ivFPA
I
United States i
Environmental Protection
Agency t
Oxidation of Arsenic (III) by
Aeration and Storage
As (III)
As(V)
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EPA/600/R-01/102
January 2002
Oxidation of Arsenic (III) by Aeration and
Storage
by
Jerry D. Lowry and Sylvia B. Lowry
Lowry Environmental Engineering, Inc.
Blue Hill, ME 04614
Contract No. 8C-R433-NTSX
Project Officer
Thomas J. Sorg
Water Supply and Water Resources Division
National Risk Management Research Laboratory
Cincinnati, OH 45268
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
Recycled/Recyclable
Printed with vegetable-based ink on
paper that contains a minimum of
50% p'ost-consumer fiber content
processed chlorine free.
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Disclaimer
The information in this document has been funded wholly or in part by the U.S. Environmental Protection
Agency. It has been subjected to the Agency's peer and administrative review, and it has been approved
for publication as an EPA document. Mention of trade names or commercial products is for explanatory
purpose only, and does not constitute endorsement or recommendation for use.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's land, air,
and water resources. Under a mandate of national environmental laws, the Agency strives to formulate
and implement actions leading to a compatible balance between human activities and the ability of natural
systems to support and nurture life. To meet this mandate, EPA's research program is providing data and
technical support for solving environmental problems today and building a science knowledge base
necessary to mange our ecological resources wisely, understand how pollutants affect our health, and
prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for investigation of
technological and management approaches for preventing and reducing risks from pollution that
threatens human health and the environment. The focus of the Laboratory's research program is on
methods for the prevention and control of pollution to air, land, and water and subsurface resources;
protection of water quality in public water systems; remediation of contaminated sites, sediments and
groundwater; prevention and control of indoor air pollution; and restoration of ecosystems. NRMRL
collaborates with both public and private sector partners to foster technologies that reduce the cost of
compliance and to anticipate emerging problems. . NRMRL's research provides solutions to
environmental problems by: developing and promoting technologies that protect and improve the
environment; advancing scientific and engineering information to support regulatory and policy decisions;
and providing the technical support and information transfer to ensure implementation of environmental
regulations and strategies at the national, state, and community levels.
This publication has been produced as part of the Laboratory's strategic long-term research plan. It is
published and made available by EPA's Office of Research and Development to assist the user
community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
III
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Abstract
A study of the effects of aeration and storage on the oxidation of arsenic(lll) was undertaken at three
utilities in the U.S. to establish the engineering significance of aeration as a potential pre-treatment
method for arsenic removal. Aeration has been referred to in the literature as a possible useful pre-
treatment method to ensure that arsenic in is the arsenic(V) state before subsequent removal by any of
several treatment processes. Since aeration a common process for treating groundwater for iron
oxidation, radon, volatile organics, carbon dioxide, and hydrogen sulfide, it is reasonable to investigate its
effectiveness for arsenic(lll) oxidation.
The results of this study clearly establish that aeration and aerobic storage do not oxidize arsenic(lll).
The major conclusion is that aeration is not effective for this purpose and should not be relied upon or
expected to contribute to the oxidation of arsenic(lll). One of the test sites in this study clearly showed
that arsenic(lll) is significantly removed by the oxidation and precipitation of iron, but this should not be
attributed to an oxidation of arsenic(lll) to arsenic(V) by dissolved oxygen. Past research has established
that iron precipitation can be partially effective for the adsorptive removal of arsenic(lll), and this is the
likely explanation for the apparent drop in arsenic(lll) at the site that had high iron.
The effect of iron precipitation on the removal of arsenic was also present in the long term storage of
aerated water in this study. When all of the iron (initial iron at = 2.7 mg/L) precipitated from the quiescent
storage water, the remaining aqueous total arsenic was entirely dissolved and in the arsenic(V) state.
The aqueous arsenic(lll) was below detection and apparently completely removed or converted by the
insoluble iron. Even in this case it is doubtful if DO was responsible for any oxidation of arsenic(lll),
because the loss directly correlated to the loss of iron precipitate and no other instance of arsenic(lll)
oxidation occurred at the other sites. In summary, the data supported the fact that iron is extremely
important in the removal of arsenic(lll), but did not support the idea that arsenic(lll) is oxidized by
aeration. This is true at least for the conditions used in this study.
While the subtleties of the results are interesting, especially for the site with high iron, it is important to
emphasize the original objective of this study, which was to establish if typical aeration and storage
methods could oxidize arsenic(lll). Based upon the results of this study, it is concluded that aeration does
not oxidize arsenic(lll) and that subsequent storage for up to five days does not result in arsenic(lll)
oxidation. Dissolved oxygen should not be considered as a candidate for arsenic(lll) oxidation; however,
aeration will continue to be considered a very effective process for the oxidation of iron. In that way,
aeration can be said to be effective in bringing about the removal of As via the oxidative precipitation of
iron.
IV
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Contents
Foreword '"
Abstract... - iv
Tables v|
Figures vii
Abbreviations v'ii
Acknowledgements ix
1.0 Introduction 1
1.1 Background 1
1.2 Objectives 2
2.0 Materials and Procedures 3
2.1 Selection of Water Supplies 3
2.2 Aeration Systems 5
2.3 Storage Container 7
2.4 Water Sampling & Data Collection 8
2.5 Analytical Procedures 9
3.0 Test Results 12
3.1 Northeast Site 12
3.2 Midwest Site 15
3.3 Southwest Site 22
4.0 Discussion and Conclusions 27
4.1 Oxidation of As(lll) by Dissolved Oxygen 27
4.2 Sample Analysis Problems 28
4.3 Conclusions 28
5.0 References 29
6.0 Appendices 30
Appendix A: Northeast Site Data 31
Appendix B: Midwest Site Data 39
Appendix C: Southwest Site Data 42
v
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Tables
Table 2-1 Raw Water Quality for the Northeast Site 3
Table 2-2 Raw Water Quality for the Midwest Site 4
Table 2-3 Raw Water Quality for the Southwest Site 5
Table 2-4 Summary of Sampling for Aeration and Storage Testing 10
Table 2-5 Analytical Method Summary 10
VI
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Figures
Figure 2-1 Spray Aeration nozzle spraying water at 0.5 Us (8 gpm) 6
Figure 2-2 Aeration Systems (bubble - center; tower - left; spray - right) 6
Figure 2-3 Forced-Draft Tray Aerators - 7
Figure 3-1 As Speciation Test Results for Well Water, Northeast Site 13
Figure 3-2 Avg. Well & Aeration DO Concentrations (2 Test Runs), Northeast Site 13
Figure 3-3 As Speciation Test Results for Bubble Aeration (Day 1), Northeast Site 14
Figure 3-4 Avg. As Results for the Well & Aerated Samples (Run 1 & 2), Northeast Site.. 14
Figure 3-5 As Speciation Test Results for the Storage Water, Northeast Site 15
Figure 3-6 As Speciation Test Results for the Blended Well Water, Midwest Site 16
Figure 3-7 Avg. Well & Aeration DO Concentrations (2 Runs), Midwest Site 17
Figure 3-8 Avg. Fe Test Results for Well & Aerated Water (Run 1), Midwest Site 17
Figure 3-9 Avg. Mn Test Results for Well & Aerated Water (Run 1), Midwest Site 18
Figure 3-10 pH Results for Well & Aerated Samples (Run 1), Midwest Site 18
Figure 3-11 Fe (total) Results for^Packed Tower (Run 1 & 2) Samples, Midwest Site 19
Figure 3-12 As Speciation Test Results for FDA Samples (Run 1), Midwest Site 20
Figure 3-13 As Speciation Results for Well & Aerated Samples (Run 1), Midwest Site.. 21
Figure 3-14 As Speciation Test Results for the Storage Samples, Midwest Site 21
Figure 3-15 Fe & Mn Concentrations for Storage Samples, Midwest Site 22
Figure 3-16 Avg. Well & Aeration DO Concentrations (2 Test Runs), Southwest Site 23
Figure 3-17 pH Results for Well & Aerated Samples (Run 1), Southwest Site 23
Figure 3-18 As Speciation Test Results for Well Samples, Southwest Site 24
Figure 3-19 As Speciation Test Results for Spray Aeration (Run 2), Southwest Site 24
Figure 3-20 Average As Results for the Well & Aerated Samples, Southwest Site 25
Figure 3-21 As Speciation Test Results for Storage Water Samples, Southwest Site 26
VII
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AA
As
A/W
BAT
BOSC
DO
EPA
gpd
HPC
IX
MCL
MHETL
MF
RO
SDWA
TOG
Abbreviations
activated alumina
arsenic
air to water ratio
best available technology
Board of Scientific Counselors
dissolved oxygen
U.S. Environmental Protection Agency
gallons per day
heterotrophic plate count
ion exchange
maximum contaminant level
Maine Health and Environmental Testing Laboratory
coagulation microfiltration
reverse osmosis
Safe Drinking Water Act
total organic carbon
Vlll
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Acknowledgements
The authors wish to extend their appreciation to the owner of the Sandy Stream community water system,
the City of Albuquerque Water Department, and the owners of the Midwest site water utility. All of the
personnel were extremely helpful. They are also grateful to Thomas Sorg who provided important review
and editorial commentary. Finally, the authors recognize the laboratory personnel at the Maine Health
and Environmental Laboratory for their extraordinary effort to complete the sample analyses for this
project.
