runoff samples 

collected at site 5

[--, not determined]

Constituent p-value

Is there a

seasonal

difference?

Nitrogen, total 0.29 No

Ammonia nitrogen, dissolved (as N) .07 Maybe

Nitrite + nitrate nitrogen, dissolved (as N) .77 No

Phosphorus, total .52 No

Phosphorus, dissolved .29 No

Orthophosphate phosphorus, dissolved (as P) .40 No

Calcium, dissolved .34 No

Magnesium, dissolved .17 No

Potassium, dissolved .30 No

Chloride, dissolved .12 No

Sulfate, dissolved .24 No

Date

Discharge 

(cubic feet 

per second)

Concentration

(milligrams

per liter)

Percent silt 

and clay 

(by weight)

04/03/97 3,230 812 98.4

04/03/97 2,380 839 99.4

04/04/97 990 841 99.7

05/16/97 750 570 --

05/17/97 225 327 --
WATER QUALITY 15

The sediment concentrations from these samples 

(shown above) are not EMCs but represent sediment 

concentrations at the time of sample collection. 

Sediment concentrations are a function of stream discharge and other factors, including rainfall duration 

and intensity and tillage practices. The relation between 

suspended sediment concentration and discharge for 

runoff samples collected at site 5 is shown in figure 12. 

The equation describing this relation is: 

C = 0.0925 x Q 1.25, (2)

where

C = sediment concentration in milligrams per liter, 

and 

Q = stream discharge in cubic feet per second.

Equation 2 was used to compute discharge-weighted 

EMCs for sediment at site 5 for the seven events for 

which runoff samples were collected for water-quality 

analysis. The sediment EMCs are shown below:

Calculated EMCs of suspended sediment in selected 

runoff samples collected at site 5 

Constituent Loads and Yields

The load of a constituent in runoff is the mass 

of a given constituent transported past a site during a 

Date Runoff

(acre-feet)

EMC

(milligrams per liter)

09/96 82 199

03/97 38 172

04/97 10,100 727

05/97 2,700 543

10/97 18,000 759

09/98 3,420 509

10/98 10,300 688

1-Naphthol

2,4,5-T

2,4-DB 2,6-Diethylaniline

Acetochlor

Acifluorfen

Alachlor

Amiben

Benfluralin

Bentazon

Bromacil Bromoxynil Butylate

Chloramben

Chlorothalonil Clopyralid

Dacthal mono-acid

Dicamba

Dichlobenil

Dichlorprop

Dinoseb

DNOC

EPTC

Esfenvalerate

Ethalfluralin

Fenuron

Linuron

MCPB

MCPA

Metribuzin

Molinate Napropamide

Neburon

Norflurazon Oryzalin

Pebulate

Picloram

Pronamide

Propachlor Propanil

Propham

Silvex

Tebuthiuron

Terbacil

Thiobencarb

Triallate

Triclopyr

Cyanazine

Glyphosate

Lindane

Prometon

Fluometuron

2,4-D

DCPA

Metolachlor

Diuron

Pendimethalin

Simazine

Trifluralin Deethylatrazine

Atrazine

HERBICIDES

0 20 40 60 80 100

DETECTION FREQUENCY, 

IN PERCENT

Figure 10. Herbicide and insecticide detection 

frequency in 29 runoff samples, 1996–98.

Aldicarb

Aldicarb sulfone

Aldicarb sulfoxide

Chlorpyrifos

DDE

Dieldrin

Disulfoton

Ethoprop

Fonofos

Methiocarb

Methomyl

Oxamyl

Permethrin

Parathion

Methyl parathion

Propoxur

Propargite

Phorate

Terbufos

3-Hydroxycarbofuran

Diazinon

Carbaryl

Azinphos-methyl

Alpha-BHC

Carbofuran

Malathion

0 20 40 60 80 100

DETECTION FREQUENCY, 

IN PERCENT

INSECTICIDES
16 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

specified time. Daily nutrient and pesticide loads in 

runoff were computed at each site from runoff and 

concentration data. For runoff events that were sampled 

and for which EMCs were determined, the daily constituent load at a particular site is (Ockerman and others, 

1999):

Ln = EMC x V x Cf, (3)

where

Ln = constituent load, in pounds per day at site n;

EMC = event-mean concentration during runoff 

event, in milligrams per liter or 

micrograms per liter;

V = daily runoff, in acre-feet; and 

Cf = conversion factor, 2.719 for concentrations 

in milligrams per liter or 0.00272 for 

concentrations in micrograms per liter. 

For unsampled events, median EMCs for samples 

collected during the study were used in equation 3 to 

compute daily loads. Runoff loads from the total study 

area are the sum of loads from sites 3, 4, and 5. Net runoff loads from the lower study area are total study area 

loads (sites 3, 4, and 5) minus loads from the upper 

study area (sites 1 and 2). Daily loads were summed to 

compute monthly and annual loads. 

Constituent yield, a measure of the load-producing characteristics of a watershed, is computed by dividing the load by the drainage area of the watershed:

Y = L / DA, (4)

where

Y = constituent yield, in pounds per acre per 

month (or year);

L = constituent load exiting the watershed, in 

pounds per month (or year); and

DA = contributing area of the watershed, in acres.

Sediment loads for the entire study area were 

computed from regression equations based on sediment 

concentration and load data collected at site 5. Sediment 

yields were computed from the estimated loads using 

equation 4.

Nutrients

Nutrient concentrations estimated from 

median EMCs (table 6) were used in equation 3 to 

compute daily loads for runoff events in which waterquality samples were not collected. Unsampled events 

account for about 5 percent (513 acre-ft of the total of 

Interquartile range (IQR)

75th percentile

Median (50th percentile)

25th percentile

(16)

Largest data value within 1.5

times the IQR above the box

Data value 1.5 to 3.0 times

the IQR outside the box

Data value more than 3.0

times the IQR outside the box

Number of samples

EXPLANATION

Smallest data value within 1.5

times the IQR below the box

100

10

1

0.1

0.01

ATRAZINE CONCENTRATION, IN MICROGRAMS PER LITER

PRE-HARVEST

(FEB.–MAY)

POST-HARVEST

(AUG.–OCT.)

(13)

(16)

Figure 11. Seasonal distribution of atrazine 

concentrations in runoff samples, 1996–98.
WATER QUALITY 17

9,920 acre-ft) of the runoff in the upper study area and 

about 7 percent (3,480 acre-ft of the total of 50,300 

acre-ft) of the runoff exiting the study area during 1996–

98. The monthly and annual loads of selected nutrients 

in runoff are listed in table 10 (at end of report), and the 

corresponding annual yields are listed in table 11 (at end 

of report). The tables list loads and yields for the upper 

and lower study area as well as for the entire study area. 

Annual runoff loads and yields of nutrients were 

highly variable, corresponding to the highly variable 

annual runoff. The total nitrogen load exiting the study 

area in 1996 was 800 lb compared to 102,000 lb in 1997 

and 81,500 lb in 1998. The average annual load of total 

nitrogen for 1996–98 was 61,400 lb. The annual total 

phosphorus loads were 180 lb in 1996, 46,600 lb in 

1997, and 21,400 lb in 1998, for an average annual load 

for 1996–98 of 22,700 lb.

The total nitrogen yields for the study area were 

0.02, 2.1, and 1.6 pounds per acre per year (lb/acre-yr) 

in 1996, 1997, and 1998, respectively. The average 

annual yield for 1996–98 was 1.2 pounds per acre 

(lb/acre). The total phosphorus yields were <0.01, 

0.94, and 0.43 lb/acre-yr in 1996, 1997, and 1998, 

respectively, for an average annual yield for 1996–98 

of 0.46 lb/acre.

Net runoff yields of total nitrogen and total phosphorus for the lower study area were compared to nutrient inputs from fertilizer applications and rainfall 

deposition, as shown in the table below:

Comparison of fertilizer applications, rainfall 

deposition, and runoff yields of nitrogen and 

phosphorus for the 40,540-acre lower study area

[In pounds per acre per year; <, less than] 

Year or

period

Nitrogen

fertilizer

application

Rainfall

deposition

of nitrogen

Runoff yield

of nitrogen

Total nitrogen

1996 45 2.1 0.02

1997 73 3.9 2.1

1998 28 3.3 1.7

1996–98 average 49 3.1 1.3

Year or

period

Phosphorus

fertilizer

application

Rainfall

deposition of

phosphorus

Runoff

yield of

phosphorus

Total phosphorus

1996 0 0.02 <0.01

1997 7.1 .04 1.0

1998 5.4 .03 .45

1996–98 average 4.2 .03 .48

C = 0.0925 x Q1.25

Sediment-discharge relation

Sample

1,000

900

800

700

600

500

400

300

200

100

0

SUSPENDED SEDIMENT CONCENTRATION, IN

MILLIGRAMS PER LITER

0 500 1,000 1,500 2,000 2,500 3,000 3,500

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 12. Relation between suspended sediment concentration (C) and discharge (Q) in runoff samples 

collected at site 5.
18 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

Nitrogen input from fertilizer is much larger 

than all other inputs or outputs of nitrogen in the 

study area. Average annual application of fertilizerbased nitrogen in the lower study area was 49 lb/acre6

compared to 3.1 lb/acre from rainfall deposition and 

1.3 lb/acre in net runoff. Annual rainfall deposition of 

nitrogen on the lower study area exceeds runoff yields 

of nitrogen exiting the study area because most of the 

rainfall does not contribute to runoff. The average 

annual net runoff yield of nitrogen exiting the lower 

study area represents about 2.5 percent of the nitrogen 

entering the lower study area as fertilizer and in rainfall 

deposition. However, the forms of nitrogen entering the 

lower study area are different from the forms exiting 

the area in runoff. Most of the nitrogen applied as fertilizer is in the form of ammonia and nitrate, and rainfall 

nitrogen also consists mainly of ammonia and nitrate 

(table 5). Annual net runoff loads of total nitrogen exiting the lower study area are primarily (about 77 percent) 

organic nitrogen (table 10). Also, the organic nitrogen is 

mostly in particulate form7 (crop residue), rather than 

dissolved organic nitrogen. 

As with nitrogen, the average annual application 

of fertilizer-based phosphorus exceeds other sources 

of phosphorus entering or exiting the lower study 

area. Average annual application of fertilizer-based 

phosphorus in the lower study area was 4.2 lb/acre. 

The fertilizer is applied as soluble orthophosphate 

phosphorus. The average annual net runoff yield of 

phosphorus exiting the lower study area was 0.48 

lb/acre (or about 11 percent of phosphorus entering as 

fertilizer and in rainfall deposition). About 21 percent 

(0.10 lb/acre) of the phosphorus exiting the study 

area is orthophosphate. Most of the phosphorus in 

runoff was particulate in nature, associated with crop 

debris and eroded soils that contain phosphorus (Hem, 

1985, p. 126). Phosphorus inputs from rainfall during 

1996–98 were relatively small, with an average annual 

deposition of 0.03 lb/acre. 

Comparisons of sources of nitrogen and phosphorus also were made for an individual runoff event (April 

1997). Various forms of nitrogen in rainfall deposition 

and runoff yields for the lower study area are shown in 

figure 13. Rainfall deposition of total nitrogen was 

0.15 lb/acre compared to runoff yield of 1.18 lb/acre. 

Similar to annual trends, most of the rainfall nitrogen 

was ammonia and nitrate while most of the runoff 

nitrogen was particulate organic nitrogen. The ammonia 

nitrogen was greater in rainfall deposition than in 

runoff yield, possibly indicating that some of the rainfall 

ammonia might undergo a transformation to other 

forms of nitrogen, such as nitrate, during the runoff 

process. 

