nation in the Pinal Creek Basin in 1984.

Research has been funded mainly by the

USGS Toxic Substances Hydrology

Program, the goal of which is to provide

earth-science information on the

behavior of toxic substances in the

Nation’s surface waters and ground

waters. The program does this through

research on the processes that control

contaminant movement and through the

development of new methods with which

to undertake this research. USGS

research in Pinal Creek Basin initially

focused on a plume of acidic ground

water in the regional alluvium, but has

broadened to include investigations of

streamflow contamination and transport

interactions between streamflow and

shallow ground water. Research at the

site is now a multidisciplinary,

collaborative effort among scientists

from the USGS, University of Arizona,

and Arizona State University. The overall

objectives are to increase scientific

U.S. Department of the Interior

U.S. Geological Survey

Figure 2. Mine tailings, which cover about 27 square kilometers in Pinal Creek Basin,

have dramatically altered the landscape. Runoff from tailings is a potential source

of acid drainage in the basin.

Research on Acidic Metal

Contaminants in Pinal Creek

Basin near Globe, Arizona

Figure 1. Ground-water and surfacewater data-collection sites, Pinal Creek

Basin, Arizona.

Bloody

Inspiration Dam

PINAL

CREEK

PINAL

CREEK

Russell Gulch

Globe

Webster Lake (dry)

10

700

09498400

300

200

Setka

Ranch

BOUNDARY

OF AQUIFER

Miami

600

100

50

50

Miami Wash

400

450 500

09498380

09498380

EXPLANATION

DATA SITES—Number

is site identifier

Observation-well group

Streamflow-gaging

station

Precipitation gage

MINE OR TAILINGS

1 2 MILES

0

0

1 2 KILOMETERS

110°55' 50' 110°45'

25'

30'

33°35'

33°20'

TanksWash

Ri ver

G ila

Salt

iv R er

STUDY

AREA

The Need for Research on

Acidic Metal Contamination

Acidic drainage from mines has been

recognized as a problem since mining

began in this country over a century ago.

The U.S. Forest Service has estimated

that more than 1,500 mining sites in the

western United States contribute to the

problem. Other estimates have placed

the total number of mine sites involved

at 20,000 to 50,000. In many instances,

acidic drainage, which consists of flows

or seeps into streams and rivers, can

adversely affect aquatic ecosystems and

degrade drinking-water quality. In order

to remediate the worst abandoned mine

sites and minimize the effect from

existing and new mines, adequate

knowledge is needed of the physical and

chemical processes that control the

movement and fate of contaminants.

The U.S. Geological Survey (USGS)

began research on acidic metal contamiWell 51

Well 402

Horseshoe

Bend Wash

Roosevelt

Lake

Pinal

Creek

Basin

understanding of the

controls on transport

of metals and other

inorganic contaminants and to develop

new methods and

models to investigate

contaminant transport.

1997

Circa 1915
Site Description

Pinal Creek Basin is in central

Arizona about 100 kilometers east of

Phoenix (fig.1). Globe and Miami are

the principal communities in the basin,

which had a population of about 18,000

in 1990. Copper has been mined in the

basin since 1882—first in underground

mines and, beginning in 1948, in openpit mines. The largest open-pit mines in

the basin are adjacent to and north of

Miami, where mines and tailings dominate the local landscape (fig. 2).

In the late 1980’s, Pinal Creek Basin

was placed in the Water Quality

Revolving Fund (WQARF), which is

the State of Arizona’s equivalent of the

U.S. Environmental Protection Agency

(USEPA) Superfund Program. At about

the same time, three mining companies

currently or formerly active in the basin

formed the “Pinal Creek Group” to

reduce contaminant sources and

remediate existing contamination.

Information from the USGS studies was

used by the Pinal Creek Group to help

develop a cost- effective plan for

remediation of the aquifer.

Hydrogeology of Pinal

Creek Basin

Russell Gulch and Bloody Tanks

Wash combine to form Miami Wash,

which is the main tributary to Pinal

Creek in the basin. These and other

tributaries drain the hills and mountains

that surround the valley occupied by

Pina l Cr e ek and Mi ami Wa sh.

