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
     BULK RATE
POSTAGE & FEES PAII
        EPA
   PERMIT No. G-35
Official Business
Penalty for Private Use S300

EPA/600/S2-86/042
              0000329   PS

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