U.S. Geological Survey

Water-Resources Investigations Report 

U.S. Department of the Interior

Ground-water quality, levels, and flow 

direction near Fort Cobb Reservoir, 

Caddo County, Oklahoma, 1998-2000

01–4076

Prepared in cooperation with the Bureau of Reclamation
U.S. Geological Survey

Water-Resources Investigations Report 

U.S. Department of the Interior

Ground-water quality, levels, and flow 

direction near Fort Cobb Reservoir, 

Caddo County, Oklahoma, 1998-2000

By C.J. Becker

01–4076

Prepared in cooperation with the Bureau of Reclamation
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

CHARLES G. GROAT, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

UNITED STATES GOVERNMENT PRINTING OFFICE: OKLAHOMA CITY 2001

District Chief

U.S. Geological Survey

Water-Resources Division

202 NW 66 St., Bldg. 7

Oklahoma City, OK 73116
CONTENTS III

CONTENTS

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

Introduction............................................................................................................................................................................1

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

Approach and methods ..............................................................................................................................................2

Description of the study area .....................................................................................................................................5

Acknowledgments ...................................................................................................................................................14

Ground-water quality ..........................................................................................................................................................14

Levels ..................................................................................................................................................................................17

Flow direction .....................................................................................................................................................................17

Discussion ...........................................................................................................................................................................18

Summary .............................................................................................................................................................................18

References cited...................................................................................................................................................................19

ILLUSTRATIONS

Figures 1-2. Maps showing:

1. Study area, Fort Cobb Reservoir, and Cobb Creek subbasin, Caddo County, Oklahoma...................................3

2. Location of swine feeding operation and wells used for this study ................................................................... 4

Figure 3. Graph showing daily mean water level in well MWR-A and daily precipitation at the Fort

Cobb Mesonet station, April 14, 1999, to May 31, 2000 ............................................................................... 11

Figures 4-6. Maps showing:

4. Potentiometric surface and direction of ground-water flow in study area, December 1998.............................12

5. Potentiometric surface and direction of ground-water flow in the study area, August 1999............................13

6. Land use and land cover in the study area, near Fort Cobb Reservoir ..............................................................15

Figure 7. Graph showing ranges of the nitrogen isotopes nitrogen-14 and nitrogen-15 expressed as δ15N parts 

per thousand relative to atmospheric nitrogen in water samples in the study area and in common

sources of nitrate..............................................................................................................................................17

TABLES

1. Location information and depth-to-water measurements for wells in the study area .......................................................6

2. Measured water properties and concentrations of major ions, nutrients, trace elements, nitrogen isotopes, 

pesticides, and bacteria counts in water samples collected April and June 2000, from observation wells 

in the study area ......................................................................................................................................................8

3. Pesticides and pesticide metabolites analyzed for in ground-water samples collected April 12, 2000, 

from observation wells BD–1 and MW–B in the study area ................................................................................10

4. Land use and land cover in the study area and the Cobb Creek subbasin ......................................................................14
IV CONTENTS

CONVERSION FACTORS AND VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)--a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called 

Sea Level Datum of 1929.

Altitude, as used in this report, refers to distance above sea level.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C).

Multiply By To obtain

Length

inch (in) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2) 2.590 square kilometer 

Volume

gallon (gal) 0.003785 cubic meter

Flow rate

foot per mile (ft/mi) 0.1894 meter per kilometer

foot per day (ft/d) 0.3048 meter per day

gallon per minute (gal/min) 0.06309 liter per second

gallon per day (gal/d) 0.003785 cubic meter per day

inch per year 25.4 millimeter per year

Mass

pound (lb) 0.4536 kilogram
CONTENTS V

EXPLANATION OF THE SITE-NUMBERING SYSTEM

Well locations in this report are specified by a local site-numbering system, which is based on the public land-survey location

in Oklahoma. The local site-numbering system consists of the township number, north or south; range number, east or west; 

and the section number. Each section is divided into four quarters, A, B, C, and D. The township numbers are north of the 

third parallel and the range numbers are west of the Indian Meridian. A section is equal to one square mile and fractional parts 

are given from larger to smaller quartered areas of the section. The final digit (1) is the sequential number of a well within the 

smallest fractional subdivision (10 acres, in the example shown) relative to other wells in the same subdivision. The diagram 

shown below illustrates the location of a well described as: 09N-12W-28 BCB 1.

T.9 N.

6

7

18

19

30

31

1

12

13

24

25

36

B A

D

A

C D

A

C D

09N-12W-28 BCB 1

R. 12 W.
 1

Ground-Water Quality, Levels, and Flow Direction

Near Fort Cobb Reservoir, Caddo County, 

Oklahoma, 1998-2000

By C.J. Becker

Abstract

Fort Cobb Reservoir in northwest Caddo 

County Oklahoma is managed by the Bureau of 

Reclamation for water supply, recreation, flood 

control, and wildlife. Excessive amounts of 

nitrogen in the watershed have the potential to 

cause long-term eutrophication of the reservoir 

and increase already elevated concentrations of 

nitrogen in the Rush Springs aquifer. The U.S. 

Geological Survey in cooperation with the Bureau 

of Reclamation studied ground water in the area 

surrounding a swine feeding operation located 

less than 2 miles upgradient from Fort Cobb 

Reservoir in Caddo County, Oklahoma. Objectives of the study were to (1) determine if the 

operation was contributing nitrogen to the ground 

water and (2) measure changes in ground-water 

levels and determine the local ground-water flow 

direction in the area surrounding the swine 

feeding operation. 