During the April 1997 event, the net runoff 

yield of total phosphorus exiting the lower study 

area was 0.41 lb/acre, including 0.08 lb/acre of dissolved orthophosphate. Rainfall deposition of total 

phosphorus was 0.004 lb/acre, including 0.003 lb/acre 

of orthophosphate. The ratio of orthophosphate to total 

phosphorus in runoff during this event is similar to that 

observed in runoff during the 1996–98 study. 

Pesticides

Runoff loads of three pesticides (atrazine plus 

deethylatrazine, diuron, and trifluralin) were computed 

similarly to loads of nutrients. Loads for unsampled 

events were computed using the seasonal median concentrations (table 9). Monthly and annual loads of the 

three pesticides are listed in table 12 (at end of report), 

and the corresponding annual yields are listed in table 

13 (at end of report). The tables list loads and yields for 

the upper and lower study area as well as for the entire 

study area. 

The net runoff yield of dissolved atrazine plus 

deethylatrazine exiting the lower study area was computed (table 13) and compared to applications of atrazine in the lower study area (table 2). During the study 

period, atrazine applications in the lower study area 

ranged from 0.26 to 0.43 lb/acre-yr as shown in the table 

below:

Comparison of applications and runoff yields of 

atrazine for the 40,540-acre lower study area 

[In pounds per acre] 6 Fertilizer application is computed as the total mass (pounds) 

of active ingredients divided by the total acreage of the lower study 

area, including 3,932 acres of rangeland, which do not actually 

receive any nitrogen applications.

7 The EMCs of dissolved organic nitrogen at all five sites 

during the April 1997 runoff event were below the minimum reporting level of 0.2 mg/L; however, total (dissolved and particulate) 

organic nitrogen concentrations at the five sites ranged from 0.77 to 

2.83 mg/L. 

Year or

period

Atrazine

application

Net runoff yield

of atrazine plus

deethylatrazine

1996 0.26 0.0003

1997 .42 .00876

1998 .43 .0003

1996–98 average .37 .0027
WATER QUALITY 19

0.15

0.02 0.04

0.14

<0.01

1.01

1.18

Rainfall deposition

Runoff yield

1.2

1.0

0.8

0.6

0.4

0.2

0

POUNDS PER ACRE

NITROGEN, AMMONIA NITROGEN, NITRITE

PLUS NITRATE

NITROGEN, ORGANIC NITROGEN, TOTAL

0.10

Figure 13. Comparison of rainfall deposition and runoff yield of nitrogen during April 2–13, 1997, lower study area.

Overall, during 1996–98, 45,500 lb of atrazine 

were applied (0.37 lb/acre-yr), and about 330 lb of dissolved atrazine plus deethylatrazine exited the lower 

study area in net runoff (about 0.0027 lb/acre-yr). The 

1996–98 net runoff yield was about 0.7 percent of the 

total atrazine applied to the cropland during that period. 

Most of the atrazine runoff loads occurred during April 

and May 1997. As a result, the 1997 runoff yield of 

atrazine was about 1.8 percent of the atrazine applied 

that year. 

Sediment 

The sediment loads for all runoff events at site 5 

were computed using equation 3 and the EMCs determined earlier. The event sediment loads are compared 

with event runoff in figure 14. The equation expressing 

the relation between sediment load and runoff is:

Ls = 56.1 x R 0.35, (5)

where 

Ls = sediment load in tons, and 

R = event runoff in acre-feet.

Equation 5 was used to estimate sediment loads 

for runoff events at sites 3 and 4. The total (sites 3, 4, 

and 5) monthly and annual loads of sediment in runoff 

exiting the study area are listed in table 14 (at end of 

report), and the corresponding annual yields are listed in 

table 15 (at end of report).

Sediment loads exiting the study area during 

runoff events varied widely, ranging from 0.90 ton in 

August 1996 to 19,000 tons in October 1997. Sediment 

loads from the study area are dominated by storm 

events—87 percent of the 45,300 tons of sediment 

transported from the study area during 1996–98 

occurred during three events (April 1997, October 1997, 

and October 1998). The annual sediment yield was 2.2 

lb/acre in 1996 compared to 1,270 lb/acre in 1997 with 

a 1996–98 average yield of 610 lb/acre-yr. 

Runoff-Weighted Average Concentrations

The runoff-weighted average concentration is 

computed by dividing the load of a constituent exiting 

the study area by the study area runoff to take into 

account the runoff at each site. Runoff-weighted 

average concentrations are considered a way to express 
20 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

the total watershed load in terms of a concentration 

value. The runoff-weighted average concentration can 

be determined for a single runoff event or for a longer 

period, such as a month or year. For example, because 

the runoff at site 5 was always much greater than the 

runoff from sites 3 and 4, the corresponding event concentrations at site 5 will be weighted more heavily in 

computing the average concentrations of constituents in 

runoff exiting the study area. Computation of annual (or 

longer) runoff-weighted average concentrations will be 

influenced more by concentration data from larger runoff events than by concentration data from smaller runoff events.

Nutrients

Study area runoff-weighted nutrient concentrations computed by month, year, and entire study period 

are listed in table 16 (at end of report). The monthly runoff-weighted concentrations generally correspond to 

single events. The annual and entire study period concentrations represent long-term runoff water quality. 

The 1996–98 average runoff-weighted concentration for total nitrogen (1.3 mg/L) is smaller than either 

the mean (1.6 mg/L) or the median (1.4 mg/L) EMC 

because of the influence of large runoff events with 

smaller concentrations. Similarly, the 1996–98 nitrite 

plus nitrate runoff-weighted concentration (0.22 mg/L) 

is smaller than either the mean (0.36 mg/L) or the 

median (0.27 mg/L) EMC. However, the total phosphorus runoff-weighted concentration (0.50 mg/L) is 

slightly larger than either the mean (0.48 mg/L) or the 

median (0.46 mg/L) EMC because much of the total 

phosphorus is associated with sediment, and larger 

sediment concentrations associated with greater runoff 

events result in the larger runoff-weighted average 

concentrations.

Pesticides

Runoff-weighted concentrations computed for 

three pesticides are listed in table 17 (at end of report). 

For EMCs less than the minimum reporting level, the 

minimum reporting level was used to compute the event 

runoff loads that then were used to compute runoffweighted average concentrations.

Although atrazine was the most frequently 

detected pesticide, the herbicide diuron had the largest 

Power equation fit

Site 5 events

20,000

15,000

10,000

5,000

0

SUSPENDED SEDIMENT LOAD, IN TONS

0 5,000 10,000 15,000 20,000

RUNOFF, IN ACRE-FEET

Figure 14. Relation between suspended sediment load and storm event runoff at site 5, 1996–98.
SUMMARY 21

runoff-weighted concentration, 2.9 micrograms per 

liter (g/L) for 1996–98. Nearly all (99 percent) of the 

diuron runoff occurred during pre-harvest events in 

1997. The April and May 1997 runoff-weighted diuron 

concentrations were 12 and 4.9 g/L, respectively. The 

1996–98 runoff-weighted atrazine plus deethylatrazine 

concentration was 2.7 g/L. Most of the atrazine runoff 

also occurred during the pre-harvest season in 1997, 

resulting in April and May 1997 runoff-weighted 

concentrations of 7.6 and 11 g/L, respectively.

SUMMARY

From January 1996 to December 1998, rainfall 

and runoff were monitored at five streamflow-gaging 

stations and one weather station in a 49,680-acre 

agricultural area in Kleberg and Nueces Counties in the 

Baffin Bay watershed of South Texas. Nineteen rainfall 

samples were collected at the weather station and analyzed for nutrients, and 29 event-composite runoff samples were collected at the streamflow stations and 

analyzed for major ions, nutrients, and pesticides.

Total rainfall for 1996–98 averaged 25.86 in. per 

year, which is less than the long-term annual average 

rainfall of 29.80 in. for the area. Historically, rainfall in 

the region can be characterized by long periods of 

below-average rainfall punctuated by extreme rainfall 

events, with the heaviest rainfall generally occurring 

during May–June and August–October. Study area rainfall during 1996–98 was typical of this pattern. Rainfall 

was below average for 1996 (12.86 in.), but extreme 

rainfall events during 1997 and 1998 resulted in aboveaverage rainfall for 1997 and near-average rainfall for 

1998.

The combination of climate, soil type, and agricultural practices typically results in runoff only after 

rainfall of 2 in. or more, depending on antecedent conditions. Extreme rainfall events are the most important 

factor affecting the amount and timing of runoff and 

associated constituent loads. During 1996, the belowaverage rainfall coupled with no daily rainfall greater 

than 1.5 in. resulted in very little runoff. During 1997, 

rainfall of 5.50 in. on Apr. 2–13, 2.54 in. on May 15–19, 

and 11.16 in. on Oct. 10–24 accounted for about 97 percent of the annual runoff. Similarly, during 1998, rainfall of 4.62 in. on Sept. 17–Oct. 4 and 5.45 in. on Oct. 

18–31 accounted for about 90 percent of the annual runoff. Overall, five rainfall events during the study period 

accounted for about 38 percent of the total rainfall and 

94 percent of the total runoff. Annual runoff coefficients 

(ratio of runoff volume to rainfall volume) were less 

than 0.01 in 1996, 0.21 in 1997, and 0.15 in 1998.

Rainfall on the 40,540-acre lower study area 

during 1996–98 deposited an average of about 3.1 

lb/acre-yr of total nitrogen compared to 49 lb/acre-yr 

from fertilizer. Rainfall on the lower study area during 

1996–98 deposited an average of about 0.03 lb/acre-yr 

of total phosphorus compared to 4.2 lb/acre-yr applied 

as fertilizer. 

Annual net runoff yields computed for total nitrogen and total phosphorus exiting the lower study area 

were slightly greater than yields computed for the entire 

49,680-acre study area. Net runoff yields of nitrogen 

exiting the lower study area, ranging from 0.02 lb/acre 

in 1996 to 2.1 lb/acre in 1997 with an average of 1.3 

lb/acre-yr, are less than that entering through rainfall. 

Total nitrogen in runoff from the lower study area is 

about 2.5 percent of the combined fertilizer and rainfall 

nitrogen entering the lower study area during 1996–98. 

While most of the nitrogen entering the study area 

through rainfall or fertilizer is ammonia and nitrate, 

most of the nitrogen in runoff exiting the study area is 

particulate organic nitrogen. The major source of the 

organic nitrogen likely is crop residue. Net runoff yields 

of phosphorus exiting the lower study area ranged from 

<0.01 lb/acre in 1996 to 1.0 lb/acre in 1997 with an 

average of 0.48 lb/acre-yr. Total phosphorus in runoff 

from the lower study area is about 11 percent of the 

combined fertilizer and rainfall (mostly fertilizer) phosphorus entering the lower study area during 1996–98. 

Phosphorus fertilizer is applied as soluble orthophosphate; however, most of the total phosphorus in runoff 

is particulate, associated with crop debris and eroded 

soils. 

Twenty-one pesticides were detected in runoff 

with varying degrees of frequency during the study. 

The herbicide atrazine and its breakdown product 

deethylatrazine were detected in all runoff event 

samples. Other frequently detected pesticides included 

trifluralin, simazine, pendimethalin, and diuron. Concentrations of these most frequently detected pesticides 

were greater during the pre-harvest period (March–

May) than during the post-harvest period (August–

October). 

