U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3111

November 2005

Prepared in cooperation with the U.S. Environmental Protection Agency and the City of Norman

Naturally Occurring Arsenic in Ground Water, Norman, 

Oklahoma, 2004, and Remediation Options for Produced Water

By S. Jerrod Smith

Printed on recycled paper

Introduction

In 2000, the U.S. Environmental 

Protection Agency (EPA) reviewed 

the arsenic drinking water standard for 

public water supplies. Considering the 

available research and statistics on the 

health effects of arsenic ingestion, the 

EPA reduced the Maximum Contaminant 

Level (MCL) for public drinking water 

from 50 micrograms per liter (μg/L) to 

10 μg/L (U.S. Environmental Protection 

Agency, 2001a). As a result of the more 

stringent standard, the EPA estimates that 

about 3,000 public water providers across 

the United States must take action to 

meet the new standard before it becomes 

effective on January 23, 2006 (U.S. Environmental Protection Agency, 2001b). 

The City of Norman (City) is one of 

several Oklahoma municipalities affected 

by the new arsenic standard. About 20 

percent of Norman’s water is supplied by 

wells completed in the Central Oklahoma 

(Garber-Wellington) aquifer; the rest is 

supplied by Lake Thunderbird (fig. 1) 

or purchased from Oklahoma City. The 

Norman well field is composed of 24 

active wells, and water produced from 

about half of the wells will not be in 

compliance with the new MCL (figs. 

2 and 3). Chemical treatment of water 

with elevated arsenic is possible, but it 

is generally cost prohibitive. Another 

costly solution is simply to abandon the 

high-arsenic wells and replace them with 

new wells in low-arsenic areas. In the 

next phase of well construction beginning 

in 2005, the City plans to construct as 

many as 30 new wells in northeast Norman (Bryan Mitchell, City of Norman, 

oral commun., 2005). The new wells will 

replace production lost to the new arsenic 

standard and add new production to keep 

pace with rapidly growing consumer 

demand. Well modification to exclude 

arsenic-bearing water from existing wells 

is a more cost-effective solution, but it 

requires a great deal of knowledge about 

local aquifer properties and individual 

well dynamics to decide which wells are 

good candidates for modification. With 

the goal of determining if well modification can be used to bring some of Norman’s high-arsenic wells into compliance 

with the new arsenic standard, the EPA 

Office of Research and Development 

(ORD) initiated a three-year research 

project in 2003 with participation from 

the U.S. Geological Survey (USGS), 

Oklahoma State University, and the City 

of Norman. The primary objectives of 

the project are to: (1) determine where 

naturally occurring arsenic is entering 

wells by collecting water samples at different depths, (2) investigate the utility of 

new methods for collecting water-quality data in a pumping well, (3) better 

understand the stratigraphy and composition of aquifer rocks, (4) assess 10 wells 

for the possibility of arsenic remediation 

by well modification, and (5) evaluate 

the effectiveness of well modification in 

bringing marginal wells into compliance 

with the new arsenic MCL. The purpose 

of this report is to describe the occurrence 

of arsenic in ground water near Norman, Oklahoma, and available options 

for reducing arsenic concentrations in 

produced ground water.

Central Oklahoma (GarberWellington) Aquifer

The City of Norman and many other 

municipalities in the Oklahoma City 

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97°45'W 97°30'W 97°15'W 97°00’W 96°45'W

35°00’N

35°15'N

35°30'N

35°45'N

36°00’N

Little

River

LINCOLN

PAYNE

LOGAN

KINGFISHER CANADIAN

POTTAWATOMIE

GRADY

MCCLAIN

CLEVELAND

OKLAHOMA

Lake

Thunderbird

Canadian

River

River

Cimarron

SEMINOLE

Albers Equal-Area Conic Projection

Geologic and well data from U.S. Geological Survey

Hydrography from Oklahoma Water Resources Board

EXPLANATION

Arsenic, total,

in micrograms per liter

! No detection - 10.00

! 10.01 - 50.00

! 50.01 - 232.00

Arsenic, dissolved, 

in micrograms per liter

# No detection - 10.00

# 10.01 - 50.00

# 50.01 - 110.00

Stream

Lake

Confined part of the

Central Oklahoma aquifer

County boundary

Central

Oklahoma

Aquifer

OKLAHOMA

City of Norman boundary

0 5 10 KILOMETERS

0 5 10 MILES

Unconfined part of the 

Central Oklahoma aquifer

Sampled wells, 1977 - 2004

Figure 1. Extent of the Central Oklahoma aquifer with well-head arsenic concentrations. The 

dark shaded part of the aquifer represents the part that is confined by the Hennessey Group. 