Unconsolidated stream alluvium and

underlying basin fill form the regional

aquifer in Pinal Creek Basin. The

alluvium is from 300 to 800 meters

wide, less than 50 meters thick, and

contains poorly sorted, cobble- to claysized material. Most material, however,

is sand size or greater. The regional

aquifer is bounded laterally and at

depth by older, less permeable rocks

that include granite, schist, and

vol c ani c rocks. Infiltr a tion of

intermittent storm runoff and snowmelt

from surrounding mountains through

permeable streambed alluvium in the

basin is a major source of recharge to

the aquifer. In the northern part of Pinal

Creek Basin, the aquifer becomes

shallower in the direction of groundcalcium, iron, manganese, copper,

aluminum, and zinc. In 1996,

concentrations of dissolved iron were

200 milligrams per liter at well 51 and

190 milligrams per liter at well 402.

Acidic ground water is neutralized

mainly through reaction with calcium

carbonate as the plume moves

downgradient. The pH increases to

between 5 and 6, and metals including

iron, copper, and zinc precipitate or

adsorb to mineral surfaces in the

aquifer. Manganese dissolves and

becomes mobile in ground water. As

ground water surfaces in the perennial

reach of Pinal Creek, the pH of

contaminated stream water increases to

about 8 as the water equilibrates with

the atmosphere and thus causes

manganese carbonate and oxide

minerals to precipitate as crusts in the

streambed.

Major changes in the chemistry of the

ground-water plume have occurred

because of the removal of the major

source of contaminants, withdrawal of

contaminated ground water, dilution

from natural recharge, better control of

surface runoff, and subsurface chemical

reactions. Concentrations of dissolved

iron in water from well 51 decreased

from 3,200 milligrams per liter in

November 1984 to 1,700 milligrams per

liter in November 1990. During that

time, ground-water levels declined

about 16 meters despite normal

amounts of rainfall in most years.

Greater-than- average winter precipitation caused water levels to rise as

much as 16 meters from January 1991

to May 1993. By June 1994, concentrations of dissolved iron in well 51 had

decreased to 200 milligrams per liter

(fig. 4).

water flow and Inspiration Dam (fig. 3).

As a result, ground water flows upward

to the stream channel of Pinal Creek and

forms perennial streamflow for

approximately 7 kilometers from near

Horseshoe Bend Wash to Inspiration

Dam. Past Inspiration Dam, Pinal Creek

flows through a narrow rock-walled

canyon to the Salt River and eventually

to Roosevelt Lake, an important watersupply reservoir for the City of Phoenix.

Nature and Extent of

Contamination

In Pinal Creek Basin, acidic mine

drainage over the past century has

resulted from oxidation of sulfide

minerals, from ore processing, and from

other mining activities in the basin.

Unlined surface-water impoundments of

a c idi c wa t e r we r e signific ant

contaminant sources. The single largest

source probably was Webster Lake,

which existed from about 1940 to 1988.

Available data indicate that from the

mid-1970’s until the lake was drained in

1988, pH of the lake water was between

2 and 3. Concentrations of dissolved

iron and sulfate were greater than 2,000

and 19,000 milligrams per liter,

respectively.

Contaminated acidic ground water

was first recognized in the regional

aquifer in the 1930's, and perennial

streamflow in Pinal Creek is known to

have been affected by contamination

since 1963 (Envirologic Systems, Inc.,

1983) and possibly earlier. A plume of

contaminated ground water has moved

about 25 kilometers through the stream

alluvium of Miami Wash and Pinal

Creek. The acidic part of the plume

contains large concentrations of sulfate,

Figure 3. Perennial

streamflow in Pinal

Creek, which consists of a mixture of

contaminated and

u n c o n t a m i n a t e d

ground water that

has discharged to

the surface, flows

over Inspiration Dam

and enters the Salt

River just above

Roosevelt Lake.
Research Themes

Two general themes have guided

research at the site. The first is the

development of new tools to assess the

transport and fate of contaminants in

the stream- aquifer system. The tools

and techniques developed at Pinal

Creek will be transferable to similar

sites affected by acidic mine drainage.

The second theme is the evaluation

of computer models to characterize the

physical and chemical processes that

occur at contaminant sites, to help

design experiments, and to assess the

effectiveness of remediation at Pinal

Creek Basin and elsewhere. A valuable

component of any modeling exercise is

uncertainty analysis. Modeling and

uncertainty analysis are necessary to

generalize research results and transfer

data to other areas affected by mining.

