U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
Hydrologic and Geochemical Factors affecting Metal Contaminant
Transport in Pinal Creek Basin near Globe, Arizona
by
James G. Brown (U.S. Geological Survey, Tucson, Ariz.) and Judson
W. Harvey (U.S. Geological Survey, Menlo Park, Calif.)
Contents
ABSTRACT
Acidic-mine drainage from copper mining in the Pinal Creek basin has
contaminated the regional aquifer. Chemical reactions neutralize the contaminant
plume as it moves downgradient. Neutralized contamination subsequently surfaces
in a perennial reach of Pinal Creek and equilibrates with the atmosphere,
causing pH to rise and manganese minerals to precipitate on the streambed.
Ongoing research at the site is focused on ground-water hydrology and geochemistry,
metal transport, and interactions between shallow ground water and streamflow.
Field and laboratory experiments will attempt to identify the reduction(s)
that balance the oxidation of iron in the aquifer. Reactive-transport simulations
of plume evolution and remediation will identify the important processes
and reactions that produced the observed conditions in the plume. Measurements
of gravity change and ground-water level change at project wells may provide
refined estimates of aquifer specific yield for use in reactive-transport
simulations. Estimates of ground-water ages by chlorofluorocarbon age-dating
techniques may improve estimates of traveltimes and reaction rates of contaminants
in ground water.
Mechanisms of trace-metal scavenging by manganese carbonates, manganese
oxides, and iron oxides in the perennial stream are being investigated,
and partitioning coefficients are being estimated to constrain hydrogeochemical
modeling. A coupled aquifer-stream mass-balance approach has been used to
constrain effective rates of solid-aqueous partitioning of manganese, iron,
nickel, copper, and cobalt. Tracer experiments are being undertaken to distinguish
reactions occurring in near-stream ground water from reactions in streamflow.
INTRODUCTION
The Pinal Creek basin, in central Arizona (fig. 1), has been an area
of large-scale copper mining since the late 1880's. Acidic drainage from
mining and related activities has contaminated the regional alluvial aquifer.
The U.S. Geological Survey began a study of the contamination with the following
objectives: (1) Define the nature and extent of ground-water and surface-water
contamination, (2) identify the reactions and processes occurring in the
contaminant plume, and (3) develop and test the ability of geochemical and
transport models to simulate observed conditions. The purpose of this paper
is to provide an overview of the site and a summary of past and ongoing
research at Pinal Creek.
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Figure 1. Area of study
GEOHYDROLOGY
The drainage area of Pinal Creek above Inspiration Dam is
516 km2. About 170 km2 of this
area is underlain by a basin-fill conglomerate and a permeable
stream alluvium, which together compose the regional
aquifer. The aquifer is bounded laterally and at depth in most
areas by relatively impermeable rocks. The aquifer thins and
becomes narrower from well site 500 (fig. 2) north to Inspiration
Dam. This thinning and narrowing decreases the volume of water
that can move and be stored in the subsurface, and sustains
perennial flow into Pinal Creek. Streamflow in Pinal Creek is
perennial in a 5-to 6 kilometer reach above Inspiration
Dam.
Basin fill ranges from unsorted and unconsolidated boulders to well-stratified
deposits of firmly cemented sand, silt, and gravel (Peterson, 1962, p. 41).
Carbonate content of the basin fill is about 1.5 percent (Eychaner, 1989,
p. 570). Horizontal hydraulic conductivity of basin fill is estimated to
range from 0.1 to 0.2 m/day (C.G. Taylor, Magma Copper Corporation., written
commun., 1987). Stream alluvium consists mainly of sand and gravel but includes
stratified deposits of clay-sized material as much as 12 m thick locally.
Horizontal hydraulic conductivity of alluvium between well sites 400 and
500 was estimated to be 260 m/d. Alluvium at well site 600 (fig. 2), through
which neutralized, contaminated water has passed, contained from 0.3- to
0.7- percent carbonate material.
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Figure 2. Generalized
hydrogeologic section of the aquifer. Line of section approximates the principal
ground-water flow line from Webster Lake to Inspiration Dam along the channels
of Webster Gulch, Bloody Tanks Wash, Miami Wash, and Pinal Creek.
Ground-water levels in stream alluvium and adjacent basin fill respond
rapidly to variations in recharge and pumpage. Water levels declined steadily
from 1986 to 1990 in response to less-than-average rainfall in the basin
and increased pumping between Miami Wash and Pinal Creek for the purposes
of remediation. The amount of ground-water decline generally decreased in
a downstream direction. Ground-water levels rose as much as 16 m (fig. 3)
adjacent to Miami Wash by February 1993 because of wetter than normal winters
in 1991 and 1993.
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Figure 3. Ground-water
levels and concentrations of iron and sulfate adjacent to Miami Wash.