Deep municipal supply wells in the confined part are most likely to exceed the U.S. Environmental Protection Agency Maximum Contaminant Level for arsenic.

Core sample of the Garber Sandstone in Norman, Oklahoma
0 0.5 1 2 MILES

0 0.5 1 2 KILOMETERS

35°18'N

35°17'N

35°16'N

35°15'N

35°14'N

35°13'N

35°12'N

97°30'W 97°29'W 97°28'W 97°27'W 97°26'W 97°25'W 97°24'W 97°23'W 97°22'W

OKLAHOMA

Map

area

EXPLANATION

! Norman wells and number

! Sampled Norman wells and number

Norman Roads

Rivers and streams

Geologic Unit

WATER

Alluvium

Terrace

Garber Sandstone

Hennessey Group

(confining unit)

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6 23

8

7

5

4

3

2

38

36 37

35

34

33

32

31

25

21

20

19

18

16

15

14

13

11

10

1

40 39

12

24

Canadian

River

River Little

STATE HWY 9 STATE HWY 9

INTERSTATE 35

Albers Equal-Area Conic Projection

Roads from Norman GIS Department

Geologic data from Oklahoma Geological Survey

Hydrography from Oklahoma Water Resources Board

20

18

Figure 2. Surficial geology and the city of Norman well field, 2003.

0.1

1

10

100

1000

1 2 3 4 5 6 7 8 10 11 12 13 14 15 16 18 19 20 21 23 24 25 31 32 33 34 35 36 37 38 39 40

(1) (2) (2) (1) (1) (1) (1) (3) (1) (1) (1) (2) (0) (1) (0) (2) (2) (2) (0) (0) (0) (0) (0) (1) (1) (1) (1) (1) (0) (2) (1) (1)

CITY OF NORMAN WELLS

ARSENIC, TOTAL, IN MICROGRAMS PER LITER

4 5 4 10 6 6 7 6 4 6 5 5 7 7 3 5 6 6 9 21 2 7 3 3 2 3 2 3 3 4 3 3

10 Mg/L

50 Mg/L

AbandonedAbandonedAbandonedAbandonedAbandonedAbandonedAbandonedAbandoned

Wells which exceed 

the old MCL

(1)

3

Arsenic sample

Number of samples

Median concentration

of detections

Number of samples

with no arsenic detected

EXPLANATION

Old Maximum Contaminant 

Level (MCL) for arsenic 

(U.S. Environmental

Protection Agency, 1986)

New Maximum Contaminant 

Level (MCL) for arsenic 

(U.S. Environmental

Protection Agency, 2001a)

Abandoned

2 Well number of sampled wells

50

500

5

0.5

Figure 3. Sample concentrations, median values, and drinking-water standards for arsenic in the Norman 

well field, 1984-2001.
metropolitan area draw water from the 

Central Oklahoma aquifer. The aquifer 

underlies about 3,000 square miles in 

Cleveland, Lincoln, Logan, Oklahoma, 

Payne, Pottawatomie, and Seminole 

Counties (Parkhurst and others, 1996; 

fig. 1). It is commonly known as the 

Garber-Wellington aquifer because the 

greatest quantities of usable water are 

in the Garber Sandstone and Wellington 

Formation of Permian age (fig. 4). These 

rock units were deposited by an ancient 

river system and consist of discontinuous, 

interbedded channel bars (sandstones) 

and floodplain deposits (mudstones) 

(Stan Paxton and others, Oklahoma State 

University, written commun., 2005). The 

complex stratigraphy of the aquifer creates a complex ground-water flow system 

and promotes chemical interactions 

between water and rock. The western part 

of the aquifer is confined or buried by the 

Permian-age Hennessey Group (fig. 1). 

Water in the confined part of the aquifer 

is generally more removed from recharge 

by precipitation than water in the unconfined part, so the effects of chemical 

interactions between water and rock are 

more prevalent. 