Nitrate concentrations (28.1 and 31.5 milligrams per liter) were largest in two ground-water 

samples from a well upgradient of the wastewater 

lagoon. Nitrate concentrations ranged from 4.30 

to 8.20 milligrams per liter in samples from 

downgradient wells. Traces of ammonia and 

nitrite were detected in a downgradient well, but 

not in upgradient wells. δ15N values indicate 

atmospheric nitrogen, synthetic fertilizer, or 

plants were the predominate sources of nitrate in 

ground water from the downgradient wells. The 

δ15N values in these samples are depleted in 

nitrogen-15, indicating that animal waste was not 

a significant contributor of nitrate.

Manganese concentrations (1,150 and 965 

micrograms per liter) in samples from a downgradient well were substantially larger than concentrations in samples from other wells, exceeding 

the secondary drinking-water standard of 50 

micrograms per liter. Larger concentrations of 

bicarbonate, magnesium, fluoride, and iron and a 

higher pH were also measured in water from a 

downgradient well. 

Ground-water levels in an observation well 

were higher from April to mid-July and lower 

during the late summer and in the fall due to a 

seasonal decrease in precipitation, increase in 

water withdrawals, and increase in evapotranspiration. Ground water near the wastewater spray 

field moved south-southeast toward Willow Creek 

along a gradient of about 50 feet per mile. 

Analysis of ground-water samples suggest 

that commercial fertilizer is contributing nitrate 

upgradient of the swine feeding operation and that 

wastewater from the lagoon is contributing 

reduced forms of nitrogen, ammonia and nitrite. 

Additional downgradient wells would be needed 

to (1) determine if the swine feeding operation is 

adding excessive amounts of nitrogen to ground 

water, (2) determine the vertical dimension of 

wastewater flow, and (3) the extent of wastewater 

downgradient of the lagoon.

INTRODUCTION

The number of confined animal feeding operations near reservoirs has increased in Oklahoma, 
2 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

resulting in increased concern about the effects of 

these operations on water quality. One water-quality 

concern in both surface water and ground water is the 

increased concentration of nitrogen. Fort Cobb Reservoir in northwest Caddo County Oklahoma is 

managed by the Bureau of Reclamation for water 

supply, recreation, flood control, and wildlife (fig. 1). 

Excessive amounts of nitrogen in the watershed have 

the potential to cause long-term eutrophication of the 

reservoir and increase already elevated concentrations 

of nitrogen in the Rush Springs aquifer.Ground water 

from the Rush Springs aquifer is the sole source of 

water for domestic use for rural homesteads in the 

watershed. Becker and Runkle (1998, pgs. 19 and 24) 

reported that 50 percent of 64 ground-water samples 

from the Rush Springs aquifer exceeded the drinkingwater standard for nitrate of 10 milligrams per liter. 

Land use in the Fort Cobb watershed is mostly 

irrigated agriculture and the long-term use of commercial fertilizer on the sandy permeable soils may be 

responsible for much of the elevated nitrate concentrations measured in the Rush Springs aquifer. The major 

sources of nitrogen in ground water are precipitation, 

fertilizer application (both commercial and manure), 

and sewage disposal. Precipitation deposits about 1.3 

to 1.7 tons of nitrogen per square mile per year in 

Caddo County (Mueller and Helsel, 1996, pg. 10). The 

application of nitrogen derived from commercial 

fertilizer in Caddo County was about 11.2 tons per 

square mile per year (1987 through 1996) (Dan Storm, 

Oklahoma State University, written commun., 2001) 

application of manure was about 5.3 tons per square 

mile per year (1997 only) (Barbara Ruddy, U.S. 

Geological Survey, written commun., 2001). 

A swine feeding operation (fig. 2) began functioning in July 1997 about two miles north of Fort 

Cobb Reservoir. The facility is a finishing operation 

for about 4,000 pigs with an average weight of 150 

pounds each (Agricultural Engineering Associates, 

1997). Wastewater from four swine barns is 

discharged into an earthen lagoon that is about 430 

feet by 320 feet and about 25 feet deep. The lagoon 

was lined with a mixture of sodium bentonite clay and 

soil 18 inches thick during construction (Ham and 

DeSutter, 2000). A water-balance study was 

conducted between May 27 and June 9, 2000, to determine the rate at which wastewater was seeping 

through the lagoon liner (Ham and DeSutter, 2000). 

The study showed an average seepage rate of 11.6 

inches per year, equivalent to an average volume of 

2,663 gallons per day. Wastewater from the lagoon is 

removed during the growing season and applied by 

center pivot irrigation to 83.4 acres in the eastern half 

of the southwest quarter of section 16 (fig. 2). About 

18,478 pounds (9.24 tons) of nitrogen in lagoon wastewater from the swine feeding operation is applied 

annually (Agricultural Engineering Associates, 1997). 

In response to concerns the U.S. Geological 

Survey in cooperation with the Bureau of Reclamation 

studied ground water in an area surrounding the swine 

feeding operation located near Fort Cobb Reservoir in 

Caddo County, Oklahoma (fig. 2). Objectives of the 

study were to (1) determine if the operation was 

contributing nitrogen to the ground water and (2) 

measure changes in ground-water levels and determine 

the local ground-water flow direction in the area 

surrounding the swine feeding operation. 

Purpose and Scope

The purpose of this report is to show groundwater quality, changes in ground-water levels, and 

direction of ground-water flow north of Fort Cobb 

Reservoir surrounding a swine feeding operation. 