Significant Findings

Research in Pinal Creek Basin has

determined that:

—Dissolved-metal contaminants in the

plume move much more slowly than the

plume water. Neutralization reactions in

the carbonate aquifer slow the rate of

advance of the plume's acidic front and Figure 5. In the perennial reach some of the stream water continually moves into

and out of an area in shallow ground water known as the hyporheic zone. This process

helps enhance removal of dissolved metals from streamflow.

Well 51

Well 101

Well 101

Well 51

Well 51

0.0

5.0

WATER LEVEL,

IN METERS BELO

W

LAND SURFACE

DISSOLVED IRON, IN

MILLIGRAMS PER LITER

10.0

15.0

20.0

5.0

2.5

3,000

2,500

2,000

1,500

1,000

500

pH

0

1984198619881990199219941996

3,500

Figure 4. Ground-water levels, pH, and

concentrations of iron in ground water

adjacent to Miami Wash, 1984 –96.

most of the dissolved metals to about

one seventh of the rate of advective

ground-water flow.

—The oxidation and precipitation of

iron is driven by the reductive

dissolution of manganese oxide

minerals. This reaction is a source of

additional acidity to the plume and a

source of dissolved manganese, which

travels ahead of the plume’s acidic

front in neutralized ground water.

—The downgradient decrease in

dissolved concentrations of copper,

cobalt, nickel, and zinc was the result

of the pH- dependent adsorption to iron

hydroxides in the aquifer.

—Geochemical modeling has revealed

the need for information on redoxreaction rates in the plume and on

carbon dioxide leaving the plume and

oxygen entering the plume in order to

better understand controls on pH and

movement of contaminants.

—The exchange of gases between the

plume and atmosphere affects the

geochemistry of the shallow part of the

plume. The major controls on

exchange of carbon dioxide and

oxygen from the plume across the

water table are (1) neutralization

reactions involving carbonate minerals

and (2) oxidation of dissolved iron

near the water table.

—After contaminated ground water

discharges to Pinal Creek, gas

exchange causes a rapid increase in pH

and dissolved oxygen in streamflow.

Continual movement of perennial

streamflow into and out of areas of

shallow ground water beneath the

stream (hyporheic zone, fig. 5) increases

the contact of streamflow with sediment

surfaces. The increased contact

stimulates precipitation of dissolved

manganese as oxide coatings on

streambed sediment.

—Chemical reactions in the hyporheic

zone are enhanced by the activity of

manganese oxidizing bacteria, which

are active in the zone because of input

o f o x y g e n a t e d st r e a m fl o w .

Approximately twenty percent of the

total load of dissolved manganese

flowing out of the drainage basin is

removed in this manner.

Precipitation of manganese in the

hyporheic zone reduces the movement

of dissolved nickel and cobalt because

manganese oxides are an excellent

sorbent for those metals. As a result, the

loads of dissolved nickel and cobalt

flowing from the basin each were

reduced because of hyporheic exchange.

—Solute tracers are useful in determining the extent of chemical reactions

in streamflow and shallow ground-water

systems. The technique, however,

sometimes has high uncertainty. A new

method was developed to improve the

design of tracer experiments to reduce

uncertainty when using tracers to

estimate rates of transport and chemical

reaction.

Well 101

ATMOSPHERE

STREAM

HYPORHEIC

GROUND WATER

Dissolved-metal

removal

Lower pH and

oxygen, higher

dissolved-metal

concentration

Increased contact of stream water

with sediment and microbes.

Higher pH and oxygen

than ground water Dissolved-metal

removal

Higher pH and oxygen

through gas exchange

Gas exch an ge

 H ydrologic exch ange

 H ydrologic exch ange
Ongoing Research

Research focuses on the processes in

the two major hydrologic regimes at the

site: ground-water transport in the

southern part of the basin and interaction

of surface-water and shallow groundwater transport in the northern part of the

basin. Ongoing and proposed research

are designed to:

— Determine the hydraulic-flow

characteristics of discrete intervals of

aquifer sediments. Contaminants move

preferentially along zones of higher

permeability. Accordingly, the

permeability of such discrete zones

determines, in part, the rate at which

contaminants move through the groundwater system.

—Using parameter estimation and

sensitivity-analysis techniques, evaluate

and determine the value of different

types of data that are used in groundwater models. Constrain the flow system

with ground-water ages obtained from

chlorofluorocarbon analyses.

