U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5136

In cooperation with the Texas State Soil and Water Conservation Board, 

Coastal Bend Bays and Estuaries Program, and 

Texas AgriLife Research and Extension Center at Corpus Christi

Hydrologic Conditions and Water Quality of Rainfall and 

Storm Runoff for Two Agricultural Areas of the Oso Creek 

Watershed, Nueces County, Texas, 2005–08
Cover: Rainfall on field of grain sorghum, September 2, 2003. 
Hydrologic Conditions and Water Quality 

of Rainfall and Storm Runoff for Two 

Agricultural Areas of the Oso Creek 

Watershed, Nueces County, Texas, 2005–08 

By Darwin J. Ockerman and Carlos J. Fernandez

In cooperation with the Texas State Soil and Water Conservation Board, 

Coastal Bend Bays and Estuaries Program, and 

Texas AgriLife Research and Extension Center at Corpus Christi

Scientific Investigations Report 2010–5136

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

This and other USGS information products are available at http://store.usgs.gov/

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Denver, CO 80225

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ockerman, D.J., and Fernandez, C.J., 2010, Hydrologic conditions and water quality of rainfall and storm runoff for 

two agricultural areas of the Oso Creek watershed, Nueces County, Texas, 2005–08: U.S. Geological Survey Scientific 

Investigations Report 2010–5136, 63 p. 
iii

Acknowledgments

Special thanks are extended to Dr. Bobby Eddleman and Carlos Correa from Texas 

AgriLife Research and Extension Service at Corpus Christi. Dr. Eddleman provided valuable 

project oversight and technical assistance, including compilation of pesticide and fertilizer 

application information obtained from producers in the study subwatersheds. Carlos Correa 

assisted with station and equipment maintenance and sample-collection activities. Thanks also 

are extended to personnel of the Nueces County Soil and Water Conservation District and the 

U.S. Department of Agriculture, Natural Resources Conservation Service (NRCS), for assisting 

in delineating the study subwatersheds and providing contacts with agricultural producers in 

the subwatersheds. John Freeman, NRCS, also assisted with compilation of information on 

application rates of fertilizers and pesticides in the study watersheds.
Blank Page
v

Contents

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

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

Previous Study and Report .................................................................................................................2

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

Description of Study Areas (Subwatersheds) ................................................................................2

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

Data Collection ..............................................................................................................................................5

Rainfall ...................................................................................................................................................5

Streamflow ............................................................................................................................................6

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

Rainfall Sampling ........................................................................................................................6

Runoff Sampling ..........................................................................................................................6

Quality Control and Assurance .................................................................................................8

Hydrologic Conditions ..................................................................................................................................9

Rainfall ...................................................................................................................................................9

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

Water Quality ...............................................................................................................................................10

Rainfall .................................................................................................................................................10

Runoff ...................................................................................................................................................14

Nutrients and Major Inorganic Ions ......................................................................................14

Suspended Sediment ...............................................................................................................19

Pesticides ...................................................................................................................................21

Bacteria ......................................................................................................................................23

Summary .......................................................................................................................................................32

References ...................................................................................................................................................33

Appendixes 

1. Quality Control and Assurance Data for Rainfall Samples .................................................39

2. Water-Quality Data for Runoff Samples Collected at West Oso Creek ............................46

3. Water-Quality Data for Runoff Samples Collected at Oso Creek Tributary ......................55

Figures

1. Map showing Coastal Bend Bays and Estuaries Program (CBBEP) area, South 

Texas, and Oso Creek watershed area, Nueces County, Texas ..........................................3

2. Chart showing typical timing of agricultural activities in Nueces County, Texas .............4

3. Photograph showing streamflow-gaging and water-quality monitoring station 

08211525, unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, 

Texas, September 19, 2006 .........................................................................................................6

4. Hydrograph showing cumulative rainfall, streamflow, and subsample-collection 

timing at station 08211517, West Oso Creek at Merrett Road near Corpus Christi, 

Texas, during storm runoff, March 14, 2007 ............................................................................7

5. Photograph showing suspended-sediment sample collection at station 08211517, 

West Oso Creek at Merrett Road near Corpus Christi, Texas, June 2, 2006 ......................8
vi

6–9. Graphs showing:

6. Mean monthly rainfall for National Weather Service station 412015, Corpus 

Christi International Airport, 1960–2008; and monthly area-weighted rainfall 

for West Oso Creek and Oso Creek tributary subwatersheds (combined), 

Nueces County, Texas, water years 2006–08 ...............................................................10

7. Rainfall/runoff response for West Oso Creek and Oso Creek tributary 

subwatersheds, Nueces County, Texas, March 12–18, 2007 .....................................11

8. Daily deposition of total nitrogen computed from event (daily) rainfall and 

sample concentrations and estimated by regression with daily rainfall, each 

relative to daily rainfall, for sampled rainfall events, Oso Creek watershed, 

Nueces County, Texas, water years 2006–08 ...............................................................17

9. Relations between suspended-sediment load and streamflow for West 

Oso Creek (station 08211517) and Oso Creek tributary (station 082115725) 

subwatersheds, Nueces County, Texas, water years 2006–08 .................................23

Tables

1. Monitoring stations providing data used for analysis of hydrologic and waterquality conditions in agricultural areas of the Oso Creek watershed, Nueces 

County, Texas ................................................................................................................................5

2. Estimated annual (water years 2006–08) rainfall for West Oso Creek and Oso 

Creek tributary subwatersheds, Nueces County, Texas, and long-term average 

annual (water years 1960–2008) rainfall measured at National Weather Service 

station 412015, Corpus Christi International Airport ............................................................12

3. Annual (water years 2006–08) runoff from West Oso Creek and Oso Creek 

tributary subwatersheds and long-term average annual (water years 1973–2008) 

runoff from Oso Creek watershed, Nueces County, Texas .................................................12

4. Rainfall, runoff, and runoff/rainfall coefficients (ratio of runoff volume to rainfall volume) for selected rainfall and runoff events during water years 2006–08, West 

Oso Creek and Oso Creek tributary subwatersheds, Nueces County, Texas ..................13

5. Event (daily) rainfall, Texas AgriLife Research at Corpus Christi weather station 

(WS1); and event concentrations of selected constituents in rainfall samples, 

Texas AgriLife Research at Corpus Christi rainfall-quality sampling station 

(08211511), Nueces County, Texas, water years 2006–08 ...................................................14

6. Summary statistics for event rainfall, Texas AgriLife Research at Corpus Christi 

weather station (WS1); and event concentrations of selected nutrients in rainfall 

samples, Texas AgriLife Research at Corpus Christi rainfall-quality sampling 

station (08211511), Nueces County, Texas, water years 2006–08 ......................................16

7. Estimated monthly and annual total nitrogen rainfall deposition on West Oso 

Creek and Oso Creek tributary subwatersheds, Nueces County, Texas, water 

years 2006–08 .............................................................................................................................18

8. Summary statistics for event concentrations of selected nutrients and major 

inorganic ions in runoff samples, West Oso Creek (station 08211517) and Oso 

Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, water 

years 2006–08 .............................................................................................................................18

9. Event-median concentrations of selected nutrients and major inorganic ions in 

runoff samples, West Oso Creek (station 08211517) and Oso Creek tributary (station 

08211525) subwatersheds, Nueces County, Texas, water years 2006–08; and results 

of Wilcoxon rank-sum tests to indicate whether event-median concentrations for 

the two subwatersheds are significantly different ..............................................................19
vii

10. Estimated monthly and annual runoff loads of selected nutrients from West Oso 

Creek (station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................20

11. Estimated annual and average annual runoff yields of selected nutrients from 

West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, water years 2006–08 ..........................................22

12. Fertilizer applications, rainfall deposition, and runoff yields of total nitrogen and 

total phosphorus for West Oso Creek and Oso Creek tributary subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................24

13. Results of analyses for samples of suspended sediment in runoff from West Oso 

Creek (station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................25

14. Regression equations relating suspended-sediment load to streamflow for 

West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, water years 2006–08 ..........................................25

15. Estimated monthly and annual runoff suspended-sediment loads, West Oso Creek 

(station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................26

16. Annual and average annual runoff yields of suspended sediment, West Oso Creek 

(station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................26

17. Pesticides used in West Oso Creek and Oso Creek tributary subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................26

18. Summary statistics of concentrations of selected pesticides in runoff samples 

from West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, water years 2006–08 ..........................................27

19. Event-median concentrations of selected pesticides in runoff samples from 

West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, by season (preharvest or harvest/

postharvest), water years 2006–08, used to estimate runoff loads for unsampled 

events ..........................................................................................................................................29

20. Estimated monthly and annual runoff loads of selected pesticides from West Oso 

Creek (station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................29

21. Estimated annual and average annual application rates and runoff yields of 

selected pesticides from West Oso Creek and Oso Creek tributary subwatersheds, 

Nueces County, Texas, water years 2006–08 ........................................................................30

22. Bacteria densities in runoff samples from West Oso Creek (station 08211517) and 

Oso Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, 

water years 2006–08 ..................................................................................................................30

23. Summary statistics of bacteria densities in runoff samples from West Oso Creek 

(station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, 

Nueces County, Texas, water years 2006–08; and Texas surface-water-quality 

standards for bacteria in Oso Bay and Oso Creek, Texas Commission on 

Environmental Quality segments 2485 and 2485A, respectively ........................................32
viii

Conversion Factors, Datum, and Water-Quality 

Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg)

ton, short (2,000 lb) 0.9072 megagram (Mg)

ton per day (ton/d) 0.9072 megagram per day (Mg/d)

Application rate

pound per acre (lb/acre) 1.121 kilogram per hectare (kg/ha)

pound per acre per year [(lb/acre)/yr] 1.121 kilogram per hectare per year [(kg/ha)/yr] 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Water-Quality Abbreviations

CFU/100 mL, colony forming units per 100 milliliters

mg/L, milligrams per liter

μg/L, micrograms per liter

μm, micrometers

μS/cm, microsiemens per centimeter at 25 degrees Celsius

mm, millimeters
Abstract 

The U.S. Geological Survey, in cooperation with the 

Texas State Soil and Water Conservation Board, Coastal Bend 

Bays and Estuaries Program, and Texas AgriLife Research 

and Extension Center at Corpus Christi, studied hydrologic 

conditions and water quality of rainfall and storm runoff of 

two primarily agricultural subwatersheds of the Oso Creek 

watershed in Nueces County, Texas. One area, the upper West 

Oso Creek subwatershed, is about 5,145 acres. The other 

area, a subwatershed drained by an unnamed tributary to Oso 

Creek (hereinafter, Oso Creek tributary), is about 5,287 acres. 

Rainfall and runoff (streamflow) were continuously monitored 

at the outlets of the two subwatersheds during the study period 

October 2005–September 2008. Seventeen rainfall samples 

were collected and analyzed for nutrients and major inorganic 

ions. Twenty-four composite runoff water-quality samples 

(12 at West Oso Creek, 12 at Oso Creek tributary) were collected and analyzed for nutrients, major inorganic ions, and 

pesticides. Twenty-six discrete suspended-sediment samples 

(12 West Oso Creek, 14 Oso Creek tributary) and 17 bacteria 

samples (10 West Oso Creek, 7 Oso Creek tributary) were 

collected and analyzed. These data were used to estimate, for 

selected constituents, rainfall deposition to and runoff loads 

and yields from the two subwatersheds. Quantities of fertilizers and pesticides applied in the two subwatersheds were 

compared with quantities of nutrients and pesticides in rainfall 

and runoff. 

For the study period, total rainfall was greater than 

average. Most of the runoff from the two subwatersheds 

occurred in response to a few specific storm periods. The 

West Oso Creek subwatershed produced more runoff during 

the study period than the Oso Creek tributary subwatershed, 

13.95 inches compared with 9.45 inches. Runoff response 

was quicker and peak flows were higher in the West Oso 

Creek subwatershed than in the Oso Creek tributary subwatershed. Total nitrogen runoff yield for the 3-year study period 

averaged 2.62 pounds per acre per year from the West Oso 

Creek subwatershed and 0.839 pound per acre per year from 

the Oso Creek tributary subwatershed. Total phosphorus yields 

from the West Oso Creek and Oso Creek tributary subwatersheds for the 3-year period were 0.644 and 0.419 pound per 

acre per year, respectively. Runoff yields of nitrogen and phosphorus were relatively small compared to inputs of nitrogen in 

fertilizer and rainfall deposition. Average annual runoff yield 

of total nitrogen (subwatersheds combined) represents about 

2.5 percent of nitrogen applied as fertilizer to cropland in the 

watershed and nitrogen entering the subwatersheds through 

rainfall deposition. Average annual runoff yield of total phosphorus (subwatersheds combined) represents about 4.0 percent 

of the phosphorus in applied fertilizer and rainfall deposition. 

Suspended-sediment yields from the West Oso Creek subwatershed were more than twice those from the Oso Creek 

tributary subwatershed. The average suspended-sediment yield 

from the West Oso Creek subwatershed was 522 pounds per 

acre per year and from the Oso Creek tributary subwatershed 

was 139 pounds per acre per year. Twenty-four herbicides and 

eight insecticides were detected in runoff samples collected 

at the two subwatershed outlets. At the West Oso Creek site, 

19 herbicides and 4 insecticides were detected; at the Oso 

Creek tributary site, 18 herbicides and 6 insecticides were 

detected. Fourteen pesticides were detected in only one sample 

at low concentrations (near the laboratory reporting level). 

Atrazine and atrazine degradation byproduct 2-chloro-4-

isopropylamino-6-amino-s-triazine (CIAT) were detected in all 

samples. Glyphosate and glyphosate byproduct aminomethylphosphonic acid (AMPA) were detected in all samples collected and analyzed during water years 2006–07 but were not 

included in analysis for samples collected in water year 2008. 

Of all pesticides detected in runoff, the highest runoff yields 

were for glyphosate, 0.012 and 0.001 pound per acre per year 

for the West Oso Creek and Oso Creek tributary subwatersheds, respectively. About 0.9 percent of glyphosate applied 

to the West Oso Creek subwatershed was detected in runoff 

and for the Oso Creek tributary subwatershed, about 0.08 percent. At both subwatershed outlet sites, most sample concentrations of fecal coliform, Escherichia coli, and Enterococci

were greater than Texas surface-water-quality standards for 

Hydrologic Conditions and Water Quality of Rainfall and 

Storm Runoff for Two Agricultural Areas of the Oso Creek 

Watershed, Nueces County, Texas, 2005–08

By Darwin J. Ockerman1 and Carlos J. Fernandez2

1

 U.S. Geological Survey, San Antonio, Texas.

2

 Texas AgriLife Research and Extension Center at Corpus Christi, Texas.
2 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

those bacteria established for the receiving waters of Oso 

Creek and Oso Bay. Because runoff and associated bacteria 

concentrations represent relatively brief and infrequent conditions, the resulting effect on Oso Creek and Oso Bay is not 

known.

Introduction

The Coastal Bend Bays and Estuaries of South Texas is 1 

of 28 estuaries or groups of estuaries in the United States that 

have been designated nationally important and as such are part 

of the National Estuaries Program of the U.S. Environmental 

Protection Agency (2008). The Coastal Bend Bays and 

Estuaries Program (CBBEP) area (fig. 1) encompasses the 12 

counties of the Coastal Bend Council of Governments extending from the land cut in the Laguna Madre north along the 

coast to the Aransas National Wildlife Refuge (Coastal Bend 

Bays and Estuaries Program, 2008). The bays and estuaries of 

the CBBEP are affected by nonpoint-source runoff from agricultural land that constitutes about 88 percent of the CBBEP 

area. 

The Oso Creek watershed drains about 234 square miles 

(mi2) to Oso Bay. Agricultural land (pasture and cropland) 

accounts for about 69 percent of the Oso Creek watershed. 

Oso Bay, along the southern shore of Corpus Christi Bay, is 

relatively small (surface area about 7 mi2) and shallow (average depth about 2.3 feet [ft]) (Quenzer and others, 1998). 

Ecologically, Oso Bay provides habitat for many plants and 

animals and is important for water purification and storm protection (Texas Commission on Environmental Quality, 2007).

Previous Study and Report

The U.S. Geological Survey (USGS), in cooperation with the Texas State Soil and Water Conservation 

Board (TSSWCB), CBBEP, and Texas AgriLife Research 

and Extension Center at Corpus Christi (formerly Texas 

Agricultural Experiment Station–Corpus Christi and hereinafter referred to as Texas AgriLife Research), conducted 

a study of hydrologic conditions and quality of rainfall and 

storm runoff of two primarily agricultural subwatersheds of 

the Oso Creek watershed (fig. 1) during the period October 

2005–September 2007. As a result of the study, a USGS 

Scientific Investigations Report (SIR) (Ockerman, 2008) was 

published documenting hydrologic conditions and water quality of rainfall and storm runoff for the agricultural study areas 

during water years 2006–07 (October 2005–September 2007). 

(A water year is defined as the 12-month period October 1, for 

any given year, through September 30, of the following year; 

the water year is designated by the calendar year in which it 

ends.)

As an extension of the 2005–07 study, the USGS, in 

cooperation with the TSSWCB, CBBEP, and Texas AgriLife 

Research continued data collection at the Oso Creek study 

subwatersheds during October 2007–September 2008 to 

further characterize hydrologic and water-quality conditions in 

the Oso Creek watershed. 

Purpose and Scope

The purpose of this report is to characterize hydrologic 

conditions and the water quality of rainfall and storm runoff 

for two primarily agricultural subwatersheds in the Oso 

Creek watershed in Nueces County. The report presents the 

results of collection and analysis of hydrologic data and 

water-quality samples of rainfall and runoff during the 3-year 

data-collection period during October 2005–September 2008. 

This report supersedes the previous report (Ockerman, 2008) 

and includes results and analysis of all data collected during 

the 2005–08 study period. Continuous rainfall and streamflow data were collected at monitoring stations at the outlets 

of the study subwatersheds. Seventeen rainfall samples were 

collected and analyzed for nutrients and major inorganic ions. 

Rainfall nutrient analyses and daily rainfall totals were used 

to estimate rainfall nutrient deposition to the study subwatersheds. Twenty-four runoff samples were collected by 

automatic sampling equipment during storm-runoff events. 

These samples were analyzed for nutrients, major inorganic 

ions, and pesticides. Twenty-six discrete samples of runoff 

suspended sediment were collected and analyzed for sediment 

size and concentration. Seventeen discrete samples of runoff 

were collected and analyzed for fecal coliform, Escherichia 

coli (E. coli), and Enterococcus bacteria. These data were 

used to estimate, for selected constituents, rainfall deposition 

to and runoff loads and yields from the study subwatersheds. 

Quantities of fertilizers and pesticides applied in the subwatersheds were compared with quantities of nutrients and 

pesticides in rainfall and runoff. Estimates of rainfall deposition and runoff constituent loads and yields for 2006–07 in the 

earlier report of the study watersheds (Ockerman, 2008) were 

revised in this report on the basis of collection and analysis of 

additional data.

Description of Study Areas (Subwatersheds)

Of the two subwatersheds studied (fig. 1), one surrounds 

the upstream reaches of West Oso Creek and drains about 

5,145 acres to Oso Bay. The other surrounds the upstream 

reaches of an unnamed tributary to Oso Creek (hereinafter, 

Oso Creek tributary) and drains about 5,287 acres to Oso 

Bay. The topography of the areas is flat with elevations 

ranging from about 15 ft above sea level where Oso Creek 

enters Oso Bay to about 65 ft above sea level in the upland 

parts of the West Oso Creek subwatershed. Because of 

negligible relief, the Oso Creek tributary subwatershed might 

include some noncontributing drainage area. The streams of 

the study subwatersheds are ephemeral, producing runoff 

lasting from a few hours to several weeks, depending on 

rainfall duration and intensity and antecedent soil moisture. 
Introduction 3

Figure 1. Coastal Bend Bays and Estuaries Program (CBBEP) area, South Texas, and Oso Creek watershed area, Nueces 

County, Texas. 

Oso Creek

West Oso Creek

Oso Bay

Corpus Christi 

Bay

FM 43

FM 1694

FM 70

FM 665 FM 763

County Hwy 67

County Hwy 54

County Hwy 24

FM 1694

FM 1694

Robstown Corpus Christi

8 1,2

6

5

7

4

3

Oso Creek tributary

97°35'30" 97°30' 97°24'30" 97°19'

27°47'30"

27°42'

27°36'30"

Base modified from U.S. Geological Survey

digital data 1:24,000 quadrangles

Universal Transverse Mercator Projection, 

Zone 14

North American Datum of 1983

1

5

3

7

Study area (subwatershed)

Boundary of Oso Creek watershed

Site description—Number referenced in table 1

Texas AgriLife Research weather station and 

U.S. Geological Survey rainfall sampling station

Long-term U.S. Geological Survey streamflowgaging station 

U.S. Geological Survey rain gage and streamflowgaging and water-quality sampling station 

National Weather Service rain gage

EXPLANATION

0 1.5 3 4.5 6 MILES

TEXAS

CBBEP area

LOCATION MAP

CHOKE CANYON 

RESERVOIR

NUECES

LAKE CORPUS

CHRISTI

GULF OF MEXICO

Corpus Christi

Bay

Copano

Bay

RIVER

Oso Creek

 watershed

 area

ARANSAS

DUVAL

BEE

KENEDY BROOKS

LIVE OAK

NUECES

MCMULLEN

KLEBERG

REFUGIO

SAN PATRICIO JIM WELLS

Aransas

National

Wildlife

Refuge

Laguna Madre

Land cut

Corpus Christi

International 

Airport
4 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

The main stem of Oso Creek maintains a continuous flow 

from wastewater discharges by the cities of Robstown and 

Corpus Christi. There are no wastewater discharges in the two 

subwatersheds.

The Oso Creek watershed area is described as having a 

subtropical, subhumid climate characterized by hot summers 

and mild, dry winters (Larkin and Bomar, 1983). Maximum 

annual rainfall tends to occur in spring, early summer, and fall 

but can occur anytime during the year. The following meteorological statistics are from the National Weather Service (NWS) 

station at the Corpus Christi International Airport (U.S. 

Department of Commerce, National Climatic Data Center, 

2006). Average annual rainfall (1971–2000) is 32.92 inches 

(in.); rainfall greater than 0.01 in. occurs, on average, 82 days 

per year. The average monthly low temperatures range from 

46.2 degrees Fahrenheit (oF) in January to 74.5 oF in August; 

average monthly high temperatures range from 66.0 oF in 

January to 93.4 oF in August; and mean annual temperature is 

71.5 oF.

Besides climate and rainfall, the type and nature of the 

soils affect the rainfall-runoff process. Victoria Association 

clays are the dominant soil in the study subwatersheds (U.S. 

Department of Agriculture, Soil Conservation Service, 1965). 

During dry periods these soils crack and absorb water rapidly. 

These natural processes act to reduce runoff. However, once 

the soils are wet, water infiltrates slowly, which increases the 

potential for runoff. Thus moisture conditions prior to the 

onset of substantial rainfall are an important factor affecting 

runoff.

Agriculture is the predominant land use within each 

of the study subwatersheds. Land use in each subwatershed 

is similar, consisting almost entirely (about 98 percent) of 

cropland. Impervious land (mostly roads) in the study subwatersheds makes up about 2 percent of the total study area (Dr. 

Bobby Eddleman, Texas AgriLife Research, written commun., 

2007). During 2006–08, the primary crops in the West Oso 

Creek and Oso Creek tributary subwatersheds were cotton and 

grain sorghum, accounting for about 92 percent of the total 

agricultural land. Corn and wheat accounted for about 6 

percent of the acreage. The remaining 2 percent was fallow 

or pasture (Dr. Bobby Eddleman, Texas AgriLife Research, 

written commun., 2007; Dr. Carlos Fernandez, Texas AgriLife 

Research, written commun., 2009). 