Water-quality constituents were measured in groundwater samples from five observation wells installed 

near the swine feeding operation. Ground-water levels 

were measured continuously in a ground-water level 

observation well near a swine feeding operation from 

April 1999 to May 2000. Depth-to-water measurements during December 1998 and August 1999 were 

used to construct two potentiometric-surface maps to 

determine the direction of ground-water flow for a 14 

square mile area bounded by Lake Creek, Willow 

Creek, State Highway 152 and Fort Cobb Reservoir. 

Approach and Methods

Five observation wells were installed and 

sampled to describe the ground-water quality in the 

area surrounding the swine feeding operation (table 1). 

Two wells, MW-B and MW-C, were installed upgradient to a depth of about 70 feet with the bottom 20 

feet of the wells screened. Three observation wells, 

BD-1, BD-2, and BD-3, were installed downgradient 

to a depth of about 55 feet with the bottom 10 feet 

screened. All five wells were completed with the
 3

Figure 1. Study area, Fort Cobb Reservoir, and Cobb Creek subbasin, Caddo County, Oklahoma.
4 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

Figure 2. Location of swine feeding operation and wells used for this study.
 5

top of the screened interval intersecting the water 

table. 

Observation wells were sampled April 5-18, 

2000, and again June 12-19, 2000, except for well BD3. Observation well BD-3 had an insufficient volume 

of water to sample because the adjacent irrigation well 

was operating. Ground water was sampled using 

methods described in Wilde and others (1999). Field 

measurements of specific conductance, pH, and 

dissolved oxygen were performed using methods 

described in Wilde and Radtke (1998). 

Water samples were analyzed for major ions, 

nutrients, and the trace elements copper, selenium, and 

zinc at the U.S. Geological Survey National Water 

Quality Laboratory in Lakewood, Colorado (table 2). 

Major ion concentrations were measured using 

methods described in Fishman and Friedman (1989) 

and in Fishman (1993). Nutrient concentrations were 

measured using methods described in Fishman (1993). 

Copper, selenium, and zinc concentrations were 

measured using methods described in Garbarino 

(1999) and Faires (1993). Water samples also were 

analyzed for the ratio of stable nitrogen isotopes 

nitrogen-15 to nitrogen-14 in nitrate (April samples 

only) at the Stable Isotope Laboratory, Boston University, Boston, Massachusetts, using methods described 

by Downs and Michener (1999). 

Water samples collected from wells BD-1 and 

MW-B on April 12, 2000, were analyzed for a suite of 

pesticides commonly used in Caddo County and pesticide metabolites (table 3). The analyses were 

performed at the U.S. Geological Survey Organic 

Geochemistry Research Laboratory in Lawrence, 

Kansas, using gas chromatography/mass spectrometry 

as described in Zimmerman and Thurman (1999) and 

liquid chromatography/mass spectrometry as 

described in Hostetler and Thurman (1999) and Ferrer 

and others (1997). 

Water samples collected in June were cultured 

for Escherichia coli, fecal streptococci, and fecal 

coliform using methods described in Myers and Wilde 

(1997). 

A ground-water level observaiton well, MWRA, was installed to measure variation in ground-water 

levels near the swine feeding operation (fig. 3). The 

well was equipped with a pressure transducer and data 

logger for depth-to-water measurements and a data 

collection platform for satellite transmission of 

measurements to the U.S. Geological Survey National 

Water Information System. Ground-water levels were 

measured continuously from April 14, 1999, to May 

31, 2000. 

Depth-to-water measurements for 30 wells 

during December 1998 and for 27 wells during August 

1999 (table 1) were used to construct potentiometricsurface maps (figs. 4 and 5). The average total depth of 

domestic wells was about 100 feet, whereas, the 

average total depth of irrigation wells was about 300 to 

350 feet. Wells in the northern part were generally 

deeper than wells in the southern part of the study 

area. The differential altitude of the land-surface near 

each well was determined with a global positioning 

system and was relative to a selected point in the study 

area and corresponding altitude from the U.S. Geological Survey 1:24,000-scale topographic quadrangle. 

Description of the Study Area

The 14 square mile study area is in the Cobb 

Creek subbasin of the Washita River drainage basin in 

northwest Caddo County and is bounded by Lake 

Creek, Willow Creek, State Highway 152, and the Fort 

Cobb Reservoir (fig. 1). Fort Cobb Reservoir was 

constructed in 1959 and has an average surface area of 

4,100 acres. A public hunting area and a State Park 

with public use areas, including a golf course, are 

adjacent to the reservoir (Oklahoma Tourism Department, 2000). Fort Cobb Reservoir is in the Central 

Flyway for migratory waterfowl. The estimated 

number of waterfowl at the reservoir averaged 66,331 

ducks and 10,527 geese a day between mid-November 

and late January during 1991-2000 (Michael O'Meilia, 

Oklahoma Department of Wildlife Conservation, 

written commun., 2001). 

The study area is a sparsely populated agricultural area, with three to four homesteads per square 

mile. There are numerous houses on the north side of 

the reservoir and the local population increases during 

the summer months because of an increase in lakerelated activities. Households and businesses in the 

study area use septic tank systems. 

Agriculture is the primary land use in the study 

area and wheat and peanuts are the principal crops 

grown. Land use and land cover information (U.S. 

