United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-86/042 June 1986
Project Summary
Dissolved Oxygen and
Oxidation-Reduction Potentials
in Ground Water
Thomas R. Holm, Gregory K. George, and Michael J. Barcelona
Water samples were collected from
various depths in a pristine sand and gravel
water table aquifer at monthly intervals
over a period of one year. Dissolved ox-
ygen concentrations were near saturation
9 feet below the water table and decreas-
ed to nearly zero at 78 feet below the
water table. Changes in the Eh values were
consistent with changes in the dissolved
oxygen concentrations. Hydrogen perox-
ide was detected in nanomolar concentra-
tions at all depths, but not on every
sampling run. Of all oxidation-reduction
potentials calculated from substituting
analytical results into the Nemst equation,
only the Fe3+/Fe2+ couple in the
deepest well agreed with the measured Eh
within 50 millivolts. For the O2/H202 and
NO3 /NH/ couples the range of poten-
tials calculated from one year's data
overlapped the range of measured Eh
values, so there was some agreement on
the average. However, for a given sampl-
ing run, the various calculated potentials
spanned several hundred millivolts, which
means that the activity ratios for the
various couples differed by many orders
of magnitude (i.e. the system was not in
redox equilibrium). The concentration pro-
files of many solutes, including dissolved
oxygen, suggest mixing of shallow and
deep ground waters. The observed con-
centration profiles were relatively constant
over the duration of the sampling.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental Re-
search Laboratory, Ada, OK, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report order-
ing information at back).
Introduction
If ground-water quality is defined in
terms of the concentrations and properties
of substances dissolved in the water, then
the presence or absence of dissolved
oxygen affects many aspects of ground-
water quality. The mobilities, reactivities,
and toxicities of many elements depend
on their oxidation states. Microbial popula-
tions are distinctly different in oxic and
anoxic waters and, therefore, the rates of
microbial degradation of organic com-
pounds are also quite different. Thus,
dissolved oxygen affects both the geo-
chemical and microbial processes which
are likely to influence water quality. In this
project we have begun the chemical char-
acterization of a shallow aquifer with
dissolved oxygen concentrations ranging
from near-saturation to near-anoxic, con-
ditions that are typical of many aquifers
which are susceptible to contamination.
The results of this study should, therefore,
be useful to microbiologists, geochemists,
and engineers studying ground-water
quality and the fate of contaminants in
ground waters.
Aquifer contamination may be partially
mitigated by natural physical processes,
but the time scale for flushing a conser-
vative substance from an aquifer is propor-
tional to the hydraulic residence time,
which can be hundreds of years in some
aquifers, and the time required for the
flushing of a hydrophobia contaminant
that is strongly sorbed by the aquifer solids
may be much longer than the residence
time. Clearly, physical processes for
aquifer self-purification can be very slow.
However, natural chemical and biological
processes, many of which are redox-
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sensitive, can also contribute to aquifer
self-purification.
In view of the importance of the redox
status of ground water and other natural
waters it is desirable to have a convenient,
reliable redox indicator. The potential of a
platinum electrode, or Eh, is often used as
a geochemical redox indicator because the
Eh of a water sample can be readily mea-
sured and, in certain aquatic environ-
ments, the Eh can be quantitatively related
to chemical speciation. However, using the
Eh as a redox potential for an entire
aquatic system is only meaningful in
systems that are in chemical equilibrium
and that contain electroactive solutes at
approximately millimolar concentrations.
Most ground waters are not in equilibrium,
and the concentrations of electroactive
species are often less than one micromolat
Despite its limitations, the Eh can be a
useful qualitiative indicator of the redox
status. Consistently measured or calcu-
lated Eh values can indicate relative redox
levels in a single system, such as suc-
cessive depths in sediment or zones in
flow systems with similar concentrations
of major electrolyte ions and electroactive
minor ions. On the other hand, comparison
of the Eh values of very different waters,
e.g., of ground waters that are well-poised
and anoxic with those that are poorly-
poised and oxic, is probably not mean-
ingful. A more reliable characterization of
the redox level of a natural water involves
a complete chemical analysis, including all
redox-active species as well as Eh. This
approach has been taken in this project.