Beginning in late July through early September, crops are 

harvested and remaining crop residue is shredded and stubble 

is plowed. Postcrop emergents are controlled with herbicides 

(in the conservation tillage systems); any residual vegetation is 

killed as well. The process of tilling and spraying the fields 

after harvest helps prepare the ground to absorb fall and 

winter rains (fig. 2) (Dr. Bobby Eddleman, Texas AgriLife 

Research, written commun., 2007). The soil is retilled or 

resprayed during September and October to destroy winter 

weeds and to prepare the seedbed for the succeeding crop. 

Preplant fertilizers and broadcast preemergent herbicides 

usually are applied during the low-rainfall months of 

November and December. From January to mid-February, 

additional tilling might be done to prepare for planting. 

Planting begins in early to late March, depending on the 

available soil moisture, soil temperature, and type of crop. 

After crops have emerged from the soil, production practices 

throughout the season consist of pesticide application and row 

cultivation. From mid- to late April through early July, few 

field operations are required for grain sorghum. In contrast, 

this period involves ongoing insect control for cotton. Grain 

sorghum usually is harvested in early to mid-July. Cotton 

harvest usually begins with application of harvest aids (defoliants) around mid-July and ends around mid-August to early 

September.

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

Shredding previous crop and 

retilling

Applying fertilizers and 

herbicides

Tilling in preparation for 

planting

Planting

Applying pesticides and 

cultivating

Controlling insects (cotton)

Harvesting (cotton)

Figure 2. Typical timing of agricultural activities in Nueces County, Texas. 
Data Collection 5

Texas Surface-Water-Quality Standards 

The Texas Commission on Environmental Quality 

(TCEQ) has designated Texas surface-water-quality standards 

(TSWQS) and appropriate uses (such as aquatic life, contact or noncontact recreation, or drinking water) for specific 

stream, estuary, and bay segments (Texas Commission on 

Environmental Quality, 2006). TSWQS have not been established for either specific streams or subwatersheds in this 

study (West Oso Creek and Oso Creek tributary). However, 

the two subwatersheds drain to Oso Creek and Oso Bay, which 

are water bodies with TSWQS. The TSWQS are defined by a 

combination of designated uses and criteria necessary to maintain the designated uses. The designated uses for Oso Bay are 

contact recreation and aquatic life. Oso Creek (TCEQ segment 

2485A) is considered an unclassified water body associated 

with a classified segment, Oso Bay (TCEQ segment 2485). 

Oso Creek, segment 2485A, includes all of Oso Creek to its 

confluence with Oso Bay. Oso Creek and Oso Bay are on the 

“2008 Texas Water Quality Inventory and 303d List” (Texas 

Commission on Environmental Quality, 2008) for elevated 

bacteria counts. Oso Bay also is listed for depressed dissolved 

oxygen concentration. 

Data Collection 

Monitoring stations providing data for this study are 

listed in table 1 and include an hourly weather station with 

rain gage operated by Texas AgriLife Research (site 1); a 

USGS rainfall-quality sampling station (site 2); two USGS 

rainfall, streamflow, and water-quality sampling stations 

located at the outlets of the study subwatersheds (sites 3, 4); a 

long-term USGS streamflow-gaging station on the main stem 

of Oso Creek (site 5); and three NWS rain gages (sites 6, 7, 8). 

Rainfall

Rainfall amounts in the Oso Creek watershed area were 

obtained from six monitoring stations (fig. 1; table 1). Hourly 

and daily rainfall data were recorded at the Texas AgriLife 

Research weather station (site 1). Two tipping-bucket rain 

gages were installed, one at each monitoring station at the 

outlet of each study subwatershed (sites 3, 4). These stations recorded 15-minute rainfall totals. Also, daily rainfall 

data were obtained from three NWS rain gages (sites 6, 7, 8). 

Daily rainfall in the West Oso Creek study subwatershed was 

estimated by using a Thiessen-weighted average (Wanielista, 

1990) of rainfall measured at sites 1, 3, and 8. Daily rainfall 

in the Oso Creek tributary subwatershed was estimated by 

using rainfall primarily from site 4. The NWS Corpus Christi 

Botanical Gardens station (site 7) was used for estimating 

daily rainfall on the Oso Creek tributary subwatershed for 

days when data were not available from site 4, including all of 

the period October 2007–September 2008. Rainfall data from 

the NWS Corpus Christi International Airport station (site 6) 

were used to represent long-term rainfall conditions in both 

subwatersheds.

Table 1. Monitoring stations providing data used for analysis of hydrologic and water-quality conditions in agricultural areas of the 

Oso Creek watershed, Nueces County, Texas.

[Texas AgriLife Research, Texas AgriLife Research and Extension Center at Corpus Christi; USGS, U.S. Geological Survey; NWS, National Weather Service; 

--, unknown]

Site 

number

(fig. 1)

Station name, number

Latitude 

(degrees 

minutes 

seconds)

Longitude 

(degrees 

minutes 

seconds)

Type of 

data

Period of 

record used

1 Weather station WS1, Texas AgriLife Research at Corpus 

Christi, Tex. 

27°46'57" 97°33'43" Hourly and daily 

rainfall

Oct. 2005–

Sept. 2008

2 USGS station 08211511, Texas AgriLife Research at Corpus 

Christi, Tex.

27°46'57" 97°33'43" Rainfall quality Oct. 2005–

Sept. 2008

3 USGS station 08211517, West Oso Creek at Merrett Road 

near Corpus Christi, Tex.

27°43'50" 97°34'37" Rainfall, streamflow, 

water quality

Oct. 2005–

Sept. 2008

4 USGS station 08211525, Unnamed Oso Creek tributary at 

Farm Road 2444 near Corpus Christi, Tex.

27°39'07" 97°26'40" Rainfall, streamflow, 

water quality

Oct. 2005–

Sept. 2008

5 USGS station 08211520, Oso Creek at Corpus Christi, Tex. 27°42'40" 97°30'06" Streamflow Oct. 1972–

Sept. 2008

6 NWS station 412015, Corpus Christi International Airport 27°46'--" 97°31'--" Hourly and daily 

rainfall

January 1972–

Sept. 2008

7 NWS station 412013, Corpus Christi Botanical Gardens 27°39'--" 97°24'--" Daily rainfall Oct. 2005–

Sept. 2008

8 NWS station 417677, Robstown, Tex. 27°47'--" 97°40'--" Daily rainfall Oct. 2005–

Sept. 2008
6 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Streamflow

Water-surface elevation (stage) was continuously 

monitored (at 15-minute intervals) at the study subwatershed 

stations (sites 3, 4, table 1) by up-looking acoustic transducers 

mounted on the streambeds (SonTek, 2010). At each station, 

relations between stage and runoff (streamflow) were developed from streamflow measurements made during various 

flow conditions during runoff events (Buchanan and Somers, 

1969). From these relations, continuous runoff was computed 

for each study subwatershed (Kennedy, 1984).

Water Quality

Water-quality samples were collected for two sources, 

rainfall and runoff. Rainfall-quality samples were collected to 

measure nutrient concentrations and compute nutrient deposition to the study subwatersheds. Runoff-quality samples 

were collected to characterize runoff quality and to estimate 

constituent loads and yields of selected nutrients, suspended 

sediment, and pesticides transported from the study subwatersheds. Also, runoff-quality samples were collected during 

runoff events and analyzed for fecal coliform, E. coli, and 

Enterococci bacteria. 

Rainfall Sampling

Rainfall samples for nutrients and major inorganic ions 

were collected at the USGS rainfall-quality sampling station (site 2, table 1) by an automatic rainfall sampler. The 

automatic rainfall sampler is equipped with a polyethylene 

bucket that is covered to prevent contamination of the bucket 

and sample when rainfall is not occurring. A moisture sensor 

activates a mechanism to uncover the sample-collection bucket 

when rainfall begins and to cover the sample when rainfall 

ends. Rainfall samples were collected as a single composite 

sample for rainfall events. Results of the analyses of the composite samples represent the average constituent concentrations during rainfall events. As soon as possible after rainfall, 

samples were retrieved from the rainfall sample collector and 

immediately placed on ice. With the use of a Teflon churn, the 

composite rainfall sample was then split into various sample 

containers for overnight shipment to the USGS National 

Water Quality Laboratory (NWQL), Lakewood, Colo., for 

analysis. Part of the sample was bottled as unfiltered samples, 

representing whole-water samples. Analyses of unfiltered 

samples provided concentrations of total phosphorus and total 

Kjeldahl nitrogen (ammonia nitrogen plus organic nitrogen). 

Some samples were filtered through 0.45-micrometer (μm) 

pore-diameter filters. Analyses of filtered samples provided 

concentrations of dissolved ammonia as N (nitrogen), nitrite 

plus nitrate nitrogen, phosphorus, and orthophosphate phosphorus. Major inorganic ion samples also were filtered by 

using a 0.45-μm pore-diameter filters. Analytical methods are 

described in Fishman (1993), Fishman and Friedman (1989), 

Patton and Truitt (2000), and U.S. Environmental Protection 

Agency (1993).

Runoff Sampling

Automatic water samplers at the runoff-monitoring stations (sites 3, 4, table 1) collected runoff samples for nutrients, 

major inorganic ions, and pesticides during storm events. 

When runoff was detected by the streamflow-gaging instrumentation (fig. 3), automatic water-quality samplers were 

activated to collect samples. Discrete aliquots (water-quality 

subsamples) were collected hourly during a period of runoff. 

Near or at the end of the runoff event, sampling was completed 

and the aliquots from each site were combined into a single 

streamflow-weighted composite sample (separate composite 

samples from each station). For example, an aliquot collected 

when streamflow measured 10 cubic feet per second (ft3/s) 

would have twice the volume in the composite sample as an 

aliquot that was collected when streamflow measured 5 ft3

/s. 

In this way, analysis of the composite sample represents the 

event-mean concentration (EMC) during runoff (Huber, 1993, 

p. 14.1). Figure 4 shows a rainfall-streamflow hydrograph at 

West Oso Creek for March 14, 2007, including the subsamplecollection timing during the event. As runoff samples were 

retrieved from the autosampler they were immediately chilled 

to preserve their composition (Komor and Hansen, 2003). 

When sample collection was complete, portions of each 

individual aliquot, based on flow at the time of aliquot collection, were combined in a Teflon churn. Samples were then 

processed from the churn and bottled as either unfiltered or 

filtered samples (Wilde and others, 2004) and submitted for 

laboratory analysis to the NWQL. Similar to rainfall samples, 

nutrient samples were processed from the churn and bottled as 

unfiltered and filtered (0.45-μm pore-diameter filters) samples 

for laboratory analysis. Major inorganic ion samples were 

filtered through 0.45-μm pore-diameter filters, and pesticide 

Figure 3. Streamflow-gaging and water-quality monitoring 

station 08211525, unnamed Oso Creek tributary at Farm Road 2444 

near Corpus Christi, Texas, September 19, 2006. 
Data Collection 7

samples were filtered through 0.7-μm pore-diameter filters. 

Analytical methods are described in Fishman (1993), Fishman 

and Friedman (1989), Furlong and others (2001), Lindley and 

others (1996), Madsen and others (2003), Patton and Truitt 

(2000), Sandstrom and others (2001), U.S. Environmental 

Protection Agency (1993), and Zaugg and others (1995).

Suspended-sediment samples also were collected during 

storm-runoff events. Suspended-sediment samples were collected as discrete samples during various streamflow conditions. Samples were collected by the flow-weighted, depthintegrating, equal-width increment (EWI) method (sampler 

is raised and lowered through the water column at equal 

intervals across the stream) (U.S. Geological Survey, 2006) 

with isokinetic samplers US DH-48 and US DH-59 (Federal 

Interagency Sedimentation Project, 2010). An isokinetic 

sampler collects a water-sediment sample from the stream 

at a rate such that the velocity of the intake nozzle is equal 

to the incident stream velocity at the nozzle entrance. The 

water-sediment sample collected is thus representative of the 

suspended-sediment load throughout the channel cross section 

and is appropriate for use in estimating sediment load carried 

by the stream (Davis, 2005). Suspended-sediment samples 

were collected by wading (fig. 5) or with bridge-suspended 

equipment during relatively large streamflows. During collection of a suspended-sediment sample at a particular station, 

samples collected at each vertical section in the stream were 

combined in a Teflon churn and processed as a composite 

sample for analysis. The composite sample represents the 

flow-weighted average suspended-sediment concentration 

during the period of sample collection. Suspended-sediment 

samples were analyzed by the USGS sediment laboratory 

in Iowa City, Iowa. Samples were analyzed for suspendedsediment concentration and sand-break analysis (Guy, 1969). 

Sand-break analysis gives the percentage of sediment by 

weight that is finer than 0.062 millimeter (mm). Particle sizes 

less than 0.062 mm are defined as silt and clay. Particles 

greater than 0.062 mm are defined as sand.

Discrete bacteria samples were collected during several 

events at each of the sampling stations. Samples were collected by Texas AgriLife Research personnel and analyzed 

by the Texas A&M University Environmental Microbiology 

Laboratory in Corpus Christi. Bacteria samples were collected 

Figure 4. Hydrograph showing cumulative rainfall, streamflow, and subsample-collection timing at station 08211517, West Oso Creek at 

Merrett Road near Corpus Christi, Texas, during storm runoff, March 14, 2007. 

Cumulative rainfall

Streamflow

Subsample-collection timing

0000

0100

0200

0300

0400

0500

0600

0700

0800

0900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

2300

0000

MARCH 14, 2007

0

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

CUMULATIVE RAINFALL, IN INCHES

STREAMFLOW, IN CUBIC FEET PER SECOND

0

10

20

30

40

50

60

70

80

90

100
8 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

as grab samples from the centroid of flow and analyzed by 

membrane filtration for fecal coliform (American Public 

Health Association, 1998), E. coli (American Public Health 

Association, 1998), and Enterococci (U.S. Environmental 

Protection Agency, 2000). The objective for bacteria sampling 

was a single sample collected as soon as possible after runoff 

began. Accordingly, samples were collected within several 

hours of the beginning of runoff and do not necessarily represent average concentrations during the entire runoff event.

Quality Control and Assurance

Quality control (QC) samples, designed to ensure the 

integrity of water-quality data analyzed in this report, represented more than 10 percent of field samples collected. QC 

samples consisted of five field blanks that were collected and 

analyzed to evaluate any contamination, as well as bias and 

variability of data, which might have resulted from sample 

collection, processing, transportation, and lab analysis. These 

samples were collected by passing USGS-certified, highly 

purified water through the same equipment used to collect and 

process routine water-quality samples. These blank samples 

were then collected, processed, and analyzed in the same way 

as routine environmental samples. 

Two field blank samples were collected at the Texas 

AgriLife Research rainfall-quality site (station 08211511) 

and analyzed for nutrients and inorganic ions. Two field blank 

samples were collected at the West Oso Creek subwatershed 

outlet site (station 08211517). The first of these samples, collected in November 2006, was analyzed for pesticides. The 

second sample, collected in October 2007, was analyzed for 

nutrients and inorganic ions. One blank sample was collected 

at the Oso Creek tributary subwatershed outlet site (station 

08211525) in March 2006 and analyzed for nutrients and inorganic ions. Field blank samples were not collected or analyzed 

for sediment or bacteria samples. QC and assurance methods 

for sediment and bacteria focused on laboratory analytical 

procedures.

Most compounds were not detected in the blank samples, 

but if detected the reported concentrations were less than or 

near the laboratory reporting level for the compounds (appendix 1). The concentration of the laboratory reporting level is 

reported with a “less than” (<) remark code (appendix 1) for 

samples in which the analyte was not detected. Concentrations 

reported by the NWQL for pesticides are flagged as “estimated” (indicated by an “E” where reported) when they are 

qualitatively identified as present, but the reported concentrations have a greater uncertainty than usual (Childress and 

others, 1999). Cases that result in an “estimated” flag include 

concentrations that are less than the laboratory reporting level 

but still detectable in the analyst’s judgment. Estimated flags 

also are associated with any detection of the atrazine degradation compounds 2-chloro-4-isopropylamino-6-amino-striazine (CIAT) and 2-chloro-6-ethylamino-4-amino-s-triazine 

(CEAT). Estimated flags are automatically applied to any 

CIAT and CEAT detections because of poor performance 

results during laboratory quality-assurance testing (U.S. 

Geological Survey, 2010). Estimated concentrations were 

included in computations of event loads and yields and for 

statistical analyses. Laboratory reporting levels for some compounds changed during the study. The NWQL updates laboratory reporting levels yearly on the basis of results of internal 

QC sample analyses.

Reported concentrations for certain nutrient constituents 

are not analyzed directly but are computed from other constituent concentrations. Computed constituents included dissolved 

(filtered) and total (unfiltered) nitrogen, dissolved and total 

organic nitrogen, and dissolved nitrate nitrogen. Laboratory 

analyses are performed explicitly for ammonia nitrogen, nitrite 

nitrogen, dissolved nitrite plus nitrate nitrogen, ammonia plus 

dissolved organic nitrogen, and ammonia plus total organic 

nitrogen. Dissolved nitrogen is computed as the sum of nitrite 

plus nitrate nitrogen and ammonia plus dissolved organic 

nitrogen (filtered); total nitrogen is computed as the sum of 

nitrite plus nitrate nitrogen and ammonia plus total organic 

nitrogen (unfiltered). Dissolved organic nitrogen is computed 

as the difference between ammonia plus dissolved organic 

nitrogen (filtered) and ammonia nitrogen alone; total organic 

nitrogen is computed as the difference between ammonia 

plus total organic nitrogen (unfiltered) and ammonia nitrogen 

alone. Nitrate nitrogen is computed as the difference between 

nitrite plus nitrate nitrogen and nitrite nitrogen alone.

QC and assurance procedures applied by the USGS 

NWQL and sediment laboratories include the determination 

and tracking of long-term method detection level, internal and 

external audits, and blind-blank and blind-spike programs that 

use standard reference materials. Laboratory quality assurance 

data and methods are documented in U.S. Geological Survey 

(2007).

Quality assurance procedures applied by the Texas 

A&M microbiology laboratory for bacteria include field split 

samples, laboratory duplicates, and method blanks (Texas 

Figure 5. Suspended-sediment sample collection at station 

08211517, West Oso Creek at Merrett Road near Corpus Christi, 

Texas, June 2, 2006. 
Hydrologic Conditions 9

Commission on Environmental Quality, 2003). Analytical 

results of laboratory duplicates and field split replicates are 

included with primary bacteria sample results.

Water-quality sample data were reviewed by a USGS data 

manager before data were entered into the USGS National 

Water Information System (NWIS) (U.S. Geological Survey, 

2008). The emphasis of the review was to verify the accuracy 

and completeness of the laboratory data, to determine whether 

laboratory QC and assurance data were within acceptable limits, and to determine whether samples were handled appropriately in the field and the laboratory. 

Hydrologic Conditions

The 3-water-year study period represents three crop 

cycles in the study subwatersheds. Water year 2006 was very 

dry, especially during the planting and preharvest period 

of February through May. In fact, some producers did not 

plant or abandoned crops because of lack of rainfall. Water 

years 2007 and 2008 were relatively typical years in terms of 

agricultural practices (Dr. Carlos Fernandez, Texas AgriLife 

Research, written commun., 2008).

Rainfall 

Long-term (1960–2008) rainfall from the NWS Corpus 

Christi International Airport station (site 6, table 1) was compared with rainfall in the subwatersheds during water years 

2006–08 (October 2005–September 2008) (table 2). Average 

annual rainfall in the Corpus Christi area during 1960–2008 

was 31.66 in. Area-weighted, average annual rainfall on the 

two study subwatersheds during water years 2006–08 was 

33.26 in. Although rainfall during 2006 was less than normal 

(29.97 in.), the period of June–August included several major 

runoff events. Rainfall in water year 2007 was above average 

(45.03 in.), with especially heavy rainfall during July 2007, 

about 15 in. Rainfall in water year 2008 was substantially less 

than average in the West Oso Creek subwatershed (19.61 in.) 

but only slightly less than average in the Oso Creek tributary 

subwatershed (29.10 in.). 

Mean (1960–2008) monthly rainfall for the NWS 

Corpus Christi International Airport station and monthly 

area-weighted rainfall for the West Oso Creek and Oso Creek 

tributary subwatersheds (combined) for October 2005–

September 2006, October 2006–September 2007, and October 

2007–September 2008 are shown in figure 6. Although study 

area rainfall was greater than average during the 3-year study 

period, monthly rainfall was less than average during 23 of the 

36 months. 

Runoff 

The USGS, in cooperation with the Texas Water 

Development Board, has operated streamflow-gaging 

station 08211520 Oso Creek at Corpus Christi, Tex., since 

1972 (site 5, fig. 1; table 1). Data from this station were used 

to estimate long-term annual runoff volumes for comparison with study subwatershed runoff volumes. Land use in 

the 90.3-mi2 watershed upstream from the gaging station is 

largely cropland, similar to that in the study subwatersheds. 

However, a substantial amount of impervious cover, associated with the town of Robstown, is located upstream from the 

station. Average streamflow during water years 1973–2008 

was 30.6 ft3/s. Excluding an estimated wastewater treatment 

discharge of about 3 million gallons per day (Mgal/d) from 

Robstown, long-term average annual runoff at the Oso Creek 

at Corpus Christi station was 3.90 in. (table 3). During water 

years 2006–08, average annual runoff at the Oso Creek at 

Corpus Christi station was greater than average, 5.82 in. So, it 

is likely that 2006–08 represents a period of greater than average runoff for the study area subwatersheds.

Runoff during water years 2006–08 corresponded to 

rainfall patterns, with rainfall and runoff events interspersed 

between considerable periods of no runoff. Selected rainfall 

and runoff events (typically rainfall greater than 1.0 in. or 

runoff greater than 0.001 in., or both) are listed in table 4, 

including dates, rainfall totals, runoff volumes, and runoff 

coefficients (ratio of runoff volume, in inches, to rainfall 

volume, in inches). Most of the minor rainfall and runoff 

events, for which study subwatersheds had less than 0.01 in. 

of runoff, are not listed separately but are included in the 

totals for October 2005–September 2008 listed in table 4. 

Table 4 also indicates whether runoff water-quality samples 

were collected during the event. Appendix 2 (West Oso 

Creek) and appendix 3 (Oso Creek tributary) list waterquality data for all sampled runoff events during water years 

2006–08.

Runoff is highly dependent on antecedent conditions. 

Rainfall of 2 to 3 in. during May 25–29, 2007, resulted in no 

runoff or only minor runoff at the subwatershed sites. Less 

rainfall during January 24–February 3, 2007, produced substantially more runoff because of antecedent wet conditions 

from substantial rainfall in early January.

Most of the runoff from the study subwatersheds occurred 

in response to a few specific storm periods (events). More than 

60 percent of the runoff from each subwatershed occurred 

during two wet periods: September 18–October 1, 2006, and 

July 2–August 8, 2007 (table 4). During both periods, multiple 

rainfall events and prolonged wet-soil conditions contributed 

to substantial runoff. 

The West Oso Creek subwatershed produced more runoff 

during the study period than the Oso Creek tributary subwatershed, 13.95 in. compared with 9.45 in. (table 4). Runoff 

response was quicker and peak flows were larger in the West 

Oso Creek subwatershed than in the Oso Creek tributary 

subwatershed. Differences in hydrologic response between 

the subwatersheds might be explained by slightly greater land 

slopes in the West Oso Creek subwatershed and possible noncontributing areas in the Oso Creek tributary subwatershed. 

The rainfall/runoff response for each subwatershed during the 

March 14–18, 2007, event is shown in figure 7.
10 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Water Quality

Rainfall

Seventeen rainfall samples were collected and analyzed for major inorganic ions and nutrients during the 

study (table 5). During the October 2005–September 2008 

study period, 89.17 in. of rainfall were recorded at the Texas 

AgriLife Research station (site 1, fig. 1). The 17 rainfallquality samples represent 31.27 in. of rainfall recorded at the 

Texas AgriLife Research station or about 35 percent of total 

rainfall during the study period. Thirteen samples were collected during rainfall for which runoff events were sampled. 