Environmental Protection Agency, 2000a) show that 

the largest percent of land in the study area and in the 

Cobb Creek subbasin is used to cultivate row crops 

and small grains (72 and 63 percent). Grassland and
6 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

Table 1. Location information and depth-to-water measurements for wells in the study area 

Identification 

number

(figure 1)

Legal description USGS site identification 

number

Well 

type

Landsurface 

altitude 

(feet 

above 

sea level)

Depth to 

water 

12/98 

(feet 

below 

land 

surface)

Depth to 

water

8/99 

(feet 

below 

land 

surface)

MW-B 09N-12W-08 DDD2 351545098285701 O 1,501 b -- --

MW-C 09N-12W-16 BAAA 2 351542098282501 O 1,495 b -- --

BD-1 09N-12W-16 CBAD 1 351506098284001 O 1,472 b -- --

BD-2 09N-12W-16 CB 1 351504098284601 O 1,475 b -- --

BD-3 09N-12W-16 C 2 351502098284001 O 1,475 b -- --

MWR-A 09N-12W-17 DDD 1 351452098290601 R 1,460.0 -- 38.18

1 09N-12W-04 CBC 1 351650098285301 S 1,541.8 60.91 61.54

2 09N-12W-04 B 1 351712098284101 I 1,540 b 55.55 --

3 09N-12W-05 CCC 1 351635098300301 D 1,508.2 47.07 47.87

4 09N-12W-08 B 1 351622098294201 I 1,501.5 36.93 42.45

5 09N-12W-08 ABB 1 351633098292601 S 1,516.3 48.04 49.36

6 09N-12W-08 DDD 1 351543098285701 D 1,506.8 55.49 55.76

7 09N-12W-08 D 1 351555098291101 I 1,505.1 50.64 51.39

8 09N-12W-09 BB 1 351628098284401 I 1,515.4 43.12 45.80

9 09N-12W-09 AAA 1 351630098275301 D 1,525.4 74.61 75.12

10 09N-12W-09 BCB 1 351618098285301 D 1,518.4 51.96 52.20

11 09N-12W-10 CBBB 1 351608098274901 D 1,517.3 82.24 82.75

12 09N-12W-16 BCB 1 351523098285501 S 1,512.4 69.04 68.65

13 09N-12W-16 C 1 351507098284101 I 1,480.5 46.05 46.93

14 09N-12W-16 CBB 1 351514098285301 S 1,492.5 56.72 58.60

15 09N-12W-17 DD 1 351454098290501 D 1,460.4 37.33 37.45

 16 1 09N-12W-17 DDC 1 351453098290701 U 1,463.8 40.09 40.16

17 09N-12W-17 D 1 351505098291401 I 1,464.7 38.07 --

18 09N-12W-17 CCC 1 351453098295801 D 1,443.4 31.37 31.82

19 09N-12W-17 CBB 1 351512098295501 D 1,458.6 36.96 37.18

20 09N-12W-18 BBC 1 351531098310101 D 1,425.8 28.70 29.26

21 09N-12W-20 CBC 1 351414098295801 D 1,427.0 43.53 43.97

Table 1. Location information and depth-to-water measurements for wells in the study area

[Well types: D, domestic; I, irrigation; O, USGS observation: R, USGS observation well with water-level recorder; S, livestock; U, unused; --, not available]
 7

22 09N-12W-20 DD 1 351411098290401 I 1,426.4 33.17 33.69

23 09N-12W-21 DAD 1 351416098275301 D 1,377.5 7.05 7.49

24 09N-12W-21 CDB 1 351409098283901 I 1,432.6 50.56 50.18

25 09N-12W-21 CBB 1 351421098285301 D 1,432.0 28.46 29.68

26 09N-12W-21 CBB 2 351421098285302 D 1,431.1 26.06 --

27 09N-12W-28 BCB 1 351341098285401 D 1,410 b 50.6 --

28 09N-12W-29 DAA 1 351328098285801 D 1,400 b 51.03 50.89

29 09N-12W-29 DAD 1 351322098285701 D 1,400 b 52.28 52.08

 30 a 09N-13W-12 DAA 1 351607098310201 D 1,467.9 61.17 64.32

 1Irrigation well was operating adjacent to well during 8/99 measurement.

bLand-surface altitude was estimated from USGS 1:24,000 topographic quadrangle.

Table 1. Location information and depth-to-water measurements for wells in the study area —Continued

Identification 

number

(figure 1)

Legal description USGS site identification 

number

Well 

type

Landsurface 

altitude 

(feet 

above 

sea level)

Depth to 

water 

12/98 

(feet 

below 

land 

surface)

Depth to 

water

8/99 

(feet 

below 

land 

surface)
8 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000 Table 2. Measured water properties and concentrations of major ions, nutrients, trace elements, nitrogen isotopes, pesticides, and bacteria 

collected April and June 2000, from observation wells in the study area Identification number Sample date Specific conduc- tance

(μs/cm)

pH

(standard 

units)

Dissolved 

oxygen 

(mg/L)

Alkalinity

(mg/L as 

CaCO3)

Total dissolved solids

(mg/L)

Calcium 

(mg/L as Ca)

Ma

(mg

Upgradient wells

MW–B 04/06/2000 225c 6.5 c 7.5 93 160 16 MW–B 06/12/2000 226c 6.6 7.2 94 163 18 MW–C 04/05/2000 467c 6.9 c 9.4 104 353 42 MW–C 06/14/2000 546c 6.9 c 8.6 107 377 45 

Downgradient wells

BD–1 04/11/2000 674c 7.6 c -- 136 365 65 BD–1 06/15/2000 690c 7.0 c 7.9 128 443 74 BD–2 04/18/2000 601c 7.5 -- 258 365 63 BD–2 06/19/2000 725c 7.4 c 4.8 342 440 87 BD–3 04/12/2000 465c 7.1 -- 107 271 44 

Potassium 

(mg/L as K)