— Use geochemical modeling to

determine appropriate simplifications

that can be made. For example, what is

the effect of assuming that the

mi n e r a l o g y o f t h e a q u if e r is

h omo g e n e o u s ? I n f o rma ti o n o n

appropriate simplifications will be

useful at similarly contaminated sites

where data are limited and may help to

determine the types of data that need to

be collected.

—Develop improved instrumentation

and more efficient data - collection

techniques to determine rates of uptake

of dissolved metals in the hyporheic

zone.

—Identify the most important physical

and chemical processes that stimulate

enhanced chemical reactions in the

hyporheic zone and thus reduce the load

of metal contaminants flowing out of the

Pinal Creek drainage basin.

—Transfer stream-tracer design methodology to the USGS Abandoned Mine

Lands Initiative. The new design strategy

will improve the efficiency of future

experiments and increase the precision of

tracer methods as a means of characterizing contamination and chemical

reactions in surface water and shallow

ground water at sites with different

characteristics from those of Pinal Creek.

—James G. Brown, Robert Brew, and

Judson W. Harvey

March 1997

USGS Fact Sheet

FS-005-97

Selected References

Brown, J.G., and Favor, Barbara, eds., 1996,

Hydrology and geochemistry of aquifer and

stream contamination related to acidic water in

Pinal Creek Basin near Globe, Arizona:

U.S. Geological Survey Water-Supply Paper

2466, 103 p.

Gellenbeck, D.J., and Hunter, Y.R., 1994,

Hydrologic data from the study of acidic

contamination in the Miami Wash–Pinal Creek

area, Arizona, water years 1992–93: U.S.

Geological Survey Open-File Report 94–508,

103 p.

Brown, J.G., and Harvey, J.W., 1996, Hydrologic

and geochemical factors affecting metalcontaminant transport in Pinal Creek basin near

Globe, Arizona, in Morganwalp, D.W., and

Aronson, D.A., eds., U.S. Geological Survey

Toxic Substances Hydrology Program—

Proceedings of the Technical Meeting,

Colorado Springs, Colorado, September 20–24,

1993: U.S. Geological Survey WaterResources Investigations Report 94 – 4015,

v. 2, p. 1035–1042.

For more information contact:

James Brown

U.S. Geological Survey

520 N. Park Avenue, Suite 221

Tucson, AZ 85719

(520) 670-6671, ext. 280

http://wwwdaztcn.usgs.gov

or

Judson Harvey

U.S. Geological Survey

National Center, MS 430

12201 Sunrise Valley Drive

Reston, VA 20192

(703) 648-5876

Sample location

10 51 302 402 503 702

Pinal Creek

at

Inspiration Dam

Table 1. Measured water chemistry of wells and surface water, Pinal Creek Basin,

spring 1996

[Values are in milligrams per liter except for pH; <, less than; — indicates no data]

Sample date June 1

pH

Calcium

Magnesium

Sodium

Potassium

Alkalinity

Total dissolved

inorganic carbon

Sulfate

Chloride

Fluoride

Silica

Iron

Manganese

Aluminum

Cadmium

Copper

Cobalt

Nickel

Strontium

Zinc

June 6 June 6 June 2 June 8 May 31 May 31

5.3

5.3

2.9

.026

2.61

.022

.67

.87

.80

1.5

1.9

460

110

62

27

24

1,800

49

67

75

6.8

2.0

.3

<.003

.001

<.005

<.001

.02

<.003

<.01

.37

.008

56

17

26

199

—

36

26

26

300

97

77

1,900

41

88

200

13

24

3.9

8.2

—

9.4

7.6

.069

—

.45

1.3

2.9

28.1

390

90

63

90

30

7.8

6.4

—

—

—

3.1

.250

.007

<.03

.21

.49

1.4

.35

45.5

<.130

4.2

5.3

—

4.9

.011

.41

.42

1.0

2.1

250

73

58

24

1,400

54

73

190

24

12

10.6

4.0

4.4

—

3.4

.049

—

.24

.81

1.5

210

38

66

20

1,200

45

75

140

<

11

<7.1

13.7

470

88

57

165

1,200

48

43

6.9

3.9

—

.4

<.009

1.1

<.005

<.003

<.03

.014

<.03

1.4

<.009

1,500

47

00

20