Five rainfall samples were collected during events for which 

no runoff occurred. These five samples were collected during 

rainfall events with about 0.5 in. of rainfall or less. These 

samples provided only enough sample volume for nutrient 

analyses. 

Summary statistics for selected rainfall nutrient concentrations are listed in table 6. Most of the nitrogen in rainfall is 

in the form of dissolved ammonia and dissolved nitrate (the 

nitrite plus nitrate analysis was predominantly nitrate in these 

samples, and nitrite was rarely detected; therefore, nitrite 

plus nitrate is referred to as nitrate in this report), which were 

Figure 6. Mean monthly rainfall for National Weather Service station 412015, Corpus Christi International Airport, 1960–2008; and 

monthly area-weighted rainfall for West Oso Creek and Oso Creek tributary subwatersheds (combined), Nueces County, Texas, water 

years 2006–08. 

MONTH

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

MEAN MONTHLY RAINFALL, IN INCHES

National Weather Service station 412015, Corpus Christi International Airport, 1960–2008

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

West Oso Creek and Oso Creek tributary subwatersheds, October 2005–September 2006

West Oso Creek and Oso Creek tributary subwatersheds, October 2006–September 2007

West Oso Creek and Oso Creek tributary subwatersheds, October 2007–September 2008

EXPLANATION
Water Quality 11

detected in all samples. Median concentrations of ammonia 

and nitrate were 0.17 and 0.12 milligram per liter (mg/L), 

respectively. Organic forms of nitrogen were detected in all 

samples at relatively low concentrations; the median concentration of total organic nitrogen was 0.08 mg/L. Nitrite 

was detected in less than one-half the samples (table 5); the 

median concentration was less than 0.002 mg/L (table 6). 

Dissolved and total phosphorus were detected in about onehalf of the samples (table 5); the median concentrations were 

less than 0.006 mg/L and less than 0.007 mg/L, respectively 

(table 6).

The deposition of rainfall constituents, in pounds per 

acre, can be defined as the product of the EMC and rainfall 

volume and a conversion factor. For rainfall events during 

which rainfall samples were collected and analyzed, daily 

deposition of a constituent was computed (using total nitrogen 

for example) as

DTN = EMCTN x R x cf, (1)

where

DTN = daily deposition of total nitrogen, in pounds per 

acre;

EMCTN = total nitrogen rainfall event-mean concentration, 

in milligrams per liter;

R = daily rainfall on the study subwatershed, in 

inches; and

cf = conversion factor of 0.2266.

For unsampled rainfall events, those for which nitrogen 

concentrations were not available, daily deposition of nitrogen was estimated from a regression equation that relates 

daily rainfall and daily nitrogen deposition. Least-squares 

regression was used with the computed event deposition from 

sampled events to develop the equation relating daily rainfall 

and daily deposition. Similar to a previous study of rainfall 

nitrogen deposition in the Coastal Bend area (Ockerman 

and Livingston, 1999), rainfall and deposition were logtransformed before performing the regression, then retransformed to original units (Helsel and Hirsch, 2002) to improve 

MAR 12 MAR 13 MAR 14 MAR 15 MAR 16 MAR 17 MAR 18

2007

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

RAINFALL, IN INCHES

0

10

20

30

40

50

60

70

80

90

100

STREAMFLOW, IN CUBIC FEET PER SECOND

West Oso Creek rainfall

Oso Creek tributary rainfall

Oso Creek tributary streamflow

West Oso Creek streamflow

Figure 7. Rainfall/runoff response for West Oso Creek and Oso Creek tributary subwatersheds, Nueces County, Texas, March 12–18, 

2007. 
12 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

the quality of the regression relation. The resulting regression 

equation is

DTN = 0.09 x R0.360, (2)

where

DTN = daily deposition of total nitrogen, in pounds per 

acre; and

R = daily rainfall on the study subwatershed, in inches.

The coefficient of determination (R-squared) for the 

regression equation was 0.22; the residual standard error was 

0.082 pound per acre (lb/acre).

The sampling periods for most rainfall-quality samples 

collected during the study ranged from 12 to 24 hours and, in 

most cases, represented the total rainfall for a day. Thus, for 

purposes of computing deposition of rainfall constituents by 

using equation 1 and for developing equation 2 relating daily 

rainfall and daily total nitrogen deposition, the event rainfall 

for each rainfall sample (table 5) was treated as a daily rainfall 

total. 

Daily total nitrogen deposition for the 17 sampled rainfall 

events, which was computed by using equation 1 with event 

(daily) rainfall and total nitrogen sample concentrations 

(representing EMCs) (total nitrogen unfiltered, table 5), and 

the corresponding daily total nitrogen deposition, which was 

estimated by using equation 2, each relative to daily rainfall, 

are shown in figure 8.

 Daily total nitrogen rainfall deposition for each subwatershed was estimated by using equation 2 with daily 

rainfall for each subwatershed; estimated monthly and annual 

deposition, computed from summation of daily deposition, 

are shown in table 7. The average (area-weighted) annual total 

nitrogen deposition for the subwatersheds over the 3-year 

study period was 4.47 pounds per acre per year [(lb/acre)/yr]. 

Rainfall deposition of total phosphorus was much less 

than that of total nitrogen. The median total phosphorus concentration for the 17 rainfall samples (table 6) was less than 

Table 2. Estimated annual (water years 2006–08) rainfall for West Oso Creek and Oso Creek tributary subwatersheds, Nueces County, 

Texas, and long-term average annual (water years 1960–2008) rainfall measured at National Weather Service station 412015, Corpus 

Christi International Airport.

[--, not available]

Subwatershed

Oct. 2005–Sept. 

2006 rainfall 

(inches)

Oct. 2006–Sept. 

2007 rainfall 

(inches)

Oct. 2007–Sept. 

2008 rainfall 

(inches)

Average annual 

rainfall, Oct. 

2005–Sept. 2008 

(inches)

Long-term average 

annual rainfall, 

1960–2008

(inches)

West Oso Creek 27.91 44.87 19.61 30.80 --

Oso Creek tributary 31.97 45.19 29.10 35.65 --

Mean (subwatershed area 

weighted) 29.97 45.03 24.77 33.26 31.66

Table 3. Annual (water years 2006–08) runoff from West Oso Creek and Oso Creek tributary subwatersheds and long-term average 

annual (water years 1973–2008) runoff from Oso Creek watershed, Nueces County, Texas. 

[--, not applicable]

Subwatershed 

or station name 

(number)

Oct. 2005–Sept. 

2006 runoff 

(inches)

Oct. 2006–Sept. 

2007 runoff 

(inches)

Oct. 2007–Sept. 

2008 runoff 

(inches)

Average annual 

study area 

runoff 

(inches)

Water years 

1973–2008 average 

annual runoff 

(inches)

West Oso Creek 

(station 08211517)

3.19 8.19 3.14 5.69 --

Oso Creek tributary 

(station 08211525)

2.60 4.67 2.17 3.64 --

Oso Creek at Corpus 

Christi, Tex.1

(station 08211520)

4.37 10.9 2.18 5.82 3.90

1

 Runoff at station 08211520 excludes an estimated annual 3 million gallons per day of wastewater discharge during the study period.
Water Quality 13

Table 4. Rainfall, runoff, and runoff/rainfall coefficients (ratio of runoff volume to rainfall volume) for selected rainfall and runoff events 

during water years 2006–08, West Oso Creek and Oso Creek tributary subwatersheds, Nueces County, Texas.

[--, not applicable] 

Event date

Subwatershed

West Oso Creek Oso Creek tributary

Rainfall

(inches)

Runoff

(inches)

Runoff/

rainfall 

coefficient

Runoff 

waterquality 

samples 

collected?

Rainfall

(inches)

Runoff

(inches)

Runoff/

rainfall 

coefficient

Runoff 

waterquality 

samples 

collected?

2005

Oct. 11–13 0.92 0 0 No 7.18 0.14 0.02 No

Nov. 16–17 .49 .01 .02 No .95 0 0 No

Nov. 26–29 2.62 .24 .09 No 3.80 .04 .01 No

2006

June 1–4 3.90 .65 .17 Yes 2.50 .08 .03 Yes

July 1–5 2.32 .01 .005 Yes 1.75 .002 .001 Yes

July 6–9 .88 .01 .01 Yes 1.73 .02 .01 Yes

July 26–27 1.34 .02 .01 No .15 0 0 No

Aug. 19–20 .60 .01 .02 No 0 0 0 No

Sept. 9–17 2.79 .01 0 Yes 2.91 .18 .06 Yes

Sept. 18–23 3.17 .96 .30 Yes 3.58 1.31 .37 Yes

Sept. 24–Oct. 1 2.82 1.29 .46 No 2.04 .85 .42 No

Oct. 22–25 1.36 0 0 No 1.01 .06 .06 No

2007

Jan. 3–7 1.68 .02 .01 Yes 2.13 .06 .03 Yes

Jan. 24–Feb. 3 2.29 .14 .06 Yes 2.40 .29 .12 Yes

Mar. 14–18 1.71 .12 .07 Yes 1.82 .08 .04 Yes

May 25–29 2.48 0 0 Yes 3.15 .02 .01 No

July 2–10 8.20 2.70 .33 Yes 10.72 3.26 .30 Yes

July 19–24 2.07 1.26 .61 No 1.76 .05 .03 No

July 26–Aug. 8 6.37 2.32 .36 No 2.13 .27 .13 No

Aug. 30–Sept. 2 2.45 0 0 No 1.05 .10 .10 Yes

Sept. 3–13 1.29 .63 .49 No 1.67 .18 .11 No

Sept. 29–Oct. 15 3.34 .32 .10 No 2.43 .46 .12 No

2008

Mar. 10–19 .75 .03 .04 Yes 1.00 .06 .12 Yes

July 23–Aug. 8 4.51 .22 .05 Yes 6.65 1.62 .12 Yes

Aug. 13–31 3.21 2.81 .88 No 7.21 .12 .12 Yes

Sept. 14–22 .18 0 0 No .50 .15 .12 No

Sept. 24–30 .14 0 0 No .84 .03 .03 No

Total—27 events 63.75 13.78 .22 -- 73.26 9.43 .13 --

Total—Oct. 2005–

Sept. 2008 

92.39 13.95 .15 -- 106.26 9.45 .09 --
14 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

the minimum laboratory reporting level, 0.008 mg/L. The use 

of equation 1 with mean annual (October 2005–September 

2008) rainfall for the subwatersheds (33.26 in., table 2) and the 

minimum laboratory reporting level to estimate an upper limit 

of annual total phosphorus deposition for the study period 

resulted in an estimate of less than 0.07 (lb/acre)/yr. 

Values for estimated monthly and annual total nitrogen 

deposition for water years 2006–07 were revised and compared with the values published in the previous report on 

hydrologic conditions and water quality of the study subwatersheds (Ockerman, 2008, table 7). Additional data collected 

during water year 2008, along with previously collected data, 

were used to revise the relation between rainfall and nitrogen 

deposition (equation 2), resulting in revised deposition values 

for water years 2006–07. Previously published values of average annual total nitrogen deposition for 2006–07 were 5.94 

and 5.06 (lb/acre)/yr for the West Oso Creek and Oso Creek 

tributary subwatersheds, respectively. The revised values of 

average annual total nitrogen deposition for 2006–07 are 5.52 

and 4.52 (lb/acre)/yr for the West Oso Creek and Oso Creek 

tributary subwatersheds, respectively.

Runoff

During June 2006–September 2008, 24 event-composite 

runoff (streamflow) samples were collected at the two subwatershed outlet monitoring sites: 12 samples at West Oso Creek 

(site 3, fig. 1) and 12 samples at Oso Creek tributary (site 4, 

fig. 1). Samples were analyzed to characterize the quality of 

storm runoff from the study subwatersheds. Results of analyses of West Oso Creek and Oso Creek tributary samples are 

shown in appendixes 2 and 3, respectively. 

Nutrients and Major Inorganic Ions

Summary statistics for event concentrations of selected 

nutrient (nitrogen and phosphorus) and major inorganic ions 

Table 5. Event (daily) rainfall, Texas AgriLife Research at Corpus Christi weather station (WS1); and event concentrations of selected 

constituents in rainfall samples, Texas AgriLife Research at Corpus Christi rainfall-quality sampling station (08211511), Nueces County, 

Texas, water years 2006–08.

[fltrd, filtered; mg/L, milligrams per liter; N, nitrogen; unfltrd, unfiltered; --, not analyzed; <, less than; E, estimated; P, phosphorus]

Sample 

date

Event 

(daily) 

rainfall 

(inches)

Calcium, 

water, 

fltrd 

(mg/L)

Magnesium, 

water, 

fltrd 

(mg/L)

Potassium, 

water, 

fltrd 

(mg/L)

Sodium, 

water, 

fltrd 

(mg/L)

Chloride, 

water, 

fltrd 

(mg/L)

Sulfate, 

water, 

fltrd 

(mg/L)

Ammonia 

+ organic 

nitrogen, 

water, 

fltrd 

(mg/L as N)

Ammonia 

+ organic 

nitrogen, 

water, 

unfltrd 

(mg/L as N)

Ammonia 

nitrogen, 

water, 

fltrd 

(mg/L as N)

May 10, 2006 1.10 1.20 1.10 0.127 0.142 1.15 2.69 1.1 1.0 0.82

May 14, 2006 .52 -- -- -- -- -- -- 2.3 1.1 .75

June 1, 2006 9.30 <.02 <.008 <.010 <.20 .16 .16 .17 E.06 .04

July 3, 2006 1.60 .14 .040 .029 .32 .56 .69 .28 .26 .22

July 6, 2006 1.30 .11 .027 .018 .20 .35 .67 .18 .18 .16

July 26, 2006 2.40 .38 .109 .046 .73 1.29 .95 .29 .20 .21

Sept. 11, 2006 .46 -- -- -- -- -- -- .13 2.0 .10

Sept. 24, 2006 1.80 .04 .010 .031 .47 .76 .64 .24 .09 .08

Oct. 14, 2006 .14 -- -- -- -- -- -- .75 .75 .71

Jan. 3, 2007 1.49 .09 .035 .024 .34 .47 .59 .17 .17 .13

Jan. 24, 2007 .51 -- -- -- -- -- -- .18 .18 .07

Mar. 14, 2007 1.19 .23 .044 .047 .39 .54 .88 .24 .24 .17

May 25, 2007 1.52 .13 .048 .046 .49 1.46 .71 .20 .20 .20

July 2, 2007 4.67 .07 .028 .014 .20 .35 .38 .08 .08 .05

Mar. 6, 2008 1.24 .68 .192 .091 1.47 2.76 2.18 .34 .35 .30

July 24, 2008 1.77 .29 .100 .109 1.32 2.56 .63 E.10 E.07 .05

Aug. 18, 2008 .26 -- -- -- -- -- -- .40 .34 .32
Water Quality 15

(calcium, potassium, chloride, and sulfate) for each subwatershed are shown in table 8. Nutrient concentrations in runoff 

were greater at the West Oso Creek site than at the Oso Creek 

tributary site. Major inorganic ion concentrations were greater 

at the Oso Creek tributary site than at the West Oso Creek 

site. 

The results of Wilcoxon rank-sum tests (Helsel and 

Hirsch, 2002), done to indicate whether event-median concentrations of respective constituents at the two subwatershed 

sites are significantly different at the .05 level, are listed in 

table 9. The tests indicate that event-median concentrations 

of selected forms of nitrogen are significantly greater at the 

West Oso Creek subwatershed. Event-median concentrations of major inorganic ions in runoff were all significantly 

greater at the Oso Creek tributary subwatershed. There was no 

significant difference in the total phosphorus median concentrations between the two subwatersheds; however, dissolved 

orthophosphate concentrations were greater in the Oso Creek 

tributary subwatershed.

The load of a constituent in runoff (streamflow) is the 

mass of a given constituent transported past a site on a stream 

during a specified period (Huber, 1993, p. 14.2). Daily nutrient 

loads were computed for the study subwatershed outlet sites 

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 computed as

L = EMC x R x cf, (3)

where

L = constituent load, in pounds per day;

EMC = event-mean concentration during runoff event, in 

milligrams per liter or micrograms per liter;

R = runoff, in acre-feet per day; and

cf = conversion factor, 2.719 for concentrations 

in milligrams per liter or 2.719 x 10-3 for 

concentrations in micrograms per liter.

Table 5. Event (daily) rainfall, Texas AgriLife Research at Corpus Christi weather station (WS1); and event concentrations of selected 

constituents in rainfall samples, Texas AgriLife Research at Corpus Christi rainfall-quality sampling station (08211511), Nueces County, 

Texas, water years 2006–08—Continued.

Sample 

date

Event 

(daily) 

rainfall 

(inches)

Nitrite + 

nitrate 

nitrogen, 

water, fltrd 

(mg/L as N)

Nitrite 

nitrogen, 

water, 

fltrd 

(mg/L 

as N)

Organic 

nitrogen, 

water, 

fltrd 

(mg/L 

as N)1

Organic 

nitrogen, 

water, 

unfltrd 

(mg/L 

as N)1

Total 

nitrogen, 

water, fltrd 

(mg/L as N)1

Total 

nitrogen, 

water, 

unfltrd 

(mg/L as N)1

Phosphorus, water, 

fltrd 

(mg/L as P)

Phosphorus, water, 

unfltrd 

(mg/L as P)

Orthophosphate 

phosphorus, 

water, fltrd 

(mg/L as P)

May 10, 2006 1.10 0.378 0.004 0.31 0.18 1.5 1.4 <0.004 0.006 <0.006

May 14, 2006 .52 .338 .003 1.6 .31 2.6 1.4 .21 .043 <.006

June 1, 2006 9.30 .029 <.002 .14 -- .20 E.09 <.004 .007 <.006

July 3, 2006 1.60 .135 <.002 .06 .04 .41 .39 .006 <.004 <.006

July 6, 2006 1.30 .122 <.002 .02 .02 .30 .30 <.004 <.004 <.006

July 26, 2006 2.40 .149 <.002 .08 -- .44 .35 .004 .005 <.006

Sept. 11, 2006 .46 .091 <.002 .03 1.9 .22 2.1 <.004 <.004 <.006

Sept. 24, 2006 1.80 .078 <.002 .16 -- .32 E.17 <.004 <.004 <.006

Oct. 14, 2006 .14 .250 .004 .01 .04 .96 1.0 .018 .028 .013

Jan. 3, 2007 1.49 .064 .002 .03 .04 .22 .24 .005 <.008 .003

Jan. 24, 2007 .51 .066 .002 .05 .10 .19 .24 <.006 <.008 <.006

Mar. 14, 2007 1.19 .092 .001 .05 .07 .31 .33 <.006 .005 <.006

May 25, 2007 1.52 .102 .002 .09 .005 .39 .30 <.006 <.008 <.006

July 2, 2007 4.67 .057 <.002 .08 .03 .19 E.13 <.006 .004 <.006

Mar. 6, 2008 1.24 .132 E.002 .11 .12 .47 .48 .008 .009 E.003

July 24, 2008 1.77 E.013 <.002 E.07 E.03 E.11 E.08 E.004 E.005 E.004

Aug. 18, 2008 .26 .134 <.002 .15 .09 .53 .47 E.005 E.007 E.005

1

 Concentration of constituent not analyzed directly but computed from other analyzed constituents.
16 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Event-median concentrations of selected nutrients in 

samples at each site (table 9) were used to estimate runoff 

loads for unsampled events at each site. During water years 

2006–08, sampled runoff events represent about 35 percent 

of total runoff from the West Oso Creek subwatershed. 

About 65 percent of the runoff was unsampled. Sampled 

and unsampled runoff percentages from the Oso Creek 

tributary subwatershed were about 76 percent and 24 percent, 

respectively. The percentage of total runoff sampled in the 

West Oso Creek subwatershed was less than that sampled in 

the Oso Creek tributary subwatershed because three of the 

four runoff events with the greatest runoff volume in West 

Oso Creek were not sampled. The three runoff events with 

the greatest runoff volume in Oso Creek tributary were 

sampled.

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

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 drainage area of the watershed, in 

acres.

Estimated monthly and annual loads of selected nutrients 

in runoff from each of the study subwatersheds are listed in 

table 10, and the corresponding annual runoff yields are listed 

in table 11. Monthly and annual nutrient loads were highly 

variable, depending on storm runoff. Large storm events 

contributed most of the runoff constituent loads. As a result 

of large storms in July 2007, about 39 percent of the total 

nitrogen runoff load and 44 percent of the total phosphorus 

runoff load for the entire study period for both subwatersheds 

occurred in this 1-month period.

Nitrogen yields from the West Oso Creek subwatershed 

were greater than yields from the Oso Creek tributary subwatershed for two reasons: (1) West Oso Creek subwatershed 

runoff was greater, and (2) nitrogen concentrations generally 

were greater for the West Oso Creek subwatershed. Average 

total nitrogen yield from the West Oso Creek subwatershed 

was 2.62 (lb/acre)/yr for the 3-year study period. Average total 

nitrogen yield from the Oso Creek tributary subwatershed for 

the 3-year period was less than one-half that for the West Oso 

Creek subwatershed, 0.839 (lb/acre)/yr. 

Average total phosphorus yield from the West Oso Creek 

subwatershed was greater than that from the Oso Creek tributary subwatershed. Although the median runoff concentrations 

for total phosphorus were about the same for each subwatershed, greater runoff at West Oso Creek resulted in greater 

runoff yields. The average annual total phosphorus yields 

for West Oso Creek and Oso Creek tributary were 0.644 and 

0.419 (lb/acre)/yr, respectively (table 11).

Table 6. Summary statistics for event rainfall, Texas AgriLife Research at Corpus Christi weather station (WS1); and event 

concentrations of selected nutrients in rainfall samples, Texas AgriLife Research at Corpus Christi rainfall-quality sampling station 

(08211511), Nueces County, Texas, water years 2006–08.

[in., inches; mg/L, milligrams per liter; --, not applicable; <, less than]

Constituent Number of samples Mean Median Minimum Maximum

Rainfall (in.) 17 1.84 1.30 0.14 9.30

Ammonia + organic nitrogen, dissolved (mg/L) 17 .42 .24 .08 2.30

Ammonia + organic nitrogen, total (mg/L) 17 .40 .20 .06 2.00

Ammonia nitrogen, dissolved (mg/L) 17 .26 .17 .04 .82

Nitrite + nitrate nitrogen, dissolved (mg/L) 17 .17 .12 .013 .378

Nitrite nitrogen, dissolved (mg/L) 17 -- <.002 <.002 .004

Nitrate nitrogen, dissolved (mg/L)1 17 .17 .12 .013 .78

Organic nitrogen, total (mg/L)2 17 .20 .08 .02 1.9

Nitrogen, total (mg/L)3 17 .56 .33 .17 2.10

Phosphorus, dissolved (mg/L) 17 <.006 <.006 <.004 .021

Phosphorus, total (mg/L) 17 -- <.007 <.004 .043

Orthophosphate phosphorus, dissolved (mg/L) 17 <.006 <.006 <.006 .013

1

 Computed as difference between nitrite plus nitrate nitrogen and nitrite nitrogen. 

2

 Computed as difference between ammonia plus total organic nitrogen (unfiltered) and ammonia nitrogen.