Sulfate 

(mg/L as SO4)

Chloride 

(mg/L as Cl)

Fluoride

(mg/L as F)

Silica 

(mg/L as 

SIO2)

Nitrite

(mg/L as N)

Nitrate nitrite

(mg/L as N)

Ammonia

(mg/L as N)

Orth

p

(mg

Upgradient wells

4.9 13 0.77 0.28 36 < 0.010 1.84 < 0.020 03.6 12 .49 .21 37 < .010 1.95 < .020 3.8 29 13 .22 28 < .010 28.1 f < .020 3.8 29 14 .19 29 < .010 31.5 f < .020 

Downgradient wells

3.9 130 32 .42 29 .022 7.38 < .020 2.6 140 33 .35 32 < .010 6.79 < .020 8.3 22 11 .47 31 .064 6.37 .254 6.6 22 16 .20 34 .029 4.30 .227 4.3 71 13 .33 38 < .010 8.20 < .020 
 9

Table 2. Measured water properties and concentrations of major ions, nutrients, trace elements, nitrogen isotopes, pesticides, an

samples collected April and June 2000, from observation wells in the study area—Continued

Iron 

(μg/L as Fe)

Manganese

(μg/L as Mn)

Selenium 

(μg/L as Se)

Zinc

(μg/L as Zn)

δ15N 

(parts per 

thousand 

relative to 

atmospheric 

nitrogen) 1

 1 Analyses were performed at the Stable Isotope Laboratory, Boston University, Boston, Massachusetts.b Analyses were performed at the U.S. Geological Survey Organic Geochemistry Research Laboratory, Lawrence, Kansas.c Field measurement.d Estimates are based on non-ideal colony count.e Less than the calculated number per 100 milliliters.f Measured concentration exceeds U.S. Environmental Protection Agency primary drinking water standard of 10 milligrams per liter (U.S. Environmental 

2000).

Pesticide 

metabolite

(μg/L) b

Escherichia

coli

(colonies per 

100 mil)

Fecal 

coliform 

(colonies per 

100 mil)

Upgradient wells

E 5.1 16 2 2.1 + 2.43 ND -- -- 

< 10 49 2 2.7 -- -- 60 d 2 d, e 

E 5.1 6.2 1 3.2 + 5.57 ND -- -- 

< 10 12 -- < 20 -- -- 5 d 2 d, e 

Downgradient wells

< 10 2.3 M 1.1 – .09 metolachlor 

ethane sulfonic 

acid 0.66 

-- -- 

< 10 2.7 E 1 3.4 -- -- 3 d 2 d, e < 10 1,150 2 8.4 – .56 ND -- -- 

 20 965 2 4.1 -- -- 10 d 8 d0o 

< 10 32 M 1.2 + 1.88 ND -- -- 
10 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

Table 3. Pesticides and pesticide metabolites analyzed for in ground-water samples collected April 12, 2000, 

from observation wells BD–1 and MW–B in the study area 

[Metolachlor ethane sulfonic acid was measured at a concentration of 0.66 microgram per liter in water from well BD–1. No other pesticides or pesticide metabolites were detected at a concentration above 0.05 micrograms per liter] 

Pesticide Pesticide metabolite

acetochlor acetochlor ethane sulfonic acid acetochlor oxanilic acid

alachlor alachlor ethane sulfonic acid alachlor oxanilic acid

ametryn 

atrazine deisopropylatrazine deethylatrazine

cyanazine cyanazine amide

dimethenamid dimethenamid ethane 

sulfonic acid

dimethenamid oxanilic acid

flufenacet flufenacet ethane sulfonic acid flufenacet oxanilic acid

metolachlor metolachlor ethane sulfonic acid metolachlor oxanilic acid

metribuzin

pendimethalin

prometon

prometryn

propachlor

propazine

simazine

terbutryn
 11

36

42363840

DEPTH TO WATER IN FEETDepth to water from land surface in feet well MWR-A, 09N-12W-17 DDC 3

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

1999 2000

051234

PRECIPITATION IN INCHES Daily precipitation at the Fort Cobb Mesonet station, 08N-12W-21

APR MAY JUN JUL AUG SEP OCT NOV DEC MAR JAN FEB 

Figure 3. Daily mean water level in well MWR-A and daily precipitation at the Fort Cobb Mesonet station, April 14, 1999
12 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

Figure 4. Potentiometric surface and direction of ground-water flow in study area, December 1998.
 13

Figure 5. Potentiometric surface and direction of ground-water flow in the study area, August 1999.
14 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

land used for pasture and hay are the next most 

common land cover and land use (fig. 6 and table 4).

The topography in the study area is characterized by permeable, wind blown sand that forms gently 

rolling hills and steep-walled canyons along Lake and 

Willow Creeks. Soils in the northwest corner of the 

study area are of the Pond Creek and Cobb association 

and are described as deep and moderately deep, loamy, 

and nearly level to sloping soils on uplands. Soils in 

the rest of the study area are of the Dougherty and 

Eufaula association and are described as deep, sandy, 

very gently sloping to rolling soils on the uplands 

(U.S. Department of Agriculture-Soil Conservation 

Service, 1973). The Pond Creek soils have moderately 

slow permeability, Cobb and Dougherty soils are 

moderately permeable, and the Eufaula soils are 

rapidly permeable. 

Privately-owned wells completed in the Rush 

Springs aquifer supply water for irrigation, stock, and 

domestic uses in the study area. Center-pivot irrigation 

is used extensively during the summer growing 

season. In 1995, about 95 percent of water withdrawals from the Rush Springs aquifer were used for 

irrigation (Tortorelli, 2000). 