NATURE AND EXTENT OF CONTAMINANTION
Leakage of mine wastewater and process water from unlined impoundments
into stream alluvium have contributed to a 15-kilometer-long acidic contaminant
plume within the alluvium along Miami Wash and Lower Pinal Creek (fig. 4).
Webster Lake, the single largest potential source of contamination, was
used to store waste and process water from 1941 until 1988, when it was
drained by the order of the U.S. Environmental Protection Agency. Contaminated
ground water removed by remedial pumping is either spread on mine tailings
to evaporate or is used at the mines.
(33)
Figure 4. Distribution
of pH in the aquifer in 1992. Line of section approximates the principal ground-water
flow line from Webster Lake to Inspiration Dam along the channels of Webster
Gulch, Bloody Tanks Wash, Miami Wash, and Pinal Creek.
Water from well 51, which is completed in the acidic core
of the plume, contained more than 500 mg/L iron (Fe),
32 mg/L copper (Cu), and 2,700 mg/L sulfate (SO4)
at a pH of 3.7 in 1992 (table 1). Between site 450 and the perennial
reach of Pinal Creek, neutralization reactions caused pH
of ground water to increase from between 4 and 5 to about 6
in 1992. In February 1993, neutralized ground water discharged
to a perennial reach that began less than 1 km above well site
500. As the neutralized ground water discharges into the stream,
gases dissolved in water reach equilibrium with the atmosphere.
In the process, carbon dioxide (CO2) degases, and pH rises to
about 8 over a distance of about 5 km. Neutralized water from
Pinal Creek at Inspiration Dam (Streamflow-gaging station 09498400)
contained 560 mg/L dissolved calcium (Ca), 2,800 mg/L dissolved
SO4, and 39 mg/L dissolved manganese (Mn) in May 1992.
Dissolved Fe and Cu concentrations were 0.01 and 0.006 mg/L
(micrograms per liter), respectively. From 1984 to 1992,
concentrations of Fe in water from well 51 decreased from 3,100
mg/L to 500 mg/L, and concentrations of SO4 decreased
from 10,000 mg/L to 2,700 mg/L (fig. 3). These decreases were
caused by (1) removal of a significant source of contamination,
(2) recharge from winter runoff in 1991-92, and (3) chemical
reactions in the subsurface.
RESEARCH IN PROGRESS
Ongoing research falls within three general categories: ground-water
geochemistry and transport, ground-water hydrology, and hydrochemical interactions
between ground water and surface water. These broad categories provide the
framework in which to discuss present research at the site.
Ground-Water Geochemistry and Transport
Although the most important reactions in the aquifer are well
understood, the redox reactions responsible for the observed
decreases in dissolved Fe have not been adequately characterized.
The results of geochemical modeling (Glynn and others, 1991)
suggest that the dissolution of Mn oxide solids could be
the principal reduction that balances Fe oxidation. Reactions
involving Mn and Fe indicate that 2 mol of Fe should precipitate
for each mole of Mn that dissolves, even though column
experiments by Stollenwerk gave the ratio of 7 to 1 (Eychaner,
1991a). This ratio suggests that (1) Mn-oxide solids are
reduced incongruently to form other Mn minerals, (2) other minerals
or aqueous species might also be reduced, and (3) the atmosphere
may be a significant source of O2 to the plume. The
incongruent dissolution of Mn-oxide solids and potential reduction
of other minerals or aqueous species to balance the redox
budget will be examined by a combination of laboratory
work and geochemical modeling.
Table 1. Chemical analyses of contaminated water in Pinal Creek
basin
[Concentrations in milligrams per liter, except as indicated; °C, degrees
Celsius; µS/cm, micromsiemens per centimeter at 25 degrees Celsius.
Water filtered through 0.45-micrometer filter before analysis except where
indicated. <, less than.]
|
Well 51 |
Well 451 |
Well 503 |
Station 09498400 |
Date sampled |
5-27-92 |
5-28-92 |
5-29-92 |
5-20-92 |
pH |
3.7 |
4.1 |
5.4 |
7.8 |
Temperature (°C) |
19.0 |
19.0 |
18.5 |
20.0 |
Specific conductance (µS/cm) |
3,740 |
3,890 |
3,620 |
3,240 |
Calcium (Ca2+) |
400 |
400 |
520 |
560 |
Magnesium (Mg2+) |
87 |
180 |
220 |
140 |
Bicarbonate (HCO3-) |
0 |
0 |
65 |
130 |
Sulfate (SO42-) |
2,700 |
2,500 |
2,200 |
2,800 |
Copper (Cu2+) |
32 |
15 |
<.020 |
0.006 |
Iron (Fe2+) |
500 |
280 |
.22 |
.01 |
Manganese (Mn2+) |
16 |
72 |
110 |
39 |
Glynn and others (1991) simulated chemical reactions
and transport in the subsurface with the model PHREEQM
and found a tendency for the predicted pH to
be higher than pH measured in the field. Exsolution
of CO2 will be measured at the leading edge of
the acidic part of the plume to determine the
contribution of this process to the pH and redox balance
and ultimately to contaminant transport. Preliminary
gas measurements of CO2 in the unsaturated-zone at
site 500 indicate a diffusion-induced fractionation
of 4.4 per mil for carbon 13C over a
distance of 2.38 m between neutralized water
at the water table and the land surface-the fractionation
expected for a steady-state upward flux of CO2
(Glynn and Busenberg, 1996). Pierre D. Glynn,
(U.S. Geological Survey, National Research Program,
Reston, Virginia), plans to measure exsolution and
estimate the flux of CO2 at or near the
leading edge of the acidic part of the plume.