Small concentrations of arsenic occur 

naturally in nearly all rocks in the aquifer 

(Mosier, 1998), but only some areas show 

elevated concentrations (greater than 10 

mg/L) in the water of the aquifer (fig. 1). 

Concentrations as great as 232 μg/L have 

been measured in the confined part of 

the aquifer in Norman (figs. 2 and 3). In 

other parts of the aquifer, arsenic concentrations in water are so small that they are 

not detectable by current technology and 

methods. Several studies by the USGS 

and others have identified a tendency 

for elevated concentrations to occur in 

the western, confined part of the aquifer 

(fig. 1). City of Norman officials also 

recognized this trend and stopped drilling 

new wells in the western half of the City. 

Since 1990, Norman well-field expansion has been almost exclusively to the 

northeast of the City, and arsenic concentrations measured in these newer wells 

are typically less than the new MCL (figs. 

2 and 3). 

The processes and conditions that 

lead to elevated arsenic in the Central 

Oklahoma aquifer are well understood. 

Much of the understanding of the aquifer 

system arose from the U.S. Geological 

Survey National Water Quality Assessment (NAWQA) program. Several 

NAWQA studies, beginning in the late 

1980s and concluding in the mid 1990s, 

determined the rock composition, water 

chemistry, and ground-water movement 

in the aquifer (Parkhurst and others, 

1996; Breit, 1998; Mosier, 1998; Schlottmann and others, 1998). The findings of 

these studies led to a comprehensive conceptual model that is useful for explaining phenomena and patterns of arsenic 

concentrations observed in the field.

Ground-water flow in the southwestern part of the Central Oklahoma aquifer 

is driven by a potentiometric high centered in south-central Oklahoma County 

(Parkhurst and others, 1996; fig. 1). In the 

Norman area, short flowpaths travel to 

the southeast through Cleveland County 

and eventually discharge to the Little 

River drainage system. Other flowpaths 

travel to the southwest, descending under 

the confining unit before turning back to 

the east (Parkhurst and others, 1996; fig. 

4). These flowpaths are longer, deeper, 

and slower, so water spends much more 

time in contact with aquifer materials.

In the Central Oklahoma aquifer, 

release of arsenic to ground water occurs 

when pH exceeds 8.5 standard units (fig. 

5). The dominant geochemical processes 

that cause the increase in pH are cation 

exchange and dissolution of dolomite 

(Parkhurst and others, 1996). Along 

long flowpaths in the confined part of 

the aquifer (fig. 4), these processes can 

elevate the pH from nearly neutral (7.0) 

to greater than 8.5, at which point arsenic 

begins to desorb from aquifer mineral 

grains and reside in the water (Schlottmann and others, 1998). 

Concentrations of arsenic in water of 

the Central Oklahoma aquifer tend to 

increase with depth. Most private wells 

in central Oklahoma draw water from 

the shallow part of the aquifer, which is 

less than 300 feet below land surface (fig. 

4). Shallow wells rarely produce water 

that exceeds the MCL, so arsenic is not 

a significant problem in domestic, stock, 

and irrigation wells. Most public-supply 

wells, on the other hand, produce water 

Lake Thunderbird

1,500

1,000

500

SEA

LEVEL

-500

Garber Sandstone and 

Wellington Formation

Chase, Council Grove, and

Admire Groups

Shallow

Deep

Deep unconfined

Shallow

confined

Hennessey Group

(Confining unit)

 Shallow unconfined

Pennsylvanian

geologic units,

undivided

BRINE

Deep

confined

0 5 10

0 5 10 MILES

KILOMETERS

ELEVATION, IN FEET ABOVE SEA LEVEL

Water table

Land surface

Base of fresh ground water

West East

Modified from Schlottmann and others (1998)

300-foot depth

Norman well field

Short flowpath

Canadian

River

Long flowpath

Note: The rock layers of the aquifer are nearly horizontal. They appear to dip steeply to 

the west because of vertical scale exaggeration. Vertical scale greatly exaggerated.