The Rush Springs aquifer crops out throughout 

the study area and is a massive to highly cross-bedded, 

flat-lying sandstone with some interbedded dolomite 

or gypsum (Becker and Runkle, 1998, pg. 7). Well 

yields vary, but some irrigation wells are reported to 

yield more than 1,000 gallons per minute (Becker and 

Runkle, 1998). Hydraulic conductivities of the Rush 

Springs aquifer have been estimated to range from 1 to 

10 feet per day (Becker, 1998, pg. 12). 

Acknowledgments

The author thanks the owner and operator of Bar 

D Swine Corporation for allowing access to property 

and wells. The author thanks the many well owners in 

the study area for allowing access to their wells and 

especially Mr. and Mrs. Eugene King, Mr. and Mrs. 

Garland Griffin, and Mr. and Mrs. Tommy Schoolcraft 

for allowing the installation of ground-water observation wells on their property.

GROUND-WATER QUALITY 

Nitrate nitrogen (referred to as nitrate) is the 

most common nitrogen compound in water, whereas, 

ammonia nitrogen and nitrite nitrogen (referred to as 

ammonia and nitrite) generally occur only in oxygendepleted water, such as wastewater from lagoons or 

septic tanks. Ammonia, the predominant nitrogen ion 

in swine wastewater, was measured in seven swine 

lagoon samples (Dana Kolpin, U.S. Geological 

Survey, written commun., 2000) at concentrations of 

620 to 2,000 milligrams per liter. Background concentrations of ammonia in ground water, based on analysis of nationally distributed ground-water studies, 

were generally less than 0.1 milligram per liter and for 

nitrate were generally less than 2.0 milligrams per liter 

(Mueller and Helsel, 1996, pgs. 14-15). It also was 

found that nitrate concentrations in agricultural areas 

were generally greater (Mueller and Helsel, 1996, pg. 

17). 

Nitrate concentrations (28.1 and 31.5 milligrams per liter) were largest in the two ground-water 

samples from upgradient well MW-C (table 2), indicating a source of nitrate upgradient of the wastewater 

lagoon. Nitrate concentrations ranged from 4.30 to 

8.20 milligrams per liter in samples from downgradient wells. Traces of ammonia and nitrite were 

detected in downgradient well BD-2, but not in upgradient wells. Ammonia was measured in samples from 

well BD-2 at concentrations of 0.254 and 0.227 milligram per liter, suggesting that wastewater from the

Table 4. Land use and land cover in the study area and the 

Cobb Creek subbasin (U.S. Environmental Protection 

Agency, 2000a)

Land use / land cover

Percent in 

the study 

area

Percent in the 

Cobb Creek 

subbasin

Water 1 2

Residential and commercial development< 1 <1

Barren earthen material < 1 <1

Deciduous, evergreen, 

and mixed forest

7 3

Shrub land 1 3

Grassland 12 23

Pasture and hay 7 6

Row crops and small 

grains

72 63

Urban and recreational 

grasses

0 <1
 15

EXPLANATION National Land Cover Data Set, 2001

U.S. Geological Survey digital data, 1:100,000

Albers Equal Area Conic projection

North American Datum of 1983 

Water

Residential and commercial development

Barren earthen materials

Deciduous, evergreen, and mixed forest

Shrubland

Grassland

Pasture and hay

Small grains and row crops

Fallow Urban and recreational grasses

Wetlands

Section line

012 0.5 Kilometers

012 0.5 Miles

Highway 152

R. 13 W. 98 31’ R. 12 W. 98 27’ 30"

35 13’ 30" 35 17’ 30" T. 9 N.

Figure 6. Land use and land cover in the study area, near Fort Cobb Reservoir. The Landsat thematic mapper 

scenes used to construct this image are from March 1987 to April 1994.
16 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

lagoon is contributing nitrogen to ground water. Nitrite 

was detected in the April sample from BD-1 (0.022 

milligram per liter) and in both samples from BD-2 

(0.064 and 0.029 milligram per liter) (table 2). The 

relatively small proportion of nitrate in water from 

well BD-2 may be attributed to the low dissolved 

oxygen content of the water. The low dissolved 

oxygen concentration indicates that the nitrification 

process may be impeded and that minimal amounts of 

ammonia and nitrite, from lagoon wastewater, are 

converted to nitrate at this location. 

Ammonia concentrations larger than 0.01 milligram per liter have been measured previously by the 

U.S. Geological Survey in the Rush Springs aquifer. 

As reported by Becker (1994, pg. 28), out of 42 wells, 

ground water from nine had ammonia concentrations 

ranging from 0.11 to 0.35 milligram per liter. All nine 

wells were more than 100 feet deep. Ground water 

from one 38-foot deep well had an ammonia concentration of 1.35 milligrams per liter. The source of 

ammonia in these wells in unknown.

Background concentration of phosphorus in 

ground water is usually less than 0.1 milligram per 

liter (Mueller and Helsel, 1996, pg. 15). Concentrations of ortho-phosphorus in ground-water samples in 

this study did not exceed a few tenths of a milligram 

per liter. There was no significant difference in orthophosphorus concentrations between upgradient and 

downgradient samples. Concentrations ranged from 

0.038 (BD-2) to 0.066 milligram per liter (MW-B and 

BD-1) (table 2). 