The testing, verification, and application of coupled geochemical-transport
models also will help answer some of the remaining questions about subsurface-contaminant
transport. General testing of reactive-transport models and the use of sensitivity
analyses to define the most important reactions will be continued by Glynn,
in cooperation with Peter Engesgaard (Technical University, Denmark) and
Kenneth L. Kipp (U.S. Geological Survey, Denver, Colorado). James G. Brown,
(U.S. Geological Survey, Tucson, Arizona), R.L. Bassett (University of Arizona,
Tucson, Arizona), and Glynn plan to use a sensitivity- analysis type of
approach to identify and determine the relative importance of chemical reactions,
dilution, and transport that may control the remediation and chemical recovery
of the regional aquifer. Brown, Bassett, and Glynn also plan to simulate
transient, reactive transport in two dimensions, accounting for redox, adsorption,
dissolution-precipitation, complexation, and hydrolysis.
Ground-Water Hydrology
Adequate simulation of reactive-solute transport through the subsurface
depends on the accuracy of estimating recharge to the aquifer and subsurface-hydrological
transport. D.R. Pool (U.S. Geological Survey, written commun., 1993) estimated
changes in storage and specific yield in the alluvium on the basis of observed
changes in ground-water level and gravity, and plans similar measurements
between Miami Wash and Pinal Creek. Average values of specific yield of
sediments that were saturated by water-level rises are from 0.16 to 0.21.
Larger specific-yield values were associated with sediments containing large
percentages of sand and gravel. Small specific yields were associated with
sediments finer than sand.
Pierre Glynn and Eurybiades Busenberg (U.S. Geological Survey, National
Research Program, Reston, Virginia) measured chlorofluorocarbon (CFC) concentrations
in ground water to estimate ground-water ages in the basin. Preliminary
results using CFC-11 for age-dating indicate that ground water from a depth
of about 18 to 27 m in alluvium upgradient from contamination sources was
recharged in 1979 (Glynn and Busenberg, 1996). Planned additional measurements
of CFC's may indicate if an assumption of steady-state flow for reactive-transport
modeling is valid. Travel-time information derived from these measurements,
combined with the results from geochemical modeling, may provide estimates
of chemical reaction-rates at the site.
Metal Transport in Shallow Ground Water
and Streamflow
The acidic ground-water plume has moved downgradient at a rate of about
0.25 km/yr over the past several decades (Eychaner, 1991a). Higher-than-normal
rainfall in 1991-93 increased ground-water levels by as much as 15 m and
moved the headwaters of perennial flow upvalley. In early 1993, acidic-contaminated
ground water in Miami Wash rose temporarily to the ground surface and discharged
directly to streamflow. The headwater of perennial streamflow has since
reestablished itself 1 km upstream from the location shown in figures 1
and 3. Increased hydraulic gradients and shortened ground-water flow paths
may cause the acidic core of the plume to reach perennial surface water
sooner than expected, depending on the transport of uncontaminated recharge
water and the degree of mixing of uncontaminated recharge water with the
contaminated water in the plume. Chemical mass-balance models and ground-water-flow
modeling, where needed, will be used by Harvey and others to investigate
the consequences of enhanced interaction between ground water and surface
water on mass-flow rates of contaminants out of the basin.
Neutralized contaminated ground water with large Mn concentrations
(table 1, well 503) is being discharged to the perennial reach
of Pinal Creek. High pCO2 and low pO2 of
contaminated ground water is modulated by gas exchange with the
atmosphere after ground water discharges to the stream. Resulting
increases in pH and pe favor Mn oxidation (Eychaner, 1991b).
Concentrations of colloidal Fe and Mn presently in streamflow
are low compared to total concentrations in solution (Harvey
and Fuller, 1996). The absence of high Mn colloid concentrations
suggests that Mn is partitioned to solid phases mostly
on surfaces of streambed solids. Colloidal transport of Fe, Mn,
and other trace metals appears in general to be only a minor
mechanism of metal transport during baseflow in Pinal Creek.