3

 Computed as sum of nitrite plus nitrate nitrogen and ammonia plus total organic nitrogen (unfiltered). 
Water Quality 17

Estimated runoff loads and yields of nitrogen and phosphorus for water years 2006–07 were revised from estimates 

published in the previous report on the study subwatersheds 

(Ockerman, 2008). Runoff-quality data collected since the 

previous report, along with previous data, were used to estimate median concentrations that were then used to compute 

runoff loads during unsampled events. This resulted in revisions to 2006–07 load and yield estimates. The previously 

published 2006–07 average annual runoff yield (weighted 

average of both study subwatersheds) for total nitrogen was 

1.78 (lb/acre)/yr. The revised estimate of average annual runoff yield for 2006–07 is 1.91 (lb/acre)/yr. 

Information on fertilizer nutrient application in study 

subwatershed croplands for 2006–08 was compiled by 

Texas AgriLife Research (Dr. Bobby Eddleman, Texas 

AgriLife Research, written commun., 2007) and the Natural 

Resources Conservation Service (John Freeman, U.S. 

Department of Agriculture, Natural Resources Conservation 

Service, Nueces County, Tex., written commun., 2009). 

Annual runoff yields of total nitrogen and total phosphorus 

were compared to nutrient inputs from fertilizer applications 

and rainfall deposition (table 12). 

Nitrogen input from fertilizer is much larger than nitrogen that either enters the study area subwatersheds through 

rainfall deposition or exits the subwatersheds in runoff. 

Average (area-weighted for both subwatersheds) annual 

application of fertilizer-based nitrogen was 64.2 (lb/acre)/

yr, compared to 4.47 (lb/acre)/yr from rainfall deposition 

Figure 8. Daily deposition of total nitrogen computed from event (daily) rainfall and sample concentrations and estimated by 

regression with daily rainfall, each relative to daily rainfall, for sampled rainfall events, Oso Creek watershed, Nueces County, Texas, 

water years 2006–08. 

0 1 2 3 4 5 6 7 8 9 10

DAILY RAINFALL, IN INCHES

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

DAILY RAINFALL DEPOSITION OF TOTAL NITROGEN, IN POUNDS PER ACRE

EXPLANATION

Daily deposition computed from event (daily) rainfall and sample (event-mean) concentration (table 5, total

nitrogen, unfiltered)

Daily deposition estimated by regression with daily rainfall
18 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Table 8. Summary statistics for event concentrations of selected nutrients and major inorganic ions in runoff samples, West Oso Creek 

(station 08211517) and Oso Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[mg/L, milligrams per liter; --, not available; <, less than laboratory reporting level]

Constituent Number of 

samples Mean Median Minimum Maximum

West Oso Creek subwatershed

Ammonia nitrogen, dissolved (mg/L) 12 -- 0.040 <0.02 0.228

Nitrite + nitrate nitrogen, dissolved (mg/L) 12 1.00 .61 .22 3.26

Ammonia + organic nitrogen, total (mg/L) 12 2.50 2.20 .75 7.0

Organic nitrogen, total (mg/L)1 12 2.40 2.07 .74 6.6

Nitrogen, total (mg/L)2 12 3.50 3.18 .97 9.0

Phosphorus, total (mg/L) 12 .71 .62 .29 1.5

Calcium, dissolved (mg/L) 11 14.2 14.4 8.9 18.4

Potassium, dissolved (mg/L) 11 4.86 4.07 3.24 9.73

Chloride, dissolved (mg/L) 11 2.33 1.61 .97 6.03

Sulfate, dissolved (mg/L) 11 1.99 1.60 .90 3.90

Oso Creek tributary subwatershed

Ammonia nitrogen, dissolved (mg/L) 12 -- .021 <.020 .172

Nitrite + nitrate nitrogen, dissolved (mg/L) 12 -- .20 <.04 1.37

Ammonia + organic nitrogen, total (mg/L) 12 1.28 1.04 .79 2.5

Organic nitrogen, total (mg/L)1 12 1.26 1.03 .78 2.3

Nitrogen, total (mg/L)2 12 1.68 1.26 .84 3.57

Phosphorus, total (mg/L) 12 .66 .61 .41 1.02

Calcium, dissolved (mg/L) 11 18.7 18.9 10.3 28.5

Potassium, dissolved (mg/L) 11 8.58 8.31 4.82 16.9

Chloride, dissolved (mg/L) 11 11.9 11.0 1.97 23.8

Sulfate, dissolved (mg/L) 11 6.98 6.23 1.30 17.4

1

 Computed as difference between ammonia plus total organic nitrogen (unfiltered) and ammonia nitrogen.

2

 Computed as sum of nitrite plus nitrate nitrogen and ammonia plus organic nitrogen, total. 

Table 7. Estimated monthly and annual total nitrogen rainfall deposition on West Oso Creek and Oso Creek tributary subwatersheds, 

Nueces County, Texas, water years 2006–08.

[In pounds per acre]

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

West Oso Creek subwatershed

2006 0.48 0.37 0.12 0.23 0.15 0.19 0.14 0.47 0.39 0.73 0.27 1.03 4.57

2007 .38 .05 .52 .68 .13 .20 .44 .53 .52 1.39 .95 .68 6.47

2008 .26 .25 .20 .54 .16 .23 .31 .23 .12 .60 .67 .19 3.76

Oso Creek tributary subwatershed

2006 .59 .43 .18 .17 0 .09 0 .31 .26 .42 .08 .94 3.48

2007 .62 0 .51 .72 .07 .24 .29 .33 .41 1.06 .76 .56 5.56

2008 .20 .27 .06 .38 .05 .18 .14 .21 .13 .60 .55 .13 3.03
Water Quality 19

and 1.72 (lb/acre)/yr in runoff. Annual rainfall deposition of 

nitrogen exceeds runoff yields because most rainfall does not 

contribute to runoff, even though for most periods of runoff, 

nitrogen runoff yields exceed rainfall nitrogen deposition. The 

average annual runoff yield of total nitrogen represents about 

2.5 percent of nitrogen applied as fertilizer and nitrogen entering the subwatersheds through rainfall deposition. Also, nearly 

all of the nitrogen applied as fertilizer and entering the subwatersheds through rainfall deposition is in the form of ammonia 

and nitrate. Nitrogen exiting the study subwatersheds in runoff 

is mostly organic nitrogen (table 11).

As with nitrogen, fertilizer inputs of phosphorus are 

much greater than phosphorus runoff yields. Average annual 

fertilizer input of phosphorus was 13.2 (lb/acre)/yr (table 12). 

Average runoff of total phosphorus was 0.530 (lb/acre)/yr, 

or about 4.0 percent of the phosphorus applied as fertilizer. 

Average phosphorus input from rainfall was relatively small, 

estimated to be less than 0.07 (lb/acre)/yr. Fertilizer is applied 

as soluble orthophosphate. Most of the runoff phosphorus also 

was in the form of orthophosphate (table 11). 

Suspended Sediment

Results of analyses for 26 samples of suspended sediment 

in runoff are listed in table 13. Twelve samples were collected 

at West Oso Creek (station 08211517), and 14 samples were 

collected at Oso Creek tributary (station 08211525). The 

samples were analyzed for concentration and for percentage 

silt and clay, by weight (particle size less than 0.062 mm). 

The sediment concentrations from these samples are not 

EMCs but represent sediment concentrations at the time of 

sample collection. Sediment concentrations, loads, and yields 

are a function of streamflow and other factors, including soil 

erodibility, rainfall intensity and duration, and crop growth 

stage and tillage practices (U.S. Department of Agriculture, 

Agricultural Research Service, 2006). Generally, suspendedsediment concentrations, loads, and yields were larger for the 

West Oso Creek subwatershed than for the Oso Creek tributary 

subwatershed. Considering all samples from both sites, the 

percentage of silt and clay was 96 or greater for 23 of the 26 

samples. 

For each subwatershed (sites 3, 4, fig. 1), a regression 

equation was developed relating suspended-sediment load 

([instantaneous] tons per day) to streamflow (cubic feet per 

second). The regression relations for each site are shown in 

figure 9. The equations and R-squared and residual standard 

error values are listed in table 14. 

From these equations, instantaneous sediment loads were 

computed for each subwatershed outlet site and aggregated to 

obtain monthly and annual loads (table 15). As with runoff and 

nutrient loads, suspended-sediment loads were highly variable 

by month. More than 75 percent of the entire sediment load 

from the study subwatersheds occurred during the 3 months of 

September 2006, July 2007, and August 2008.

Table 9. Event-median concentrations of selected nutrients and major inorganic ions in runoff samples, West Oso Creek (station 

08211517) and Oso Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08; and results of 

Wilcoxon rank-sum tests to indicate whether event-median concentrations for the two subwatersheds are significantly different.

[mg/L, milligrams per liter]

Constituent

Subwatershed event-median concentration

(mg/L) p-value

Concentrations 

significantly different 

West Oso Creek Oso Creek tributary at .05 level?

Ammonia nitrogen, dissolved 0.040 0.021 .03 Yes

Nitrite + nitrate nitrogen, dissolved .61 .20 .02 Yes

Ammonia + organic nitrogen, total 2.20 1.04 .01 Yes

Organic nitrogen, total1 2.07 1.03 .01 Yes

Nitrogen, total2 3.18 1.26 .01 Yes

Phosphorus, total .62 .61 .34 No

Orthophosphate phosphorus, dissolved .284 .418 .0068 Yes

Calcium, dissolved 14.4 18.9 .02 Yes

Potassium, dissolved 4.07 8.31 .0009 Yes

Chloride, dissolved 1.61 11.0 .0001 Yes

Sulfate, dissolved 1.60 6.23 .001 Yes

1

 Computed as difference between ammonia plus total organic nitrogen (unfiltered) and ammonia nitrogen.

2

 Computed as sum of nitrite plus nitrate nitrogen and ammonia plus total organic nitrogen (unfiltered). 
20 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Annual and average annual suspended-sediment yields 

for the subwatersheds (table 16) were computed from the 

loads and watershed drainage areas. Suspended-sediment 

yields from the West Oso Creek subwatershed were more than 

three times greater than those from the Oso Creek tributary 

subwatershed. The average suspended-sediment yield from 

the West Oso Creek subwatershed was 522 (lb/acre)/yr. The 

average suspended-sediment yield from the Oso Creek tributary subwatershed was 139 (lb/acre)/yr. One reason the West 

Oso Creek sediment yield is greater is that the subwatershed 

produced more runoff than the Oso Creek tributary subwatershed. Also, because runoff response is more rapid for the 

West Oso Creek subwatershed than for the Oso Creek tributary 

subwatershed, larger peak flows can generate higher stream 

velocities and, potentially, greater sediment concentrations and 

loads. The West Oso Creek monitoring station is located at the 

edge of an agricultural field with no pasture buffer or structural controls to mitigate sediment runoff to the monitoring 

site. Also, part of West Oso Creek upstream from the monitoring site is not a well-defined channel, but a watercourse that 

flows through (over) cultivated cropland. In contrast, the Oso 

Creek tributary upstream from the monitoring site on that 

stream is surrounded by a buffer area of grassland along the 

sides of the channel.

Table 10. Estimated monthly and annual runoff loads of selected nutrients from West Oso Creek (station 08211517) and Oso Creek 

tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[In pounds]

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

West Oso Creek subwatershed

Water year 2006

Ammonia nitrogen, 

dissolved

0 11.4 0 0 0 0 0 0 13 1.1 0.5 105 131

Nitrite + nitrate nitrogen, 

dissolved

0 175 0 0 0 0 0 0 190 14 5.2 1,350 1,730

Organic nitrogen, total 0 601 0 0 0 0 0 0 910 65 17 4,760 6,350

Nitrogen, total 0 910 0 0 0 0 0 0 1,140 84 26 5,030 7,190

Phosphorus, total 0 177 0 0 0 0 0 0 356 20 5.3 1,580 2,140

Orthophosphate 

phosphorus, dissolved

0 81 0 0 0 0 0 0 127 7.5 2.5 703 884

Water year 2007

Ammonia nitrogen, 

dissolved

.1 0 0 11 0 27 0 .4 0 189 6.8 41 275

Nitrite + nitrate nitrogen, 

dissolved

1.1 0 0 583 .4 105 0 2.2 0 2,550 138 620 4,000

Organic nitrogen, total 3.9 0 0 251 .1 566 0 7.4 0 11,000 467 2,135 14,400

Nitrogen, total 5.8 0 0 851 .5 688 0 10 0 16,400 718 3,230 21,900

Phosphorus, total 1.1 0 0 102 .1 205 0 2.9 0 4,460 140 630 5,540

Orthophosphate 

phosphorus, dissolved

.5 0 0 48 0 48 0 1.0 0 2,650 63 285 3,100

Water year 2008

Ammonia nitrogen, 

dissolved

3.4 0 0 0 0 15 0 0 0 8.8 131 0 158

Nitrite + nitrate nitrogen, 

dissolved

52 0 0 0 0 75 0 0 0 89 2,000 .2 2,220

Organic nitrogen, total 180 0 0 0 0 243 0 0 0 531 6,770 .8 7,720

Nitrogen, total 273 0 0 0 0 332 0 0 0 405 10,400 1.2 11,400

Phosphorus, total 53 0 0 0 0 11 0 0 0 147 2,030 .2 2,240

Orthophosphate 

phosphorus, dissolved

24 0 0 0 0 11 0 0 0 87 916 .1 1,040
Water Quality 21

Estimated runoff loads of suspended sediment for water 

years 2006–07 were revised from estimates published in the 

previous report on the study subwatersheds (Ockerman, 2008). 

Additional suspended-sediment data collected since the 

previous report were used to revise the relations used to 

estimate suspended-sediment loads (fig. 9). The 2006–07 

average annual runoff yield for suspended sediment in the 

West Oso Creek subwatershed was revised to 608 (lb/acre)/

yr, compared with the previous estimate of 582 (lb/acre)/

yr. The 2006–07 average annual runoff yield for suspended 

sediment in the Oso Creek tributary subwatershed was revised 

to 159 (lb/acre)/yr, compared with the previous estimate of 

257 (lb/acre)/yr. 

Pesticides

Runoff samples were analyzed for a suite of 88 pesticides. The pesticide analysis schedules requested from 

the USGS NWQL included some pesticides not used in the 

subwatersheds, and some pesticides used in the subwatersheds were not included in the 88 requested analyses because 

some analyses were not available through the NWQL. Also, 

a complete list of pesticides applied by agricultural users 

in the subwatersheds was not available at the beginning of 

the study. Texas AgriLife Research (Dr. Bobby Eddleman, 

Texas AgriLife Research, written commun., 2007), with 

assistance from the Natural Resources Conservation Service 

Table 10. Estimated monthly and annual runoff loads of selected nutrients from West Oso Creek (station 08211517) and Oso Creek 

tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–07—Continued.

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

Oso Creek tributary subwatershed

Water year 2006

Ammonia nitrogen, 

dissolved

3.3 1.0 0 0 0 0 0 0 2.3 0.3 0 63 70

Nitrite + nitrate nitrogen, 

dissolved

33 9.9 0 0 0 0 0 0 24 1.3 0 559 627

Organic nitrogen, total 168 51 0 0 0 0 0 0 135 25 0 2,530 2,910

Nitrogen, total 206 62 0 0 0 0 0 0 162 26 0 3,520 3,980

Phosphorus, total 100 30 0 0 0 0 0 0 88 17 0 1,450 1,680

Orthophosphate 

phosphorus, dissolved

69 21 0 0 0 0 0 0 57 12 0 877 1,040

Water year 2007

Ammonia nitrogen, 

dissolved

1.9 0 0 9.0 .3 16 0 .6 0 74 3.3 12 117

Nitrite + nitrate nitrogen, 

dissolved

18 0 0 311 6.2 95 0 14.8 0 662 28 107 1,240

Organic nitrogen, total 90 0 0 426 7.4 228 0 26 0 4,070 165 590 5,600

Nitrogen, total 110 0 0 749 13.7 333 0 23.1 0 4,660 190 714 6,790

Phosphorus, total 53 0 0 190 3.3 93 0 18 0 2,780 80 343 3,560

Orthophosphate 

phosphorus, dissolved

37 0 0 118 2.2 35 0 6.5 0 2,330 58 237 2,820

Water year 2008

Ammonia nitrogen, 

dissolved

4.5 0 0 0 0 13 0 0 0 38 3.6 4.6 64

Nitrite + nitrate nitrogen, 

dissolved

45 0 0 1.1 0 101 0 0 0 96 30 43 316

Organic nitrogen, total 232 0 0 .6 0 147 0 .1 0 1,830 165 221 2,600

Nitrogen, total 284 0 0 .7 0 258 0 .1 0 1,600 146 271 2,560

Phosphorus, total 138 0 0 .3 0 75 0 0 0 975 93 131 1,410

Orthophosphate 

phosphorus, dissolved

95 0 0 .2 0 41 0 0 0 776 82 90 1,080
22 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

(John Freeman, U.S. Department of Agriculture, Natural 

Resources Conservation Service, Nueces County, Tex., written 

commun., 2009), compiled information on pesticide application rates for the study subwatersheds during 2005–08. The 

pesticides that were reported as applied in the study subwatersheds during 2005–08 are listed in table 17. Table 17 also 

indicates applied pesticides that were not included in the 

laboratory analyses of runoff samples.

Pesticides detected in runoff samples and concentration 

summary statistics are listed in table 18. Pesticide analytical results for all samples are listed in appendixes 2 and 3. 

Laboratory analyses indicated detection of 32 pesticide compounds (24 herbicides and 8 insecticides) in 24 runoff samples 

collected from the two subwatershed outlet sites combined. 

At the West Oso Creek site, 23 pesticides were detected (19 

herbicides and 4 insecticides) and at the Oso Creek tributary 

site, 24 pesticides were detected (18 herbicides and 6 insecticides). Of the 32 pesticides detected during the study, 14 were 

detected only once, either at the West Oso Creek site or the 

Oso Creek tributary site. All of these 14 single detections were 

at low concentrations (near the minimum laboratory reporting 

level). Two herbicides were detected in all samples from both 

sites: atrazine and atrazine degradation byproduct CIAT. Also, 

glyphosate and glyphosate degradation byproduct aminomethylphosphonic acid (AMPA) were detected in all samples 

collected during water years 2006–07. Glyphosate and AMPA 

were not included in the sample analysis for samples collected 

during water year 2008. Herbicides 2,4-D and pendimethalin 

were detected in more than 50 percent of all samples at both 

sites. 

Pesticide runoff loads and yields were computed by using 

equations 3 and 4, respectively, for atrazine, glyphosate, and 

pendimethalin. For sampled events, sample concentrations 

were used to estimate runoff loads. For unsampled runoff, 

seasonal event-median concentrations (table 19) were used 

to estimate runoff loads. The preharvest season comprises 

January–June; the harvest/postharvest season comprises July–

September. No samples were collected from October through 

December. Monthly and annual estimates of selected pesticide 

runoff loads are listed in table 20.

Annual and average annual application rates and runoff 

yields for selected pesticides are listed in table 21. Similar to 

results for nutrients and suspended sediment, runoff yields of 

the herbicides atrazine, glyphosate, and pendimethalin were 

greater for the West Oso Creek subwatershed than for the 

Oso Creek tributary subwatershed. Of all pesticides detected 

in runoff, the highest runoff yields were for glyphosate, 

0.012 (lb/acre)/yr for the West Oso Creek subwatershed and 

0.001 (lb/acre)/yr for the Oso Creek tributary subwatershed. 

Average annual runoff yields of atrazine were 0.002 and 

0.0003 (lb/acre)/yr, respectively, from the West Oso Creek and 

Oso Creek tributary subwatersheds. Comparison of applications and runoff yields indicates that, for water years 2006–08, 

about 0.9 percent of glyphosate applied to the West Oso Creek 

Table 11. Estimated annual and average annual runoff yields of selected nutrients from West Oso Creek (station 08211517) and Oso 

Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08. 

 [In pounds per acre per year]

Nutrient Water year 2006 Water year 2007 Water year 2008 Water years 2006–08 

average

West Oso Creek subwatershed (5,145 acres)

Ammonia nitrogen, dissolved 0.026 0.053 0.031 0.037

Nitrite + nitrate nitrogen, dissolved .336 .778 .432 .515

Organic nitrogen, total 1.23 2.80 1.50 1.84

Nitrogen, total 1.39 4.26 2.22 2.62

Phosphorus, total .416 1.08 .435 .644

Orthophosphate phosphorus, dissolved .172 .602 .202 .325

Oso Creek tributary subwatershed (5,287 acres)

Ammonia nitrogen, dissolved .013 .022 .012 .016

Nitrite + nitrate nitrogen, dissolved .119 .235 .060 .138

Organic nitrogen, total .550 1.06 .492 .701

Nitrogen, total .753 1.28 .484 .839

Phosphorus, total .318 .673 .267 .419

Orthophosphate phosphorus, dissolved .197 .533 .204 .311
Water Quality 23

subwatershed croplands was detected in runoff. For the Oso 

Creek tributary subwatershed, about 0.08 percent of applied 

glyphosate was detected in runoff. The percentages of applied 

atrazine in runoff were about 0.4 and 0.05, respectively, in 

the West Oso Creek and Oso Creek tributary subwatersheds. 

Applied pendimethalin in runoff was estimated to be 0.08 and 

0.008 percent, respectively, in the West Oso Creek and Oso 

Creek tributary subwatersheds.

Estimated runoff loads and yields of selected pesticides for water years 2006–07 were revised from estimates 

published in the previous report on the study subwatersheds 

(Ockerman, 2008). Runoff-quality data collected since the previous report, along with previous data, were used to estimate 

median concentrations that were then used to compute runoff 

loads during unsampled events. This resulted in revisions 

to 2006–07 load and yield estimates. The 2006–07 average 

annual runoff yield for atrazine was unchanged for the West 

Oso Creek subwatershed. The 2006–07 average annual runoff 

yield for atrazine for the Oso Creek tributary subwatershed 

was revised to 0.00023 (lb/acre)/yr, compared with a previous 

estimate of 0.00012 (lb/acre)/yr. No additional sample data 

were collected for glyphosate since the previous study, so 

runoff load and yield estimates for glyphosate did not change 

from the previous report.

Bacteria

Seventeen bacteria samples were collected during the 

study, 10 from the West Oso Creek subwatershed outlet site 

and 7 from the Oso Creek tributary subwatershed outlet site 

(table 22). Summary statistics of bacteria sample analysis 

results are shown in table 23. Table 23 also shows TSWQS for 

bacteria in Oso Bay and Oso Creek, TCEQ segments 2485 and 

2485A, respectively (Texas Commission on Environmental 

Quality, 2006).

Similar to other constituents, bacteria densities were 

greater at the West Oso Creek subwatershed than at the 

Oso Creek tributary subwatershed. Bacteria loads and yields 

were not estimated because bacteria samples were collected 

near the beginning of runoff events, and concentrations 

100 150 200

0

50

100

150

200

250

250

300

SUSPENDED-SEDIMENT LOAD, IN TONS PER DAY

STREAMFLOW, IN CUBIC FEET PER SECOND

0 50

West Oso Creek suspended-sediment relation

Oso Creek tributary suspended-sediment relation

West Oso Creek suspended-sediment loads

computed from samples

Oso Creek tributary suspended-sediment loads

computed from samples 

Figure 9. Relations between suspended-sediment load and streamflow for West Oso Creek (station 08211517) and Oso Creek tributary 

(station 082115725) subwatersheds, Nueces County, Texas, water years 2006–08. 
24 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

might not be representative of bacteria conditions throughout the runoff events. Most sample bacteria densities 

exceeded TSWQS (table 23). The TSWQS listed in table 

23 apply to Oso Bay and Oso Creek and are not technically 

applicable to the study subwatersheds. Also, because runoff 

and associated bacteria densities represent relatively brief 

and infrequent conditions, the resulting effect on downstream receiving waters (Oso Bay and Oso Creek) is not 

known. However, the relatively large bacteria densities (compared to TSWQS) indicate that runoff from these subwatersheds has the potential to contribute bacteria to Oso Bay and 

Oso Creek. 