Materials and Methods
Four monitoring wells were installed in
the Sand Ridge State Forest near Havana,
Illinois, in the Havana Lowlands water table
aquifer. Three wells were completed at
depths of 35, 50, and 65 feet in October,
1984, and the fourth well was completed
at 104 feet in September, 1985. A deter-
gent- and acid-cleaned Teflon positive-
displacement bladder pump and Teflon
tubing were installed in each well.
For pH, Eh, specific conductance, and
temperature measurements, ground water
was pumped from the well and through a
flow-cell. When all parameters reached
stable values (i.e., less than 0.05 pH units,
10 mV, or 10 pS cm"1 change in suc-
cessive casing volumes) the water that
was being pumped from that point on was
considered to be representative of the
aquifer water. These stable values were
recorded.
After well flushing, unfiltered water
samples were collected for organic car-
bon, ammonia, hydrogen peroxide,
hydrogen sulfide, and dissolved oxygen
determinations. Mass transfer calculations
indicated that oxygen diffusing through
the sampling tubing probably did not ap-
preciably contaminate any of the samples
from the 35-, 50-, or 65-foot wells but
may have contaminated the samples from
the 104-foot well.
After collection of the unfiltered
samples, filtered samples were collected
for determination of alkalinity, metals, and
anions. In-line filtration was used to pre-
vent air contact. Standard preservatives
and holding times were used to preserve
and store the water samples. Field blanks
were collected by filtering deionized water.
Determinations of the unstable solutes,
dissolved oxygen (DO), alkalinity, and
hydrogen peroxide were performed im-
mediately after sample collection. In water
samples from 35, 50, and 65 feet, DO was
determined using the azide modification
of the Winkler method. In samples from
104 feet a field colorimetric method
(Chemetrics Inc., Calverton, VA) was used
to determine DO.
Hydrogen peroxide concentrations were
determined using the fluorimetric scopo-
letin/horseradish peroxidase method. Rea-
gent blanks were run substituting distill-
ed, deionized, and freshly distilled water
for ground water. Hydrogen peroxide con-
centrations in the blanks were equal to or
up to three times greater than those of the
samples for many sampling runs. Varying
the reagent concentrations had no effect
on the blanks. The H202 signal in the
blanks was eliminated by addition of the
enzyme prior to the scopoletin. Storing
distilled water did not increase the concen-
trations of H2O2 over those in freshly
distilled water. The stoichiometry of the
reaction in distilled water blanks was close
to the accepted value of 1.0, which sug-
gests that side reactions were not respon-
sible for fluorescence quenching. There-
fore, the measured quenching in the
reagent blanks was due to H2O2 present
in the distilled water and was not intro-
duced with the reagents or produced by
the analytical method.
Blanks have not been addressed in the
literature on this method. However, a num-
ber of authors have suggested that H2O2
can be produced in deionized distilled
water as a result of microbial activity,
photochemical reactions, or by sparging
with air. It is likely that hydrogen peroxide
is a normal trace component of our distil-
led water, produced in the distillation pro-
cess or carried over from the feed water
and varying with inputs of radiation and
dissolved oxygen. Therefore, while a large >
fraction of our H2O2 determinations do '
not satisfy the conventional analytical
criterion of low blanks, we feel that we
have positively identified H202 in ground
waters from the Sand Ridge site.
Manual colorimetric methods were used
to determine ammonia in all samples and
iron in samples from 35, 50, and 65 feet.
Manganese determinations in the 35, 50,
and 65-foot samples were performed
using cathodic stripping and anodic strip-
ping voltammetry. For the 9/19/85 and
10/17/85 samples, Fe and Mn were deter-
mined by atomic absorption spectrophoto-
metry. Automated adaptations of standard
colorimetric methods were used to deter-
mine orthoposphate, dissolved silica, sul-
fate, nitrite, and nitrate. Chloride was
determined by automated potentiometric
titration. Volatile and nonvolatile organic
carbon fractions were determined by wet
oxidation and infrared CO2 detection.
Results and Discussion
The profiles of DO and Eh are shown in
Figure 1. The shallowest ground waters
were nearly saturated with DO and the
deepest waters were nearly anoxic. The
measured DO concentration of 0.2 mg
L "1 in the 104-foot samples may have
been an artifact of sampling (see below).