The ratio of the stable nitrogen isotopes 

nitrogen-15 to nitrogen-14 in nitrate, reported as δ15N 

in parts per thousand relative to atmospheric nitrogen, 

helps differentiate animal waste (which includes waste 

from humans and swine) from other nitrate sources 

such as synthetic fertilizer. Most synthetic nitrogen 

fertilizers are derived from atmospheric nitrogen (N2), 

which is depleted in nitrogen-15 (Kreitler, 1975). As a 

result, the ratio of nitrogen-15 to nitrogen-14 in nitrate 

leached from synthetic fertilizers is similar to atmospheric nitrogen, with δ15N values ranging from -3 to 

+2 parts per thousand (fig. 7). Nitrate leached from 

soils is relatively enriched with nitrogen-15, with 

δ15N values ranging from +2 to +8 parts per thousand. 

Nitrate derived from animal wastes is more enriched 

with nitrogen-15, with δ15N values ranging from +10 

to +20 parts per thousand (Kreitler, 1975). 

Ground water from upgradient well MW-B had 

a δ15N value (2.43 parts per thousand) slightly above 

the range for atmospheric nitrogen and synthetic fertilizer, which may represent a mixture of nitrate from 

these sources and other sources enriched with 

nitrogen-15, such as soils and plants. Nitrate in ground 

water from upgradient well MW-C was the most 

enriched with nitrogen-15, with a δ15N value of 5.57 

parts per thousand. This value for δ15N is within a 

range encompassing soil and plant sources of nitrate. 

The nitrate in ground water from MW-C may be from 

soil and plant sources or a mixture of sources that 

include fertilizer and animal waste. δ15N values indicate atmospheric nitrogen, synthetic fertilizer, or 

plants were the predominate sources of nitrate in 

ground water from the downgradient wells BD-1, BD2, and BD-3 (fig. 7) (table 2). The δ15N values in 

these samples are depleted in nitrogen-15, indicating 

that animal waste was not a significant contributor of 

nitrate. 

Dissolved oxygen concentrations, measured in 

June 2000, ranged from 4.8 to 9.4 milligrams per liter, 

with the lowest concentration of oxygen measured in 

the water sample from downgradient well BD-2 (table 

2). Manganese concentrations (1,150 and 965 micrograms per liter) in samples from well BD-2 were 

substantially larger than concentrations in samples 

from other wells, exceeding the secondary drinkingwater standard of 50 micrograms per liter (U.S. Environmental Protection Agency, 2000b). The larger 

concentrations were probably related to the low 

dissolved oxygen content of the water. Solubility of 

manganese can increase as the oxygen content of the 

water decreases. Larger concentrations of bicarbonate, 

magnesium, fluoride, and iron and a higher pH were 

also measured in water from well BD-2. Dissolved 

solids concentration in samples from the upgradient 

wells, MW-B and MW-C, ranged from 160 to 377 

milligrams per liter. The dissolved solids concentration in water from downgradient wells ranged from 

271 to 443 milligrams per liter (table 2). All samples 

had dissolved solids concentrations less than the 

secondary drinking-water standard of 500 milligrams 

per liter (U.S. Environmental Protection Agency, 

2000b).

Pesticide analyses showed metolachlor ethane 

sulfonic acid, a metabolite of metolachlor, was 

measured at a concentration of 0.66 microgram per 

liter in water from well BD-1 (table 3). Metolachlor is 

a broad spectrum herbicide used for general weed
 17

PLANTS

ANIMAL WASTE

SOILS

SYNTHETIC FERTILIZER

ATMOSPHERE

BD-1

BD-2 BD-3

MW-B

MW-C

δ15N, parts per thousand relative to atmospheric nitrogen

 -10 -5 0 5 10 15 20

Upgradient wells

MW-B

MW-C

Downgradient wells

BD-1

BD-2

BD-3

Figure 7. Ranges of the nitrogen isotopes nitrogen-14 and nitrogen-15 expressed as δ15N 

parts per thousand relative to atmospheric nitrogen in water samples and in common 

sources of nitrate (from Kreitler, 1975).

control on many crops. No other pesticides or pesticide metabolites were detected at a concentration 

above 0.05 microgram per liter in water from wells 

BD-1 and MW-3. Pesticides and pesticide metabolites 

analyzed for are shown on table 3. 

Ground-water samples collected in June 2000 

were analyzed for Escherichia coli, fecal coliform, and 

fecal streptococci. Escherichia coli bacteria were 

counted at populations less than 10 colony forming 

units per 100 milliliters in water from wells MW-C, 

BD-1, and BD-2 and were counted at 60 colony 

forming units per 100 milliliters in water from well 

MW-B. Fecal coliform, and fecal streptococci bacteria 

were counted at populations less than 10 colony 

forming units per 100 milliliters in water from wells 

(table 2). 

LEVELS

Ground-water levels at well MWR-A were 

higher from April to mid-July and lower during the 

late summer and in the fall due to a seasonal decrease 

in precipitation, increase in water withdrawals, and 

increase in evapotranspiration. Ground-water levels 

varied 2.1 feet over the time period measured, with the 

highest in June (37.6 feet below land surface) and 

lowest in September (39.7 feet below land surface) 

(fig. 3). Ground-water levels are shown on a 

hydrograph with daily precipitation measured by the 

Oklahoma Climatological Survey at a Mesonet station 

near the Fort Cobb Reservoir dam (figs. 1 and 3) (Amy 

Cameron, Oklahoma Climatological Survey, written 

commun., 2000). The Mesonet station is south of the 

reservoir and about 6.5 miles south of well MWR-A.

Irrigation wells were operating adjacent wells 

17 and 30, during August 1999 measurements (table 

1). A comparison of depth to water measurements 

from December 1998 and August 1999 shows that 

pumpage from irrigation wells had a minimal effect on 

water levels indicating a high storage coefficient and 

transmissivity for the aquifer. 