Table 12. Fertilizer applications, rainfall deposition, and runoff yields of total nitrogen and total phosphorus for West Oso Creek and 

Oso Creek tributary subwatersheds, Nueces County, Texas, water years 2006–08. 

[lb/acre, pounds per acre; (lb/acre)/yr, pounds per acre per year; <, less than]

Subwatershed and period Fertilizer application Rainfall deposition Runoff yield

Total nitrogen

West Oso Creek

Water year 2006, lb/acre 71.3 4.57 1.39

Water year 2007, lb/acre 65.6 6.47 4.26

Water year 2008, lb/acre 59.6 3.76 2.22

2006–08 average, (lb/acre)/yr 65.5 4.93 2.62

Oso Creek tributary

Water year 2006, lb/acre 68.7 3.48 .753

Water year 2007, lb/acre 62.9 5.56 1.28

Water year 2008, lb/acre 57.5 3.03 .484

2006–08 average, (lb/acre)/yr 63.0 4.02 .839

Area-weighted average, both subwatersheds, 2006–08, (lb/acre)/yr 64.2 4.47 1.72

Total phosphorus

West Oso Creek

Water year 2006, lb/acre 15.4 <.06 .416

Water year 2007, lb/acre 14.3 <.09 1.08

Water year 2008, lb/acre 11.1 <.04 .435

2006–08 average, (lb/acre)/yr 13.6 <.07 .644

Oso Creek tributary

Water year 2006, lb/acre 14.6 <.06 .318

Water year 2007, lb/acre 12.9 <.09 .673

Water year 2008, lb/acre 10.8 <.06 .267

2006–08 average, (lb/acre)/yr 12.8 <.07 .419

Area-weighted average, both subwatersheds, 2006–08, (lb/acre)/yr 13.2 <.07 .530
Water Quality 25

Table 13. Results of analyses for samples of suspended sediment in runoff from West Oso Creek (station 08211517) and Oso Creek 

tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[ft3/s, cubic feet per second; mg/L, milligrams per liter]

Sample collection date Collection time Discharge 

(ft3/s)

Concentration 

(mg/L)

Percentage silt and clay, 

by weight

West Oso Creek subwatershed

June 1, 2006 1400 129 745 96

1140 27 447 99

1600 24 315 97

July 3, 2006 1717 .81 1,400 99

Jan. 4, 2007 1145 2.8 1,005 100

1530 2.5 1,070 100

Jan. 25, 2007 0900 9.2 383 97

Mar. 14, 2007 1430 10 2,560 99

July 2, 2007 1845 108 234 72

July 3, 2007 0945 230 136 73

Mar. 11, 2008 0445 1.1 2,720 99

July 24, 2008 1300 70 762 99

Oso Creek tributary subwatershed

June 1, 2006 1710 25 923 97

Sept. 15, 2006 1400 22.4 293 99

1505 22.5 311 99

1730 22.1 234 99

1930 20.3 239 96

Sept. 16, 2006 0730 13.5 163 96

Jan. 4, 2007 1415 8.1 484 97

Jan. 25, 2007 0700 24.8 200 100

Mar. 14, 2007 1700 10 1,504 98

July 3, 2007 1930 32 50 99

July 4, 2007 0830 56 76 100

Aug. 30, 2007 0800 11 44 99

Mar. 11, 2008 1400 3.8 882 99

July 24, 2008 1700 109 64 87

Table 14. Regression equations relating suspended-sediment load to streamflow for West Oso Creek (station 08211517) and Oso Creek 

tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[L, load in tons per day; Q, streamflow in cubic feet per second]

Subwatershed Equation Coefficient of determination

(R-squared)

Residual standard error

(tons per day)

West Oso Creek L = 1.82 x Q0.93 0.80 71

Oso Creek tributary L = 0.71 x Q0.92 .64 20
26 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Table 15. Estimated monthly and annual runoff suspended-sediment loads, West Oso Creek (station 08211517) and Oso Creek tributary 

(station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[In tons]

Water 

year

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

West Oso Creek subwatershed

2006 0 74 0 0 0 0 0 0 187 9.2 1.3 627 898

2007 .6 0 0 54 .04 36 0 1.0 0 1,827 63 253 2,230

2008 25 0 0 0 0 10 0 0 0 66 798 0 899

Oso Creek tributary subwatershed

2006 16 5.8 0 0 0 0 0 0 10 2.7 0 266 300

2007 10 0 0 45 1.0 11 0 2.4 0 399 14 60 542

2008 26 0 0 0 0 9 0 0 0 178 21 23 257

Table 16. Annual and average annual runoff yields of suspended sediment, West Oso Creek (station 08211517) and Oso Creek tributary 

(station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08. 

[In pounds per acre per year]

Subwatershed Water year 2006 Water year 2007 Water year 2008 Water years 

2006–08 average

West Oso Creek 349 867 350 522

Oso Creek tributary 113 205 97 139

Table 17. Pesticides used in West Oso Creek and Oso Creek 

tributary subwatersheds, Nueces County, Texas, water years 

2006–08. 

[* indicates chemical not included in runoff-sample laboratory analysis]

Herbicides Insecticides Defoliants, desicants, 

growth regulators

2,4-D *Acephate *Ethephon

Atrazine *Acetamiprid *Mepiquat chloride

Glyphosate *Cyfluthrin *Mepiquat pentaborate

Glufosinate-ammonium *Deltamethrin *Thidiazuron

*Prosulfuron Dicrotophos *Thidiazuron + diuron

Pendimethalin Imidacloprid *Paraquat chloride

Prometryn Malathion

Trifluralin Myclobutanil1

Propoxur2

*Thiamethoxam

Zeta-cypermethrin

1

 Fungicide used primarily for residential applications.

2

 Insecticide used primarily for residential applications.
Water Quality 27

Table 18. Summary statistics of concentrations of selected pesticides in runoff samples from West Oso Creek (station 08211517) and 

Oso Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[μg/L, micrograms per liter; --, not computed; <, less than]

Pesticide Number of 

samples

Number of 

detections

Mean 

(μg/L)

Median 

(μg/L)

Minimum 

(μg/L)

Maximum 

(μg/L)

West Oso Creek subwatershed

Herbicides

2,4-D 12 8 -- 0.09 <0.02 6.24

2-Chloro-4-isopropylamino-6-aminos-triazine (CIAT)12 12 0.07 .05 .015 .18

2-Chloro-6-ethylamino-4-amino-striazine (CEAT)12 2 <.08 <.08 <.08 <.08

2-Hydroxy-4-isopropylamino-6-

ethylamino-s-triazine (OIET)

12 11 .17 .11 <.04 .529

Acifluorfen 12 1 <.06 <.06 <.028 <.06

Aldicarb sulfoxide 12 1 <.04 <.04 <.04 <.04

Aminomethylphosphonic acid 

(AMPA)

10 10 5.44 3.60 1.13 13.2

Atrazine 12 12 .98 .120 .02 8.90

Benfluralin 12 1 <.01 <.01 <.01 <.01

Dimethyl tetrachloroterephthalate 

(DCPA)

12 2 <.003 <.003 <.003 .004

Diuron 12 2 <.04 <.04 <.01 .08

Dinoseb 12 1 <.04 <.04 <.04 <.04

Glufosinate 10 2 -- <.14 <.14 .58

Glyphosate 10 10 17.6 8.51 1.81 53.5

Metolachlor 12 4 -- <.006 <.010 .228

Metribuzin 12 1 <.028 <.028 <.028 <.028

Pendimethalin 12 9 -- .33 <.020 13.7

Simazine 12 4 -- <.006 <.005 .022

Trifluralin 12 4 -- <.009 <.009 .05

Insecticides

Dicrotophos 12 1 <.08 <.08 <.08 <.08

Imidacloprid 12 1 -- <.060 <.020 .086

Myclobutanil 12 1 -- <.0334 <.033 <.033

Propoxur 12 1 -- <.004 <.004 <.005
28 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Table 18. Summary statistics of concentrations of selected pesticides in runoff samples from West Oso Creek (station 08211517) and 

Oso Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08—Continued.

Pesticide Number of 

samples

Number of 

detections

Mean 

(μg/L)

Median 

(μg/L)

Minimum 

(μg/L)

Maximum 

(μg/L)

Oso Creek tributary subwatershed

Herbicides

2,4-D 12 8 -- <0.05 <0.02 1.23

2-Chloro-4-isopropylamino-6-aminos-triazine (CIAT)12 12 0.107 .062 .009 .562

2-Chloro-6-ethylamino-4-amino-striazine (CEAT)12 3 -- <.08 <.08 .11

2-Hydroxy-4-isopropylamino-6-

ethylamino-s-triazine (OIET)

12 12 .257 .200 .021 .902

3,4 Dichloroaniline 12 1 -- <.004 <.004 .006

Aminomethylphosphonic acid 

(AMPA)

8 8 .90 .60 .35 1.94

Atrazine 12 12 1.94 .254 .012 10.5

Bromacil 12 1 -- <.04 <.02 .05

Carbaryl 12 4 <.02 <.02 <.02 <.02

Dimethyl tetrachloroterephthalate 

(DCPA)

12 1 <.003 <.003 <.003 .004

Dicamba 12 1 -- <.04 <.04 .97

Diuron 12 3 -- <.04 <.02 .10

Glyphosate 8 8 2.56 1.08 .59 10.6

Metolachlor 12 3 -- <.010 <.006 .008

Pendimethalin 12 8 -- .040 <.012 .144

Simazine 12 5 -- <.006 <.005 .064

Terbuthylazine 12 1 -- <.01 <.01 .02

Trifluralin 12 4 -- <.009 <.009 .054

Insecticides

Azinphos-methyl 9 1 <.05 <.05 <.05 <.08

Dicrotophos 12 1 <.08 <.08 <.08 <.08

Malathion 12 2 -- <.016 <.016 2.64

Fipronil sulfide 12 1 -- <.013 <.013 <.013

Malaoxon 12 1 -- <.039 <.039 .049

Myclobutanil 12 2 -- <.033 <.010 <.033
Water Quality 29

Table 19. Event-median concentrations of selected pesticides in runoff samples from West Oso Creek (station 08211517) and Oso 

Creek tributary (station 08211525) subwatersheds, Nueces County, Texas, by season (preharvest or harvest/postharvest), water years 

2006–08, used to estimate runoff loads for unsampled events.

[In micrograms per liter]

Pesticide

Season median concentration 

Median concentration, 

all samples Preharvest 

(Jan.–June)

Harvest/postharvest 

(July–Sept.)

West Oso Creek subwatershed

Atrazine 0.198 0.097 0.120

Glyphosate 5.10 12.6 8.51

Pendimethalin 1.28 .210 .334

Oso Creek tributary subwatershed

Atrazine 1.88 .194 .254

Glyphosate 1.34 1.08 1.08

Pendimethalin .091 .020 .040

Table 20. Estimated monthly and annual runoff loads of selected pesticides from West Oso Creek (station 08211517) and Oso Creek 

tributary (station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08.

[In pounds]

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

West Oso Creek subwatershed

Water year 2006

Atrazine 0 0.028 0 0 0 0 0 0 0.755 0.002 0.001 0.182 0.97

Glyphosate 0 3.60 0 0 0 0 0 0 1.37 .581 .104 21.5 27.2

Pendimethalin 0 .06 0 0 0 0 0 0 .015 .043 .002 .728 .85

Water year 2007

Atrazine .0002 0 0 .010 0 .034 0 .0004 0 28.4 .023 .102 28.6

Glyphosate .023 0 0 .448 0 .703 0 .153 0 92.5 2.85 12.8 109

Pendimethalin .0004 0 0 .483 0 .003 0 .007 0 1.20 .05 .21 1.95

Water year 2008

Atrazine .008 0 0 0 0 .035 0 0 0 .023 .317 0 .38

Glyphosate 1.08 0 0 0 0 .466 0 0 0 3.18 41.2 0 45.9

Pendimethalin .018 0 0 0 0 .015 0 0 0 .064 .70 0 .78

Oso Creek tributary subwatershed

Water year 2006

Atrazine .032 .010 0 0 0 0 0 0 .170 .007 0 .086 .30

Glyphosate .176 .053 0 0 0 0 0 0 .055 .014 0 2.34 2.64

Pendimethalin .003 .001 0 0 0 0 0 0 .002 .001 0 .134 .14

Water year 2007

Atrazine .017 0 0 .009 0 .897 0 .004 0 .82 .091 .170 2.01

Glyphosate .096 0 0 .335 .005 .364 0 .024 0 8.24 .234 .677 10.0

Pendimethalin .002 0 0 .056 .001 .013 0 0 0 .085 .063 .071 .290

Water year 2008

Atrazine .044 0 0 0 0 .774 0 0 0 .69 .06 .10 1.67

Glyphosate .242 0 0 0 0 .099 0 0 0 2.06 .24 .23 2.88

Pendimethalin .005 0 0 0 0 .005 0 0 0 .04 .06 .004 .11
30 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Table 21. Estimated annual and average annual application rates and runoff yields of selected pesticides from West Oso Creek and 

Oso Creek tributary subwatersheds, Nueces County, Texas, water years 2006–08.

[In pounds per acre per year]

Pesticide

Water year 2006 Water year 2007 Water year 2008 Water years 2006–08 

average

Application Runoff yield Application Runoff yield Application Runoff yield Application Runoff yield 

West Oso Creek subwatershed (5,145 acres)

Atrazine 0.54 0.0002 0.58 0.006 0.52 0.00007 0.55 0.002

Glyphosate 1.40 .005 1.85 .021 .86 .009 1.37 .012

Pendimethalin .39 .0002 .36 .0004 .37 .0002 .37 .0003

Oso Creek tributary subwatershed (5,287 acres)

Atrazine .54 .00006 .67 .0004 .49 .0003 .57 .0003

Glyphosate 1.42 .0005 1.65 .002 .72 .0005 1.26 .001

Pendimethalin .38 .00003 .27 .00005 .50 .00002 .38 .00003

Area-weighted average, both subwatersheds, 2006–08 

Atrazine .56 .001

Glyphosate 1.31 .006

Pendimethalin .38 .0002

Table 22. Bacteria densities in runoff samples from West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, water years 2006–08—Continued.

Sample collection date Collection time Fecal coliform

(CFU/100 mL)

E. coli

(CFU/100 mL)

Enterococci

(CFU/100 mL)

West Oso Creek subwatershed

July 7, 2006

(primary sample) 1015 320 46 59,000

(lab duplicate) 1015 -- -- 107,000

(average concentration) 1015 320 46 83,000

July 27, 2006

(primary sample) 0935 31,000 4,700 127,000

(field split sample) 0935 42,000 9,000 154,000

(lab duplicate of primary sample) 0935 -- -- 145,000

(average concentration) 0935 36,500 6,850 142,000

Sept. 18, 2006

(primary sample) 0918 60,000 84,000 96,000

(field split sample) 0918 46,000 100,000 88,000

(average concentration) 0918 53,000 92,000 92,000

Jan. 4, 2007

(primary sample) 1154 6,000 7,600 21,000

(lab duplicate) 1154 14,000 -- --

(average concentration) 1154 10,000 7,600 21,000

Jan. 25, 2007

(primary sample) 0900 44,000 39,000 64,000

(lab duplicate) 0900 45,000 -- --

(average concentration) 0900 44,500 39,000 64,000

Table 22. Bacteria densities in runoff samples from West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, water years 2006–08.

[CFU/100 mL, colony forming units per 100 milliliters; E. coli, Escherichia coli; --, not analyzed for]
Water Quality 31

Table 22. Bacteria densities in runoff samples from West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525) 

subwatersheds, Nueces County, Texas, water years 2006–08—Continued.

Sample collection date Collection time Fecal coliform

(CFU/100 mL)

E. coli

(CFU/100 mL)

Enterococci

(CFU/100 mL)

West Oso Creek subwatershed—Continued

May 25, 2007

(primary sample) 1512 304 391 5,200

(lab duplicate) 1512 761 -- --

(average concentration) 1512 532 391 5,200

July 7, 2007

(primary sample) 1150 -- 15,600 45,000

(field split sample) 1150 -- 5,800 83,000

(lab duplicate of primary sample) 1150 -- 7,200 49,000

(average concentration) 1150 -- 9,530 59,000

Aug. 30, 2007

(primary sample) 1540 82,000 12,500 145,000

(field split sample) 1540 69,000 69,000 143,000

(lab duplicate of primary sample) 1540 -- 30,000 --

(average concentration) 1540 75,500 37,200 144,000

July 24, 2008

(primary sample) 1015 10,200 1,550 127,000

(field split sample) 1015 9,800 1,125 197,000

(average concentration) 1015 10,000 1,338 162,000

Aug. 18, 2008

(primary sample) 0935 <63 250 11,600

(field split sample) 0935 <63 125 11,500

(average concentration) 0935 <63 188 11,600

Oso Creek tributary subwatershed

July 7, 2006

(primary sample) 1110 145 80 4,800

(field split sample) 1110 253 77 5,200

(lab duplicate) 1110 -- 53 --

(lab duplicate of primary sample) 1110 83 -- --

(average concentration) 1110 160 70 5,000

Sept. 15, 2006

(primary sample) 1000 3,600 16,000 25,000

(field split sample) 1000 8,000 21,000 28,000

(lab duplicate of primary sample) 1000 -- -- 25,000

(average concentration) 1000 5,800 18,500 26,000

Jan. 4, 2007

(primary sample) 1217 4,300 5,300 31,000

(field split sample) 1217 2,780 3,000 39,000

(average concentration) 1217 3,540 4,150 35,000

Jan. 25, 2007

(primary sample) 0920 14,900 4,100 22,000

July 2, 2007

(primary sample) 1150 -- 5,000 75,000

July 24, 2008 (primary sample) 1100 1,600 933 71,000

(field split sample) 1100 -- -- 83,000

(average concentration) 1100 1,600 933 77,000

Aug. 18, 2008

(primary sample) 1015 <63 188 10,800

(field split sample) 1015 <63 313 5,800

(average concentration) 1015 <63 250 8,300
32 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Summary

During water years 2006–08 (October 2005–September 

2008), the U.S. Geological Survey (USGS), in cooperation 

with the Texas State Soil and Water Conservation Board, 

Coastal Bend Bays and Estuaries Program (CBBEP), and 

Texas AgriLife Research and Extension Center at Corpus 

Christi, studied hydrologic conditions and water quality of 

rainfall and storm runoff of two primarily agricultural subwatersheds of the Oso Creek watershed in Nueces County, 

Texas. One area, the upper West Oso Creek subwatershed, is 

about 5,145 acres. The other area, a subwatershed drained by 

an unnamed tributary to Oso Creek (hereinafter, Oso Creek 

tributary), is about 5,287 acres.

Rainfall and runoff (streamflow) were continuously monitored at USGS stations at the outlets of the study subwatersheds during the 3-year study. Seventeen rainfall samples were 

collected and analyzed for nutrients and major inorganic ions. 

Twenty-four composite runoff samples were collected and 

analyzed for nutrients, major inorganic ions, and pesticides. 

Twenty-six discrete suspended-sediment samples were collected and analyzed, and 17 bacteria samples were collected 

and analyzed for fecal coliform, Escherichia coli (E. coli), and 

Enterococcus. These rainfall, streamflow, and water-quality 

data were used to estimate loads of selected constituents 

entering the study subwatersheds in rainfall and exiting the 

subwatersheds in runoff. Data on applications of fertilizers 

to croplands in the study subwatersheds were compiled and 

compared with quantities of nutrients in rainfall deposition 

and runoff yields, and data on applications of pesticides were 

compiled and compared with quantities of pesticides in runoff 

yields.

For the study period, total rainfall was greater than average, although annual rainfall was less than average in each of 

water years 2006 and 2008, and monthly rainfall was less than 

average during 23 of the 36 months. Runoff in the subwatersheds during water years 2006–08 corresponded to rainfall 

patterns, with periods of runoff interspersed between considerable periods of no runoff. Most of the runoff from the two subwatersheds occurred in response to a few temporally isolated 

storm periods. More than 60 percent of the runoff from each 

subwatershed occurred during two wet periods: September 

18–October 1, 2006, and July 2–August 8, 2007. During both 

of these periods, multiple rainfall events and prolonged wetsoil conditions contributed to substantial runoff. The West Oso 

Creek subwatershed produced more runoff during the study 

period than the Oso Creek tributary subwatershed, 13.95 in. 

compared with 9.45 in. Runoff response was quicker and peak 

flows were larger in the West Oso Creek subwatershed than in 

the Oso Creek tributary subwatershed.

Table 23. Summary statistics of bacteria densities in runoff samples from West Oso Creek (station 08211517) and Oso Creek tributary 

(station 08211525) subwatersheds, Nueces County, Texas, water years 2006–08; and Texas surface-water-quality standards for bacteria 

in Oso Bay and Oso Creek, Texas Commission on Environmental Quality segments 2485 and 2485A, respectively.

[CFU/100 mL, colony forming units per 100 milliliters; TSWQS, Texas surface-water-quality standard; <, less than; E. coli, Escherichia coli]

Bacteria Number of 

samples

Mean 

(CFU/100 mL)

Median 

(CFU/100 mL)

Minimum 

(CFU/100 mL)

Maximum 

(CFU/100 mL)

TSWQS1

(CFU/100 mL)

for receiving waters 

of Oso Bay and 

Oso Creek

West Oso Creek subwatershed

Fecal coliform 9 25,600 10,000 <63 75,500 2200/400

E. coli 10 19,400 7,220 46 92,000 3

126/394

Enterococci 10 78,400 73,500 5,200 162,000 4

35/89

Oso Creek tributary subwatershed

Fecal coliform 6 4,340 2,570 <63 14,900 2200/400

E. coli 7 4,720 4,100 70 18,500 3

126/394

Enterococci 7 35,500 26,000 5,000 77,000 4

35/89

1

 Texas Commission on Environmental Quality (2006).

2

 Geometric mean of fecal coliform should not exceed 200 CFU/100 mL; single samples should not exceed 400 CFU/100 mL (freshwater, contact recreation 

criteria).

3

 Geometric mean of E. coli should not exceed 126 CFU/100 mL; single samples should not exceed 394 CFU/100 mL (freshwater, contact recreation criteria).

4

 Geometric mean of Enterococci should not exceed 35 CFU/100 mL; single samples should not exceed 89 CFU/100 mL (saltwater, contact recreation 

criteria).
References 33

Most of the nitrogen in rainfall was in the form of dissolved ammonia and dissolved nitrate, which were detected 

in all samples. Median concentrations of ammonia and nitrate 

were 0.17 and 0.12 mg/L, respectively. Organic forms of 

nitrogen were detected in all samples at relatively low concentrations; the median value of total organic nitrogen was 0.08 

mg/L. Nitrite was detected in less than one-half of the samples 

at low concentrations; the median concentration was less than 

0.002 mg/L. Total phosphorus was detected in about one-half 

of the samples at relatively low concentrations; the median 

concentration was less than 0.007 mg/L.

Based on rainfall volumes in the study subwatersheds 

and sample concentrations, daily total nitrogen and total 

phosphorus deposition were computed for each subwatershed. 

The average (area-weighted) annual total nitrogen deposition 

for both subwatersheds over the 3-year study period was 4.47 

(lb/acre)/yr, compared with less than 0.07 (lb/acre)/yr for total 

phosphorus. 

Statistical tests (Wilcoxon rank-sum tests) indicate 

significant differences in median values of event-mean runoff 

concentrations between the two subwatersheds. Nitrogen 

concentrations in runoff were greater at the West Oso Creek 

site than at the Oso Creek tributary site. Major inorganic ion 

concentrations were greater at the Oso Creek tributary site 

than at the West Oso Creek site. There was no significant 

difference in total phosphorus concentrations between the 

subwatersheds. 