Atrazine plus deethylatrazine and diuron contributed the greatest loads of pesticides in runoff. Larger 

concentrations are detected in spring; thus, spring runoff events have the potential to deliver considerably 

larger pesticide loads than fall events. Most of the 

atrazine runoff loads were during April and May 1997. 
22 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

About 1.8 percent of the atrazine applied to the lower 

cropland study area during 1997 exited the cropland 

in runoff. The 1996–98 runoff yield of atrazine plus 

deethylatrazine exiting the lower study area was about 

0.7 percent of the total atrazine application.

Sediment loads exiting the study area during 

runoff events varied widely, ranging from 0.90 ton in 

August 1996 to 19,000 tons in October 1997. Sediment 

loads from the study area are dominated by storm 

events—of the 45,300 tons of sediment transported 

from the study area during 1996–98, 87 percent was 

transported during three events (April 1997, October 

1997, and October 1998). The annual sediment yield 

was 2.2 lb/acre in 1996 compared to 1,270 lb/acre in 

1997 with a 1996–98 average yield of 610 lb/acre-yr. 

Runoff-weighted average concentrations (constituent load exiting study area divided by study area runoff) for the entire study area were computed for selected 

nutrients and pesticides for month, year, and entire 

study period. Because runoff-weighted average concentrations assign more weight to the sample concentrations at sites or during events where greater runoff 

occurred, they might best characterize water quality 

from the study area. The study area runoff-weighted 

concentrations of total nitrogen and total phosphorus for 

1996–98 were 1.3 and 0.50 mg/L, respectively. Runoffweighted concentrations of total nitrogen were less than 

either the mean or the median EMC. Runoff-weighted 

concentrations of total phosphorus were slightly greater 

than either the mean or the median EMC. The runoffweighted atrazine plus deethylatrazine concentration 

for 1996–98 was 2.7 g/L.

SELECTED REFERENCES

Armstrong, N.E., and Ward, G.H., 1998, Analysis of point 

source discharges (including oil field brine discharges) 

in the Corpus Christi Bay National Estuary Program 

study area: Corpus Christi Bay National Estuary Program, CBNEP–30, 175 p.

Baird, F.C., Dybala, T.J., Jennings, Marshall, and Ockerman, 

D.J., 1996, Characterization of nonpoint sources and 

loadings to the Corpus Christi Bay National Estuary 

Program study area: Texas Natural Resource Conservation Commission, Corpus Christi Bay National Estuary 

Program CCBNEP–05, 226 p.

Coastal Bend Bays and Estuaries Program, 1998, Coastal 

Bend bays plan: Austin, Texas Natural Resource Conservation Commission.

Helsel, D.R., and Hirsch, R.M., 1992, Studies in environmental science 49—Statistical methods in water resources: 

Amsterdam, Elsevier, 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.

Ockerman, D.J., and Livingston, C.W., 1999, Nitrogen concentrations and deposition in rainfall at two sites in the 

Coastal Bend area, south Texas, 1996–98: U.S. Geological Survey Fact Sheet FS–146–99, 6 p.

Ockerman, D.J., Petri, B.L., and Slattery, R.N., 1999, Stormwater runoff for selected watersheds in the Edwards 

aquifer recharge zone, Bexar County, Texas, 1996–98: 

U.S. Geological Survey Fact Sheet FS–172–98, 6 p.

Patino, Eduardo, and Ockerman, D.J., 1997, Computation 

of mean velocity in open channels using acoustic 

velocity meters: U.S. Geological Survey Open-File 

Report 97–220, 12 p.

Soil Conservation Service, 1992, Soil survey, Nueces County, 

Texas: U.S. Department of Agriculture, 65 p.

State of Texas, 1997, Site-specific uses and criteria for classified segments: Texas Administrative Code, Title 30, Part 

1, Chap. 307, Rule 307.10, App. A.

Texas Natural Resource Conservation Commission, 1996, 

1994 Regional assessment of water quality in the Nueces 

Coastal Basins: Austin, Texas Natural Resource Conservation Commission AS–035, 567 p.

______1999, Texas Surface Water Quality Standards, 

accessed December 31, 1999, at URL 

http://www.tnrcc.state.tx.us/water/quality/standards/

index.html

Texas State Data Center, 1998, 1997 Total population estimates for Texas places, accessed Feb. 27, 2001, at URL 

http://txsdc.tamu.edu/pepp/1997_txpopest_place.html

Wanielista, M.P., 1990, Hydrology and water quantity 

control: New York, Wiley, 565 p.

Ward, G.H., 1997, Processes and trends of circulation within 

the Corpus Christi Bay National Estuary Program study 

area: Corpus Christi Bay National Estuary Program 

CCBNEP–21, 286 p.

Wells, F.C., Gibbons, W.J., and Dorsey, M.E., 1990, Guidelines for collection and field analysis of water-quality 

samples from streams in Texas: U.S. Geological Survey 

Open-File Report 90–127, 79 p.
Table 2 23

Table 2

Table 1

Table 1. Selected characteristics of data-collection sites 

[USGS, U.S. Geological Survey; TAES, Texas Agricultural Experiment Station; --, not applicable; NOAA, National Oceanic 

and Atmospheric Administration]

Table 2. Applications of fertilizers and selected pesticides on 40,540-acre lower study area, 1996–98

[Actual planted area ranged from 36,128 to 36,168 acres]

Site

no.

(fig. 1)

Responsible

agency

USGS

station

no.

Station name Type of data collected

Period of

operation

1 TAES, USGS 08212500 Upper Madero Canal near Kingsville, Tex. Rainfall, streamflow, water quality 1996–98

2 TAES, USGS 08212600 Chiltipin Creek near Kingsville, Tex. Rainfall, streamflow, water quality 1996–98

3 TAES, USGS 08212700 Pinto Creek Canal near Kingsville, Tex. Rainfall, streamflow, water quality 1996–98

4 TAES, USGS 08212800 Lower Madero Canal near Kingsville, Tex. Rainfall, streamflow, water quality 1996–98

5 TAES, USGS 08212900 Tunas Creek near Kingsville, Tex. Rainfall, streamflow, water quality 1996–98

-- TAES -- Kleberg County–2 Rainfall

Rainfall quality

1988–present

1996–98

-- NOAA -- Kingsville Rainfall 1949–present

Chemical Application (pounds)

1996 1997 1998 Average

Fertilizer

Nitrogen 1,800,000 2,950,000 1,130,000 1,960,000

Phosphorus 0 289,000 220,000 170,000

Pesticide

Acephate 614 10,900 998 4,170

Atrazine 10,700 17,200 17,600 15,200

Azinphos-methyl 3,040 1,080 0 1,370

Carbofuran 1,130 0 4,140 1,760

2,4-D 1,060 13,600 18,100 10,900

Dimate 2,750 4,190 1,410 2,780

Diuron 760 21,800 19,600 14,100

Ethephon 11,900 13,600 5,050 10,200

Glyphosate 17,900 30,100 48,100 32,000

Malathion 63,300 87,100 70,700 73,700

MSMA 2,890 174 900 1,320

Paraquat 0 1,430 401 611

Thidiazuron 16,800 16,400 4,560 12,600

Trifluralin 16,900 18,200 0 11,700
24 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Table 3 Table 3. Rainfall, runoff volumes, and runoff coefficients for runoff events, 1996

–98

[Total study area runoff is sum of runoff from sites 3, 4, and 5; --, no runoff; <, less than] 

Event date

Study area

rainfall

(inches)

Runoff

(acre-feet) 

Site 1 Site 2 Site 3 Site 4 Site 5 T

stud

05/11–12/96 0.62 -- 14 -- -- 110 08/23–25/96 1.74 -- -- 4.7 -- -- 08/31–09/01/96 1.20 -- 1.0 .5 -- -- 09/20–21/96 1.55 2.1 4.2 .7 -- 82 03/12–13/97 1.47 -- -- -- -- 38 04/02–13/97 5.50 820 590 410 130 10,100 1005/09–11/97 1.53 -- 24 -- -- 180 05/15–19/97 2.54 71 110 82 34 2,700 205/21–26/97 1.34 -- 67 80 10 610 05/28–30/97 .40 -- -- 10 3.0 -- 09/24–27/97 1.91 -- -- 15 -- 11 

10/10–24/97 11.16 3,620 1,570 620 320 18,000 1802/14–15/98 1.23 -- 4.0 1.0 -- 63 02/21–22/98 .37 -- -- 18 -- -- 09/17–10/04/98 4.62 43 222 540 23 3,420 310/06–17/98 2.41 23 30 160 6.8 540 10/18–31/98 5.45 1,550 1,050 640 100 10,300 1111/08–22/98 1.19 73 36 150 21 780 Total of 18 events 46.23 6,200 3,720 2,730 650 46,900 50
Table 4 25

Table 4

Table 5

Table 4. Summary statistics of selected nutrient concentrations for 19 rainfall samples, 1996–98

[In milligrams per liter; --, not determined; <, less than] Table 5. Monthly and annual rainfall deposition of nitrogen and phosphorus,1996-98[In pounds per acre; <, less than]

Nutrient Mean Median Minimum Maxi

Nitrogen, total 0.36 0.37 0.03 0.Ammonia nitrogen, dissolved (as N) .18 .15 .01 .Nitrate nitrogen, dissolved (as N) .12 .10 .03 .Nitrite nitrogen, dissolved (as N) -- .001 <.001 .Organic nitrogen, total (as N) .05 .02 <.1 .Phosphorus, total .01 .005 .001 .Phosphorus, dissolved -- .004 <.001 .Orthophosphate phosphorus, dissolved (as P) -- <.002 <.001 .

Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov

Nitrogen, total

1996 0 0 0.02 0.19 0.10 0.12 0.03 0.53 0.54 0.16 0.21 1997 .18 .37 .54 .39 .60 .09 .09 .20 .37 .73 .32 1998 .15 .40 .25 .05 .01 .06 .06 .49 .86 .49 .36 

Ammonia nitrogen

1996 0 0 .01 .10 .06 .08 .02 .26 .32 .10 .12 1997 .12 .22 .27 .24 .31 .06 .05 .09 .29 .21 .18 1998 .09 .23 .14 .02 .01 .04 .03 .26 .44 .23 .20 

Nitrate nitrogen

1996 0 0 .01 .07 .03 .04 .01 .17 .18 .06 .07 1997 .07 .13 .18 .13 .19 .03 .03 .08 .14 .20 .12 1998 .05 .13 .09 .02 <.01 .02 .02 .17 .31 .17 .13 

Phosphorus, total

1996 0 0 <.001 .002 .003 <.001 <.001 .008 .006 .001 .002 1997 <.001 .001 .008 .004 .007 <.001 .001 .005 .006 .011 .002 1998 .001 .004 .001 <.001 0 <.001 <.001 .005 .007 .009 .002 
26 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

Table 7

Table 6

Table 6. Summary statistics of selected major ion and nutrient concentrations in runoff samples, 1996–98, and 

selected regulatory water-quality criteria 

[In milligrams per liter; does not include major ion data from site 5 (sub-tidal site). TSWQS, Texas Surface Water Quality 

Standard; --, not applicable; <, less than]