Figure 4. Generalized hydrogeologic-section of the Central Oklahoma aquifer in the vicinity of Norman. The Norman well field produces water 

from the Garber Sandstone and Wellington formation in the deep confined and deep unconfined part of the aquifer.
from the deep aquifer system (greater 

than 300 feet below land surface). Norman municipal wells, which are typically 

600 to 800 feet deep, tap either the deep 

confined or deep unconfined aquifer 

system (fig. 4). Wells open to sandstone 

zones in the deep confined system (like 

those in western Norman) are more likely 

to exceed the arsenic MCL than wells 

completed in the unconfined system 

(Schlottmann and others, 1998). However, arsenic concentrations in water are 

sometimes only elevated in one or two 

zones, usually near the bottom of the well 

(fig. 6). If these zones can be sealed off 

or isolated from production, the well will 

produce water with lesser concentrations 

of arsenic. The Central Oklahoma aquifer 

is ideal for this remediation strategy 

because contaminated sandstone zones 

are naturally separated or compartmentalized by relatively impermeable mudstone 

zones (fig. 6).

Identifying candidate wells 

The best candidates for successful 

remediation are those wells that have 

marginal arsenic concentrations (slightly 

greater than 10 μg/L) and high waterproduction rates (greater than 200 gallons 

per minute). These wells are most likely 

to benefit from isolation of a single, higharsenic zone and are the least likely to 

suffer from loss of production. 

To determine the changes in water 

contribution and contaminant concentrations at different depths in each well, 

the USGS is using a newly developed 

“well flowmeter and down-hole sampler" 

(Izbicki and others, 1999), referred to 

in this report as the down-hole sampler. 

The down-hole sampler can estimate the 

contribution of water from perforated 

zones and collect water samples at various depths in a pumping well (fig. 6). 

The USGS down-hole sampler is essentially a slim, high-pressure hose that can 

be raised and lowered between the pump 

column and well casing using a motorized hose reel (fig. 7). A line counter at 

the surface reports the depth of the hose 

outlet, which is equipped with check 

valves that allow water to enter but not 

exit the hose. The new method is considerably less expensive than standard methods of depth-dependent sampling, and it 

requires less down-time of the well. In 

terms of data quality, the greatest advantage of the USGS down-hole sampler is 

that all data collection is performed under 

true production conditions. 

Low-cost remediation options 

involving well modification

Based on data collected using the 

USGS down-hole sampler, EPA’s Ground 

Water and Ecosystems Restoration Division in Ada, Oklahoma, and the City 

of Norman are evaluating two low-cost 

options for arsenic remediation in produced water. Because the USGS downhole sampler has shown that greater 

arsenic concentrations tend to be found at 

deeper zones in a well, simply raising the 

pump intake may be an effective remediation strategy. If the pump intake is moved 

farther from the contaminated zone, the 

well may produce a lesser proportion of 

water from the contaminated zone. This 

simple strategy is advantageous because 

it is reversible, it may not sacrifice production, and it involves minimal down 

time for the well. However, this method 

may be just a temporary solution because 

the arsenic concentration in produced 

water may fluctuate with seasonal or prolonged changes in aquifer water levels.

Another remediation strategy is zonal 

isolation, which involves cementing in or 

plugging an arsenic-contributing zone. 

This strategy is advantageous because 

it ensures that a contaminated zone will 

no longer contribute any arsenic to the 

well. This technique is best suited for use 

in gun-perforated wells with a cement 

annulus, but it also could be applied in 

screened wells with a gravel-pack annulus. Zonal isolation also is best suited for 

use in aquifers where contributing zones 

are hydraulically separated by relatively 

impermeable units. 

Relevance and benefits

With the prospect of identifying 

individual wells for remediation using 

the USGS down-hole sampler, the City of 

0

5

10

15

20

25

30

35

40

45

50

55

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

PH, IN STANDARD UNITS

ARSENIC, DISSOLVED, IN MICROGRAMS PER LITER

Arsenic sample

Old arsenic MCL

(U.S. Environmental

Protection Agency, 

1986)

New arsenic MCL

(U.S. Environmental

Protection Agency, 

2001a)

PH = 8.5

Figure 5. Relation between dissolved arsenic concentration and pH for wells in Cleveland 

County, Oklahoma, 1977-2004.
Norman may be able to avoid the costly 

installation of some additional wells, 

conveyance infrastructure, and treatment 

technologies. Most importantly, the City 

may be able to decrease arsenic exposure to citizens and protect them against 

potentially unnecessary costs associated 

with treatment. Neighboring municipalities may eventually share the benefits of 

these new strategies for detecting and 

reducing contaminants in the Central 

Oklahoma aquifer. Furthermore, this 

understanding also may be transferable 

to other water suppliers in the United 

States that draw contaminated water from 

multilayered aquifers.