IX
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1.0 Introduction
1.1 Background
The 1996 Safe Drinking Water Act (SDWA) amendments required EPA to propose a revised arsenic MCL
bv January 2000 and to finalize it by January, 2001. The amendments also required that EPA develop
an arsenic (As) research strategy to support the revised MCL, and a .draft of that plan was prepared in
December 1996. The plan identifies the research needed to revise the MCL, and the technologies that
are available or anticipated for the removal of As. On June 20, 2000, the EPA published in the Federa
Register a notice of proposed rule making to lower the MCL to 0.005 mg/L. Comments were also sought
on 3 10 and 20 M9/L limits. On Monday, January 22, 2001, the Final As Rule was published in the
Federal Register. On May 22, 2001, the EPA extended the effective date for the Arsenic Rule from May
22 2001 to February 22, 2002. The effective date for the final arsenic regulation was previously delayed
for 60 days on March 23, 2001, to May 22, 2001. The current standard of 50 ug/L remains the applicable
arsenic drinking water standard until the 2006 compliance date for the January 2001 final rule. It is
expected that the revised As MCL will be significantly lower than the current 50 ug/L.
Arsenic can be found in drinking water supplies at concentrations ranging from a few Mg/L to several
ma/L Arsenic in water can occur in four oxidation states; however, it is normally found as an anion with
acid characteristics in only the trivalent (arsenite) and pentavalent (arsenate) forms. These two oxidation
states are referred to as As(lll) and As(V). A given- groundwater may have As(lll) and or As(V),
deoendinq upon the specific oxidation/reduction characteristics and pH of the water. There are two
primary reasons that the oxidation state of As is important: 1) As(lll) is a greater health concern compared
to As(V), and 2) treatment process efficiency is less for As(lll) than for As(V).
The removal of As from drinking water has been studied in the laboratory and in the field. There are
existing As removal plants that routinely remove As to low levels, and As(V) is relatively easy to remove
by a variety of processes, including ion exchange (IX), reverse osmosis (RO), coagulation + microfiltration
(MF), conventional coagulation, iron precipitation/filtration, lime softening, and activated alumina (AA)_
None of these treatment processes are reliably effective for the removal of As(lll). Oxidation is required
to convert As(lll) to As(V), if it is to be effectively removed by any of the processes listed above.
Very little data exist on the effectiveness of various processes to oxidize As(lll) to As(V) One of the
major recommendations of an EPA Board of Scientific Counselors (BOSC) rev.ew of the EPA dra
research plan was that the Agency conduct a specific research task on the methods for oxidation of As(lll)
to As(V) In response to this recommendation, EPA funded two research projects on the oxidation of
As(lll) This research project specifically focuses on one aspect of As(lll) oxidation, namely the degree of
oxidation by dissolved oxygen that may occur during aeration and storage A second project .s currency
studying the effectiveness of seven other potential oxidation methods. As(lll) can be easily oxidized by
contacting the water with a strong oxidant such as chlorine, ozone, or potassium permanganate Other
methods using hydrogen peroxide or oxygen (via aeration) have been mentioned as possibilities bu
definitive research has not been conducted to date. During the 1994 EPA Arsemc Treatrnenl.Workshop
it was concluded that "aeration has been reported in a few instances to oxidize As(lll) to As(V); however,
the kinetics of aeration with respect to arsenic are poorly understood. Further, it was noted that Aeration
has not been shown to be a reliable and effective process; therefore, further research is warranted.
Despite a lack of evidence regarding the efficacy of As(lll) by aeration, it continues to be considered as a
possibility, in summary, there is no clear evidence that aeration is a potential method for the oxidation of
As(lll) In fact the little data that exist indicate that aeration may not be effective in this application.
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1.2 Objectives
Storage and aeration are common aspects of many drinking water treatment schemes. Storage can be
utilized before and after treatment, so it may be an important step prior to further treatment to remove As
Aeration is commonly used for the removal of carbon dioxide, volatile organic compounds, and hydrogen
suifide. Greater application of aeration will result in the future because aeration is a Best Available
Technology (BAT) for the removal of radon. Radon is due to be regulated in the near future and aeration
is the most cost effective method of removal. Storage/decay may also play a significant role in the
reduction of radon.
The objective of this project was to investigate the effectiveness of storage and aeration to oxidize of
As(lll) to As(V) in drinking water systems. Field testing at three different sites that have groundwater
containing As(lll) was used to study the effects of aeration and storage on the oxidation of As(lll) to
As(V). Each site was visited for several days to test three different types of aeration and to study the
effects of storage over a period of 5 days.
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2.0 Materials and Procedures
2.1 Selection of Water Supplies
Three (3) sites were selected based upon the following criteria:
one site each from the Northeast (low sulfate/TDS), the Midwest (moderate sulfate/TDS), and the
Southwest (high sulfate/TDS) Note that the actual water quality at available sites had varying
amounts of TDS and sulfate ranging from low to moderate.
Total As concentration greater than 20 M9/U with As(III) being at least 50 percent of the total As
present
groundwater with no chemical oxidant added prior to the testing point in the process train
pH in the range of 7 to 9
the total dissolved solids (TDS) in the range of 100 to 1,000 mg/L
the total organic carbon (TOG) in the range of 0.5 to 5.0 mg/L
2.1.1 Northeast Site
The Northeast Site was a water utility in Unity, ME. Previous experience at this site had determined that
the As present was As(III) and that sulfates were relatively low. Further, the As(lll) level was known to be
in the range of 100 ug/L and the supply met all the other criteria for selection. An As speciation test was
conducted to confirm past historical data. A summary of pertinent raw water quality for this site during the
two days of testing is given in Table 2-1.