The highest Eh values were measured in
the shallow oxic ground waters and the
lowest Eh values were measured in the
nearly anoxic deep waters. Dissolved
oxygen concentrations fluctuated up to
1.5 mg L ~1 between some sampling runs
at all depths (Figure 2), which is greater
than the uncertainty in the Winkler titra-
tion; therefore the observed fluctuations
were not an artifact of the analyses. Most
of the Eh values in the three shallow wells
varied between +330 and +430 mV
(Figure 3) with a fairly constant difference
of less than 50 mV between the Eh values
at 35 and 65 feet.
Iron and Mn concentrations were below
the detection limits of approximately 1 ng
L~1 in all samples from the 35-, 50-, and
65-foot wells. In the samples from the
104-foot well, Fe and Mn concentrations
were approximately 0.5 and 0.2 mg L"1,
respectively. Assuming that most of the
dissolved Fe is in the ferrous form and
using the published rate law for ferrous
iron oxidation by dissolved oxygen, the
half-life of ferrous iron in the aquifer
should be less than two hours, which is
much shorter than the hydraulic residence
time. Therefore, the DO measured in the
104-foot samples was probably the result
of oxygen contamination during sampling.
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0
-10-
-20-
-30-
-40-
-50-
-70 -
-50-
-50 -
-100 -
-110
4 6
+ Dissolved Oxygen m/L
10
12
0.2
0.4
0.6
OEhfV)
Figure 1.
Concentrations of dissolved oxygen and Eh values in ground waters of Havana
lowlands aquifer, 9/19/85.
11
10 .
9 -
8 -
I"
100
200
300
Time (days)
Figure 2.
Time series graph of dissolved oxygen concentrations in ground waters of Havana
lowlands. D 35, + 50, O 65 feet.
The time series graph of hydrogen per-
oxide is shown in Figure 4. The highest
concentrations were measured on 1/17/85.
The 2/19/85 concentrations were some-
what lower than those observed on 1/17.
There was no consistent profile for H2O2.
The 02/H202 couple may be a signifi-
cant participant in the redox chemistry of
certain natural waters, but its influence
has not been studied. In the Sand Ridge
ground waters, H202 concentrations may
be comparable to those of other electroac-
tive solutes. For example, dissolved Fe con-
centrations were below the detection limit
of approximately 2 \HQ L~1, or 36 nM,
which is similar to H2O2 concentrations
measured on some sampling runs. In prin-
ciple, the oxidizing power of dissolved
oxgen can be controlled by the kinetics of
its reduction, if the rate of reduction of
H202 is slower than the rate of its forma-
tion, then the potential is effectively that
of O2/H202, and O2becomes a weaker
oxidant than if it were directly reduced
to H20.
There is now evidence that H2O2 is
formed and accumulates, in the photooxi-
dation of organic compounds in surface
and ground waters. However, the presence
of H202 in untreated ground water has
not previously been reported. Since H202
concentrations in precipitation are fre-
quently orders of magnitude greater than
in ground water due to atmospheric
photoproduction, rain water may also be
a source of peroxide in ground water (i.e.
relict H202 from recharge). Despite the
wide-ranging sampling and detailed
analysis of H202 photoaccumulation
rates, no measurements of H2O2 in
ground waters prior to irradiation have
been reported. The few reported ground
water H202 measurements were below
detection limits.
The concentration profiles of nitrate and
orthophosphate (Figure 5) were represen-
tative of most sampling runs. Nitrate con-
centrations decreased with depth, while
phosphate concentrations increased. The
decrease to a nearly undetectable nitrate
concentration in the 104-foot well is con-
sistent with the sharp decrease in DO and
Eh between 65 and 104 feet. However,
reduced nitrogen species, nitrite or am-
monia, were not detected. Nitrate concen-
trations tended to decrease with time
(Figure 6), while phosphate concentra-
tions showed no temporal trend.
The concentration profiles of DO,
NO3", and, possibly, o-POA had two
inflection points. Such profiles can be
produced by diffusion/dispersion. The con-
ceptual model is that upgradient from the
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600
0 100 200 300
Time (days)
D35 +50 O 65 A 104
Figure 3. Time series graph of Eh values of ground waters of Havana lowlands. D 35, + 50,
65, A 104 feet.
roo
200
Time (days)
300
400
Figure 4. Time series graph of hydrogen peroxide concentrations in ground waters of Havana
lowlands. D 35, + 50, 65 feet.
monitoring wells' two layers of ground nitrate, are separated by a thin transitional
water, i.e. oxic recharge waters that leach region. Diffusion and dispersion causes
nitrate from fertilized fields and deep the transitional layer to broaden and pro-
anoxic ground waters that are low in duces the observed profile. It may be
possible to estimate the average vertical
dispersion coefficient in the aquifer by
modeling the concentration profiles of DO,
nitrate, and other solutes.