Runoff loads and yields were computed for selected 

nutrients. Total nitrogen runoff yield for the 3-year study 

period averaged 2.62 (lb/acre)/yr from the West Oso Creek 

subwatershed and 0.839 (lb/acre)/yr from the Oso Creek 

tributary subwatershed. Total phosphorus yields from the West 

Oso Creek and Oso Creek tributary subwatersheds for the 

3-year period were 0.644 and 0.419 (lb/acre)/yr, respectively. 

Runoff yields of nitrogen and phosphorus were relatively 

small compared to inputs of nitrogen in fertilizer and rainfall 

deposition. Average annual runoff yield of total nitrogen (subwatersheds combined) represents about 2.5 percent of nitrogen 

applied as fertilizer and nitrogen entering the subwatersheds 

through rainfall deposition. Average annual runoff yield of 

total phosphorus (subwatersheds combined) represents about 

4.0 percent of the phosphorus in applied fertilizer and rainfall 

deposition. 

Suspended-sediment yields from the West Oso Creek 

subwatershed were more than three times those from the 

Oso Creek tributary subwatershed. The average suspendedsediment yield from the West Oso Creek subwatershed was 

522 (lb/acre)/yr. The average suspended-sediment yield from 

the Oso Creek tributary subwatershed was 139 (lb/acre)/yr. 

Thirty-two pesticide compounds (24 herbicides and 

8 insecticides) were detected in runoff samples collected from 

the two subwatersheds. At the West Oso Creek subwatershed, 

23 pesticides were detected (19 herbicides and 4 insecticides). 

At the Oso Creek tributary subwatershed, 24 pesticides were 

detected (18 herbicides and 6 insecticides). Of the 32 pesticides detected during the study, 14 of the pesticides were 

detected in only one sample (at either one or the other 

subwatershed site). All of these 14 detections were at low 

concentrations (near the minimum laboratory reporting level). 

Two herbicides were detected in all samples: atrazine and 

atrazine degradation byproduct 2-chloro-4-isopropylamino-6-

amino-s-triazine (CIAT). Glyphosate and glyphosate byproduct aminomethylphosphonic acid (AMPA) were detected in 

all samples collected during water years 2006–07. However, 

these compounds were not included in the analyses of samples 

collected during water year 2008. Herbicides 2,4-D and 

pendimethalin were detected in more than 50 percent of the 

samples. 

Runoff yields of the herbicides glyphosate, atrazine, and 

pendimethalin were greater for the West Oso Creek subwatershed than for the Oso Creek tributary subwatershed. Of all 

pesticides detected in runoff, the highest runoff yields were for 

glyphosate: 0.012 (lb/acre)/yr for the West Oso Creek subwatershed and 0.001 (lb/acre)/yr for the Oso Creek tributary 

subwatershed. Comparison of applications and runoff yields 

indicates that, for water years 2006–08, about 0.9 percent 

of glyphosate applied to the West Oso Creek subwatershed 

croplands was detected in runoff. For the Oso Creek tributary 

subwatershed, about 0.08 percent of applied glyphosate was 

detected in runoff. 

At both subwatershed outlet sites, most sample concentrations of fecal coliform, E. coli, and Enterococci were 

greater than Texas surface-water-quality standards for those 

bacteria established for the receiving waters of Oso Creek and 

Oso Bay. Because runoff and associated bacteria densities represent relatively brief and infrequent conditions, the resulting 

effect on Oso Bay and Oso Creek is not known. However, the 

relatively large bacteria densities (compared to Texas surfacewater-quality standards) indicate that runoff from the study 

subwatersheds is a potential source of bacteria for Oso Bay 

and Oso Creek. 

Estimates of rainfall nitrogen deposition and runoff loads 

and yields of nutrients, suspended sediment, and pesticides 

for 2006–07 from the previous report of the study subwatersheds were revised, based on the collection and analysis of 

additional data during the 2008 water year. The new data were 

used to revise the relations used to estimate deposition and 

loads during unsampled events, resulting in revised estimates 

of deposition, loads, and yields.

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Blank Page
Appendixes 1–3
Blank Page
Appendix 1 39

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08.

[unfltrd, unfiltered; μS/cm, microsiemens per centimeter at 25 degrees Celsius; fltrd, filtered; mg/L, milligrams per liter; E, estimated, <, less than; --, not analyzed; N, nitrogen; P, phosphorus; mg/L, micrograms per liter; CIAT, 2-chloro-4-isopropylamino-6-amino-s-triazine; CEAT, 2-chloro-6-ethylamino-4-aminos-triazine; OIET, 2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine; DCPA, dimethyl tetrachloroterephthalate; MCPA, 4-chloro-2-methylphenoxy acetic 

acid; MCPB, 4-(2-methyl-4-chlorophenoxy)butyric acid]

Date Sample type

pH, water, 

unfltrd, lab

(standard units)

Specific 

conductance,

unfltrd, lab

(μS/cm)

Calcium,

water, fltrd

(mg/L)

Magnesium,

water,

fltrd

(mg/L)

Potassium,

water,

fltrd

(mg/L)

Sodium,

water,

fltrd

(mg/L)

Chloride,

water,

fltrd

(mg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 Blank 8.5 3 0.11 E0.005 E0.005 0.26 <0.01

Oct. 25, 2007 Blank 6.1 5 <.04 <.02 .006 <.12 <.01

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank -- -- -- -- -- -- --

Oct. 25, 2007 Blank 5.8 5 <.04 <.02 .007 <.12 <.01

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 Blank 9.5 14 <.04 .02 .03 <.12 <.01

Date Sample type

Fluoride,

water,

fltrd

(mg/L)

Silica,

water,

fltrd

(mg/L)

Sulfate,

water,

fltrd

(mg/L)

Ammonia

+ organic-N,

water, fltrd

(mg/L as N)

Ammonia

+ organic-N,

water, unfltrd

(mg/L as N)

Ammonia,

water,

fltrd

(mg/L as N)

Nitrite

+ nitrate,

water, fltrd

(mg/L as N)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 Blank 0.03 -- <0.01 <0.10 <0.10 <0.01 <0.016

Oct. 25, 2007 Blank <.01 <0.02 <.01 E.08 <.02 <.02 <.016

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank -- -- -- -- -- -- --

Oct. 25, 2007 Blank <.01 <.02 <.01 -- -- <.02 <.016

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 Blank .03 .03 <.01 -- -- <.01 <.016

Date Sample type

Nitrite,

water,

fltrd

(mg/L as N)

Organic

nitrogen,

water,

fltrd

(mg/L as N)

Organic

nitrogen,

water,

unfltrd

(mg/L as N)

Total

nitrogen,

water,

fltrd

(mg/L as N)

Total

nitrogen,

water,

unfltrd

(mg/L as N)

Orthophosphate,

water,

fltrd

(mg/L as P)

Phosphorus,

water,

unfltrd

(mg/L as P)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 Blank <0.002 -- -- -- -- <0.006 <0.004

Oct. 25, 2007 Blank <.002 -- -- -- -- <.006 <.008

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank -- -- -- -- -- -- --

Oct. 25, 2007 Blank <.002 -- -- -- -- <.006 --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 Blank E.001 -- -- -- -- <.006 --

Appendix 1
40 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08—Continued.

Date Sample type

1-Naphthol,

water,

fltrd

(μg/L)

2,4-D

methylester,

water, fltrd

(μg/L)

2,4-D,

water,

fltrd

(μg/L)

2,4-DB,

water,

fltrd

(μg/L)

2,6-Diethylaniline,

water, fltrd

(μg/L)

2-Chloro2,6-diethyl

acetanilide,

water, fltrd

(μg/L)

CIAT,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank <0.09 <0.19 <0.04 <0.02 <0.006 <0.006 <0.014

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

CEAT,

water,

fltrd

(μg/L)

2-Ethyl-6-

methylaniline,

water, fltrd

(μg/L)

OIET,

water,

fltrd

(μg/L)

3,4-Dichloroaniline,

water, fltrd

(μg/L)

3-Hydroxy

carbofuran,

water, fltrd

(μg/L)

4-Chloro2-methyl

phenol,

water, fltrd

(μg/L)

Acetochlor,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank <0.08 <0.010 <0.08 <0.004 <0.008 <0.005 <0.006

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Acifluorfen,

water,

fltrd

(μg/L)

Alachlor,

water,

fltrd

(μg/L)

Aldicarb

sulfone,

water, fltrd

(μg/L)

Aldicarb

sulfoxide,

water, fltrd

(μg/L)

Aldicarb,

water,

fltrd

(μg/L)

Atrazine,

water,

fltrd

(μg/L)

Azinphosmethyl oxon,

water, fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank <0.06 <0.005 <0.08 <0.04 <0.04 <0.007 <0.04

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --
Appendix 1 41

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08—Continued.

Date Sample type

Azinphosmethyl,

water, fltrd

(μg/L)

Bendiocarb,

water,

fltrd

(μg/L)

Benfluralin,

water,

fltrd

(μg/L)

Benomyl,

water,

fltrd

(μg/L)

Bensulfuron

methyl,

water, fltrd

(μg/L)

Bentazon,

water,

fltrd

(μg/L)

Bromacil,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank <0.08 <0.04 <0.01 <0.02 <0.06 <0.02 <0.04

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Bromoxynil,

water,

fltrd

(μg/L)

Carbaryl,

water,

fltrd

(μg/L)

Carbofuran,

water,

fltrd

(μg/L)

Chloramben

methyl ester,

water, fltrd

(μg/L)

Chlorimuron,

water, fltrd

(μg/L)

Chlorodiaminos-triazine,

water, fltrd

(μg/L)

Chlorpyrifos oxon,

water, fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank <0.12 <0.02 <0.06 <0.10 <0.08 <0.12 <0.06

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Chlorpyrifos,

water,

fltrd

(μg/L)

cisPermethrin,

water, fltrd

(μg/L)

Clopyralid,

water,

fltrd

(μg/L)

Cycloate,

water,

fltrd

(μg/L)

Cyfluthrin,

water,

fltrd

(μg/L)

Cypermethrin,

water, fltrd

(μg/L)

Dacthal

mono-acid,

water, fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 20, 2006 Blank <0.005 <0.01 <0.06 <.06 <0.053 <0.046 <0.02

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --
42 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08—Continued.

Date Sample type

DCPA,

water,

fltrd

(μg/L)

Desulfinylfipronil,

water, fltrd

(μg/L)

Diazinon,

water,

fltrd

(μg/L)

Dicamba,

water,

fltrd

(μg/L)

Dichlorprop,

water,

fltrd

(μg/L)

Dicrotophos,

water,

fltrd

(μg/L)

Dieldrin,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.003 <0.012 <0.005 <0.08 <0.04 <0.08 <0.009

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Dimethoate,

water,

fltrd

(μg/L)

Dinoseb,

water,

fltrd

(μg/L)

Diphenamid,

water,

fltrd

(μg/L)

Diuron,

water,

fltrd

(μg/L)

Ethion

monoxon,

water, fltrd

(μg/L)

Ethion,

water,

fltrd

(μg/L)

Fenamiphos

sulfone,

water, fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.006 <0.04 <0.04 <0.04 <0.021 <0.016 <0.053

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Fenamiphos

sulfoxide,

water, fltrd

(μg/L)

Fenamiphos,

water,

fltrd

(μg/L)

Fenuron,

water,

fltrd

(μg/L)

Desulfinylfipronil

amide,

water, fltrd

(μg/L)

Fipronil

sulfide,

water, fltrd

(μg/L)

Fipronil

sulfone,

water, fltrd

(μg/L)

Fipronil,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.04 <0.03 <0.04 <0.029 <0.013 <0.024 <0.016

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --
Appendix 1 43

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08—Continued.

Date Sample type

Flumetsulam,

water,

fltrd

(μg/L)

Fluometuron,

water, fltrd

(μg/L)

Fonofos,

water,

fltrd

(μg/L)

Glufosinate,

water,

fltrd

(μg/L)

Glyphosate,

water,

fltrd

(μg/L)

Hexazinone,

water,

fltrd

(μg/L)

Imazaquin,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.06 <0.04 <0.006 <0.14 <0.15 <0.026 <0.04

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Imazethapyr,

water,

fltrd

(μg/L)

Imidacloprid,

water, fltrd

(μg/L)

Iprodione,

water,

fltrd

(μg/L)

Isofenphos,

water,

fltrd

(μg/L)

Linuron,

water,

fltrd

(μg/L)

Malaoxon,

water,

fltrd

(μg/L)

Malathion,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.04 <0.06 <0.026 <0.011 <0.04 <0.039 <0.016

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

MCPA,

water,

fltrd

(μg/L)

MCPB,

water,

fltrd

(μg/L)

Metalaxyl,

water,

fltrd

(μg/L)

Methidathion,

water,

fltrd

(μg/L)

Methiocarb,

water,

fltrd

(μg/L)

Methomyl,

water,

fltrd

(μg/L)

Methyl

paraoxon,

water, fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.06 <0.2 <0.04 <0.007 <0.04 <0.06 <0.02

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --
44 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08—Continued.

Date Sample type

Methyl

parathion,

water, fltrd

(μg/L)

Metolachlor,

water,

fltrd

(μg/L)

Metribuzin,

water,

fltrd

(μg/L)

Metsulfuron,

water,

fltrd

(μg/L)

Myclobutanil,

water,

fltrd

(μg/L)

N-(4-Chlorophenyl)-N’-

methylurea

(μg/L)

Neburon,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.008 <0.010 <0.012 <0.14 <0.033 <0.06 <0.02

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Nicosulfuron,

water, fltrd

(μg/L)

Norflurazon,

water,

fltrd

(μg/L)

Oryzalin,

water,

fltrd

(μg/L)

Oxamyl,

water,

fltrd

(μg/L)

Pendimethalin,

water, fltrd

(μg/L)

Phorate

oxon,

water, fltrd

(μg/L)

Phorate,

water,

fltrd

(μg/L))

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.10 <0.04 <0.04 <0.04 <0.02 <0.03 <0.02

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Phosmet

oxon,

water, fltrd

(μg/L)

Phosmet,

water,

fltrd

(μg/L)

Picloram,

water,

fltrd

(μg/L)

Prometon,

water,

fltrd

(μg/L)

Prometryn,

water,

fltrd

(μg/L)

Propyzamide,

water, fltrd

(μg/L)

Propham,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.051 <0.008 <0.12 <0.01 <0.006 <0.004 <0.06

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --
Appendix 1 45

Appendix 1. Quality control and assurance data for rainfall samples collected at Texas AgriLife Research at Corpus Christi (station 

08211511); and for runoff samples collected at West Oso Creek (station 08211517) and Oso Creek tributary (station 08211525), Nueces 

County, Texas, water years 2006–08—Continued.

Date Sample type

Propiconazole,

water, fltrd

(μg/L)

Propoxur,

water,

fltrd

(μg/L)

Siduron,

water,

fltrd

(μg/L)

Simazine,

water,

fltrd

(μg/L)

Sulfometuron,

water, fltrd

(μg/L)

Tebuthiuron,

water,

fltrd

(μg/L)

Terbacil,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.06 <0.04 <0.04 <0.006 <0.06 <0.02 <0.04

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Date Sample type

Terbufos

oxon sulfone,

water, fltrd

(μg/L)

Terbufos,

water,

fltrd

(μg/L)

Terbuthylazine,

water, fltrd

(μg/L)

Tribuphos,

water,

fltrd

(μg/L)

Triclopyr,

water,

fltrd

(μg/L)

Trifluralin,

water,

fltrd

(μg/L)

Dichlorvos,

water,

fltrd

(μg/L)

Station 08211511—Texas AgriLife Research at Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211517—West Oso Creek at Merrett Road near Corpus Christi, Tex.

Nov. 11, 2006 Blank <0.045 <0.012 <0.009 <0.035 <0.04 <0.009 <0.013

Oct. 25, 2007 -- -- -- -- -- -- -- --

Station 08211525—Unnamed Oso Creek tributary at Farm Road 2444 near Corpus Christi, Tex.

Mar. 28, 2006 -- -- -- -- -- -- -- --
46 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08.

[unfltrd, unfiltered; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; CaCO3, calcium carbonate; fltrd, filtered; --, not 

analyzed; N, nitrogen; <, less than; P, phosphorus; μg/L, micrograms per liter; CIAT, 2-chloro-4-isopropylamino-6-amino-s-triazine; E, estimated; CEAT, 

2-Chloro-6-ethylamino-4-amino-s-triazine; OIET, 2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine; DCPA, Dimethyl tetrachloroterephthalate; MCPA, 

4-chloro-2-methylphenoxy acetic acid; MCPB, 4-(2-methyl-4-chlorophenoxy)butyric acid]

Date

pH, water, 

unfltrd, field

(standard 

units)

pH, water, 

unfltrd, lab 

(standard 

units)

Specific 

conductance,

unfltrd, lab

(μS/cm)

Specific 

conductance,

unfltrd, field

(μS/cm)

Hardness,

water 

(mg/L as 

CaCO3)

Calcium,

water, fltrd

(mg/L)

Magnesium,

water,

fltrd

(mg/L)

Potassium,

water,

fltrd

(mg/L)

June 1–2, 2006 6.6 7.7 88 89 29 10.4 0.821 3.48

July 1–2, 2006 6.8 8.0 147 149 51 18.4 1.34 4.07

July 6, 2006 7.5 8.0 135 130 49 17.6 1.33 4.15

Sept. 9–10, 2006 7.4 8.3 124 122 38 13.5 .958 3.24

Sept. 18–19, 2006 7.2 8.0 139 130 50 17.8 1.38 6.57

Jan. 3–5, 2007 7.0 8.3 125 142 40 14.2 1.16 3.86

Jan. 24–25, 2007 6.8 7.6 136 141 45 15.7 1.49 4.24

Mar. 14–15, 2007 8.2 8.0 127 104 -- -- -- --

May 25, 2007 7.3 8.3 98 85 30 10.7 .807 3.60

July 2–3, 2007 6.7 7.3 86 77 26 8.91 1.20 6.74

Mar. 10–11, 2008 -- 8.3 139 127 41 14.4 1.17 3.73

July 24, 2008 7.6 6.8 133 129 43 15.1 1.32 9.73

Date

Sodium,

water,

fltrd

(mg/L)

Chloride,

water,

fltrd

(mg/L)

Fluoride,

water,

fltrd

(mg/L)

Silica,

water,

fltrd

(mg/L)

Sulfate,

water,

fltrd

(mg/L)

Ammonia + 

organic-N,

water, fltrd

(mg/L as N)

Ammonia + 

organic-N,

water,

unfltrd

(mg/L as N)

Ammonia,

water,

fltrd

(mg/L as N)

June 1–2, 2006 2.33 1.17 0.1 6.4 0.9 0.42 1.2 0.017

July 1–2, 2006 7.38 1.58 .4 15.4 2.2 .49 2.1 .027

July 6, 2006 5.82 1.61 .3 13.6 1.6 .75 2.4 .032

Sept. 9–10, 2006 6.65 1.54 .5 11.2 1.5 2.7 3.3 .121

Sept. 18–19, 2006 4.23 2.72 .2 10.9 1.3 .47 1.4 .039

Jan. 3–5, 2007 6.33 2.24 .27 8.4 2.64 .56 2.3 .228

Jan. 24–25, 2007 7.80 3.11 .20 8.1 3.65 .47 1.3 .041

Mar. 14–15, 2007 -- -- -- -- -- .65 4.3 .200

May 25, 2007 2.87 .97 .19 8.2 1.39 .55 2.8 .148

July 2–3, 2007 1.81 1.52 .13 8.6 .93 .46 .75 <.020

Mar. 10–11, 2008 7.58 3.16 .40 9.8 3.90 1.1 7.0 .398

July 24, 2008 3.43 6.03 .15 13.4 1.86 .59 1.2 <.020

Appendix 2
Appendix 2 47

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

Nitrate,

water,

fltrd

(mg/L as N)1

Nitrite + 

nitrate,

water, fltrd

(mg/L as N)

Nitrite,

water,

fltrd

(mg/L as N)

Organic

nitrogen,

water, fltrd

(mg/L as N)1

Organic

nitrogen,

water, unfltrd

(mg/L as N)1

Total

nitrogen,

water, fltrd

(mg/L as N)1

Total

nitrogen,

water, unfltrd

(mg/L as N)1

Orthophosphate,

water, fltrd

(mg/L as P)

June 1–2, 2006 0.23 0.25 0.017 0.41 1.2 0.67 1.5 0.167

July 1–2, 2006 .44 .49 .049 .47 2.1 .98 2.6 .216

July 6, 2006 .36 .38 .026 .72 2.3 1.1 2.8 .264

Sept. 9–10, 2006 .65 .71 .058 2.6 3.2 3.4 4.0 .159

Sept. 18–19, 2006 .49 .51 .017 .43 1.4 .97 1.9 .251

Jan. 3–5, 2007 2.01 2.19 .178 .33 2.1 2.7 4.5 .310

Jan. 24–25, 2007 3.13 3.26 .123 .43 1.3 3.7 4.6 .252

Mar. 14–15, 2007 .72 .78 .057 .45 4.2 1.4 5.1 .350

May 25, 2007 .73 .77 .042 .40 2.6 1.3 3.5 .342

July 2–3, 2007 .21 .22 .008 .44 .73 .68 .98 .463

Mar. 10–11, 2008 1.95 2.04 .089 .70 6.6 3.1 9.0 .303

July 24, 2008 .33 .34 .009 -- -- .94 1.6 .332

1

 Concentration of constituent not analyzed directly but computed from other analyzed constituents.