Table 2-1 . Raw Water Quality for the Northeast Site (No. of samples)
Parameter
Total As, mg/L (5)
Dissolved As, mg/L (5)
As (III), mg/L (5)
Sulfate. mq/L(1)
Alkalinity. ma/L as CaCCX (5)
Dissolved Oxygen, mg/L (4)
Total Iron, mq/L (5)
Total Manganese, mg/L (5)
Calcium, mg/L (5)
Hardness, mq/L as CaCO, ,
Chloride, mq/L (5)
Sodium. mq/L (5)
Total Dissolved Solids, mg/L (5)
Total Organic Carbon, mq/L (5)
Temperature, degrees C (10)
PH(10)
Average Value
0.104
0.104
0.099
12
88
1.3
0.040
0.056
28
87
11
7
126
ND 1
11
7.69
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The well at the Northeast site has a flow rate of 3.78 Us (60 gpm) and there is an ion exchange (IX)
treatment system that has operated for 10 years. A trace of hydrogen sulfide odor is detectable in the
raw water (less than detection at 0.05 mg/L with Hach No. 2238-01 test kit), indicating that the water is in
a reduced state. A single well is pumped directly into hydropneumatic tanks or through the treatment
system to distribution. The treatment system includes an oxidizing filter followed by an IX bed and is
located between the pressure tanks and distribution. All source water for aeration and storage testing
was untreated well water.
2.1.2 Midwest Site
The Midwest site has a multiple well source of groundwater, and has a lime softening system that
removes hardness and Fe. An aeration step prior to lime softening is used to remove CO2 to raise pH,
and oxidize Fe present in the raw water. Historical water quality data for this site had established that a
significant portion of the As present was As(lll). The blend of the wells being pumped determines the level
of As present. The typical level of As(III) in the blend was known to be in the range of 30 - 40 ug/L and
the supply met the other criteria for selection. A summary of pertinent water quality for this site during the
two days of testing is given in Table 2-2. All source water for testing at this site was untreated water
taken from a tap in a main water line before aeration.
Table 2-2. Raw Water Quality for the Midwest Site3 (No. of samples)
Parameter
Total As, mg/L (5)
Dissolved As, mg/L (5)
As (III), mg/L (5)
Sulfate, mg/L (1)
Alkalinity, mg/L as CaCO, (4)
Dissolved Oxygen, mg/L (4)
Total Iron, mg/L (5)
Total Manganese, mg/L (5)
Calcium, mg/L (5)
Hardness, mg/L as CaCO, (5)
Chloride, mg/L (5)
Sodium, mg/L (5)
Total Dissolved Solids, mg/L (5)
Total Organic Carbon, mg/L (5)
Temperature, degrees C (4)
PH (4)
Average Value
0.041
0.036
0.032
0 -4
416
0.3
2.68
0.100
89
340
23
33
460
2.5
22
7.0
a - values vary depending on blend of wells pumped
2.1.3 Southwest Site
A single well (Walker) in the City of Albuquerque, NM water system was selected as the Southwest Site.
Historical water quality data for this well had determined that a significant portion of the As present was
As(lll). Further, the As(lll) level was known to be in the range of 0.015 mg/L and the supply met the other
criteria for selection. A separate As speciation test was done, even though others had speciated this well
supply several times over the previous 8-year period. The result of this speciation test confirmed the
previous values for total As and As(lll), further establishing that the As concentrations are relatively
constant. The levels for total As, dissolved As, and As(lll) were 0.035, 0.033, and 0.015 mg/L,
respectively. A summary of pertinent water quality for this site during the two day testing is given in Table
2-3.
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Total As. ma/L (6)
Dissolved As, mg/L (6)
As (III). ma/L (5)
Sulfate, mq/L (1)
Alkalinity, mq/L as CaCCX (6)
Dissolved Oxygen, mq/L (5)
Total Iron. ma/L (6)
Total Manganese, mq/L (6)
Calcium, mq/L (6)
Hardness. ma/L as CaCO, (6)
Chloride. ma/L (6)
Sodium. ma/L (6)
Total Dissolved Solids, mg/L (6)
Total Organic Carbon, mg/L (6)
Temperature, degrees C (6)
PH (6)
Average Value
0.033
0.015
35
141
5.3
ND 0.02
0.020
39
120
84
78
370
<1
28
7.8
Walker well is pumped directly into the distribution system with no treatment provided. AH source water for
testing was taken from taps in the main water line between the well and the distribution system. The well
was pumped continuously at a flow rate of 150 L/min (2,380 gpm) during the entire test period.
2.2 Aeration Systems
2.2.1 Spray Aeration
A fabricated spray aeration system was used at the Northeastern and Southwest sites and was operated
at 0.35 Us (5.6 gpm) and 0.50 Us (8 gpm), respectively. The system consisted of a hose with a spray
nozzle on the end, and the water was discharged into a 65-gal polyethylene tank having a 10 lid that was
removed during testing. The tank continually drained during testing, thereby serving as an chamber to
contain the water spray. The spray nozzle produced a helical pattern as shown in Figure 2-1. Other than
the flow rate, there are no specific design parameters to describe the spray aeration system.
2.2.2 Staged Bubble Aeration
A three-stage bubble aeration system3 was tested at all three sites. The system consisted of the aeration
vessel with a 1.08 m3/min (38 cfm) regenerative blower6 providing air to three individual aerators, one per
stage Static water depth was 15" and the flow was by gravity through the vessel at a flow of 0.63 Us (10
qpm)) at the three sites. The air to water (AM/) ratio was 25. The AW ratio for = 90 percent oxygen
saturation with this system would be approximately 15 at .10 degrees C, so the system was conservatively
designed with respect to oxygen transfer. The bubble aeration system is shown in Figure 2-2.
a manufactured by Lowry Systems, Inc.
b Model R103 manufactured by Gast, Inc.
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Figure 2-1 - Spray Aeration nozzle spraying water at 0.5 Us (8 gpm)
Figure 2-2 -Aeration Systems (bubble - center;
Tower - left; spray - right)
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2.2.3 Packed Tower Aeration
A 15.2 cm (6") diameter stainless steel counter-current packed tower was fabricated for this project and
tested at all three sites. Overall tower height was 3.65 m (12 ft) and packing depth was 3.05 m (10 ft).
Tower packing0 was the loose-fill type. The flow rate to the tower ranged from 0.50 L/s (8 gpm) to 0.63
Us (10 gpm). The air was supplied by a 1.19 m7min (42 cfm) regenerative blower (same as used for
bubble aeration, at a different pressure/flow). The liquid loading rate was approximately 14.3 L/m3-min
(40.7 gpm/ft2). A photo of the packed tower is shown in Figure 2-2.
2.2.4 Forced-Draft Tray Aeration
A forced-draft tray aerator was used in lieu of the spray system at the Midwest site. This aerator was a
part of the treatment train at this site and presented an opportunity to document a full-scale operation.
The aerator operated at 47.3 - 56.7 L/s (750 - 900 gpm) during our testing period. A photo of the forced
draft aerator is shown in Figure 2-3.
Figure 2-3 - Forced-Draft Tray Aerators
2.3 Storage Container
The storage container0 used at all three locations was a 56.7 L (15 gal) polyethylene carboy. The carboy
was filled with raw water from a hose at the beginning of the site visit and allowed to sit in a quiescent
state for the duration of the time spent at each site. Two different approaches were used in doing the
storage experiments: 1) at the Southwest site, the storage container was filled with unaerated well water,
1" Polypro Tri-Packs manufactured by Tri-Mer Corp.
' Nalgene brand
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and 2) at the Midwest and Northeast sites the water was aerated prior to filling the storage container
aeration. At the end of the test period (varied by site - approx. 48 hrs), enough water was transferred into
3.78 L (1 gal) or 1-L polyethylene containers for later samplings necessary to complete the five-day
storage experiment. These samples were transferred back to the office for processing at the specified
storage times. Samples were taken from a plastic spigot installed in the side of the carboy at 3" off the
bottom.