1 Texas Natural Resource Conservation Commission (1999).

Table 7. Pesticides applied on lower study area, 1996–98

[X, application; --, no application

Major ion or nutrient

No. of

samples

Mean Median Maximum Minimum

TSWQS

aquatic life

protection1

TSWQS

human

health

protection1

Calcium, dissolved 20 17.1 16.2 30.0 10.0 -- --

Magnesium, dissolved 20 1.46 1.25 2.7 .90 -- --

Potassium, dissolved 20 3.4 2.9 7.1 1.4 -- --

Sulfate, dissolved 20 6.7 3.9 20 1.3 -- --

Chloride, dissolved 20 7.0 3.8 32 .90 -- --

Nitrogen, total 29 1.6 1.4 5.3 .49 -- --

Ammonia nitrogen, dissolved (as N) 29 .06 .03 .26 <.015 -- --

Nitrite + nitrate nitrogen, dissolved (as N) 29 .36 .27 1.5 .12 10 --

Ammonia + organic nitrogen, total (as N) 29 1.2 1.0 3.8 .34 -- --

Phosphorus, total 29 .48 .46 1.2 .03 -- 100

Phosphorus, dissolved 29 .13 .12 .36 .02 -- --

Orthophosphate phosphorus, dissolved (as P) 29 .13 .11 .37 .02 -- --

Pesticide 1996 1997 1998

Included

in sample

analysis

Pesticide 1996 1997 1998

Included

in sample

analysis

Herbicides Insecticides

Atrazine X X X Yes Acephate X X X No

Bromoxynil X X X No Azinphos-methyl X X X Yes

Clethodin X X X No Carbofuran X X X Yes

Cyanazine X X X Yes Dimethoate X X X Yes

2,4-D X X X Yes Endosulfan X X X Yes

Dicamba X X X Yes Esfenvalerate X X X Yes

Dimate X X X No Imidacloprid X X X No

Diuron X X X Yes Lambda cyhalothrin -- X -- No

Fenoxaprop-P-ethyl -- X -- No Malathion X X X Yes

Glyphosate X X X Yes Oxamyl -- X -- Yes

Halosulfuron-methyl -- X X No Tralomethrin X X X No

MSMA X X X No zeta-Cypermethrin X X X No

Paraquat -- X X No Defoliants, growth regulators

Pendimethalin X X X Yes Ethephon X X X No

Pyrithiobac-sodium X X X No Mepiquat chloride -- X -- No

Quizalofop-P-ethyl X -- X No Thidiazuron X X X No

Triclopyr X X X Yes

Trifluralin X X X Yes
Table 9 27

Table 9

Table 8

Table 8. Summary statistics of selected pesticide concentrations in runoff samples, 1996–98, and selected 

regulatory water-quality criteria

[In micrograms per liter; TSWQS, Texas Surface Water Quality Standard; --, not determined; <, less than]

1

 Texas Natural Resource Conservation Commission (1999).

2

 Water and fish.

3

 Freshwater chronic criteria.

4 Saltwater acute criteria.

Table 9. Median concentrations of selected pesticides in runoff samples grouped by seasonal category

[In micrograms per liter; <, less than] 

Pesticide

No. of

detections in

29 samples

Mean Median Maximum Minimum

TSWQS

aquatic life

protection1

TSWQS

human health

protection1

Herbicides

Atrazine, dissolved 29 4.76 0.61 47 0.03 -- --

Deethylatrazine, dissolved 29 .24 .12 2.4 .007 -- --

Cyanazine, dissolved 5 -- <.004 .05 <.004 -- --

2,4-D, dissolved 6 -- <.035 .87 <.035 -- 270 

DCPA, dissolved 6 -- <.002 .004 <.002 -- --

Diuron, dissolved 13 -- <.02 16.7 <.02 370 --

Fluometuron, dissolved 5 -- <.035 1.0 <.035 -- --

Glyphosate, dissolved 2 -- <5.0 20.4 <5.0 -- --

Lindane, dissolved 3 -- <.004 .014 <.004 4.16 2.20

Metolachlor, dissolved 10 -- <.002 .07 <.002 -- --

Pendimethalin, dissolved 15 -- <.004 .26 <.004 -- --

Prometon, dissolved 3 -- <.018 .05 <.018 -- --

Simazine, dissolved 18 -- .005 .24 <.005 -- --

Triclopyr, dissolved 1 -- <.05 .14 <.05 -- --

Trifluralin, dissolved 23 -- .008 .21 <.02 -- --

Insecticides --

Azinphos-methyl, dissolved 1 -- <.001 .08 <.001 -- --

alpha-BHC, dissolved 2 -- <.002 .015 <.002 -- --

Carbaryl, dissolved 1 -- <.003 .011 <.003 4613 --

Carbofuran, dissolved 2 -- <.003 .02 <.003 -- --

Diazinon, dissolved 1 -- <.002 <.02 <.002 -- --

Malathion, dissolved 8 -- <.005 .035 <.005 3.01 --

Pesticide

Pre-harvest

(Feb.–May)

Post-harvest

(Aug.–Oct.)

Median, all

samples

Atrazine, dissolved 5.4 0.14 0.61 

Deethylatrazine, dissolved .21 .03 .12

Diuron, dissolved 2.3 <.02 <.02

Pendimethalin, dissolved .04 <.004 <.004

Simazine, dissolved .04 <.005 .003

Trifluralin, dissolved .080 .005 .008
28 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Table 10 Table 10. Monthly and annual loads of selected nutrients in runoff, 1996

–981 

1 Sums of monthly loads might not equal annual loads because of rounding.

Nutrient

Runoff loads, in pounds, exiting 49,680-acre study area

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Nitrogen, total 0 0 0 0 420 0 0 30 350 0 Ammonia nitrogen, dissolved (as N) 0 0 0 0 9.0 0 0 1.0 30 0 Nitrite + nitrate nitrogen, dissolved (as 

N)

0 0 0 0 78 0 0 3.0 160 0 

Ammonia + organic nitrogen, total 

(as N)

0 0 0 0 290 0 0 27 190 0 

Phosphorus, total 0 0 0 0 130 0 0 7.0 40 0 Orthophosphate phosphorus, dissolved 

(as P)

0 0 0 0 32 0 0 1.0 27 0 

1997Nitrogen, total 0 0 50 53,400 12,100 0 0 0 150 36,000 Ammonia nitrogen, dissolved (as N) 0 0 4.0 1,310 300 0 0 0 3.0 6,350 Nitrite + nitrate nitrogen, dissolved (as 

N)

0 0 16 6,700 2,870 0 0 0 13 7,250 

Ammonia + organic nitrogen, total 

(as N)

0 0 35 46,700 8,860 0 0 0 140 28,900 

Phosphorus, total 0 0 4.0 19,000 3,910 0 0 0 50 23,600 Orthophosphate phosphorus, dissolved 

(as P)

0 0 2.0 4,150 1,260 0 0 0 7.0 5,650 

1998Nitrogen, total 0 320 0 0 0 0 0 0 22,500 55,000 Ammonia nitrogen, dissolved (as N) 0 7.0 0 0 0 0 0 0 1,480 1,240 Nitrite + nitrate nitrogen, dissolved 

(as N)

0 62 0 0 0 0 0 0 3,390 8,590 

Ammonia + organic nitrogen, total 

(as N)

0 220 0 0 0 0 0 0 16,800 45,600 

Phosphorus, total 0 100 0 0 0 0 0 0 5,870 14,700 Orthophosphate phosphorus, dissolved 

(as P)

0 24 0 0 0 0 0 0 1,310 2,400 
Table 10 29

Table 10. Monthly and annual loads of selected nutrients in runoff, 1996–981—Continued 

1 Sums of monthly loads might not equal annual loads because of rounding.

Nutrient Runoff loads, in pounds, from 9,140-acre upper study area

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Nitrogen, total 0 0 0 0 54 0 0 4.0 28 0 Ammonia nitrogen, dissolved 

(as N)

0 0 0 0 1.1 0 0 .10 .30 0 

Nitrite + nitrate nitrogen, 

dissolved (as N)

0 0 0 0 10 0 0 .70 9.3 0 

Ammonia + organic nitrogen, 

total (as N)

0 0 0 0 38 0 0 4.0 19 0 

Phosphorus, total 0 0 0 0 17 0 0 1.0 7.8 0 Orthophosphate phosphorus, 

dissolved (as P)

0 0 0 0 4.1 0 0 .40 3.8 0 

1997Nitrogen, total 0 0 0 5,910 800 0 0 0 0 9,240 Ammonia nitrogen, dissolved 

(as N)

0 0 0 110 17 0 0 0 0 290 

Nitrite + nitrate nitrogen, 

dissolved (as N)

0 0 0 830 150 0 0 0 0 2,080 

Ammonia + organic nitrogen, 

total (as N)

0 0 0 4,320 590 0 0 0 0 7,170 

Phosphorus, total 0 0 0 2,150 310 0 0 0 0 2,900 Orthophosphate phosphorus, 

dissolved (as P)

0 0 0 720 130 0 0 0 0 1,580 

1998Nitrogen, total 0 16 0 0 0 0 0 0 1,020 10,400 Ammonia nitrogen, dissolved 

(as N)

0 .30 0 0 0 0 0 0 15 220 

Nitrite + nitrate nitrogen, 

dissolved (as N)

0 3.0 0 0 0 0 0 0 310 1,940 

Ammonia + organic nitrogen, 

total (as N)

0 11 0 0 0 0 0 0 690 7,190 

Phosphorus, total 0 3.0 0 0 0 0 0 0 270 2,750 Orthophosphate phosphorus, 

dissolved (as P)

0 1.0 0 0 0 0 0 0 73 790 
30 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Table 10. Monthly and annual loads of selected nutrients in runoff, 1996

–981—Continued 

1 Sums of monthly loads might not equal annual loads because of rounding.

Nutrient

Net runoff loads, in pounds, from 40,540-acre lower study area

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Nitrogen, total 0 0 0 0 360 0 0 26 320 0 Ammonia nitrogen, dissolved (as N) 0 0 0 0 7.9 0 0 .90 30 0 Nitrite + nitrate nitrogen, dissolved 

(as N)

0 0 0 0 68 0 0 2.3 150 0 

Ammonia + organic nitrogen, total 

(as N)

0 0 0 0 250 0 0 23 170 0 

Phosphorus, total 0 0 0 0 120 0 0 6.0 32 0 Orthophosphate phosphorus, dissolved 

(as P)

0 0 0 0 28 0 0 .60 23 0 

1997Nitrogen, total 0 0 50 47,500 11,300 0 0 0 150 26,800 Ammonia nitrogen, dissolved (as N) 0 0 4.0 1,200 280 0 0 0 3.0 6,070 Nitrite + nitrate nitrogen, dissolved 

(as N)

0 0 16 5,870 2,720 0 0 0 13 5,160 

Ammonia + organic nitrogen, total 

(as N)

0 0 35 42,400 8,260 0 0 0 140 21,800 

Phosphorus, total 0 0 4.0 16,900 3,600 0 0 0 50 20,700 Orthophosphate phosphorus, dissolved 

(as P)

0 0 2.0 3,430 1,140 0 0 0 7.0 4,100 

1998Nitrogen, total 0 310 0 0 0 0 0 0 21,500 44,600 Ammonia nitrogen, dissolved (as N) 0 6.7 0 0 0 0 0 0 1,470 1,020 Nitrite + nitrate nitrogen, dissolved 

(as N)

0 59 0 0 0 0 0 0 3,080 6,650 

Ammonia + organic nitrogen, total 

(as N)

0 210 0 0 0 0 0 0 16,100 38,400 

Phosphorus, total 0 99 0 0 0 0 0 0 5,600 11,900 Orthophosphate phosphorus, dissolved 

(as P)

0 23 0 0 0 0 0 0 1,240 1,610 
Table 11 31

Table 11

Table 11. Annual and average annual yields of selected nutrients in runoff,1996–98[In pounds per acre per year; <, less than]

Nutrient 1996 1997 1998 1996

ave

Runoff yields from 49,680-acre study area

Nitrogen, total 0.02 2.1 1.6 1.2Ammonia nitrogen, dissolved (as N) <.01 .16 .06 .0Nitrite + nitrate nitrogen, dissolved (as N) <.01 .34 .26 .2Ammonia + organic nitrogen, total (as N) .01 1.7 1.3 1.0Phosphorus, total <.01 .94 .43 .4Orthophosphate phosphorus, dissolved (as P) <.01 .22 .08 .