Date

Phosphorus,

water,

fltrd

(mg/L as P)

Phosphorus,

water,

unfltrd

(mg/L as P)

1-Naphthol,

water,

fltrd

(μg/L)

2,4-D

methylester,

water, fltrd

(μg/L)

2,4-D,

water,

fltrd

(μg/L)

2,4-DB,

water,

fltrd 

(μg/L)

2,6-Diethylaniline,

water, fltrd

(μg/L)

2-Chloro2’,6’-diethyl

acetanilide,

water, fltrd

(μg/L)

CIAT,

water,

fltrd

(μg/L)

June 1–2, 2006 0.189 0.47 <0.09 <0.190 E0.06 <0.02 <0.006 <0.006 E0.112

July 1–2, 2006 .25 .80 <.09 <.638 E6.24 <.02 <.006 <.006 E.048

July 6, 2006 .28 .66 <.09 <.190 E3.35 <.02 <.006 <.006 E.030

Sept. 9–10, 2006 .27 .75 <.09 <.190 <.04 <.02 <.006 <.006 E.109

Sept. 18–19, 2006 .27 .57 <.09 <.190 .59 <.02 <.006 <.006 E.029

Jan. 3–5, 2007 .35 .78 <.09 <.200 E.12 <.02 <.006 <.006 E.015

Jan. 24–25, 2007 .257 .52 <.09 <.200 .16 <.02 <.006 <.006 E.020

Mar. 14–15, 2007 .367 1.52 <.09 <.200 .04 <.02 <.006 <.006 E.021

May 25, 2007 .40 1.00 <.09 <.200 <.04 <.02 <.006 <.006 E.049

July 2–3, 2007 .49 .59 <.09 <.200 <.04 <.02 <.006 <.006 E.179

Mar. 10–11, 2008 .33 .29 <.04 <.040 .28 <.02 <.006 <.010 E.138

July 24, 2008 .365 .57 <.04 <.040 <.02 <.02 <.006 <.010 E.149
48 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

CEAT,

water,

fltrd

(μg/L)

2-Ethyl-6-

methylaniline

water,

fltrd

(μg/L)

OIET,

water,

fltrd

(μg/L)

3,4-Dichloroaniline

water,

fltrd

(μg/L)

3-Hydroxy

carbofuran,

water,

fltrd

(μg/L)

4-Chloro2-methyl

phenol,

water,

fltrd

(μg/L)

Acetochlor,

water,

fltrd

(μg/L)

Acifluorfen,

water,

fltrd

(μg/L)

Alachlor,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.08 <0.010 0.242 <0.004 <0.008 <0.005 <0.006 E0.008 <0.005

July 1–2, 2006 <.08 <.010 .100 <.004 <.008 <.005 <.006 <.028 <.005

July 6, 2006 <.08 <.010 .111 <.004 <.008 <.005 <.006 <.028 <.005

Sept. 9–10, 2006 E.04 <.010 E.302 <.004 <.008 <.005 <.006 <.028 <.005

Sept. 18–19, 2006 <.08 <.010 .106 <.004 <.008 <.005 <.006 <.028 <.005

Jan. 3–5, 2007 <.08 <.010 E.063 <.004 <.020 <.005 <.006 <.006 <.005

Jan. 24–25, 2007 <.08 <.010 E.065 <.004 <.020 <.005 <.006 <.006 <.005

Mar. 14–15, 2007 <.08 <.010 .105 <.004 <.020 <.005 <.006 <.006 <.005

May 25, 2007 <.08 <.010 .149 <.006 <.020 <.005 <.006 <.006 <.005

July 2–3, 2007 E.07 <.010 .529 <.004 <.020 <.005 <.006 <.006 <.005

Mar. 10–11, 2008 <.08 <.010 <.040 <.006 <.040 <.005 <.006 <.040 <.006

July 24, 2008 <.08 <.010 .247 <.006 <.040 <.005 <.006 <.040 <.006

Date

Aldicarb

sulfone,

water,

fltrd

(μg/L)

Aldicarb

sulfoxide,

water, 

fltrd

(μg/L)

Aldicarb,

water,

fltrd

(μg/L)

Aminomethylphosphonic

acid,

water, 

fltrd

(μg/L)

Atrazine,

water,

fltrd

(μg/L)

Azinphosmethyl 

oxon,

water, 

fltrd

(μg/L)

Azinphosmethyl,

water,

fltrd

(μg/L)

Bendiocarb,

water,

fltrd

(μg/L)

Benfluralin,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.02 E0.006 <0.15 1.34 0.996 <0.04 <0.050 <0.08 <0.010

July 1–2, 2006 <.02 <.100 <.15 13.2 .097 <.04 <.050 <.08 <.010

July 6, 2006 <.02 <.100 <.15 4.69 .053 <.04 <.050 <.08 <.010

Sept. 9–10, 2006 <.02 <.100 <.15 10.3 .192 <.04 <.050 <.08 <.010

Sept. 18–19, 2006 <.02 <.100 <.15 1.13 .030 <.04 <.050 <.08 <.010

Jan. 3–5, 2007 <.08 <.040 <.04 2.50 .022 <.04 <.080 <.04 E.007

Jan. 24–25, 2007 <.08 <.040 <.04 1.33 .056 <.04 <.080 <.04 <.010

Mar. 14–15, 2007 <.08 <.040 <.04 1.60 .252 <.04 <.080 <.04 <.010

May 25, 2007 <.08 <.040 <.04 10.1 .144 <.04 <.080 <.04 <.010

July 2–3, 2007 <.08 <.040 <.04 8.20 8.90 <.04 <.080 <.04 <.010

Mar. 10–11, 2008 <.08 <.060 <.12 -- E.939 <.04 <.120 <.04 <.010

July 24, 2008 <.08 <.060 <.12 -- .091 <.06 <.120 <.04 <.010
Appendix 2 49

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

Benomyl

water,

fltrd

(μg/L)

Bensulfuron

methyl,

water,

fltrd

(μg/L)

Bentazon,

water,

fltrd

(μg/L)

Bromacil,

water,

fltrd

(μg/L)

Bromoxynil,

water,

fltrd

(μg/L)

Carbaryl,

water,

fltrd

(μg/L)

Carbofuran,

water,

fltrd

(μg/L)

Chloramben

methyl 

ester,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.022 <0.02 <0.02 <0.02 <0.04 <0.02 <0.016 <0.02

July 1–2, 2006 <.022 <.02 <.02 <.02 <.04 <.02 <.016 <.02

July 6, 2006 <.022 <.02 <.02 <.02 <.04 <.02 <.016 <.02

Sept. 9–10, 2006 <.022 <.02 <.02 <.02 <.04 <.02 <.016 <.02

Sept. 18–19, 2006 <.022 <.02 <.02 <.02 <.04 <.02 <.016 <.02

Jan. 3–5, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

Jan. 24–25, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

Mar. 14–15, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

May 25, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

July 2–3, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

Mar. 10–11, 2008 <.040 <.06 <.04 <.02 <.12 <.04 <.020 <.10

July 24, 2008 <.040 <.06 <.04 <.02 <.12 <.04 <.020 <.10

Date

Chlorimuron,

water,

fltrd

(μg/L)

Chlorodiaminos-triazine,

water, 

fltrd

(μg/L)

Chlorpyrifos 

oxon,

water,

fltrd

(μg/L)

Chlorpyrifos

water,

fltrd

(μg/L)

cisPermethrin,

water,

fltrd

(μg/L)

Clopyralid,

water,

fltrd

(μg/L)

Cycloate,

water,

fltrd

(μg/L)

Cyfluthrin,

water,

fltrd

(μg/L)

Cypermethrin,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.032 <0.04 <0.06 <0.006 <0.006 <0.07 <0.01 <0.053 <0.046

July 1–2, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

July 6, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

Sept. 9–10, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

Sept. 18–19, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

Jan. 3-5, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

Jan. 24–25, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

Mar. 14–15, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

May 25, 2007 <.080 <.12 <.06 <.011 <.010 <.06 <.06 <.053 <.046

July 2–3, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

Mar. 10–11, 2008 <.080 -- <.06 <.005 <.010 <.06 <.02 <.016 <.014

July 24, 2008 <.080 -- <.06 <.005 <.010 <.06 <.02 <.016 <.014
50 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

Dacthal

mono-acid,

water, 

fltrd

(μg/L)

DCPA,

water,

fltrd

(μg/L)

Desulfinylfipronil,

water,

fltrd

(μg/L)

Diazinon,

water,

fltrd

(μg/L)

Dicamba,

water,

fltrd

(μg/L)

Dichlorprop,

water,

fltrd

(μg/L)

Dicrotophos,

water,

fltrd

(μg/L)

Dieldrin,

water,

fltrd

(μg/L)

Dimethoate,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.03 <0.003 <0.012 <0.005 <0.04 <0.03 <0.08 <0.009 <0.006

July 1–2, 2006 <.03 <.003 <.012 <.005 <.04 <.03 <.08 <.009 <.006

July 6, 2006 <.03 <.003 <.012 <.005 <.04 <.03 <.08 <.009 <.006

Sept. 9–10, 2006 <.03 <.003 <.012 <.005 <.04 <.03 <.08 <.009 <.006

Sept. 18–19, 2006 <.03 E.004 <.012 <.005 <.04 <.03 <.08 <.009 <.006

Jan. 3–5, 2007 <.02 <.003 <.012 <.005 <.08 <.04 <.08 <.009 <.006

Jan. 24–25, 2007 <.02 <.003 <.012 <.005 <.08 <.04 <.08 <.009 <.006

Mar. 14–15, 2007 <.02 E.004 <.012 <.005 <.08 <.04 <.08 <.009 <.006

May 25, 2007 <.02 <.003 <.012 <.005 <.08 <.04 <.08 <.009 <.006

July 2–3, 2007 <.02 <.003 <.012 <.005 <.08 <.04 E.04 <.009 <.006

Mar. 10–11, 2008 <.02 <.003 <.012 <.005 <.04 <.02 <.08 <.009 <.006

July 24, 2008 <.02 <.003 <.012 <.005 <.04 <.02 <.08 <.009 <.006

Date

Dinoseb,

water,

fltrd

(μg/L)

Diphenamid,

water,

fltrd

(μg/L)

Diuron,

water,

fltrd

(μg/L)

Ethion

monoxon,

water,

fltrd

(μg/L)

Ethion,

water,

fltrd

(μg/L)

Fenamiphos

sulfone,

water,

fltrd

(μg/L)

Fenamiphos

sulfoxide,

water,

fltrd

(μg/L)

Fenamiphos,

water,

fltrd

(μg/L)

Fenuron,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.04 <0.01 <0.01 <0.02 <0.016 <0.053 <0.04 <0.03 <0.10

July 1–2, 2006 <.04 <.01 <.02 <.02 <.016 <.053 <.04 <.03 <.10

July 6, 2006 <.04 <.01 .03 <.02 <.016 <.053 <.04 <.03 <.10

Sept. 9–10, 2006 <.04 <.01 <.02 <.02 <.016 <.053 <.04 <.03 <.10

Sept. 18–19, 2006 <.04 <.01 <.02 <.02 <.016 <.053 <.04 <.03 <.10

Jan. 3–5, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Jan. 24–25, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Mar. 14–15, 2007 <.04 <.04 E.04 <.02 <.016 <.053 <.04 <.03 <.04

May 25, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

July 2–3, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Mar. 10–11, 2008 <.04 <.04 .08 <.02 <.016 <.053 <.20 <.03 <.04

July 24, 2008 <.04 <.04 <.04 <.02 <.016 <.053 <.20 <.03 <.04
Appendix 2 51

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

Desulfinylfipronil

amide,

water, fltrd

(μg/L)

Fipronil

sulfide,

water,

fltrd

(μg/L)

Fipronil

sulfone,

water,

fltrd

(μg/L)

Fipronil,

water,

fltrd

(μg/L)

Flumetsulam,

water,

fltrd

(μg/L)

Fluometuron,

water,

fltrd

(μg/L)

Fonofos,

water,

fltrd

(μg/L)

Glufosinate,

water,

fltrd

(μg/L)

Glyphosate,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.029 <0.013 <0.024 <0.016 <0.04 <0.02 <0.005 E0.120 1.81

July 1–2, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 51.2

July 6, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 11.8

Sept. 9–10, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 30.3

Sept. 18–19, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 2.00

Jan. 3–5, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 5.10

Jan. 24–25, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 2.10

Mar. 14–15, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 E5.22

May 25, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 .580 53.5

July 2–3, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 E12.6

Mar. 10–11, 2008 <.029 <.013 <.024 <.020 <.06 <.04 <.010 -- --

July 24, 2008 <.029 <.013 <.024 <.020 <.06 <.04 <.010 -- --

Date

Hexazinone,

water,

fltrd

(μg/L)

Imazaquin,

water,

fltrd

(μg/L)

Imazethapyr,

water,

fltrd

(μg/L)

Imidacloprid,

water,

fltrd

(μg/L)

Iprodione,

water,

fltrd

(μg/L)

Isofenphos,

water,

fltrd

(μg/L)

Linuron,

water,

fltrd

(μg/L)

Malaoxon,

water,

fltrd

(μg/L)

Malathion,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.026 <0.04 <0.04 <0.020 <0.026 <0.011 <0.01 <0.039 <0.027

July 1–2, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 <.027

July 6, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 <.027

Sept. 9–10, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 <.027

Sept. 18–19, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 <.027

Jan. 3–5, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

Jan. 24–25, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

Mar. 14–15, 2007 <.026 <.04 <.04 .086 <.026 <.011 <.04 <.039 <.016

May 25, 2007 <.026 <.04 <.04 <.078 <.026 <.011 <.04 <.039 <.016

July 2–3, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

Mar. 10–11, 2008 <.008 <.04 <.04 <.060 <.01 <.006 <.02 <.020 <.016

July 24, 2008 <.008 <.04 <.04 <.060 <.01 <.006 <.02 <.020 <.016
52 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

MCPA,

water,

fltrd

(μg/L)

MCPB,

water,

fltrd

(μg/L)

Metalaxyl,

water,

fltrd

(μg/L)

Methidathion,

water,

fltrd

(μg/L)

Methiocarb,

water,

fltrd

(μg/L)

Methomyl,

water,

fltrd

(μg/L)

Methyl

paraoxon,

water,

fltrd

(μg/L)

Methyl

parathion,

water,

fltrd

(μg/L)

Metolachlor,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.07 <0.10 <0.065 <0.009 <0.034 <0.070 <0.02 <0.015 <0.006

July 1–2, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 <.006

July 6, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 <.006

Sept. 9–10, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 <.006

Sept. 18–19, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 <.006

Jan. 3–5, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 <.010

Jan. 24–25, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 .228

Mar. 14–15, 2007 <.06 <.20 <.011 <.009 <.040 <.060 <.02 <.008 .082

May 25, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 <.013

July 2–3, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 E.008

Mar. 10–11, 2008 <.06 <.06 <.02 <.004 <.040 <.120 <.01 <.008 E.012

July 24, 2008 <.06 <.06 <.02 <.004 <.040 <.120 <.01 <.008 <.025

Date

Metribuzin,

water,

fltrd

(μg/L)

Metsulfuron,

water,

fltrd

(μg/L)

Myclobutanil,

water,

fltrd

(μg/L)

N-(4-Chlorophenyl)-N’-

methylurea

(μg/L)

Neburon,

water,

fltrd

(μg/L)

Nicosulfuron,

water,

fltrd

(μg/L)

Norflurazon,

water,

fltrd

(μg/L)

Oryzalin,

water,

fltrd

(μg/L)

Oxamyl,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.028 <0.07 <0.033 <0.04 <0.01 <0.04 <0.02 <0.02 <0.05

July 1–2, 2006 <.028 <.07 <.033 <.04 <.01 <.04 <.02 <.02 <.05

July 6, 2006 <.028 <.07 <.033 <.04 <.01 <.04 <.02 <.02 <.05

Sept. 9–10, 2006 <.028 <.07 <.033 <.04 <.01 <.04 <.02 <.02 <.05

Sept. 18–19, 2006 <.028 <.07 <.033 <.04 <.01 <.04 <.02 <.02 <.05

Jan. 3–5, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Jan. 24–25, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Mar. 14–15, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

May 25, 2007 <.012 <.14 E.013 <.06 <.02 <.10 <.04 <.04 <.04

July 2–3, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Mar. 10–11, 2008 E.022 <.14 <.010 <.12 <.02 <.10 <.02 <.04 <.12

July 24, 2008 <.012 <.14 <.010 <.12 <.02 <.10 <.02 <.04 <.12
Appendix 2 53

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

Pendimethalin,

water,

fltrd

(μg/L

Phorate

oxon,

water,

fltrd

(μg/L)

Phorate,

water,

fltrd

(μg/L)

Phosmet

oxon,

water,

fltrd

(μg/L)

Phosmet,

water,

fltrd

(μg/L)

Picloram,

water,

fltrd

(μg/L)

Prometon,

water,

fltrd

(μg/L)

Prometryn,

water,

fltrd

(μg/L)

Propyzamide,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.022 <0.03 <0.055 <0.05 <0.008 <0.03 <0.01 <0.006 <0.004

July 1–2, 2006 <.022 <.03 <.055 -- -- <.03 <.01 <.006 <.004

July 6, 2006 2.66 <.03 <.055 -- -- <.03 <.01 <.006 <.004

Sept. 9–10, 2006 13.7 <.03 <.055 <.05 <.008 <.03 <.01 <.006 <.004

Sept. 18–19, 2006 .210 <.03 <.055 <.05 <.008 <.03 <.01 <.006 <.004

Jan. 3–5, 2007 6.53 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

Jan. 24–25, 2007 2.14 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

Mar. 14–15, 2007 <.020 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

May 25, 2007 2.46 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

July 2–3, 2007 .100 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

Mar. 10–11, 2008 E.417 <.03 <.040 <.05 <.008 <.12 <.01 <.006 <.004

July 24, 2008 .252 <.03 <.040 <.06 <.008 <.12 <.01 <.006 <.004

Date

Propham,

water,

fltrd

(μg/L)

Propiconazole,

water,

fltrd

(μg/L)

Propoxur,

water,

fltrd

(μg/L)

Siduron,

water,

fltrd

(μg/L)

Simazine,

water,

fltrd

(μg/L)

Sulfometuron,

water,

fltrd

(μg/L)

Tebuthiuron,

water,

fltrd

(μg/L)

Terbacil,

water,

fltrd

(μg/L)

Terbufos

oxon

sulfone,

water, fltrd

(μg/L)

June 1–2, 2006 <0.030 <0.01 E0.005 <0.02 0.009 <0.090 <0.02 <0.026 <0.04

July 1–2, 2006 <.030 <.01 <.008 <.02 <.005 <.090 <.02 <.026 <.04

July 6, 2006 <.030 <.01 <.008 <.02 <.005 <.090 <.02 <.026 <.04

Sept. 9–10, 2006 <.030 <.01 <.008 <.02 <.010 <.090 <.02 <.026 <.04

Sept. 18–19, 2006 <.030 <.01 <.008 <.02 <.005 <.090 <.02 <.026 <.04

Jan. 3–5, 2007 <.060 <.06 <.040 <.04 <.006 <.060 <.02 <.040 <.04

Jan. 24–25, 2007 <.060 <.06 <.040 <.04 <.006 <.060 <.02 <.040 <.04

Mar. 14–15, 2007 <.060 <.06 <.040 <.04 E.006 <.060 <.02 <.040 <.04

May 25, 2007 <.060 <.06 <.040 <.04 <.006 <.060 <.02 <.040 <.04

July 2–3, 2007 <.060 <.06 <.040 <.04 .022 <.060 <.02 <.040 <.04

Mar. 10–11, 2008 <.040 <.04 <.040 <.02 E.017 <.060 <.02 <.040 <.04

July 24, 2008 <.040 <.04 <.040 <.02 <.006 <.060 <.02 <.040 <.04
54 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 2. Water-quality data for runoff samples collected at West Oso Creek (station 08211517), Nueces County, Texas, water years 

2006–08—Continued.

Date

Terbufos,

water,

fltrd

(μg/L)

Terbuthylazine,

water,

fltrd

(μg/L)

Tribuphos,

water,

fltrd

(μg/L)

Triclopyr,

water,

fltrd

(μg/L)

Trifluralin,

water,

fltrd

(μg/L)

Dichlorvos,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.02 <0.01 <0.035 <0.03 <0.009 <0.01

July 1–2, 2006 <.02 <.01 <.035 <.03 <.009 <.01

July 6, 2006 <.02 <.01 <.035 <.03 .028 <.01

Sept. 9–10, 2006 <.02 <.01 <.035 <.03 <.012 <.01

Sept. 18–19, 2006 <.02 <.01 <.035 <.03 <.009 <.01

Jan. 3–5, 2007 <.01 <.01 <.035 <.04 <.009 <.01

Jan. 24–25, 2007 <.01 <.01 <.035 <.04 <.009 <.01

Mar. 14–15, 2007 <.01 <.01 <.035 <.04 .020 <.01

May 25, 2007 <.01 <.01 <.035 <.04 E.050 <.01

July 2–3, 2007 <.01 <.01 <.035 <.04 <.009 <.01

Mar. 10–11, 2008 <.02 <.01 <.035 <.04 E.019 <.01

July 24, 2008 <.02 <.01 <.035 <.04 <.009 <.01
Appendix 3 55

Appendix 3

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08.

[unfltrd, unfiltered; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; CaCO3, calcium carbonate; fltrd, filtered; 

--, not analyzed; N, nitrogen; E, estimated; <, less than; P, phosphorus; μg/L, micrograms per liter; CIAT, 2-chloro-4-isopropylamino-6-amino-s-triazine; 

CEAT, 2-Chloro-6-ethylamino-4-amino-s-triazine; OIET, 2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine; DCPA, dimethyl tetrachloroterephthalate; 

MCPA, 4-chloro-2-methylphenoxy acetic acid; MCPB, 4-(2-methyl-4-chlorophenoxy)butyric acid]

Date

pH, water, 

unfltrd,

field

(standard 

units)

pH, water, 

unfltrd,

lab 

(standard 

units)

Specific 

conductance,

unfltrd, lab

(μS/cm)

Specific 

conductance,

unfltrd, field

(μS/cm)

Hardness,

water 

(mg/L as 

CaCO3)

Calcium,

water, 

fltrd

(mg/L)

Magnesium,

water,

fltrd

(mg/L)

Potassium,

water,

fltrd

(mg/L)

June 1–2, 2006 6.9 7.4 156 152 48 16.4 1.74 7.64

July 6–7, 2006 7.1 7.4 178 175 60 20.2 2.37 8.31

Sept. 9–15, 2006 6.7 7.6 153 154 55 18.9 1.85 7.67

Sept. 18–19, 2006 6.9 7.4 111 120 41 14.0 1.35 4.82

Jan. 3–4, 2007 6.5 8.0 215 220 56 19.1 2.09 5.83

Jan. 24–25, 2007 6.8 7.5 196 203 57 19.1 2.22 5.26

Mar. 14–15, 2007 8.2 7.8 256 246 -- -- -- --

July 3–4, 2007 6.5 6.4 153 144 30 13.5 1.66 8.35

Aug. 30–31, 2007 7.0 7.3 198 192 26 17.7 2.18 9.32

Mar. 11, 2008 -- 7.9 270 248 83 28.5 2.95 10.7

July 24, 2008 -- 7.0 115 -- 32 10.3 1.56 9.53

Aug. 18, 2008 7.5 7.5 246 243 82 27.6 3.12 16.9

Date

Sodium,

water,

fltrd

(mg/L)

Chloride,

water,

fltrd

(mg/L)

Fluoride,

water,

fltrd

(mg/L)

Silica,

water,

fltrd

(mg/L)

Sulfate,

water,

fltrd

(mg/L)

Ammonia + 

organic-N,

water,

fltrd

(mg/L as N)

Ammonia + 

organic-N,

water,

unfltrd

(mg/L as N

Ammonia,

water,

fltrd

(mg/L as N)

June 1–2, 2006 7.26 5.99 0.15 15.4 4.28 0.67 1.5 0.026

July 6–7, 2006 9.16 6.58 .20 22.8 4.88 .55 1.2 .012

Sept. 9–15, 2006 6.15 3.20 .15 19.9 2.13 1.2 .92 .028

Sept. 18–19, 2006 3.46 1.97 .13 11.0 1.30 .66 .89 .021

Jan. 3–4, 2007 14.7 19.5 .12 11.0 12.8 .43 1.4 .024

Jan. 24–25, 2007 15.7 17.2 .14 12.4 11.0 .48 .99 .021

Mar. 14–15, 2007 -- -- -- -- -- 1.1 2.5 .171

July 3–4, 2007 9.75 11.0 .10 16.0 6.23 .64 .97 E.013

Aug. 30–31, 2007 13.1 21.6 E.10 18.3 7.63 .71 1.1 <.020

Mar. 11, 2008 15.6 23.8 .23 12.2 17.4 .90 2.2 .172

July 24, 2008 4.26 7.92 E.06 12.4 2.65 .58 .79 <.020

Aug. 18, 2008 8.42 11.7 .15 29.2 6.46 1.0 .91 <.020
56 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

Nitrate,

water,

fltrd

(mg/L as N)1

Nitrite + 

nitrate,

water,

fltrd

(mg/L as N)

Nitrite,

water,

fltrd

(mg/L as N)

Organic

nitrogen,

water,

fltrd

(mg/L as N)1

Organic

nitrogen,

water,

unfltrd

(mg/L as N)1

Total

nitrogen,

water,

fltrd

(mg/L as N)1

Total

nitrogen,

water,

unfltrd

(mg/L as N)1

Orthophosphate,

water,

fltrd

(mg/L as P)

June 1–2, 2006 0.22 0.27 0.058 0.64 1.5 0.94 1.8 0.663

July 6–7, 2006 -- <.06 .002 .54 1.1 -- -- .525

Sept. 9–15, 2006 .18 .20 .016 1.2 .89 1.4 1.1 .429

Sept. 18–19, 2006 .19 .20 .005 .63 .87 .85 1.1 .229

Jan. 3–4, 2007 .49 .50 .015 .40 1.3 .93 1.9 .331

Jan. 24–25, 2007 .80 .81 .014 .46 .97 1.3 1.8 .276

Mar. 14–15, 2007 .86 1.0 .144 .92 2.4 2.1 3.5 .366

July 3–4, 2007 .12 .13 .008 .40 .96 .77 1.1 .557

Aug. 30–31, 2007 .16 .17 .009 .70 1.1 .88 1.2 .311

Mar. 11, 2008 1.31 1.37 .177 .72 2.0 2.3 3.5 .550

July 24, 2008 .05 .05 .012 -- -- .63 .84 .406

Aug. 18, 2008 -- <.04 .011 -- -- -- -- .494

1

 Concentration of constituent not analyzed directly but computed from other analyzed constituents.