2.4 Water Sampling & Data Collection
2.4.1 General Field Procedures
In preparation for- a site visit for testing, necessary treatment equipment and supplies were shipped or
transported to the site. Upon arrival, the equipment was set up, the 15 gallon storage volume was
transferred into the storage container (see above), and the "Pre-test" samples were taken and processed.
D.O. and pH were measured and As speciation separations were performed, while the equipment was
flow-calibrated and allowed to run at steady state for approximately 30 to 60 min. Calibration for flow was
done by the bucket and stopwatch method, using a calibrated 5-gal container.
Aeration and storage testing was performed at each site, as described below (Sections 2.4.3 and 2.4.4).
At the end of the on-site aeration and storage testing the equipment was dismantled and packed for
shipment to the next site or to equipment storage. After the Midwest site, it was necessary to clean the
tower packing and the bubble aeration system before it was shipped to the Southwest site. Cleaning was
necessary because of iron precipitation at the Midwest site. Cleaning was accomplished by soaking all of
the packing in a 3:1 solution of water and muriatic acid. The cleaned packing was completely free of all
traces of visible Fe.
2.4.2 Pre and Post Test Well Sampling
In addition to the As testing required to determine speciation, each water source was characterized before
and after each set of concurrent aeration tests for pH, alkalinity, dissolved oxygen, iron (dissolved and
total), manganese (dissolved and total), TOC, calcium, magnesium, total hardness, chloride, sodium,
TDS, and heterotrophic plate count (HPC). A total of 5, 5, and 6 well samples were taken at the
Northeast, Midwest, and Southwest sites, respectively. HPC samples numbered 1, 4, and at the
Northeast, Midwest, and Southwest sites, respectively.
All pre and post test well samples were taken at spigots in the well supply piping. Samples to be
processed on-site were collected in a single 800 ml polyethylene beaker, The pH, Fe and Mn, and As
samples were all taken from the single sample volume. D.O. samples were collected in standard clear
glass BOD bottles. Remaining samples for laboratory analysis were taken in laboratory supplied 250-ml
polyethylene wide-mouth bottles or in 1-L collapsible polyethylene containers.
2.4.3 Aeration Sampling
The aeration testing was designed to be conducted over a 6-hr period, with treated water samples
collected at 30-min intervals. Each water sample was analyzed for pH, As (total, dissolved, and III),
dissolved oxygen, total and dissolved iron, and total and dissolved manganese. Each aeration test was
repeated for a total of two (2) test runs per site per aeration method. Separation of dissolved from total As
and As(III) from As(V) was accomplished with an on-site As speciation procedure, as detailed in Section
2.5.2.
8
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Testing was done for all three aeration methods concurrently, for each day and at each site. Samples
were collected and the As separations, pH, and D.O. were completed within the 30-min interval when
possible For the D.O. measurements, it was often times impossible to complete the settling and titration
steps before the next sampling round. In those cases, additional D.O. bottles were used to collect the
samples on time; however, at certain times it was not possible to do the D.O. every 30 min and some
samples were not taken.
Aeration samples were taken at the discharge end of the process. For the bubble, tower and spray
methods this was at the discharge fitting or hose. For the forced-draft aerator the sample was taken from
a spigot at the bottom of the device. Samples were collected in a single 800 ml polyethylene beaker, The
pH, Fe and Mn, and As samples were all taken from the single sample volume. D.O. samples were
collected in standard clear glass BOD bottles.
The aeration methods afforded different times of actual contact. The bubble aeration gave 3.4 min of
contact at 0.63 L/s (10 gpm), and the tower and spray methods gave several seconds (estimate). The
residence time for the forced-draft aerator is not known.
2.4.4 Storage Sampling
It was intended that storage water samples would be.taken at 1, 6, 12, 24, 36, 48, 72, and 120 hr and
analyzed for arsenic, dissolved oxygen, iron (dissolved and total), manganese (dissolved and total) and
temperature. Actual times varied as dictated by travel schedule and the aeration sample processing
activities. The arsenic analyses included total and dissolved As, A(lll), and As(V). Separation of
dissolved from total As and As(lll) from As(V) was accomplished with an on-site As speciation procedure,
as detailed in Section 2.5.2.
Storage sampling was done without shaking or mixing the contents of the storage container. At the
Midwest site, significant Fe precipitation occurred during storage, which settled in the container and
adhered to the container sides. Mixing was not done to better depict the quiescent storage conditions
typical of actual facilities.
2.4.5 Summary of Sampling
Table 2-4 is a summary of the samples collected for aeration and storage testing.
2.5 Analytical Procedures
2.5.1 Chemical Analyses
Dissolved oxygen, pH, filter separations, and As speciation separations were conducted and recorded on-
site due the instability of these samples over the time required in transport to the laboratory. The HPC
samples were either conducted by a local laboratory near the site (Northeast site), or at the laboratory of
the participating water utility (Midwest and Southwest sites). At the Northeastern site the testing occurred
through a weekend and it was not possible to do all of the HPC testing originally planned because the
hold time was 24-hr maximum and the available laboratories were not open for sample receiving within
the hold time. All of the other samples were analyzed at the Maine Health and Environmental Testing
Laboratory (MHETL). Table 2-5 is a summary of the analytical methods used in this study.
-------
Table 2-4. Summary of Sampling for Aeration and Storaqe Testin
Parameter
Total As
Dissolved As
As(lll)&As(V)
Total Fe
Dissolved Fe
Total Mn
Dissolved Mn
Chloride
TDS
Calcium
Sodium
Hardness
Alkalinity
TOC
HPC
D.O.
PH
Temperature
Notes
Pre & Post
Samples
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
4- 6 sets/ site
Aeration
Samples
V
V
V
V
V
V
V
_
V
V
V
12 sets in 6-hr
& duplicated
per method/site
Storage
Samples
V
V
V
V
V
V
V
_
_
_
_
_
A/-
V
V
8 sets/site over
5-days
Note: Actual time and number of samples taken are in Appendix A, B, & C
Table 2-5. Analytical Method Summary
Analysis
As
Fe
Mn
D.O.
Sulfate
PH
Alk.
Hardness
TOC
Ca
Na
Cl
HPC
TDS
Method
ICAP
AA
ICAP
ICAP
electrometric
titration
ICAP
IR
ICAP
ICAP
filter
Method No.
200.7
200.7
200.7
4500-OC
375.2
2320B
200.7
531 OB
200.7
200.7
325.2
9215
2540C
Reference
EPA°
EPA"
EPAa
EPAa
Std. Methods"
EPA
Std. Methods"
EPA
Std. Methods0
EPA
EPA
EPA
Std. Methods"
Std. Methods"
Detection
Limit, mg/L
0.002
0.002
0.0001
0.50
2
..
0 by definition
0.1
1.0
0.01
0.1
3
20
b - "Standard Methods for the Examination
c - "Standard Methods for the Examination
" d - EP A-XXX
of Water and Wastewater," 19* ed., 1995
of Water and Wastewater," 20* ed., 1998
10
-------
2.5.2 As Speciation
Water samples were speciated for As according to the anion exchange separation method published by
Ficklin4 and further refined by Edwards5, et. al. Purolite A300E resin was converted from the chloride form
to the acetate form using the procedure given by Ficklin. A deviation form the Ficklin method is that the
resin had a mesh size of 16 x 50.
As speciation kits were prepared for each site and transported to the sites by automobile (Northeast) or
by air (Midwest and Southwest). The field kits were prepared and used, following the method
summarized by Batelle6. The syringe filter (Nalgene nylon membrane) rating was 0.45 u. The "total As"
sample was collected in an 800 mL polyethylene beaker. A portion of that volume was syringed and
filtered into the "dissolved As" bottle, and a portion of that volume was poured through the IX resin column
(Bio-Rad chromatography column - 20 ml) and became the "As(lll)" sample.