Runoff yields from 9,140-acre upper study area

Nitrogen, total .01 1.8 1.3 1.0Ammonia nitrogen, dissolved (as N) <.01 .05 .03 .0Nitrite + nitrate nitrogen, dissolved (as N) <.01 .34 .25 .2Ammonia + organic nitrogen, total (as N) .01 1.3 .90 .7Phosphorus, total <.01 .59 .34 .3Orthophosphate phosphorus, dissolved (as P) <.01 .27 .10 .

Net runoff yields from 40,540-acre lower study area

Nitrogen, total .02 2.1 1.7 1.3Ammonia nitrogen, dissolved (as N) <.01 .19 .06 .0Nitrite + nitrate nitrogen, dissolved (as N) .01 .34 .26 .2Ammonia + organic nitrogen, total (as N) .01 1.8 1.4 1.Phosphorus, total <.01 1.0 .45 .4Orthophosphate phosphorus, dissolved (as P) <.01 .21 .08 .
32 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Table 12 Table 12. Monthly and annual loads of selected pesticides in runoff, 1996

–981 

1 Sums of monthly loads might not equal annual loads because of rounding.

Pesticide Runoff loads, in pounds, exiting 49,680-acre study area

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Atrazine + deethylatrazine, 

dissolved

0 0 0 0 1.6 0 0 0.01 10.7 0 

Diuron, dissolved 0 0 0 0 .70 0 0 0 .05 0 Trifluralin, dissolved 0 0 0 0 .023 0 0 0 .004 0 

1997Atrazine + deethylatrazine, 

dissolved

0 0 .10 220 110 0 0 0 .01 8.4 

Diuron, dissolved 0 0 .50 340 49 0 0 0 0 1.0 Trifluralin, dissolved 0 0 0 3.7 .90 0 0 0 0 .33 

1998Atrazine + deethylatrazine, 

dissolved

0 1.2 0 0 0 0 0 0 4.7 7.5 

Diuron, dissolved 0 .53 0 0 0 0 0 0 .25 .91 Trifluralin, dissolved 0 .06 0 0 0 0 0 0 .06 .14 

Pesticide

Runoff loads, in pounds, from 9,140-acre upper study area

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Atrazine + deethylatrazine, 

dissolved

0 0 0 0 0.21 0 0 <0.01 0.02 0 

Diuron, dissolved 0 0 0 0 .09 0 0 0 .01 0 Trifluralin, dissolved 0 0 0 0 .003 0 0 0 .001 0 

1997Atrazine + deethylatrazine, 

dissolved

0 0 0 17 7.4 0 0 0 0 2.8 

Diuron, dissolved 0 0 0 5.0 1.0 0 0 0 0 .30 Trifluralin, dissolved 0 0 0 .15 .05 0 0 0 0 .15 

1998Atrazine + deethylatrazine, 

dissolved

0 .06 0 0 0 0 0 0 .08 1.2 

Diuron, dissolved 0 .03 0 0 0 0 0 0 .01 .23 Trifluralin, dissolved 0 .001 0 0 0 0 0 0 .001 .13 
Table 13 33

Table 13

Table 12. Monthly and annual loads of selected pesticides in runoff, 1996–981—Continued 

1 Sums of monthly loads might not equal annual loads because of rounding.

Table 13. Annual and average annual yields of selected pesticides in runoff, 1996–98

[In pounds per acre per year; <, less than] 

Pesticide Net runoff loads, in pounds, from 40,540-acre lower study area

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Atrazine + deethylatrazine, 

dissolved

0 0 0 0 1.4 0 0 0 10.7 0 

Diuron, dissolved 0 0 0 0 .61 0 0 0 .04 0 Trifluralin, dissolved 0 0 0 0 .02 0 0 0 .003 0 

1997Atrazine + deethylatrazine, 

dissolved

0 0 .10 200 100 0 0 0 .01 5.6 

Diuron, dissolved 0 0 .50 330 47.9 0 0 0 0 .74 Trifluralin, dissolved 0 0 0 3.6 .95 0 0 0 0 .18 

1998Atrazine + deethylatrazine, 

dissolved

0 1.1 0 0 0 0 0 0 4.6 6.3 

Diuron, dissolved 0 .50 0 0 0 0 0 0 .24 .68 Trifluralin, dissolved 0 .06 0 0 0 0 0 0 .06 .01 

Pesticide 1996 1997 1998

1996–

avera

Runoff yields from 49,680-acre study area

Atrazine + deethylatrazine, dissolved 0.0002 0.0068 0.0003 0.00Diuron, dissolved <.0001 .0078 <.0001 .00Trifluralin, dissolved <.0001 .0001 <.0001 <.00

Runoff yields from 9,140-acre upper study area

Atrazine + deethylatrazine, dissolved <.0001 .0030 .0002 .00Diuron, dissolved <.0001 .0007 <.0001 .00Trifluralin, dissolved <.0001 <.0001 <.0001 <.00

Net runoff yields from 40,540-acre lower study area

Atrazine + deethylatrazine, dissolved .0003 .0076 .0003 .00Diuron, dissolved <.0001 .0094 <.0001 .00Trifluralin, dissolved <.0001 .0001 <.0001 <.00
34 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Table 15 Table 14 Table 14. Monthly and annual loads of sediment in runoff, study area, 1996

–98 

[In tons] Table 15. Annual and average annual yields of sediment in runoff, study area, 1996

–98

[In pounds per acre per year] 

Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. 

1996 0 0 0 0 32 0 0 0.90 22 0 0 1997 0 0 8.9 10,200 2,390 0 0 0 4.6 19,000 0 1998 0 20 0 0 0 0 0 0 2,700 10,400 440 

1996 1997 1998 1996

–98

average

2.2 1,270 550 610
Table 16 35

Table 16

Table 16. Study area runoff-weighted concentrations of selected nutrients, 1996–98 

[In milligrams per liter]

Nutrient Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Nitrogen, total 0 0 0 0 1.4 0 0 2.1 1.5 0 Ammonia nitrogen, dissolved (as N) 0 0 0 0 .03 0 0 .07 .13 0 Nitrite + nitrate nitrogen, dissolved (as N) 0 0 0 0 .27 0 0 .21 .71 0 Ammonia + organic nitrogen, total (as N) 0 0 0 0 1.0 0 0 1.9 .84 0 Phosphorus, total 0 0 0 0 .46 0 0 .50 .18 0 Orthophosphate phosphorus, dissolved (as P) 0 0 0 0 .11 0 0 .07 .12 0 1997Nitrogen, total 0 0 .48 1.8 1.2 0 0 0 2.2 .70 Ammonia nitrogen, dissolved (as N) 0 0 .04 .05 .03 0 0 0 .04 .12 Nitrite + nitrate nitrogen, dissolved (as N) 0 0 .15 .23 .28 0 0 0 .18 .14 Ammonia + organic nitrogen, total (as N) 0 0 .34 1.6 .88 0 0 0 1.9 .56 Phosphorus, total 0 0 .04 .66 .39 0 0 0 .71 .46 Orthophosphate phosphorus, dissolved (as P) 0 0 .02 .14 .13 0 0 0 .10 .11 1998Nitrogen, total 0 1.9 0 0 0 0 0 0 2.1 1.7 Ammonia nitrogen, dissolved (as N) 0 .04 0 0 0 0 0 0 .14 .04 Nitrite + nitrate nitrogen, dissolved (as N) 0 .36 0 0 0 0 0 0 .31 .27 Ammonia + organic nitrogen, total (as N) 0 1.3 0 0 0 0 0 0 1.6 1.4 Phosphorus, total 0 .59 0 0 0 0 0 0 .54 .46 Orthophosphate phosphorus, dissolved (as P) 0 .14 0 0 0 0 0 0 .12 .08 

Nutrient 1996–98 average

Nitrogen, total 1.3Ammonia nitrogen, dissolved (as N) .08Nitrite + nitrate nitrogen, dissolved (as N) .22Ammonia + organic nitrogen, total (as N) 1.1Phosphorus, total .50Orthophosphate phosphorus, dissolved (as P) .11
36 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Table 17 Table 17. Study area runoff-weighted concentrations of selected pesticides, 1996

–98 

[In micrograms per liter; <, less than]

Pesticide Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

1996

Atrazine + deethylatrazine, dissolved 0 0 0 0 5.6 0 0 0.57 48 0 Diuron, dissolved 0 0 0 0 2.3 0 0 <.02 <.001 0 Trifluralin, dissolved 0 0 0 0 .08 0 0 .007 0 0 1997Atrazine + deethylatrazine, dissolved 0 0 .1 7.6 11 0 0 0 .17 .16 Diuron, dissolved 0 0 .5 12 4.9 0 0 0 <.02 .019 Trifluralin, dissolved 0 0 0 .13 .10 0 0 0 <.005 .006 1998Atrazine + deethylatrazine, dissolved 0 5.2 0 0 0 0 0 0 .45 .23 Diuron, dissolved 0 1.8 0 0 0 0 0 0 .08 <.02 Trifluralin, dissolved 0 .06 0 0 0 0 0 0 .009 .002 

Pesticide 1996

–98 average

Atrazine + deethylatrazine, dissolved 2.7Diuron, dissolved 2.9Trifluralin, dissolved .4
App

Hydrologic Conditions and Water Quality 

in an Agricultural Area in Kleberg and 

Nueces Counties, Texas, 1996–98

Appendix I

Appendix I—

Rainfall Sample Analyses
I–2 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 Texas Agricultural Experiment Station samples [in., inches; mg/L, milligrams per liter; <, less than; --, not measured] Date Rain- fall (in.) Nitrogen, total (mg/L as N) Nitrogen, nitrate, dissolved (mg/L as N) Nitrogen, nitrite, dissolved (mg/L as N) Nitrogen, ammonia, dissolved (mg/L as N) Nitrogen, ammonia + organic, dissolved (mg/L as N

05/11/96 0.47 0.67 0.17 <0.01 0.41 0.4 08/09/96 .19 .52 .22 <.001 .245 .3 08/21/96 .61 .35 .20 .002 .141 <.2 08/23/96 .90 .47 .064 .002 .20 <.2 09/20/96 1.63 .27 .069 <.001 .20 .3 11/24/96 .28 .61 .11 <.001 .324 .5 12/15/96 .31 .54 .14 <.001 .301 .3 03/11/97 .65 .41 .1 .01 .14 .3 04/02/97 1.73 .03 .033 <.001 .057 <.2 04/03/97 3.51 .04 .026 <.001 .089 <.2 05/09/97 1.03 .31 .169 <.001 .182 <.2 05/16/97 2.02 .20 .117 .002 .146 <.2 08/23/97 .40 .91 .45 .01 .09 .21 09/22/97 2.74 .19 .06 .002 .109 <.2 09/24/97 1.96 .38 .10 .001 .276 .26 10/11/97 5.52 .18 .04 <.001 .01 <.2 02/14/98 1.18 .52 .14 .002 .302 .34 09/23/98 1.50 .12 .066 .001 .051 <.1 10/18/98 2.21 .11 .062 .001 .053 <.1 
Appe