Calculating Redox Potentials
The redox couples considered were
O2/H2O2, Fe(lll)/Fe(ll), NO3VNH4+
Mn(lll)/Mn(ll) and S042VHS~. For each
couple a range of potentials was
calculated by substituting analytical values
of the various solutes into the Nernst
equation. These calculated potentials
were then compared to the observed
range of Eh values. As an example, the
maximum calculated 02/H202 potential
was for water samples collected from the
35-foot well with a DO of 9 mg L-1 and
pH of 7.5. The minimum potential was for
65-foot samples with DO of 3 mg L~1
and pH of 8. The DO concentrations mea-
sured in the 104-foot well were not con-
sidered because the values were question-
able. A range of H2O2 concentrations of
0.1-10.0 nM was assumed. The calculated
DO/H2O2 potentials ranged from +430 to
+ 535 mV, which overlaps the observed
range of most of the Eh measurements for
the 35- to 65-foot ground waters.
For the N03~/NH4+ couple only
nitrate was detected. As a result, the
Nernst equation becomes an inequality
(equation 1)
(E(mV) > 884 + 7.08 (log (N03~) -
10 pH - log DC) (1)
where the standard potential at 12°C is
884 mV, braces indicate ionic activities
and DL is the detection limit for ammonia
nitrogen (O.O5 mg L"). The calculated
lower bound for NO3VNH4+ potentials
was +324 mV, which is consistent with
the observed range of Eh values. The
calculated potentials would fall in the
observed Eh range if the NH4+ concen-
tration were less than 7 ng L"1. In order
for NO3"/NH4+ potential to be consistent
with the observed Eh, NO3~ must be
below detection and NH/ must be de-
tected. However, NH/ was not detected.
Thus, the NO3VNH4+couple was appar-
ently not in equilibrium in the deep
waters.
Iron and manganese were below detec-
tion in all samples collected from the 35-,
50-, and 65-foot wells. However, hydrous
oxides of Fe and Mn were extracted from
aquifer and sand samples. Thus, lower
bounds for calculated potentials of
couples involving these metals were
calculated assuming equilibrium with the
oxides. For the couple Fe(OH)3/Fe2+ the
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0
-10
-20
-30
-40
| -60
-80 -
-90-
-100 -
-110
0.00 0.02 0.04 0.06
Concentration (mM)
+ Phosphate* 100
0.08
0.10
D Nitrate
Figure 5. Profile of nitrate and orthophosphate concentrations in ground waters of Havana
lowlands. 9/19/8S. D nitrate, + orthophosphate.
100
200
Time (days)
300
Figure 6.
Time series graph of nitrate concentrations in ground waters of Havana lowlands.
D 35, + 50, 65 feet.
Nernst equation becomes the inequality
given by equation 2
E(mV) > 756 + 56.6
(4.88 - 3 pH - log DL) (2)
where a detection limit of 1 ^g L 1 is
assumed for Fe determination by the fer-
rozine method, all soluble Fe is assumed
to be Fe2+, and 4.88 is the log of the
solubility product of Fe(OH)3. (Note that
the only variable in equations ispH.)The
lower bound for the Fe(OH)3/Fe2+ poten-
tial was estimated to be +155 mV, which
is not close to but is consistent with the
observed Eh values. The calculated
Fe(OH)3-Fe3+ potential would fall in the
observed Eh range if the Fe2+ concentra-
tions were less than 20 ng L"1, and Fe3+
levels were in equilibrium with Fe(OH)3
as assumed. The calculated Fe(OH)3/Fe2+
potential in the 104-foot ground waters
was +38 mV. An uncertainty in the value
of the solubility product of Fe(OH)3 of one
logarithmic unit, a pH reading that was
high by 0.33 pH units, or a combination of
smaller uncertainties in these parameters
could account for the disagreement be-
tween the calculated and measured po-
tentials. The solubility product determined
at 25°C was used for the potential
calculation because the enthalpy change
of the reaction was not listed, which
precluded a temperature correction. Thus,
the calculated Fe(OH)3/Fe2+ potential
agrees reasonably well with Eh.