11
-------
3.0 Test Results
All test results are tabulated in Appendix A, B, and C for the Northeast, Midwest, and Southwest
respectively.
3.1 Northeast Site
3.1.1. Aeration
The bubble aeration, packed tower aeration, and spray aeration systems were operated at 0.63 Us (10 0
gpm), 0.45 Us (7.14 gpm), and 0.35 Us (5.6 gpm), respectively, on both days of testing. Flow was
reasonably steady at ± 5 percent and was checked before and after each 6-hr test period.
Testing on Day 1 was hectic due to this being the first site setup. Testing commenced late in the day and
was terminated at 5 hr, rather than the planed 6 hr, due to the lateness of the day and our proximity to
nearby residents. One (1) DO and several pH measurements were missed, as it was difficult to keep up
with the 30-min sampling schedule. Day 2 proceeded more smoothly as efficiencies were improved and a
better system of sample processing developed. After the experience at this site, we decided to not collect
separate Fe and Mn sample containers, since the total As and dissolved As (bottles were preserved and
would give the total and dissolved Fe/Mn from the results of the ICAP analysis).
The well water quality for this site is in line with what might be expected for a groundwater containing
appreciable As(lll). The DO was low at an average of 1.3 mg/L, with two samples showing approximately
0.75 mg/L. There was a noticeable hydrogen sulfide odor in the raw water, although the hydrogen sulfide
was below a detection limit of 0.05 mg/L. The total As averaged 0.104 mg/L in the five (5) "pre" and
"post" observations. The raw water As levels are summarized in Figure 3-1. Note that the dissolved As
appeared to be slightly greater than the total As; however, the difference between means was not
statistically significant (~ = 0.05). Therefore the total and dissolved As in the well were considered to be
equal. The dissolved As was 94 percent As(lll) and the difference between the mean values for
dissolved and As(lll) was significant (~ = 0.05). This indicated that there was approximately 6 percent
As(V) present in the well water.
The A/W ratio for bubble and tower aeration was 28.4 and 44, respectively. The spray was operated at a
lower flow rate because it was not as an efficient method of aeration. In all three cases, the DO was
raised to near the saturation level of 11.0 mg/L at 11 degrees C. This is illustrated in Figure 3-2. Clearly,
even at. the lower flow rate the spray device was not as efficient is transferring oxygen. The spray would
have been more effective if a fan had been used to continually replenish the air contained in the semi-
open spray chamber. In any event, it can be stated that all three aeration methods provided a maximum
opportunity for the oxidation of As(III), as the DO levels were high and the mixing of air and water was
very good.
An example of results from an aeration test is shown in Figure 3-3, for Run 1 of the bubble aeration unit
The results show that aeration had no effect on the oxidation of As(lll). The aeration processes were
operated in a steady state mode, therefore, one would not expect any temporal change in As levels
Thus, all of the samples would be expected to be duplicates over time, with each subsequent sample
reinforcing the same observation. Relatively constant As(lll) levels in the raw and treated water are
illustrated in Figure 3-3. A statistical analysis of the bubble aeration data for Run 1 showed that there was
no significant (°= = 0.05) difference between the well and aerated As(lll) levels. In addition, there was no
significant (~ = 0.05) difference between the total and dissolved As levels in the aerated samples.
The constancy of As(lll) levels is best illustrated in Figure 3-4, where all of the raw and aerated average
sample results are presented. These results show that aeration did not oxidize As(lll) at this site.
12
-------
0.120
Water Sample
Day of Collection
Figure 3-1. As Speciation Test Results for Well Water, Northeast Site
11 -
H f\ '
10 -
9"
-
8'
-
6"
-
5'
-
3'
-
2'
-
-
-
- f ---!>
Well
(5)
-- ~*
^
' "* X £j"
" .-₯ "
tf"^"*
-a- '%*
'It*'***
"\V i"
<->
~~ " &
-~ ~
Bubble
(21)
j
c
P^* i ""/ -,££
J^^Pt
J,f^r-i-__
-,- ,- -
x'"' '
' - ~:*/
Tower
(22)
-
./**" ,~-
- Iw ^
-s^L.-- -"
" ^ *
- __ -
Spray
;(22)
Water Source
(no. of samples)
Figure 3-2. Avg. Well & Aeration DO Concentrations (2 Test Runs), Northeast Site\
13
-------
0.120
0.110
0.100
0.090 -
0.080 -
0.070 -
0.060 -
0.050 -
0.040 -
0.030
0.020 -
0.010-
0.000 -
0.
^""^ ^-S^-cTj -fy«^+~zr»*I?=* -CK^'~U~ JS
Total As
a Soluble As
6 As(lll)
0 1.0 2.0 3.0 4.0 5.0 fi
Time, Hr
Figure 3-3. As Speciation Test Results for Bubble Aeration (Day 1), Northeast Site
0.120
Total As
n Soluble As
aAs(lll)
Raw
(5)
Bubble Tower
(22) (21)
Sample Source
(no. of samples)
Spray
(21)
Figure 3-4. Avg. As Results for Well & Aerated Samples (Run 1 & 2), Northeast Site
14
-------
3.1.2 Storage
The results from the storage test are summarized in Figure 3-5. The results do not show a general trend
of oxidation of As(lll). Considering the fact that appreciable DO (7 to 8+ mg/L) was present during
storage, it appears conditions of aerobic storage did not oxidize As(lll). The average level of As(lll) was
several percent lower than in the other As(lll) samples from the aeration experiments; however, with the
lack of any progression of oxidation over time this is not considered important.
3.2 Midwest Site
3.2.1 Aeration
The forced draft aeration (Unit No. 4), bubble aeration, and packed tower aeration systems were operated
at 56.7 Us (900 gpm), 0.63 Us (10.0 gpm), and 0.50 Us (8.0 gpm), respectively, on both days of testing.
Flow to the bubble and tower units was reasonably steady at ± 5 percent and was checked before and
after each 6-hr test period for the two small aeration systems. The flow rate for the forced draft aerator
was monitored by the existing plant flow metering equipment.
The raw water quality for this site is in line with what might be expected for a groundwater containing a
significant fraction of As(III). The DO was nearly zero with-an average of 0.3 mg/L. Iron and manganese
were very high at 2.76 mg/L and 0.155 mg/L, respectively. Essentially all of the raw water Fe and Mn was
in the dissolved form.
0.120
0.100
0.080
0.060
0.040
0.020 -
0.000
Samples taken at 6,
12, & 18 hr were
collected in the field
Avg. DO = 7.88 mg/L
Avg. Total As = 0.108 mg/L
Avg. Soluble As = 0.104 mg/L
Avg. As(lll) = 0.095 mg/L
-Total As
-Soluble As
As(lll)
0
24
i
72
120
48 72 96
Time, Hr
Figure 3-5. As Speciation Test Results for the Storage Water, Northeast Site
144
15
-------
The raw water As levels for Day 1 and Day 2 are summarized in Figure 3-6. On Day 1 the As levels were
very consistent, with a total As average of 0.041 mg/L. The dissolved As averaged 0.0355 mg/L (77.5
percent) and the As(lll) was 0.0315 mg/L (83.9 percent of dissolved As). The Day 2 As level was
significantly lower than on Day 1 due to a change in the blend of well water being pumped. On Day 1,
Wells 7 (higher As), 8, and 9 were pumped and starting at midnight on Day 2, only Wells 8 and 9 were
pumped. In addition to being lower overall, the Day 2 As levels fluctuated during the 6-hr testing period.