Date

Phosphorus,

water,

fltrd

(mg/L as P)

Phosphorus,

water,

unfltrd

(mg/L as P)

1-Naphthol,

water,

fltrd

(μg/L)

2,4-D

methylester,

water,

fltrd

(μg/L)

2,4-D,

water,

fltrd

(μg/L)

2,4-DB,

water,

fltrd

(μg/L)

2,6-Diethylaniline,

water,

fltrd

(μg/L)

2-Chloro2’,6’-diethyl

acetanilide,

water, fltrd

(μg/L)

CIAT,

water,

fltrd

(μg/L)

June 1–2, 2006 0.69 0.98 <0.09 <0.190 <0.04 <0.02 <0.006 <0.006 E0.184

July 6–7, 2006 .55 .74 <.09 <.190 .27 <.02 <.006 <.006 E.089

Sept. 9–15, 2006 .53 .61 <.09 <.190 E.03 <.02 <.006 <.006 E.029

Sept. 18–19, 2006 .26 .45 <.09 <.190 E.01 <.02 <.006 <.006 E.021

Jan. 3–4, 2007 .38 .61 <.09 <.200 E.06 <.02 <.006 <.006 E.011

Jan. 24–25, 2007 .30 .43 <.09 <.200 .09 <.02 <.006 <.006 E.009

Mar. 14–15, 2007 .42 .98 <.09 .208 E1.23 <.02 <.006 <.006 E.119

July 3–4, 2007 .59 .66 <.09 <.200 <.04 <.02 <.006 <.006 E.049

Aug. 30–31, 2007 .33 .41 <.09 <.200 <.04 <.02 <.006 <.006 E.014

Mar. 11, 2008 .61 1.02 <.04 <.040 .14 <.02 <.006 <.010 E.562

July 24, 2008 .44 .51 <.04 <.040 <.02 <.02 <.006 <.010 E.118

Aug. 18, 2008 .51 .55 <.04 <.040 .49 <.02 <.006 <.010 E.074
Appendix 3 57

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

CEAT,

water,

fltrd

(μg/L)

2-Ethyl-6-

methylaniline,

water, fltrd

(μg/L)

OIET,

water,

fltrd

(μg/L)

3,4-Dichloroaniline,

water,

fltrd

(μg/L)

3-Hydroxy

carbofuran,

water,

fltrd

(μg/L)

4-Chloro2-methyl

phenol,

water, fltrd

(μg/L)

Acetochlor,

water,

fltrd

(μg/L)

Acifluorfen,

water,

fltrd

(μg/L)

Alachlor,

water,

fltrd

(μg/L)

June 1–2, 2006 0.11 <0.010 0.420 <0.004 <0.008 <0.005 <0.006 <0.028 <0.005

July 6–7, 2006 E.07 <.010 .517 <.004 <.008 <.005 <.006 <.028 <.005

Sept. 9–15, 2006 <.08 <.010 .272 <.004 <.008 <.005 <.006 <.028 <.005

Sept. 18–19, 2006 <.08 <.010 .037 <.004 <.008 <.005 <.006 <.028 <.005

Jan. 3–4, 2007 <.08 <.010 E.043 <.004 <.020 <.005 <.006 <.006 <.005

Jan. 24–25, 2007 <.08 <.010 E.023 <.004 <.020 <.005 <.006 <.006 <.005

Mar. 14–15, 2007 <.08 <.010 .902 <.004 <.020 <.005 <.006 <.006 <.005

July 3–4, 2007 <.08 <.010 .101 <.004 <.020 <.005 <.006 <.006 <.005

Aug. 30–31, 2007 <.08 <.010 .021 <.004 <.020 <.005 <.006 <.006 <.005

Mar. 11, 2008 .08 <.010 .346 <.006 <.040 <.005 <.006 <.040 <.006

July 24, 2008 <.08 <.010 .135 <.006 <.040 <.005 <.006 <.040 <.006

Aug. 18, 2008 <.08 <.010 .266 E.006 <.040 <.005 <.006 <.040 <.006

Date

Aldicarb

sulfone,

water,

fltrd

(μg/L)

Aldicarb

sulfoxide,

water, 

fltrd

(μg/L)

Aldicarb,

water,

fltrd

(μg/L)

Aminomethylphosphonic

acid, water, 

fltrd

(μg/L)

Atrazine,

water,

fltrd

(μg/L)

Azinphosmethyl 

oxon,

water,

fltrd

(μg/L)

Azinphosmethyl,

water,

fltrd

(μg/L)

Bendiocarb,

water,

fltrd

(μg/L)

Benfluralin,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.02 <0.100 <0.15 0.420 1.88 <0.04 <0.050 <0.08 <0.010

July 6–7, 2006 <.02 <.100 <.15 1.10 .313 <.04 <.050 <.08 <.010

Sept. 9–15, 2006 <.02 <.100 <.15 .350 .042 <.04 <.050 <.08 <.010

Sept. 18–19, 2006 <.02 <.100 <.15 .670 .022 <.04 <.050 <.08 <.010

Jan. 3–4, 2007 <.08 <.040 <.04 1.78 .021 <.04 <.080 <.04 <.010

Jan. 24–25, 2007 <.08 <.040 <.04 .410 .021 <.04 <.080 <.04 <.010

Mar. 14–15, 2007 <.08 <.040 <.04 1.94 9.42 <.04 <.080 <.04 <.010

July 3–4, 2007 <.08 <.040 <.04 .530 .194 <.04 <.080 <.04 <.010

Aug. 30–31, 2007 <.08 <.040 <.04 -- .012 <.04 .038 <.04 <.010

Mar. 11, 2008 <.08 <.060 <.12 -- 10.5 <.04 <.120 <.04 <.010

July 24, 2008 <.08 <.060 <.12 -- .363 <.06 <.120 <.04 <.010

Aug. 18, 2008 <.08 <.060 <.12 -- .464 <.04 <.120 <.04 <.010
58 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

Benomyl,

water,

fltrd

(μg/L)

Bensulfuron

methyl,

water, fltrd

(μg/L)

Bentazon,

water,

fltrd

(μg/L)

Bromacil,

water,

fltrd

(μg/L)

Bromoxynil,

water,

fltrd

(μg/L)

Carbaryl,

water,

fltrd

(μg/L)

Carbofuran,

water,

fltrd

(μg/L)

Chloramben

methyl ester,

water, fltrd

(μg/L)

June 1–2, 2006 <0.022 <0.02 <0.02 <0.02 <0.04 <0.02 <0.016 <0.02

July 6–7, 2006 <.022 <.02 <.02 .05 <.04 <.02 <.016 <.02

Sept. 9–15, 2006 <.022 <.02 <.02 <.02 <.04 <.02 <.016 <.02

Sept. 18–19, 2006 <.022 <.02 <.02 <.02 <.04 <.02 <.016 <.02

Jan. 3–4, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

Jan. 24–25, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

Mar. 14–15, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

July 3–4, 2007 <.020 <.06 <.02 <.04 <.12 <.02 <.060 <.10

Aug. 30–31, 2007 <.020 <.06 <.02 <.04 <.12 .010 <.060 <.10

Mar. 11, 2008 <.040 <.06 <.04 <.02 <.12 E.010 <.020 <.10

July 24, 2008 <.040 <.06 <.04 <.02 <.12 E.016 <.020 <.10

Aug. 18, 2008 <.040 <.06 <.04 <.02 <.12 E.060 <.020 <.10

Date

Chlorimuron,

water,

fltrd

(μg/L)

Chlorodi-aminos-triazine,

water, fltrd

(μg/L)

Chlorpyrifosoxon,

water, 

fltrd

(μg/L)

Chlorpyrifos,

water,

fltrd

(μg/L)

cisPermethrin,

water,

fltrd

(μg/L)

Clopyralid,

water,

fltrd

(μg/L)

Cycloate,

water,

fltrd

(μg/L)

Cyfluthrin,

water,

fltrd

(μg/L)

Cypermethrin,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.032 <0.04 <0.06 <0.005 <0.006 <0.07 <0.01 <0.053 <0.046

July 6–7, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

Sept. 9–15, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

Sept. 18–19, 2006 <.032 <.04 <.06 <.005 <.006 <.07 <.01 <.053 <.046

Jan. 3–4, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

Jan. 24–25, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

Mar. 14–15, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

July 3–4, 2007 <.080 <.12 <.06 <.005 <.010 <.06 <.06 <.053 <.046

Aug. 30–31, 2007 <.080 -- <.06 <.005 <.010 <.06 <.06 <.053 <.046

Mar. 11, 2008 <.080 -- <.06 <.005 <.010 <.06 <.02 <.016 <.014

July 24, 2008 <.080 -- <.06 <.005 <.010 <.06 <.02 <.016 <.014

Aug. 18, 2008 <.080 -- <.06 <.005 <.010 <.06 <.02 <.016 <.014
Appendix 3 59

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

Dacthal

mono-acid,

water,

fltrd

(μg/L)

DCPA,

water,

fltrd

(μg/L)

Desulfinylfipronil,

water,

fltrd

(μg/L)

Diazinon,

water,

fltrd

(μg/L)

Dicamba,

water,

fltrd

(μg/L)

Dichlorprop,

water,

fltrd

(μg/L)

Dicrotophos,

water,

fltrd

(μg/L)

Dieldrin,

water,

fltrd

(μg/L)

Dimethoate,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.03 <0.003 <0.012 <0.005 <0.04 <0.03 <0.08 <0.009 <0.006

July 6–7, 2006 <.03 <.003 <.012 <.005 <.04 <.03 <.08 <.009 <.006

Sept. 9–15, 2006 <.03 <.003 <.012 <.005 <.04 <.03 <.08 <.009 <.006

Sept. 18–19, 2006 <.03 E.004 <.012 <.005 <.04 <.03 <.08 <.009 <.006

Jan. 3–4, 2007 <.02 <.003 <.012 <.005 <.08 <.04 <.08 <.009 <.006

Jan. 24–25, 2007 <.02 <.003 <.012 <.005 <.08 <.04 <.08 <.009 <.006

Mar. 14–15, 2007 <.02 <.003 <.012 <.005 .97 <.04 <.08 <.009 <.006

July 3–4, 2007 <.02 <.003 <.012 <.005 <.08 <.04 E.03 <.009 <.006

Aug. 30–31, 2007 <.02 <.003 <.012 <.005 <.08 <.04 <.08 <.009 <.006

Mar. 11, 2008 <.02 <.003 <.012 <.005 <.04 <.02 <.08 <.009 <.006

July 24, 2008 <.02 <.003 <.012 <.005 <.04 <.02 <.08 <.009 <.006

Aug. 18, 2008 <.02 <.003 <.012 <.005 <.04 <.02 <.08 <.009 <.006

Date

Dinoseb,

water,

fltrd

(μg/L)

Diphenamid,

water,

fltrd

(μg/L)

Diuron,

water,

fltrd

(μg/L)

Ethion

monoxon,

water, 

fltrd

(μg/L)

Ethion,

water,

fltrd

(μg/L)

Fenamiphos

sulfone,

water, 

fltrd

(μg/L)

Fenamiphos

sulfoxide,

water, 

fltrd

(μg/L)

Fenamiphos,

water,

fltrd

(μg/L)

Fenuron,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.04 <0.01 E0.02 <0.02 <0.016 <0.053 <0.04 <0.03 <0.10

July 6–7, 2006 <.04 <.01 .03 <.02 <.016 <.053 <.04 <.03 <.10

Sept. 9–15, 2006 <.04 <.01 <.02 <.02 <.016 <.053 <.04 <.03 <.10

Sept. 18–19, 2006 <.04 <.01 <.02 <.02 <.016 <.053 <.04 <.03 <.10

Jan. 3–4, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Jan. 24–25, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Mar. 14–15, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

July 3–4, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Aug. 30–31, 2007 <.04 <.04 <.04 <.02 <.016 <.053 <.04 <.03 <.04

Mar. 11, 2008 <.04 <.04 .10 <.02 <.006 <.053 <.20 <.03 <.04

July 24, 2008 <.04 <.04 <.04 <.02 <.006 <.053 <.20 <.03 <.04

Aug. 18, 2008 <.04 <.04 <.04 <.02 <.006 <.053 <.20 <.03 <.04
60 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

Desulfinylfipronil

amide,

water, fltrd

(μg/L)

Fipronil

sulfide,

water,

fltrd

(μg/L)

Fipronil

sulfone,

water,

fltrd

(μg/L)

Fipronil,

water,

fltrd

(μg/L)

Flumetsulam,

water,

fltrd

(μg/L)

Fluometuron,

water,

fltrd

(μg/L)

Fonofos,

water,

fltrd

(μg/L)

Glufosinate,

water,

fltrd

(μg/L)

Glyphosate,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.029 <0.013 <0.024 <0.016 <0.04 <0.02 <0.005 <0.140 0.610

July 6–7, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 .650

Sept. 9–15, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 1.51

Sept. 18–19, 2006 <.029 <.013 <.024 <.016 <.04 <.02 <.005 <.140 .590

Jan. 3–4, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 2.02

Jan. 24–25, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 .650

Mar. 14–15, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 3.82

July 3–4, 2007 <.029 <.016 <.024 <.016 <.06 <.04 <.006 <.140 10.7

Aug. 30–31, 2007 <.029 .005 <.024 <.016 <.06 <.04 <.006 -- --

Mar. 11, 2008 <.029 <.013 <.024 <.020 <.06 <.04 <.010 -- --

July 24, 2008 <.029 <.013 <.024 <.020 <.06 <.04 <.010 -- --

Aug. 18, 2008 <.029 <.013 <.024 <.020 <.06 <.04 <.010 -- --

Date

Hexazinone,

water,

fltrd

(μg/L)

Imazaquin,

water,

fltrd

(μg/L)

Imazethapyr,

water,

fltrd

(μg/L)

Imidacloprid,

water,

fltrd

(μg/L)

Iprodione,

water,

fltrd

(μg/L)

Isofenphos,

water,

fltrd

(μg/L)

Linuron,

water,

fltrd

(μg/L)

Malaoxon,

water,

fltrd

(μg/L)

Malathion,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.026 <0.04 <0.04 <0.020 <0.026 <0.011 <0.01 <0.039 <0.027

July 6–7, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 E.018

Sept. 9–15, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 <.027

Sept. 18–19, 2006 <.026 <.04 <.04 <.020 <.026 <.011 <.01 <.039 <.027

Jan. 3–4, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

Jan. 24–25, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

Mar. 14–15, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

July 3–4, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 <.039 <.016

Aug. 30–31, 2007 <.026 <.04 <.04 <.060 <.026 <.011 <.04 .049 2.64

Mar. 11, 2008 <.008 <.04 <.04 <.060 <.01 <.006 <.02 <.020 <.016

July 24, 2008 <.008 <.04 <.04 <.060 <.01 <.006 <.02 <.020 <.016

Aug. 18, 2008 <.008 <.04 <.04 <.060 <.01 <.006 <.02 <.020 <.016
Appendix 3 61

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

MCPA,

water,

fltrd

(μg/L)

MCPB,

water,

fltrd

(μg/L)

Metalaxyl,

water,

fltrd

(μg/L)

Methidathion,

water,

fltrd

(μg/L)

Methiocarb,

water,

fltrd

(μg/L)

Methomyl,

water,

fltrd

(μg/L)

Methyl

paraoxon,

water, 

fltrd

(μg/L)

Methyl

parathion,

water, 

fltrd

(μg/L)

Metolachlor,

water, 

fltrd

(μg/L)

June 1–2, 2006 <0.07 <0.10 <0.065 <0.009 <0.034 <0.070 <0.02 <0.015 <0.006

July 6–7, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 E.006

Sept. 9–15, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 <.006

Sept. 18–19, 2006 <.07 <.10 <.007 <.009 <.034 <.070 <.02 <.015 <.006

Jan. 3–4, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 <.010

Jan. 24–25, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 E.008

Mar. 14–15, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 E.007

July 3–4, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 <.010

Aug. 30–31, 2007 <.06 <.20 <.007 <.009 <.040 <.060 <.02 <.008 <.010

Mar. 11, 2008 <.06 <.06 E.014 <.004 <.040 <.120 <.01 <.008 <.010

July 24, 2008 <.06 <.06 <.007 <.004 <.040 <.120 <.01 <.008 <.011

Aug. 18, 2008 <.06 <.06 <.020 <.004 <.040 <.120 <.01 <.008 <.010

Date

Metribuzin,

water,

fltrd

(μg/L)

Metsulfuron,

water,

fltrd

(μg/L)

Myclobutanil,

water,

fltrd

(μg/L)

N-(4-Chlorophenyl)-N’-

methylurea

(μg/L)

Neburon,

water,

fltrd

(μg/L)

Nicosulfuron,

water,

fltrd

(μg/L)

Norflurazon,

water,

fltrd

(μg/L)

Oryzalin,

water,

fltrd

(μg/L)

Oxamyl,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.028 <0.07 <0.033 <0.04 <0.01 <0.04 <0.02 <0.02 <0.05

July 6–7, 2006 <.028 <.07 E.012 <.04 <.01 <.04 <.02 <.02 <.05

Sept. 9–15, 2006 <.028 <.07 E.010 <.04 <.01 <.04 <.02 <.02 <.05

Sept. 18–19, 2006 <.028 <.07 <.033 <.04 <.01 <.04 <.02 <.02 <.05

Jan. 3–4, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Jan. 24–25, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Mar. 14–15, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

July 3–4, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Aug. 30–31, 2007 <.012 <.14 <.033 <.06 <.02 <.10 <.04 <.04 <.04

Mar. 11, 2008 .021 <.14 <.010 <.12 <.02 <.10 <.02 <.04 <.12

July 24, 2008 <.012 <.14 <.010 <.12 <.02 <.10 <.02 <.04 <.12

Aug. 18, 2008 <.012 <.14 <.010 <.12 <.02 <.10 <.02 <.04 <.12
62 Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

Pendimethalin,

water,

fltrd

(μg/L)

Phorate

oxon,

water,

fltrd

(μg/L)

Phorate,

water,

fltrd

(μg/L)

Phosmet

oxon,

water,

fltrd

(μg/L)

Phosmet,

water,

fltrd

(μg/L)

Picloram,

water,

fltrd

(μg/L)

Prometon,

water,

fltrd

(μg/L)

Prometryn,

water,

fltrd

(μg/L)

Propyzamide,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.022 <0.03 <0.055 <0.05 <0.008 <0.03 <0.01 <0.006 <0.004

July 6–7, 2006 .027 <.03 <.055 -- -- <.03 <.01 <.006 <.004

Sept. 9–15, 2006 .131 <.03 <.055 <.05 <.008 <.03 <.01 <.006 <.004

Sept. 18–19, 2006 .052 <.03 <.055 <.05 <.008 <.03 <.01 <.006 <.004

Jan. 3–4, 2007 .091 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

Jan. 24–25, 2007 .144 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

Mar. 14–15, 2007 .133 <.03 <.020 <.05 <.008 -- <.01 <.006 <.004

July 3–4, 2007 <.020 <.03 <.020 <.05 <.008 <.12 <.01 <.006 <.004

Aug. 30–31, 2007 <.020 <.03 <.020 -- -- <.12 <.01 <.006 <.004

Mar. 11, 2008 .071 <.03 <.040 <.05 <.008 <.12 <.01 <.006 <.004

July 24, 2008 .019 <.03 <.040 <.06 <.008 <.12 <.01 <.006 <.004

Aug. 18, 2008 <.012 <.03 <.040 <.05 <.008 <.12 <.01 <.006 <.004

Date

Propham,

water,

fltrd

(μg/L)

Propiconazole,

water,

fltrd

(μg/L)

Propoxur,

water,

fltrd

(μg/L)

Siduron,

water,

fltrd

(μg/L)

Simazine,

water,

fltrd

(μg/L)

Sulfometruron,

water,

fltrd

(μg/L)

Tebuthiuron,

water,

fltrd

(μg/L)

Terbacil,

water,

fltrd

(μg/L)

Terbufos

oxon

sulfone,

water, 

fltrd

(μg/L)

June 1–2, 2006 <0.030 <0.01 <0.008 <0.02 0.019 <0.090 <0.02 <0.026 <0.04

July 6–7, 2006 <.030 <.01 <.008 <.02 E.007 <.090 <.02 <.026 <.04

Sept. 9–15, 2006 <.030 <.01 <.008 <.02 <.005 <.090 <.02 <.026 <.04

Sept. 18–19, 2006 <.030 <.01 <.008 <.02 <.005 <.090 <.02 <.026 <.04

Jan. 3–4, 2007 <.060 <.06 <.040 <.04 E.005 <.060 <.02 <.040 <.04

Jan. 24–25, 2007 <.060 <.06 <.040 <.04 <.006 <.060 <.02 <.040 <.04

Mar. 14–15, 2007 <.060 <.06 <.040 <.04 .064 <.060 <.02 <.040 <.04

July 3–4, 2007 <.060 <.06 <.040 <.04 <.006 <.060 <.02 <.040 <.04

Aug. 30–31, 2007 <.060 <.06 <.040 <.04 <.006 <.060 <.02 <.040 <.04

Mar. 11, 2008 <.040 <.04 <.040 <.02 E.022 <.060 <.02 <.040 <.04

July 24, 2008 <.040 <.04 <.040 <.02 <.006 <.060 <.02 <.040 <.04

Aug. 18, 2008 <.040 <.04 <.040 <.02 <.006 <.060 <.02 <.040 <.04
Appendix 3 63

Appendix 3. Water-quality data for runoff samples collected at Oso Creek tributary (station 08211525), Nueces County, Texas, water 

years 2006–08—Continued.

Date

Terbufos,

water,

fltrd

(μg/L)

Terbuthylazine,

water,

fltrd

(μg/L)

Tribuphos,

water,

fltrd

(μg/L)

Triclopyr,

water,

fltrd

(μg/L)

Trifluralin,

water,

fltrd

(μg/L)

Dichlorvos,

water,

fltrd

(μg/L)

June 1–2, 2006 <0.02 <0.01 <0.035 <0.03 <0.009 <0.01

July 6–7, 2006 <.02 <.01 <.035 <.03 <.009 <.01

Sept. 9–15, 2006 <.02 <.01 <.035 <.03 <.009 <.01

Sept. 18–19, 2006 <.02 <.01 <.035 <.03 .011 <.01

Jan. 3–4, 2007 <.01 <.01 <.035 <.04 E.007 <.01

Jan. 24–25, 2007 <.01 <.01 <.035 <.04 <.009 <.01

Mar. 14–15, 2007 .02 .02 <.035 <.04 .054 <.01

July 3–4, 2007 <.01 <.01 <.035 <.04 <.009 <.01

Aug. 30–31, 2007 <.01 <.01 <.035 <.04 <.009 <.01

Mar. 11, 2008 <.02 <.01 <.035 <.08 .026 <.01

July 24, 2008 <.02 <.01 <.035 <.08 <.009 <.01

Aug. 18, 2008 <.02 <.01 <.035 <.08 <.009 <.01

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Ockerman and Fernandez—Hydrologic Conditions and Water Quality of Rainfall and Storm Runoff for Two Agricultural Areas, Oso Creek Watershed—SIR 2010–5136

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I SBN 978- 1- 4113- 2949- 2

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