The role of Fe colloids in trace-metal transport will likely
increase if the acidic core of the contaminated plume reaches
Pinal Creek.
The character and chemical composition of manganese precipitates on the
streambed at Pinal Creek vary considerably (Lind and Hem, 1993). J.D. Hem
and C.J. Lind (U.S. Geological Survey, Menlo Park, California) are continuing
their studies of chemical controls on manganese mineral formation at the
site. Mn carbonate precipitation on streambed sediments appears to be favored
initially in upstream reaches; Mn oxide precipitation becomes more important
as pH and availability of dissolved oxygen both increase farther downstream
(Hem and Lind, 1996). Fe and Mn oxides and carbonate minerals on the streambed
scavenge other dissolved trace metals, including nickel (Ni), copper (Cu),
and cobalt (Co), from surface flow. Trace metals have an decreasing affinity
for Fe oxides, Mn oxides, and carbonate minerals, in decreasing order (Lind
and Anderson, 1992). The greater abundance of Mn solid phases compared to
Fe solid phases suggests that trace metal partitioning may be controlled
by reactions with Mn phases. Chris Fuller (U.S. Geological Survey, Menlo
Park, California) is investigating the relative roles of trace-metal coprecipitation
and adsorption on Mn phases on the streambed as mechanisms for attenuation
of dissolved Ni, Cu, and Co in streamflow (Fuller, 1996). The goals are
to determine dominant chemical mechanisms of trace-metal partitioning to
solid phases and to estimate partitioning coefficients for improved hydrochemical
modeling.
Harvey and others (1996) showed that significant partitioning of Mn and
Fe occurs between solid and aqueous phases in shallow ground water before
ground water is discharged into Pinal Creek. A gain in Fe and loss of Mn
from solution occurs in ground water that discharges laterally though the
stream-channel banks at Pinal Creek; in contrast, ground water that discharges
vertically through the creek bottom is unaltered by chemical reaction. A
coupled mass balance for 500 m of stream and the surrounding aquifer in
November 1992 demonstrated the importance of reactions in shallow ground
water on downstream water chemistry in Pinal Creek.
Water exchange between streamflow and streamwater-filled flow paths in
streambed sediments was investigated at Pinal Creek by a bromide-tracer
experiment. Exchange between streamflow and ground water affects the contact
area and contact times of streamflow with streambed solids. Rates of Mn
oxidation and trace-metal sorption on streambed surfaces, therefore, may
be sensitive to surface water-ground water hydrological exchange. Preliminary
data are being used by Harvey, Fuller, and Conklin (University of Arizona,
Tucson, Arizona) to design future experiments at Pinal Creek. Aspects of
identifying the structure of improved hydrochemical models for Pinal Creek
and of estimating model parameters and associated uncertainties are outlined
in Wagner and Harvey (1996).
SUMMARY
Acidic drainage from copper mining has contaminated the regional alluvial
aquifer in the Pinal Creek basin. The acidic contamination is neutralized
along the flowpath by reactions with aquifer materials and native ground
water. Neutralized water discharges in a perennial reach of Pinal Creek;
as the water equilibrates with the atmosphere, pH increases and Mn minerals
precipitate on the streambed.
Ongoing research at the site is focused on ground-water geochemistry,
hydrology, and solute transport and interactions between ground water and
perennial streamflow. Field and laboratory experiments will attempt to identify
the reduction(s) that balance the oxidation of Fe in the aquifer. Two-dimensional
reactive-transport simulations of plume evolution and remediation will identify
the important processes and reactions that produced the observed conditions
in the plume.
Measurements of changes in gravity and levels of ground water at project
wells may provide refined estimates of aquifer specific yield for use in
reactive-transport simulations. Estimates of ground-water ages obtained
by CFC age-dating techniques may provide improved estimates of ground-water
traveltimes and, when combined with results from geochemical modeling, provide
estimates of geochemical-reaction rates at the site.
Field, laboratory, and modeling studies of contaminant transport
are being conducted to improve understanding of the controls
on metal transport in near-stream ground water and
in streamflow. Previous work showed that Mn precipitates are
variable and complex, and that precipitation is generally controlled
by CO2 outgassing and reareation, which affects pH
and the solubility of Mn solids. Mechanisms of trace-metal scavenging
by Mn carbonates, Mn oxides, and Fe oxides are being investigated,
and partitioning coefficients are being estimated to improve
hydrogeochemical modeling. Tracer experimentation and a coupled
aquifer-stream mass-balance approach are being used to
determine transport components and to constrain net effective rates
of solid-aqueous partitioning of Mn, Fe, Ni, Cu and Co in the
system.
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