A sharp rise in concentration occurred toward the end of the test period when the pumping schedule
changed back to Wells 7, 8 , and 9. For this reason, the remainder of the result presentation focuses on
Day 1, as those data present the clearest view of the aeration results. Day 2 data show the identical
result as Day 1, except with a fluctuating inlet As level, and are tabulated in Appendix B.
All results are grab samples
of the blended well water
0.010
0.000
Pre 2 Post 1
Day 1 Day 2
Water Sample
Day of Collection
Figure 3-6. As Speciation Test Results for the Blended Well Water, Midwest Site
The DO levels for the raw and aerated water are illustrated in Figure 3-7. The A/W ratio for bubble and
tower aeration was 28.4 and 39.0, respectively. The exact design parameters for the forced draft aerator
were not known, but it is known that this device is not as effective as the other two methods of aeration.
The DO was raised to near the saturation level of 8.7 mg/L at 22.2 degrees C by the bubble (8.21 mg/L)
and tower (8.28 mg/L) aerators. The forced draft aerator raised the DO to an average of 7.39 mg/L. In
any event, it can be stated that all three aeration methods provided a maximum opportunity for the
oxidation of As(lll), as the DO levels were relatively high and the mixing of air and water was very good.
Due to the high Fe and Mn present in the raw water and their possible related importance, it is useful to
present those data before looking at the As data. Figures 3-8 and 3-9 summarize the Fe and Mn test
results, respectively. Aeration had a dramatic effect on the solubility of the Fe for this water supply and
oxidized over 98 percent of the dissolved Fe to paniculate Fe. The effect for Mn was much less, as would
be expected from basic process chemistry at a pH of approximately 8.0 in the aerated waters. Figure 3-
10 show the pH data for the raw and aerated samples on Day 1.
16
-------
Well
(4)
FDA Bubble
(24) , (24)
Water Sample
(no. of samples)
Tower
(22)
Figure 3-7. Avg. Well & Aeration DO Concentrations (2 Runs), Midwest Site
CD
U_
a Total Fe
a Soluble Fe
FDA Bubble
(11) 02)
Water Sample
.(no. of samples)
Tower
(12)
Figure 3-8. Avg. Fe Test Results for Well & Aerated Water (Run 1), Midwest Site
17
-------
.180
Water Sample
(no. of samples.)
Figure 3-9. Avg. Mn Test Results for Well & Aerated Water (Run 1), Midwest Site
9.00 -3
800J
7 00 :
6.00 ^
5.00^
4.00-
3.00^
2.00 ^
1 00 ;
1 \J\J .
000^
h- g >8e ^-----tfi tff
i m -
Raw pH average = 6.97
8
- $
i- .u. %- -. _ Jfr. ,_ .
H i i-
acr\ A
A Bubble
X Tower
i
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Time, hi
Figure 3-10. pH Results for Well & Aerated Samples (Run 1), Midwest Site
18
-------
The total Fe should remain the same through aeration unless Fe is being removed from the water.
Because aeration without subsequent downstream sedimentation or filtration normally would not show a
removal of Fe - only an oxidation effect - it was surprising to observe that the packed tower appeared to
actually reduce the total Fe. This is illustrated in Figure 3-8. Upon termination of the testing, the tower was
examined and the packing media was found to be loaded with precipitated Fe. Mass calculations show
that the Fe removed by the packing could have resulted in the lower Fe exiting the tower. For example, at
an Fe removal of 0.75 mg/L and a flow rate of 0.50 Us (8 gpm), only 8.18 grams of Fe would have been
captured by the packing media. The bubble unit also showed a similar coating of Fe, but to a much lesser
degree. The Fe data for the packed tower aerator are summarized in Figure 3-11. The packed tower data
on Day 1 at first showed no removal, but as the testing period proceeded removal progressively
increased. The Day 2 tower data showed even more removal of Fe by the tower packing and the removal
leveled off at approximately 1.0 mg/L across the unit. The trend of progressive removal by the packing is
clear. The last data point is representative of an interruption in the normal flow through the treatment unit
and the test on the tower was terminated early.
The As speciation test results for Run 1 of the forced-draft tray aerator are presented in Figure 3-12.
These results are typical of a 6-hr aeration test for this site, and the bubble and packed tower results were
very similar. The As levels are reasonably consistent for the 6-hr test period. Because the aeration
processes were operated in a steady state mode, temporal change in As levels would not be expected as
long as the well levels remained consistent. Therefore, all of the samples would be expected to be
duplicates over time, with each subsequent sample reinforcing the same observation.
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0
Average raw total Fe = 2.685 mg/L
Problem w/tower discharge
and test was terminated early
DAY1
DAY 2
4 68
Time in Operation, hr
10
12
Figure 3-11. Fe (total) Results for Packed Tower (Run 1 & 2) Samples, Midwest Site
19
-------
0.045 ]
0.040 \
0.035 \
0.030 1
0.025 j
0.020 |
0.015 I
0.01 OJ
0 005 -
^/\^v/vy
0 000 :
^* ^-^+^+^^
*~~~-~+^ * ^*--
D"" ~* *^T^- ji ^*f Ch*1*^^ ^^JT^ T"U^
A-^_ ^^^^r, ^-"ffl""0^*^ *-*'
4 * *
n Soluble As
A. Ac/llh
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Time, hr
HI
9
4.5 5.0 5.5 6.0
Figure 3-12. As Speciation Test Results for FDA Samples (Run 1), Midwest Site
All of the raw and aerated water As results for Day 1 are summarized in Figure 3-13. A comparison
between the raw and aerated results show a significant drop in the level of As(lll) after aeration. These
results are very typical of other iron removal sites reported in the literature, where aeration is used to
oxidize dissolved Fe*2. This oxidation causes precipitation of Fe+3 and the removal of As associated with
Fe.
The reduction in dissolved As(lli) across the aeration step averaged 22 percent. The real question is
whether the loss of dissolved As(lll) was: 1) a result of As(III) oxidation to As(V) followed by adsorption,
and/or 2) an adsorptive removal of part of the As(lll) directly by the precipitated Fe. This will be discussed
further in Section 4.
3.2.2 Storage
The arsenic results for the storage test are summarized in Figure 3-14. These results reflect the effect of
precipitated Fe settling in the container, which is shown in Figure 3-15. A lesser fraction of the Mn also
precipitated and settled during storage. Samples after 36 hr had less Fe and As than previous samples,
and this effect became more pronounced with time. Essentially all of the precipitated Fe settled from the
water after 36 hrs. Insoluble Fe was left in the storage and sample containers used for transport after
Day 2, as the Fe settled and coated the walls of the containers. The samples at 48-hr and beyond
showed less than = 30 percent of the original total As and less than 3 percent of the total initial Fe at the
beginning of storage. All of the As (0.012 mg/L) was in the dissolved form by the end of the storage
period and almost ail of the dissolved As was As(lll). By 48-hr As (III) was below the detection limit of
0.003 mg/L.