Hydrologic Conditions and Water Quality 

in an Agricultural Area in Kleberg and 

Nueces Counties, Texas, 1996–98

Appendix II

Appendix II—

Runoff Event-Mean Concentrations
II–2 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

p, g

Phosphorus, orthophosphate, dissolved mg/L as P .28 .20 .17 .09

Calcium, dissolved mg/L as Ca 26 16.2 16.3 25.2

Magnesium, dissolved mg/L as Mg 2 1.2 1.2 2.7

Sodium, dissolved mg/L as Na 6.6 4.7 4.9 11.8

Potassium, dissolved mg/L as K 7 3.8 3.8 7.1

Chloride, dissolved mg/L as Cl 6.3 1.0 1.3 13.2

Sulfate, dissolved mg/L as SO4 9.4 1.7 2.3 5.7

Fluoride, dissolved mg/L as F .2 .3 .2 .2

Silica, dissolved mg/L as SiO2 17 11.6 11.0 16.7

Boron, dissolved g/L as B 81 49.8 57 91

Iron, dissolved g/L as Fe 10 <3 5.2 <3.0

Dissolved solids mg/L 152 89 84 149.00

Suspended solids mg/L 192 581 615 123

Specific conductance S/cm 206 139 125 163

Alkalinity mg/L as CaCO3 80 76 70 94

pH standard units 7.3 -- 7.9 --

2,4-DB g/L <.035 <.035 <.035 <.035

2,4-D g/L <.035 <.035 <.035 <.035

2,4,5-T g/L <.035 <.035 <.035 <.035

3-Hydroxycarbofuran g/L <.014 <.014 <.014 <.014

Acetochlor g/L <.002 <.002 <.002 <.002

Amiben g/L <.011 <.011 <.011 <.011

Aldicarb g/L <.016 <.016 <.016 <.016

Aldicarb sulfone g/L <.016 <.016 <.016 <.016

Aldicarb sulfoxide g/L <.021 <.021 <.021 <.021

Acifluorfen g/L <.035 <.035 <.035 <.035

Atrazine g/L .06 5.8 10.8 .18

Deethylatrazine g/L .008 .06 .73 .03

Alachlor g/L <.002 <.002 <.002 <.002

Azinphos-methyl g/L <.001 <.001 <.001 <.001

Bentazon g/L <.014 <.014 <.014 <.014

Bromacil g/L <.035 <.035 <.035 <.035

Benfluralin g/L <.002 <.002 <.002 <.002

Bromoxynil g/L <.035 <.035 <.035 <.035

Butylate g/L <.002 <.002 <.002 <.002

Carbofuran g/L <.028 <.028 <.028 <.028

Carbaryl g/L <.003 <.003 <.003 <.003

Carbofuran g/L <.003 <.003 <.003 <.003

Chloramben g/L <.011 <.011 <.011 <.011

Chlorpyrifos g/L <.004 <.004 <.004 <.004

Cyanazine g/L <.004 <.004 <.004 <.004

Clopyralid g/L <.05 <.05 <.05 <.05

Chlorothalonil g/L <.035 <.035 <.035 <.035

Dacthal mono-acid g/L <.017 <.017 <.017 <.017

Dicamba g/L <.035 <.035 <.035 <.035

2,6-Diethylaniline g/L <.003 <.003 <.003 <.003

Diuron g/L <.02 .58 .06 <.02
Appe

EPTC g/L <.002 <.002 <.002 

Esfenvalerate g/L <.019 <.019 <.019 

Ethalfluralin g/L <.004 <.004 <.004 

Ethoprop g/L <.003 <.003 <.003 

Fluometuron g/L <.035 .91 .57 

Fenuron g/L <.013 <.013 <.013 

Fonofox g/L <.003 <.003 <.003 

BHC alpha g/L <.002 .015 .003 

Lindane g/L <.004 .014 <.004 

Linuron g/L <.002 <.002 <.002 

MCPA g/L <.05 <.05 <.05 

MCPB g/L <.035 <.035 <.035 

Malathion g/L .035 <.005 <.005 

Methyl parathion g/L <.006 <.006 <.006 

Metolachlor g/L <.002 .007 .070 

Metribuzin g/L <.004 <.004 <.004 

Molinate g/L <.004 <.004 <.004 

Methomyl g/L <.017 <.017 <.017 

Methiocarb g/L <.026 <.026 <.026 

Napropamide g/L <.003 <.003 <.003 

Norflurazon g/L <.024 <.024 <.024 

Neburon g/L <.015 <.015 <.015 

1-Naphthol g/L <.007 <.007 <.007 

Oryzalin g/L <.019 <.019 <.019 

Oxamyl g/L <.018 <.018 <.018 

Parathion g/L <.004 <.004 <.004 

Pebulate g/L <.004 <.004 <.004 

Pendimethalin g/L <.004 <.004 <.004 

Permethrin g/L <.005 <.005 <.005 

Phorate g/L <.002 <.002 <.002 

Prometon g/L <.018 <.018 .05 

Pronamide g/L <.003 <.003 <.003 

Propanil g/L <.004 <.004 <.004 

Propachlor g/L <.007 <.007 <.007 

Propargite g/L <.013 <.013 <.013 

Propoxur g/L <.035 <.035 <.035 

Propham g/L <.035 <.035 <.035 

Picloram g/L <.035 <.035 <.035 

Silvex g/L <.021 <.021 <.021 

Simazine g/L <.005 .039 .07 

Thiobencarb g/L <.002 <.002 <.002 

Tebuthiuron g/L <.01 <.01 <.01 

Terbacil g/L <.007 <.007 <.007 

Terbufos g/L <.013 <.013 <.013 

Triallate g/L <.001 <.001 <.001 

Trifluralin g/L <.002 <.002 <.002 

Triclopyr g/L <.05 <.05 <.05 

Glyphosate g/L <5 <10 <5 
II–4 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

Calcium, dissolved mg/L as Ca 24 16.4 30 14.2 18.0 10.7

Magnesium, dissolved mg/L as Mg 2.3 1.2 2.5 1.0 1.3 .9

Sodium, dissolved mg/L as Na 20 4.2 10.7 3.6 8.1 5.9

Potassium, dissolved mg/L as K 3 3.4 4.9 2.8 4.5 2.5

Chloride, dissolved mg/L as Cl 23 2.8 32 3.2 3.2 11.5

Sulfate, dissolved mg/L as SO4 12 2.2 5.7 1.9 1.6 4.1

Fluoride, dissolved mg/L as F .6 .2 .2 .2 .3 .29

Silica, dissolved mg/L as SiO2 12 10.0 10.8 8.4 12.8 9.7

Boron, dissolved g/L as B 139 42.5 51 40 76 55

Iron, dissolved g/L as Fe <3 <3 <3 3.5 <3.0 <10

Dissolved solids mg/L 148 80 139 73 94 234

Suspended solids mg/L 764 442 360 610 222 524

Specific conductance S/cm 265 123 251 108 152 99

Alkalinity mg/L as CaCO3 85 62 66 60 71 77

pH standard units 7.8 -- 7.8 8.0 -- 8.3

2,4-DB g/L <.035 <.035 <.035 <.035 <.035 <.24

2,4-D g/L <.035 <.035 <.035 <.035 <.035 <.15

2,4,5-T g/L <.035 <.035 <.035 <.035 <.035 <.035

3-Hydroxycarbofuran g/L <.014 <.014 <.014 <.014 <.014 <.014

Acetochlor g/L <.002 <.002 <.002 <.002 <.002 <.002

Amiben g/L <.011 <.011 <.011 <.011 <.011 <.011

Aldicarb g/L <.016 <.016 <.016 <.016 <.016 <.55

Aldicarb sulfone g/L <.016 <.016 <.016 <.016 <.016 <.10

Aldicarb sulfoxide g/L <.021 <.021 <.021 <.021 <.021 <.021

Acifluorfen g/L <.035 <.035 <.035 <.035 <.035 <.035

Atrazine g/L 1.93 2.41 4.80 12.6 .14 .07

Deethylatrazine g/L .12 .13 2.40 .30 .02 .07

Alachlor g/L <.002 <.002 <.002 <.002 <.002 <.002

Azinphos-methyl g/L <.001 <.001 .04 <.001 <.001 <.001

Bentazon g/L <.014 <.014 <.014 <.014 <.014 <.014

Bromacil g/L <.035 <.035 <.035 <.035 <.035 <.035

Benfluralin g/L <.002 <.002 <.002 <.002 <.002 <.002

Bromoxynil g/L <.035 <.035 <.035 <.035 <.035 <.035

Butylate g/L <.002 <.002 <.002 <.002 <.002 <.002

Carbofuran g/L <.028 <.028 <.028 <.028 <.028 <.028

Carbaryl g/L <.003 <.003 <.003 <.003 <.003 <.003

Carbofuran g/L <.003 <.003 <.003 <.003 <.003 <.003

Chloramben g/L <.011 <.011 <.011 <.011 <.011 <.42

Chlorpyrifos g/L <.004 <.004 <.004 <.004 <.004 <.004

Cyanazine g/L <.004 .05 <.004 <.004 <.004 <.004

Clopyralid g/L <.05 <.05 <.05 <.05 <.05 <.23

Chlorothalonil g/L <.035 <.035 <.035 <.035 <.035 <.48

Dacthal mono-acid g/L <.017 <.017 <.017 <.017 <.017 <.017

Dicamba g/L <.035 <.035 <.035 <.035 <.035 <.035

2,6-Diethylaniline g/L <.003 <.003 <.003 <.003 <.003 <.003

Diuron g/L .46 2.3 2.8 1.3 <.02 <.02

Dinoseb g/L <.035 <.035 <.035 <.035 <.035 <.035
Appe

Esfenvalerate g/L <.019 <.019 <.019 <.019 <.019 

Ethalfluralin g/L <.004 <.004 <.004 <.004 <.004 

Ethoprop g/L <.003 <.003 <.003 <.003 <.003 

Fluometuron g/L <.035 1.0 .18 .65 <.035 

Fenuron g/L <.013 <.013 <.013 <.013 <.013 

Fonofox g/L <.003 <.003 <.003 <.003 <.003 

BHC alpha g/L <.002 <.002 <.002 <.002 <.002 

Lindane g/L <.004 .01 <.004 <.004 <.004 

Linuron g/L <.002 <.002 <.002 <.002 <.002 

MCPA g/L <.05 <.05 <.05 <.05 <.05 

MCPB g/L <.035 <.035 <.035 <.035 <.035 

Malathion g/L <.005 <.005 <.005 <.005 <.005 

Methyl parathion g/L <.006 <.006 <.006 <.006 <.006 

Metolachlor g/L <.002 <.002 .006 .005 <.005 

Metribuzin g/L <.004 <.004 <.004 <.004 <.004 

Molinate g/L <.004 <.004 <.004 <.004 <.004 

Methomyl g/L <.017 <.017 <.017 <.017 <.017 

Methiocarb g/L <.026 <.026 <.026 <.026 <.026 

Napropamide g/L <.003 <.003 <.003 <.003 <.003 

Norflurazon g/L <.024 <.024 <.024 <.024 <.024 

Neburon g/L <.015 <.015 <.015 <.015 <.015 

1-Naphthol g/L <.007 <.007 <.007 <.007 <.007 

Oryzalin g/L <.019 <.019 <.019 <.019 <.019 

Oxamyl g/L <.018 <.018 <.018 <.018 <.018 

Parathion g/L <.004 <.004 <.004 <.004 <.004 

Pebulate g/L <.004 <.004 <.004 <.004 <.004 

Pendimethalin g/L <.004 .26 .09 .19 <.004 

Permethrin g/L <.005 <.005 <.005 <.005 <.005 

Phorate g/L <.002 <.002 <.002 <.002 <.002 

Prometon g/L <.018 <.018 <.018 <.018 <.018 

Pronamide g/L <.003 <.003 <.003 <.003 <.003 

Propanil g/L <.004 <.004 <.004 <.004 <.004 

Propachlor g/L <.007 <.007 <.007 <.007 <.007 

Propargite g/L <.013 <.013 <.013 <.013 <.013 

Propoxur g/L <.035 <.035 <.035 <.035 <.035 

Propham g/L <.035 <.035 <.035 <.035 <.035 

Picloram g/L <.035 <.035 <.035 <.035 <.035 

Silvex g/L <.021 <.021 <.021 <.021 <.021 

Simazine g/L .007 .03 .06 .17 <.005 

Thiobencarb g/L <.002 <.002 <.002 <.002 <.002 

Tebuthiuron g/L <.01 <.01 <.01 <.01 <.01 

Terbacil g/L <.007 <.007 <.007 <.007 <.007 

Terbufos g/L <.013 <.013 <.013 <.013 <.013 

Triallate g/L <.001 <.001 <.001 <.001 <.001 

Trifluralin g/L .11 .09 .09 .09 .01 

Triclopyr g/L <.05 <.05 <.05 <.05 <.05 

Glyphosate g/L <5 <10 <5 <5 <5 
II–6 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