The lower bound for calculated MnOOH
(Manganite)/Mn2+potentials in the 35- to
65-foot ground waters was calculated as
for Fe(OH)3/Fe2+. The resulting value of
+570 mV is greater than all but two
extreme Eh measurements. In order for
the calculated Mn potential to fall in the
measured Eh range the Mn2* concentra-
tion would have to be greater than 40 mg
L~1, which would have been easily de-
tectable. Assuming the presence of
MnOOH at 104 feet, the calculated
MnOOH/MN2+ potential is +474 mV,
which is much higher than the observed
value of +100 mV. Apparently the
MnOOH-Mn2+couple is not at equilibrium
in the aquifer.
Measured and calculated potentials are
compared in Figure 7. The best agreement
between calculated and measured poten-
tials was for Fe in the nearly anoxic
104-foot well. The potentials of the other
couples differed from the Eh by amounts
that were not possible to explain by ex-
perimental uncertainty. In the shallower,
oxic wells, the range of potentials calcu-
lated for the O2-H202 couple overlapped
the range of Eh values measured over one
year. However, for individual samples, the
differences between calculated and mea-
sured potentials correspond to differences
in the ratio of O2 to H202 concentrations
of many orders of magnitude. Similar lack
of agreement between measured poten-
tials and potentials calculated from
analytical data has been noted in an ex-
amination of published ground water data
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600
500
400
300
200
roo
Eh 02/H20
NOl/NHl
(35-65) \f35-65)
— Fe(OH)3/Fe**
Figure 7.
(35-65)
JL
Eh
-(104)
FefOHk/Fe*
-(104)
Comparison of Eh values and
calculated redox potentials of
ground waters of Havana low-
lands ground waters.
water system is not in redox equilibrium
at any depth sampled because calculated
redox potentials for several couples differ
by up to hundreds of millivolts, corres-
ponding to differences in activity quotients
of orders of magnitude. The concentration
profile of DO is fairly constant and is con-
sistent with mixing of layers of oxic and
anoxic water. Modeling the DO profile may
help understand mixing in the aquifer.
Recommendations
The field studies should be expanded to
include very anoxic ground waters in a
similar hydrologic environment, preferably
in the same aquifer. Solid oxidants and
reductants in the aquifer should be studied
to determine their role in redox processes.
Geochemical extractions can give an esti-
mate of the aquifer oxidizing or reducing
capacity. Redox titrations of aquifer sed-
iments can estimate the redox buffer cap-
acity of the aquifer system. Respirometry
experiments may estimate kinetics of
aquifer redox processes. Characterization
of the organic matter in ground water is
essential to understanding aquifer redox
processes in both oxic and anoxic waters.
This characterization should include com-
plexation of electroactive metals, which
may influence measured Eh values, and
molecular weight and functional group
determinations, which can indicate poten-
tial substrates for microbial respiration.
(Linberg, R.D. and D.D. Runnels. 1984.
Science 225:925-927). In certain special
situations, eg. anoxic waters at low pH
values, measured Eh values have been
found to correspond to speciation of one
or more elements. However, in oxic waters
at near neutral pH, the measured Eh is
usually far from that calculated from
elemental speciation. The probable reason
for the lack of agreement between
measured and calculated Eh is that the
system is not in redox equilibrium.
Conclusions
The relative redox status of ground
water at the Sand Ridge site is related to
the dissolved oxygen concentration as
shown by Eh measurements and chemical
speciation calculations. The Eh qualitative-
ly indicates the relative redox status of the
ground waters, i.e. the lower the DO the
lower the Eh. In the deepest waters sam-
pled, the Eh is quantitatively related to Fe
speciation. However, for other redox
couples and at other depths, there is no
quantitative relationship between redox
speciation and Eh. The aquifer-ground
U. S. GOVERNMENT PRINTING OFFICE:1986/646-l 16/20848
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Thomas R. Holm. Gregory K. George, and Michael J. Barcelona are with Illinois
State Water Survey. Champaign. IL 61820-7407.
Bert E. Bledsoe is the EPA Project Officer (see below).
The complete report, entitled "Dissolved Oxygen and Oxidation-Reduction
Potentials in Ground Water,"{Order No. PB 86-179 678/AS; Cost: $11.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
P.O.Box 1198
Ada, OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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