20
-------
O>
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
n Total As
a Soluble As
nAs(lll)
Well
(2)
Bubble
(11) (12)
Water Sample
(no. of samples)
Figure 3-13. As Speciation Results for Well & Aerated Samples (Run 1), Midwest Site
0.050 |
0.045 \
0.040 ^
0.035 {
0.030 '
0.025 \
0.020 j
0.015^
0.010^
0.005 i
0.000 ;
-Total As
-Soluble As
- -A- -As(lll)
Precipitated Fe Settled in Container
Average DO = 4.5 (5.5 - 4.0)
0
ND at 0.003 mg/L
0
24
48 72 96
Time, hr
120
144
Figure 3-14. As Speciation Test Results for the Storage Samples, Midwest Site
21
-------
3.000 ;
2.500 -
2.000 -
1.500 -
1.000-
0.500 -
0.000
0 24 48 72 96
Time, hr
n 0.200
0.180
0.160
--0.140
-0.120
--0.100
Precipitated Fe Settled in Container - - 0.080
--0.060
- - 0.040
--0.020
--0.000
Average DO = 4.5 (5.5 - 4.0)
O>
cf
:E
5
.2
120 144
Figure 3-15. Fe & Mn Concentrations for Storage Samples, Midwest Site
3.3 Southwest Site
3.3.1 Aeration
The bubble aeration, packed tower aeration, and spray aeration systems were operated at 0.63 Us (1*0.0
gpm), 0.50 Us (8.0 gpm), and 0.50 L/s (8.0 gpm), respectively, on both days of testing. Flow was
reasonably steady at ± 5 percent and was checked before and after each 6-hr test period. The well
water DO was greater than 5 mg/L. This was considered to be relatively high for a groundwater
containing appreciable As(lll). The DO levels in the raw and aerated samples are presented in Figure 3-
16. The A/W ratio for bubble and packed tower aeration was 28.4 and 44, respectively. In all three
cases, the DO measured was significantly lower than expected, as compared to the saturation level of 7.8
mg/L at 28 degrees C. The exact cause of this was not known, but the precipitation steps in the DO
analysis were difficult and notably time consuming. On the second day, the DO was measured on only a
few samples. Despite the problems with the DO analysis, it is certain that all three aeration methods
provided a maximum opportunity for the oxidation of As(lll), as the DO levels were high and the mixing of
air and water was very good.
The aeration units raised the raw water pH to more than 8.0, as shown in Figure 3-17. The pattern of
aerated pH values was similar to that for DO, as would be expected. The magnitude of pH rise is a
function of the degree of carbon dioxide removal, given that the alkalinity was the same in each case. All
of the systems raised the pH from the raw value of 7.8 to over 8.0. The range of aerated pH was 8.01 to
8.52.
The total As averaged 0.035 mg/L in the five (5) "pre" and "post" test observations. The As was 93.1
percent dissolved and 45.4 percent of the dissolved fraction was As(lll). The raw water As levels are
summarized in Figure 3-18.
22
-------
/.UU:
6f\r\
.00:
*
5 fifi
.UU :
"o> A on '
is' H-.UU :
O fifi
Q O.UU -
LJ r\ fifi ^
^i.UU
1 nn '
l .UU :
Ofifi
.UU H
.. . , iw
w;; ^"T
"*C ^s"-"**,
''>'>" '-₯j>y - - v/
W- ^-<"
^ .
, ^
Well
(5)
.^rf ':.A
%/' ,/J>
^^^" ^k
&'-$
i^f^s
^>A
Bubble
(14)
"^
3y ~ -.
"
->- ;r'J
-. i^'i
=ii
' ',.,?>'
Tower
(14)
XJWf -' '
4>,
. .."<»
T-^ *-" ^ .
^^-~ '"
**"-""*,
\ v ^
. t,'tf
« f lft
t ,s,* I
IK'-- '
Spray
(14)
Water Sample
(no. of samples)
Figure 3-16. Avg. Well & Aeration DO Concentrations (2 Test Runs), Southwest Site
a.uvj :
8.00^
7.00;
6.00;
s.ooj
4.00^
3.00 I
2.00 \
n nn :
U LI- j| "U u y -B u y u m A .
Well pH ranged from 7.74 - 8.0
0.0 2.0 4.0 6.0
Time, hr
o Bubble
Tower
A Spray
8.0
Figure 3-17. pH Results for Well & Aerated Samples (Run 1), Southwest Site
23
-------
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005 H
0.000
Pre 2 Pre 3
Day 1 Day 2
Water Sample
Day of Collection
Post 2
Day1
Posts
Day 2
Figure 3-18. As Speciation Test Results for Weil Samples, Southwest Site
The results of the packed tower aeration As results for Day 2 are shown in Figure 3-19. These data
show that an oxidation of As(lll) by aeration did not occur. Because the aeration processes were operated
In a steady state mode, temporal change in As levels would not be expected as long as the well levels
remained consistent. Therefore, all of the samples would be expected to be duplicates over time, with
each subsequent sample reinforcing the same observation. This is the result that was observed, with the
As(III) levels in the raw and treated waters remaining relatively constant.
The lack of an aeration effect on As(lll) is best illustrated in Figure 3-20, where all of the raw and
aerated sample results are presented. These results clearly demonstrate that aeration did not oxidize
As(lll) at this site.
U.UHU
0.035
0.030 -
0.025 -
Onon -
.(Jt-U
0.015 -
0.010-
0.005 -
0 000-
0
/N. Avg. Sol. As in Well = 32.6
.- Q ds. _4> «-£» ^V n*-*^ Q O O
O Soluble As
--As(lll)
Avy. As(lll) in Well = I4.c
0 2.0 4.0 6.0 8.
Time, Hr
Figure 3-19. As Speciation Test Results for PTA (Run 2), Southwest Site
24
-------
Total As
D Soluble As
BAs(IH)
Well
(5)
Bubble Tower
(24,23,12) (24,24,1
Water Sample
(no. of samoles)
Spray
(24,24,11)
Figure 3-20. Avg. As Results for the Well & Aerated Samples, Southwest Site
3.3.2 Speciation Problem
A problem was experienced at the Southwest site due to a change in the method of speciation used
in the field. The normal speciation procedure used at the other sites used the resin column once, or at
most two times, before it was discarded. For a variety of reasons thought to be valid at the time of
testing, the resin columns were not changed between each sample. All of the prepared resin columns
lost their liquid fraction in transit due to the changes in air pressure that occur in air travel. This was
aggravated by the dry climate in Albuquerque. The columns had to be re-packed to rid the resin of air
pockets and this was time consuming, especially when it was discovered during the testing. A decision
was made to use the columns for several samples, because it was believed at the time that the capacity
of the column was very high in comparison to the throughput. Previous experience with the chloride-form
resin speciation had shown this to be the case7. Later, the sample test results made it clear that this
change in procedure caused some invalid As(lll) measurements.
In addition to the misconception about column capacity, two other changes were made that
exacerbated the column capacity problem. -First, slightly more nitric and sulfuric acid was added to the
bottles to offset the inevitable loss of acid that occurred anytime the empty sample bottles were shipped.
The problem was first noticed at the Midwest site and confirmed by the utility personnel at that site, who
also received similar samples by air that were associated with another EPA research project. They
reported that their sample bottles always leaked acid in transit. Secondly, the analytical laboratory
requested that additional As(lll) sample be provided for analysis so the volume of sample poured through
the resin column was increased. These changes decreased the capacity of the column because the
sulfuric acid limits the throughput at only =18 BV under normal acid loading5.
The failure to recognize the BV limitation due to selecting sulfuric acid for acidification of the
dissolved As sample in the As speciation method led to the loss of some of the Southwest Site As(lll)
data. Unfortunately the analytical laboratory used all of the extra dissolved sample for this site due to
repeated ICAP runs on these samples. This was a separate problem thought to be related to silica
precipitation in the analysis that resulted in repeated costly repairs to their ICAP instrument. Had this
25
-------
analytical problem not taken the extra dissolved As sample volume, the resin treatment step could have
been repeated to produce another As(lll) sample. The dissolved As(lll) samples were finally successfully
run by graphite furnace AA.
Even with the loss of some of the sample results, there are ample data that clearly demonstrate the
inability of aeration to oxidize As for the Southwest site. The criteria for discounting samples was the
number of times the column was used. If a column was used one or two times, then the results were
taken to be valid because sulfate breakthrough should not have been a factor. Any test done with a
column that had already been used two times previously, was deemed invalid. Most of those tests did
show an Arsenic peaking effect.