, g

Magnesium, dissolved mg/L as Mg 1.3 -- 1.3 1.0 1.4 1.62 -- 1

Sodium, dissolved mg/L as Na 23 -- 5.0 6.0 13 6.89 -- 8

Potassium, dissolved mg/L as K 2.2 -- 2.5 2.0 2.2 4.63 -- 2

Chloride, dissolved mg/L as Cl 7.8 -- 1.1 .9 1.6 2.27 -- 1

Sulfate, dissolved mg/L as SO4 4.2 -- 2.6 1.3 2.6 1.67 -- 3

Fluoride, dissolved mg/L as F .5 -- .2 .2 .4 .20 -- 

Silica, dissolved mg/L as SiO2 4.5 -- 7.0 7.2 9.7 11.60 -- 8

Boron, dissolved g/L as B 169 -- 46.9 58 124 70.13 -- 78

Iron, dissolved g/L as Fe <3 -- <3 4.2 11 <3 -- <10

Dissolved solids mg/L 152 -- 81 70 -- .00 -- 120

Suspended solids mg/L 1,150 -- 743 565 1,140 531.84 -- 1,980

Specific conductance S/cm 258 -- 122 104 124 150.79 -- 115

Alkalinity mg/L as CaCO3 75 -- 73 62 75 87.47 -- 135

pH standard units 7.8 -- -- 8.0 8.1 7.99 -- 8

2,4-DB g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

2,4-D g/L <.035 .25 <.035 <.035 <.035 <.035 .19 <

2,4,5-T g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

3-Hydroxycarbofuran g/L <.014 <.014 <.014 <.014 <.014 <.014 <.014 <

Acetochlor g/L <.002 <.002 <.002 <.002 <.002 <.002 <.002 <

Amiben g/L <.011 <.011 <.011 <.011 <.011 <.011 <.011 <

Aldicarb g/L <.016 <.016 <.016 <.016 <.016 <.016 <.016 <

Aldicarb sulfone g/L <.016 <.016 <.016 <.016 <.016 <.016 <.016 <

Aldicarb sulfoxide g/L <.021 <.021 <.021 <.021 <.021 <.021 <.021 <

Acifluorfen g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

Atrazine g/L .61 .05 5.4 1.5 .15 .07 .04 

Deethylatrazine g/L .009 .007 .05 .15 .018 .01 .01 

Alachlor g/L <.002 <.002 <.002 <.002 <.002 <.002 <.002 <

Azinphos-methyl g/L <.001 <.001 <.001 .08 <.001 <.001 <.001 <

Bentazon g/L <.014 <.014 <.014 <.014 <.014 <.014 <.014 <

Bromacil g/L .06 <.035 <.035 <.035 <.035 <.035 <.035 <

Benfluralin g/L <.002 <.002 <.002 <.002 <.002 <.002 <.002 <

Bromoxynil g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

Butylate g/L <.002 <.002 <.002 <.002 <.002 <.002 <.002 <

Carbaryl g/L <.003 <.003 <.003 <.003 <.003 <.003 <.003 <

Carbofuran g/L <.003 <.003 <.003 <.003 <.003 <.003 <.003 <

Chloramben g/L <.011 <.011 <.011 <.011 <.011 <.011 <.011 <

Chlorpyrifos g/L <.004 <.004 <.004 <.004 <.004 <.004 <.004 <

Cyanazine g/L .023 <.004 <.004 <.004 <.004 <.004 <.004 <

Clopyralid g/L <.05 <.05 <.05 <.05 <.05 <.05 <.05 <

Chlorothalonil g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

Dacthal mono-acid g/L <.017 <.017 <.017 <.017 <.017 <.017 <.017 <

Dicamba g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

2,6-Diethylaniline g/L <.003 <.003 <.003 <.003 <.003 <.003 <.003 <

Diuron g/L <.02 <.02 16.7 4.1 <.02 <.02 <.02 <

Dinoseb g/L <.035 <.035 <.035 <.035 <.035 <.035 <.035 <

Dichlorprop g/L <.032 <.032 <.032 <.032 <.032 <.032 <.032 <
Appe

Ethalfluralin g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Ethoprop g/L <.003 <.003 <.003 <.003 <.003 <.003 <.

Fluometuron g/L <.035 <.035 <.035 <.035 <.035 <.035 <.

Fenuron g/L <.013 <.013 <.013 <.013 <.013 <.013 <.

Fonofox g/L <.003 <.003 <.003 <.003 <.003 <.003 <.

BHC alpha g/L <.002 <.002 <.002 <.002 <.002 <.002 <.

Lindane g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Linuron g/L <.002 <.002 <.002 <.002 <.002 <.002 <.

MCPA g/L <.05 <.05 <.05 <.05 <.05 <.05 <.

MCPB g/L <.035 <.035 <.035 <.035 <.035 <.035 <.

Malathion g/L .032 <.004 <.005 <.005 <.005 <.005 <.

Methyl parathion g/L <.006 <.006 <.006 <.006 <.006 <.006 <.

Metolachlor g/L <.002 <.002 .004 <.002 <.002 <.002 .

Metribuzin g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Molinate g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Methomyl g/L <.017 <.017 <.017 <.017 <.017 <.017 <.

Methiocarb g/L <.026 <.026 <.026 <.026 <.026 <.026 <.

Napropamide g/L <.003 <.003 <.003 <.003 <.003 <.003 <.

Norflurazon g/L <.024 <.024 <.024 <.024 <.024 <.024 <.

Neburon g/L <.015 <.015 <.015 <.015 <.015 <.015 <.

1-Naphthol g/L <.007 <.007 <.007 <.007 <.007 <.007 <.

Oryzalin g/L <.019 <.019 <.019 <.019 <.019 <.019 <.

Oxamyl g/L <.018 <.018 <.018 <.018 <.018 <.018 <.

Parathion g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Pebulate g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Pendimethalin g/L <.004 <.004 .02 .01 <.004 <.004 .

Permethrin g/L <.005 <.005 <.005 <.005 <.005 <.005 <.

Phorate g/L <.002 <.002 <.002 <.002 <.002 <.002 <.

Prometon g/L <.018 <.018 <.018 <.018 <.018 <.018 <.

Pronamide g/L <.003 <.003 <.003 <.003 <.003 <.003 <.

Propanil g/L <.004 <.004 <.004 <.004 <.004 <.004 <.

Propachlor g/L <.007 <.007 <.007 <.007 <.007 <.007 <.

Propargite g/L <.013 <.013 <.013 <.013 <.013 <.013 <.

Propoxur g/L <.035 <.035 <.035 <.035 <.035 <.035 <.

Propham g/L <.035 <.035 <.035 <.035 <.035 <.035 <.

Picloram g/L <.035 <.035 <.035 <.035 <.035 <.035 <.

Silvex g/L <.021 <.021 <.021 <.021 <.021 <.021 <.

Simazine g/L <.005 <.005 .011 .03 <.005 .003 .

Thiobencarb g/L <.002 <.002 <.002 <.002 <.002 <.002 <.

Tebuthiuron g/L <.01 <.01 <.01 <.01 <.01 <.01 <.

Terbacil g/L <.007 <.007 <.007 <.007 <.007 <.007 <.

Terbufos g/L <.013 <.013 <.013 <.013 <.013 <.013 <.

Triallate g/L <.001 <.001 <.001 <.001 <.001 <.001 <.

Trifluralin g/L .008 <.002 .21 .11 <.002 .008 .

Triclopyr g/L <.05 <.05 <.05 <.05 <.05 <.05 <.

Glyphosate g/L <5 20.4 <10 <10 <5 <5 18 
II–8 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