FLOW DIRECTION

General ground-water flow direction during 

December 1998 and August 1999 was from north to 

south toward Fort Cobb Reservoir and toward Lake 

and Willow Creeks along the east and west sides of the 

study area (figs. 4 and 5). The potentiometric surface 

gradient was about 30 feet per mile where the land 
18 Ground-Water Quality, Levels, and Flow Direction Near Fort Cobb Reservoir, Caddo County, Oklahoma, 1998-2000

surface is relatively flat and increased to about 40 to 50 

feet per mile where the land surface is steeper adjacent 

to Lake and Willow Creeks. Ground water near the 

wastewater spray field moved south-southeast toward 

Willow Creek along a gradient of about 50 feet per 

mile. 

DISCUSSION

Analysis of ground-water samples suggest that 

commercial fertilizer is contributing nitrate upgradient 

of the swine feeding operation and that wastewater 

from the lagoon is contributing reduced forms of 

nitrogen, ammonia and nitrite. However, the number 

of wells sampled and the number of ground-water 

samples collected are insufficient to determine the 

swine feeding operation long- term contribution of 

nitrogen to ground water. 

The extent of wastewater movement is 

unknown, as a vertical ground-water flow component 

may cause wastewater to flow below the screened 

interval in downgradient wells. Additional downgradient wells would be needed to (1) determine if the 

swine feeding operation is adding excessive amounts 

of nitrogen to ground water, (2) determine the vertical 

dimension of wastewater flow, and (3) the extent of 

wastewater downgradient of the lagoon. 

SUMMARY

Fort Cobb Reservoir in northwest Caddo County 

Oklahoma is managed by the Bureau of Reclamation 

for water supply, recreation, flood control, and wildlife. Excessive amounts of nitrogen in the watershed 

have the potential to cause long-term eutrophication of 

the reservoir and increase already elevated concentrations of nitrogen in the Rush Springs aquifer. The U.S. 

Geological Survey in cooperation with the Bureau of 

Reclamation studied ground water in the area 

surrounding a swine feeding operation located less 

than 2 miles upgradient from Fort Cobb Reservoir in 

Caddo County, Oklahoma. 

Water-quality parameters were measured in 

ground-water samples from five observation wells 

installed near the swine feeding operation. Groundwater levels were measured in a ground-water level 

monitoring well near the swine feeding operation from 

April 1999 to May 2000. Depth-to-water measurements during December 1998 and August 1999 were 

used to construct two potentiometric-surface maps to 

determine the direction of ground-water flow for a 14 

square mile area bounded by Lake Creek, Willow 

Creek, State Highway 152, and Fort Cobb Reservoir.

Nitrate concentrations (28.1 and 31.5 milligrams per liter) were largest in the two ground-water 

samples from upgradient well MW-C. Nitrate concentrations ranged from 4.30 to 8.20 milligrams per liter 

in samples from downgradient wells. Traces of 

ammonia and nitrite were detected in downgradient 

well BD-2, but not in upgradient wells. 

Ground water from upgradient well MW-B had 

a δ15N value (2.43 parts per thousand) slightly above 

the range for atmospheric nitrogen and synthetic fertilizer, which may represent a mixture of nitrate from 

these sources and other sources enriched with 

nitrogen-15, such as soils and plants. Nitrate in ground 

water from upgradient well MW-C was the most 

enriched with nitrogen-15, with a δ15N value of 5.57 

parts per thousand. The nitrate in ground water from 

MW-C may be from soil and plant sources or a 

mixture of sources that include fertilizer and animal 

waste. δ15N values indicate atmospheric nitrogen, 

synthetic fertilizer, or plants were the predominate 

sources of nitrate in ground water from the downgradient wells BD-1, BD-2, and BD-3. 

Manganese concentrations (1,150 and 965 

micrograms per liter) in samples from well BD-2 were 

substantially larger than concentrations in samples 

from other wells, exceeding the secondary drinkingwater standard of 50 micrograms per liter. Larger 

concentrations of bicarbonate, magnesium, fluoride, 

and iron and a higher pH were also measured in water 

from well BD-2. 

Pesticide analyses showed metolachlor ethane 

sulfonic acid, a metabolite of metolachlor, was 

measured at a concentration of 0.66 microgram per 

liter in water from well BD-1 

Ground-water levels at well MWR-A were 

higher from April to mid-July and lower during the 

late summer and in the fall due to a seasonal decrease 

in precipitation, increase in water withdrawals, and 

increase in evapotranspiration. Ground-water levels 

varied 2.1 feet over the time period measured, with the 

highest in June (37.6 feet below land surface) and 

lowest in September (39.7 feet below land surface) 

Ground water near the wastewater spray field 

moved south-southeast toward Willow Creek along a 

gradient of about 50 feet per mile. 
 19

Analysis of ground-water samples suggest that 

commercial fertilizer is contributing nitrate upgradient 

of the swine feeding operation and that wastewater 

from the lagoon is contributing reduced forms of 

nitrogen, ammonia and nitrite. However, the number 

of wells sampled and the number of ground-water 

samples collected are insufficient to determine the 

swine feeding operation long-term contribution of 

nitrogen to ground water. Additional downgradient 

wells would be needed to (1) determine if the swine 

feeding operation is adding excessive amounts of 

nitrogen to ground water, (2) determine the vertical 

dimension of wastewater flow, and (3) the extent of 

wastewater downgradient of the lagoon. 

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