Selected References

Breit, G.N., The diagenetic history of 

Permian rocks in the Central Oklahoma aquifer, in Christenson, S.C., and 

Havens, J.S., eds., Ground-water-quality assessment of the Central Oklahoma aquifer, Oklahoma-Results of 

investigations: U.S. Geological Survey 

Water-Supply Paper 2357-A, p. 45-70. 

Izbicki, J.A., Christensen, A.H., Hanson, 

R.T., 1999, U.S. Geological Survey 

combined well-bore flow and depthdependent water sampler: U.S. Geological Survey Fact Sheet, FS-196-99.

700

ND 10 20 30 40 ARSENIC V CONCENTRATION, 

IN MICROGRAMS PER LITER 

Sandstone

Mudstone

Formation water

and flow direction

Pumping

water level

350

400

450

500

550

600

650

0

Submersible pump 

intake and motor

Perforations

Sample hose Steel casing

Pump bowls

Pump column

Well-head discharge

Arsenic V

Water-Quality

Samples

DEPTH, IN FEET BELOW LAND SURFACE

PH

PH = 8.5

8.0 8.5 9.0 PH, IN STANDARD UNITS

USGS well 

flowmeter and 

down-hole

sampler

Not to scale

Cement

annulus

Figure 6. Depth-dependent sampling in Norman using the USGS well flowmeter 

and down-hole sampler in a hypothetical supply well. The dashed line connecting 

arsenic samples indicates no detection (ND) of the constituent. Note the identification of an arsenic-contaminated zone at 665 feet below land surface. The 

bottom 30 feet of this well could be plugged to exclude contaminated water from 

produced water.

Figure 7. The USGS well flowmeter and downhole sampler. Motorized hose reel with orange 

hose shown on the right.
Mosier, E.L., Geochemical characterization of solid-phase materials in the 

Central Oklahoma aquifer, in Christenson, S.C., and Havens, J.S., eds., 

Ground-water-quality assessment of 

the Central Oklahoma aquifer, Oklahoma-Results of investigations: U.S. 

Geological Survey Water-Supply Paper 

2357-A, p. 71-106.

Parkhurst, D.L., Christenson, S.C., and 

Breit, G.N., 1996, Ground-water-quality assessment of the Central Oklahoma aquifer, Oklahoma-Geochemical 

and geohydrologic investigations: U.S. 

Geological Survey Water-Supply Paper 

2357-C, 101 p.

Schlottmann, J.L., Mosier, E.L., and 

Breit, G.N., 1998, Arsenic, chromium, 

selenium, and uranium in the Central 

Oklahoma aquifer, in Christenson, 

S.C., and Havens, J.S., eds., Groundwater-quality assessment of the Central 

Oklahoma aquifer, Oklahoma-Results 

of investigations: U.S. Geological 

Survey Water-Supply Paper 2357-A, p. 

119-179. 

U.S. Environmental Protection Agency, 

1986, Maximum contaminant levels (subpart B of part 141, National 

interim primary drinking water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100-149, revised 

as of July 1, 1986, p. 374.

U.S. Environmental Protection Agency, 

2001a, National primary drinking 

water regulations; Arsenic and clarifications to compliance and new source 

contaminants monitoring: final rule: 

Federal Register 66, no. 14, p. 6976-

7066.

U.S. Environmental Protection Agency, 

2001b, Technical fact sheet: Final rule 

for arsenic in drinking water, from 

URL http://www.epa.gov/OGWDW/

ars/ars_rule_techfactsheet.html, 

accessed April 18, 2003.

Welch, A.H., Watkins, S.A., Helsel, D.R., 

and Focazio, M.J., 2000, Arsenic in 

ground-water resources of the United 

States: U.S. Geological Survey Fact 

Sheet FS-063-00, 4 p.

For additional information contact:

S. Jerrod Smith or Scott Christenson

U.S. Geological Survey

Oklahoma Water Science Center

202 NW 66 St., Bldg. 7

Oklahoma City, OK 73116

(405) 810-4400

Oklahoma Water Science Center home page: http://ok.water.usgs.gov

Figure 8. Test hole drilling at sunset in Norman, Oklahoma, October 2004. Photograph 

taken by S. Jerrod Smith, U.S. Geological Survey.