Calcium, dissolved mg/L as Ca 10.2 10.4 20 10

Magnesium, dissolved mg/L as Mg 1.1 1.1 2.12 1

Sodium, dissolved mg/L as Na 13.3 18.2 25 22

Potassium, dissolved mg/L as K 1.5 1.4 5.0 1.7

Chloride, dissolved mg/L as Cl 4.8 5.3 12.2 4.3

Sulfate, dissolved mg/L as SO4 10.4 19.9 20.4 20

Fluoride, dissolved mg/L as F .3 .3 .4 .52

Silica, dissolved mg/L as SiO2 5.9 6.6 15.7 8.2

Boron, dissolved g/L as B 112 153 218 240

Iron, dissolved g/L as Fe <3 <3 <3.0 <10

Dissolved solids mg/L 81 100 155 167

Suspended solids mg/L 992 1,190 867 1,220

Specific conductance S/cm 131 164 248 208

Alkalinity mg/L as CaCO3 54 58 87 160

pH standard units 8.2 8.2 -- 8.4

2,4-DB g/L <.035 <.035 <.035 <.24

2,4-D g/L <.035 <.035 <.035 .12

2,4,5-T g/L <.035 <.035 <.035 <.035

3-Hydroxycarbofuran g/L <.014 <.014 <.014 <.014

Acetochlor g/L <.002 <.002 <.002 <.002

Amiben g/L <.011 <.011 <.011 <.011

Aldicarb g/L <.016 <.016 <.016 <.55

Aldicarb sulfone g/L <.016 <.016 <.016 <.10

Aldicarb sulfoxide g/L <.021 <.021 <.021 <.021

Acifluorfen g/L <.035 <.035 <.035 <.05

Atrazine g/L 20.9 1.70 .17 .03

Deethylatrazine g/L .39 .60 .06 .016

Alachlor g/L <.002 <.002 <.002 <.002

Azinphos-methyl g/L <.001 <.001 <.001 <.001

Bentazon g/L <.014 <.014 <.014 <.014

Bromacil g/L <.035 <.035 <.035 <.035

Benfluralin g/L <.002 <.002 <.002 <.002

Bromoxynil g/L <.035 <.035 <.035 <.035

Butylate g/L <.002 <.002 <.002 <.002

Carbofuran g/L <.028 <.028 <.028 <.028

Carbaryl g/L <.003 <.003 <.003 <.003

Carbofuran g/L <.003 <.003 <.003 <.003

Chloramben g/L <.011 <.011 <.011 <.42

Chlorpyrifos g/L <.004 <.004 <.004 <.004

Cyanazine g/L <.004 <.004 <.004 <.004

Clopyralid g/L <.05 <.05 <.05 <.05

Chlorothalonil g/L <.035 <.035 <.035 <.48

Dacthal mono-acid g/L <.017 <.017 <.017 <.017

Dicamba g/L <.035 <.035 <.035 <.035

2,6-Diethylaniline g/L <.003 <.003 <.003 <.003

Diuron g/L .67 .3 <.02 <.23

Dinoseb g/L <.035 <.035 <.035 <.035
Appe

Esfenvalerate g/L <.019 <.019 <.019 

Ethalfluralin g/L <.004 <.004 <.004 

Ethoprop g/L <.003 <.003 <.003 

Fluometuron g/L <.035 <.035 <.035 

Fenuron g/L <.013 <.013 <.013 

Fonofox g/L <.003 <.003 <.003 

BHC alpha g/L <.002 <.002 <.002 

Lindane g/L <.004 <.004 <.004 

Linuron g/L <.002 <.002 <.002 

MCPA g/L <.05 <.05 <.05 

MCPB g/L <.035 <.035 <.035 

Malathion g/L <.005 <.005 <.005 

Methyl parathion g/L <.006 <.006 <.006 

Metolachlor g/L <.002 <.002 <.002 

Metribuzin g/L <.004 <.004 <.004 

Molinate g/L <.004 <.004 <.004 

Methomyl g/L <.017 <.017 <.017 

Methiocarb g/L <.026 <.026 <.026 

Napropamide g/L <.003 <.003 <.003 

Norflurazon g/L <.024 <.024 <.024 

Neburon g/L <.015 <.015 <.015 

1-Naphthol g/L <.007 <.007 <.007 

Oryzalin g/L <.019 <.019 <.019 

Oxamyl g/L <.018 <.018 <.018 

Parathion g/L <.004 <.004 <.004 

Pebulate g/L <.004 <.004 <.004 

Pendimethalin g/L .01 <.004 <.004 

Permethrin g/L <.005 <.005 <.005 

Phorate g/L <.002 <.002 <.002 

Prometon g/L <.018 <.018 <.018 

Pronamide g/L <.003 <.003 <.003 

Propanil g/L <.004 <.004 <.004 

Propachlor g/L <.007 <.007 <.007 

Propargite g/L <.013 <.013 <.013 

Propoxur g/L <.035 <.035 <.035 

Propham g/L <.035 <.035 <.035 

Picloram g/L <.035 <.035 <.035 

Silvex g/L <.021 <.021 <.021 

Simazine g/L .24 .04 .005 

Thiobencarb g/L <.002 <.002 <.002 

Tebuthiuron g/L <.01 <.01 <.01 

Terbacil g/L <.007 <.007 <.007 

Terbufos g/L <.013 <.013 <.013 

Triallate g/L <.001 <.001 <.001 

Trifluralin g/L .01 .03 .005 

Triclopyr g/L <.05 <.05 <.05 

Glyphosate g/L -- -- <5 
II–10 Hydrologic Conditions and Water Quality in an Agricultural Area in Kleberg and Nueces Counties, Texas, 1996–98 

p, pp, 

dissolved

g

Calcium, dissolved mg/L as Ca 150 465 19.5 22 33.2 30 19

Magnesium, dissolved mg/L as Mg 88 264 3.0 5.4 10.5 7.2 3.7

Sodium, dissolved mg/L as Na 890 2,159 25.5 44 92.8 68 25

Potassium, dissolved mg/L as K 11 34 3.4 3.0 6.3 4.6 4.9

Chloride, dissolved mg/L as Cl 1,700 5,288 39.3 82 176 110 30

Sulfate, dissolved mg/L as SO4 270 724 16.2 18 35.1 42 15

Fluoride, dissolved mg/L as F .3 .2 .2 .2 .2 .3 .2

Silica, dissolved mg/L as SiO2 5.2 2.6 8.5 7.6 13.1 9.7 10

Boron, dissolved g/L as B 1,100 2,454 90.5 105 229 201 125

Iron, dissolved g/L as Fe 23 40 4.7 <3 5.6 <10 5.4

Dissolved solids mg/L 3,140 9,410 153 217 417 323 188

Suspended solids mg/L 668 7.5 626 583 529 1,170 1,050

Specific conductance S/cm 5,770 15,937 276 410 775 591 270

Alkalinity mg/L as CaCO3 48 51 59 53.7 79 76 130

pH standard units 7.5 8.0 -- 7.7 -- 7.9 7.9

2,4-DB g/L <.035 <.035 <.035 <.035 <.035 <.24 <.2

2,4-D g/L .69 <.035 <.035 <.035 <.035 .6 .8

2,4,5-T g/L <.035 <.035 <.035 <.035 <.035 <.035 <.0

3-Hydroxycarbofuran g/L <.014 <.014 <.014 <.014 <.014 <.014 <.0

Acetochlor g/L <.002 <.002 <.002 <.002 <.002 <.002 <.0

Amiben g/L <.011 <.011 <.011 <.011 <.011 -- --

Aldicarb g/L <.016 <.016 <.016 <.016 <.016 <.55 <.5

Aldicarb sulfone g/L <.016 <.016 <.016 <.016 <.016 <.10 <.1

Aldicarb sulfoxide g/L <.021 <.021 <.021 <.021 <.021 <.021 <.0

Acifluorfen g/L <.035 <.035 <.035 <.035 <.035 <.035 <.0

Atrazine g/L 47.0 .72 7.31 12.6 .14 .24 .1

Deethylatrazine g/L .75 .18 .21 .28 .024 .17 .1

Alachlor g/L <.002 <.002 <.002 <.002 <.002 <.002 <.0

Azinphos-methyl g/L <.001 <.001 <.001 <.001 <.001 <.001 <.0

Bentazon g/L <.014 <.014 <.014 <.014 <.014 <.014 <.0

Bromacil g/L <.035 <.035 <.035 <.035 <.035 <.035 <.0

Benfluralin g/L <.002 <.002 <.002 <.002 <.002 <.002 <.0

Bromoxynil g/L <.035 <.035 <.035 <.035 <.035 <.035 <.0

Butylate g/L <.002 <.002 <.002 <.002 <.002 <.002 <.0

Carbaryl g/L <.003 <.003 <.003 <.003 <.003 <.008 .0

Carbofuran g/L <.003 .02 .005 <.003 <.003 <.003 <.0

Chloramben g/L <.011 <.011 <.011 <.011 <.011 <.42 <.4

Chlorpyrifos g/L <.004 <.004 <.004 <.004 <.004 <.004 <.0

Cyanazine g/L <.004 <.004 .015 <.004 .008 <.004 <.0

Clopyralid g/L <.05 <.05 <.05 <.05 <.05 <.23 <.2

Chlorothalonil g/L <.035 <.035 <.035 <.035 <.035 <.48 <.4

Dacthal mono-acid g/L <.017 <.017 <.017 <.017 <.017 <.017 <.0

Dicamba g/L <.035 <.035 <.035 <.035 <.035 <.035 <.0

2,6-Diethylaniline g/L <.003 <.003 <.003 <.003 <.003 <.003 <.0

Diuron g/L .21 4.88 11.57 5.74 <.02 <.02 <.0

Dinoseb g/L <.035 <.035 <.035 <.035 <.035 <.035 <.0
Appen

Esfenvalerate g/L <.019 <.019 <.019 <.019 <.019 <.019 

Ethalfluralin g/L <.004 <.004 <.004 <.004 <.004 <.004 

Ethoprop g/L <.003 <.003 <.003 <.003 <.003 <.003 

Fluometuron g/L <.035 <.035 <.035 <.035 <.035 <.035 

Fenuron g/L <.013 <.013 <.013 <.013 <.013 <.013 

Fonofox g/L <.003 <.003 <.003 <.003 <.003 <.003 

BHC alpha g/L <.002 <.002 <.002 <.002 <.002 <.002 

Lindane g/L <.004 <.004 <.004 <.004 <.004 <.004 

Linuron g/L <.002 <.002 <.002 <.002 <.002 <.018 

MCPA g/L <.05 <.05 <.05 <.05 <.05 <.17 

MCPB g/L <.035 <.035 <.035 <.035 <.035 <.14 

Malathion g/L .03 <.005 <.005 <.005 <.005 .032 

Methyl parathion g/L <.006 <.006 <.006 <.006 <.006 <.006 

Metolachlor g/L <.002 .01 .003 <.002 <.002 <.002 

Metribuzin g/L <.004 <.004 <.004 <.004 <.004 <.004 

Molinate g/L <.004 <.004 <.004 <.004 <.004 <.004 

Methomyl g/L <.017 <.017 <.017 <.017 <.017 <.017 

Methiocarb g/L <.026 <.026 <.026 <.026 <.026 <.026 

Napropamide g/L <.003 <.003 <.003 <.003 <.003 <.003 

Norflurazon g/L <.024 <.024 <.024 <.024 <.024 <.024 

Neburon g/L <.015 <.015 <.015 <.015 <.015 <.015 

1-Naphthol g/L <.007 <.007 <.007 <.007 <.007 -- 

Oryzalin g/L <.019 <.019 <.019 <.019 <.019 <.31 

Oxamyl g/L <.018 <.018 <.018 <.018 <.018 <.018 

Parathion g/L <.004 <.004 <.004 <.004 <.004 <.004 

Pebulate g/L <.004 <.004 <.004 <.004 <.004 <.004 

Pendimethalin g/L <.004 .01 .028 .02 <.004 .014 

Permethrin g/L <.005 <.005 <.005 <.005 <.005 <.005 

Phorate g/L <.002 <.002 <.002 <.002 <.002 <.002 

Prometon g/L <.018 <.018 <.018 <.018 <.018 <.018 

Pronamide g/L <.003 <.003 <.003 <.003 <.003 <.003 

Propanil g/L <.004 <.004 <.004 <.004 <.004 <.004 

Propachlor g/L <.007 <.007 <.007 <.007 <.007 <.007 

Propargite g/L <.013 <.013 <.013 <.013 <.013 <.013 

Propoxur g/L <.035 <.035 <.035 <.035 <.035 <.035 

Propham g/L <.035 <.035 <.035 <.035 <.035 <.035 

Picloram g/L <.035 <.035 <.035 <.035 <.035 <.05 

Silvex g/L <.021 <.021 <.021 <.021 <.021 <.021 

Simazine g/L <.005 <.005 .09 .23 .003 <.005 

Thiobencarb g/L <.002 <.002 <.002 <.002 <.002 <.002 

Tebuthiuron g/L <.01 <.01 <.01 <.01 <.01 <.01 

Terbacil g/L <.007 <.007 <.007 <.007 <.007 <.007 

Terbufos g/L <.013 <.013 <.013 <.013 <.013 <.013 

Triallate g/L <.001 <.001 <.001 <.001 <.001 <.001 

Trifluralin g/L .02 .013 .12 .08 .006 .001 

Triclopyr g/L <.05 <.05 <.05 <.05 <.05 <.25 

Glyphosate g/L <5 <5 <10 <5 <5 <5 
Ockerman and Petri— Hydrologic Conditions and Water 

 Kleberg and Nuec

District Chief U.S. Geological Survey 8027 Exchange Dr.Austin, TX 78754–4733