3.3.3 Storage
The storage results for the Southwest site are summarized in Figure 3-21. The data do not support
an oxidation of As(lll) as no general trend of oxidation was measured. The higher As(lll) samples were
typical (= 0.015 mg/L) of what was measured in the raw and aerated water samples, and it is the samples
at the beginning and end of the storage test that are atypically low (=0.010 mg/L). In general, these data
do not indicate a significant oxidation by DO and storage.
0.040
0.035 i
0.030 '
0.025 i
0.020 j
0.015;
0.010^
0.005 i
o.ooo q
-»-Total As
Soluble As
-As(lll)
24
48
96
120
72
Time, hr
Figure 3-21. As Speciation Test Results for Storage Water Samples, Southwest Site
144
26
-------
4. 0 Discussion and Conclusions
4.1 Oxidation of As(IH) by Dissolved Oxygen
4.1.1 Oxidation by Aeration
The conditions set up in this research were conducive to the oxidation of As(III) by DO. Dissolved
oxygen and mixing of oxygen and water were equal to or greater than the normal design parameters for
the transfer of oxygen into water. The data at the Northeast and the Southwest test sites support the
hypothesis that As(lll) is not oxidized by oxygen. Relatively high DO did not appear to cause any
measurable oxidation of As(lll), both in the aeration tests and the storage tests. The Northeast site had
very high dissolved As(lll) and there was no measurable oxidation by aeration and/or storage. The water
at this site was relatively cold at 11 degrees C, so it was speculated that a higher temperature might
increase the rate, if any, of As(III) oxidation by dissolved oxygen. The higher temperature condition was
tested at the Southwest site, where the well water temperature was 28 degrees C. There was no
measurable oxidation of As(lll) due to aeration for the higher temperature condition.
The Midwest site provided an interesting observation in that there was an apparent significant
reduction in As(lll) as a result of aeration. However, this reduction in As(IIl) is thought to be entirely
associated with the oxidation and precipitation of Fe. It is believed that As(lll) is partially removed by
precipitated iron, as has been reported in the literature.8'9'10'11 It should be noted that at a pH of over 8.0,
similar to that reached by the aeration systems in this study, some (= of the As(lll) would exist as the
charged As(OH);. Lastly, this study did not show the mechanism for the As(lll) reduction at the Midwest
site, so it is a possibility that the As(lll)was converted to As(V) and removed by iron. In any event, the
most important factor appeared to be the precipitation of Fe and not the presence of DO during aeration.
While the subtleties of arsenic removal associated with Fe precipitation are interesting, it is important
to emphasize that the original objective was to establish if typical aeration methods could oxidize As(lll).
The results of this study show that aeration, or more specifically DO, is not effective in this regard.
Aeration should not be considered as a candidate for As(lll) oxidation; however, it will continue to be
considered a very effective process for the oxidation of Fe. In that way, aeration can be said to be
effective in bringing about the adsorptive removal of As via the oxidative precipitation of Fe.
4.1.2 Oxidation of As(lll) During Storage
The storage holding times under aerobic conditions were typical of what would occur in clearwells,
standpipes, and/or reservoirs. These conditions should have resulted in some degree of oxidation of
As(lll), if aeration is a potential method for oxidation. At the Northeast and Southwest sites, there was no
evidence of oxidation of As(lll) by DO under the conditions of storage in this study.
The importance of Fe precipitation and associated As(lll) removal from the aqueous phase was
noted in the storage testing for the Midwest site, but there was no determination made for the actual
mechanism of removal. After 36 hr of storage, approximately 97 percent of the Fe had settled in the
storage and sample containers. Total As and As(lll) were reduced concurrently with Fe precipitation.
The remaining As in the dissolved phase was As(V), indicating a conversion of As(lll) to As(V) or a
preferential removal of As(lll) over As(V). The later is considered unlikely, so it is speculated that an
oxidation did occur in the aqueous phase. However, there were no data that indicated an oxidation of
As(lll) specifically, or solely by DO. Other possibilities for conversion include oxidation by manganese
oxides, ferric oxyhydroxides, or microbial oxidation.2 Based upon the storage results at the other test
sites, it is believed that the apparent As(lll) conversion was not simply a result of DO and time during
storage.
27
-------
4.2 Sample Analysis Problems
In addition to the speciation problems in the field at the Southwest Site, there was a major problem
that the analytical laboratory experienced with the dissolved As speciation samples from that site. The
dissolved As sample was preserved with sulfuric acid as a part of the specified method. It was concluded
by the laboratory that something contained in the chemistry of these samples, interacting with the sulfuric
acid, caused the ICAP method to fail. Physically, a deposit of an acid "white precipitate" ruined
replaceable parts of the ICAP instrument and it was at least one (1) month before the problem was finally
attributed entirely to the sample and not the instrument. This was particularly unfortunate for three (3)
reasons: 1) it delayed all of the As analyses from the Midwest and the Southwest sites, 2) almost all of the
dissolved As sample volume from the Southwest site was used, which could have been used to generate
another As(lll) sample to replace the invalid As(lll) samples due to As peaking in the resin column, and 3)
the dissolved Fe data were not useable because the laboratory used the ICAP As samples for the total
and dissolved Fe, respectively. Fortunately, all but one (1) of the dissolved As samples from the
Southwest site were finally run on the graphite furnace, with good results.
Aside from the problems with the ICAP analysis for the Southwest site, the analytical results for this
project appear to be very good. The results allow the objectives of the study to be met and the
conclusions are well supported.
4.3 Conclusions
Based upon the results from this study and for the conditions under which is was conducted, the
following conclusion are made:
1. Aeration, and specifically DO, did not cause an oxidation of As(III).
2. DO and a storage time of five (5) days did not cause an oxidation of As(lll) in low Fe waters.
3. Fe oxidation and precipitation brought about by DO in aeration and storage processes can remove
a significant fraction of As, presumably as As(lll).
4. A complete precipitation and removal of Fe under the storage conditions of the Midwest site
apparently caused or was concurrent with a conversion of As(lll) to As(V) in the dissolved phase.
From an engineering perspective, the results of this study lead to the conclusion that aeration should
not be considered for the oxidation of As(lll). -Further, long contact with DO, as afforded by water storage,
should not be considered effective in the oxidation of As(lll). With respect to the oxidation of As(lll), there
appears to be no benefit from contacting water with DO.
28
-------
1.
5. References
USEPA, Summary of Arsenic Treatment Workshop. Office of Ground Water and Drinking Water,
USEPA, Contract No. 68-C3-0365, July 18,1994. ..
2. Hering, J.G., and Chiu, V.Q., The Chemistry of Arsenic: Treatment implications of Arsenic
Speciation and Occurrence, prepared for the USEPA Office of Drinking Water Research Workshop
on The Treatment of Arsenic in Drinking Water, San Antonio, TX, February 25,1998.
3. Runnells, D.D., Skoda, R.E., Kempton, J.H., Lindberg, D.A., and D.A. Bright, Redox Chemistry of
Aqueous Arsenic, Selenium, and Iron, With Applications to Equilibrium Geochemical Modeling.
Electric Power Research Institute Report TR-103451 December 1993.
4. Ficklin, W.H., Separation of Arsenic(lll) and Arsenic(V) in Ground Waters by Ion Exchange.
Talanta, Vol. 30, No. 9, pp 64-77 (1983).
5. Edwards, M, Patel, S., McNeil!, L, Chen, H., Eaton, A.D., Antweiler, R.C., and Taylor, H.E.,
Considerations in Arsenic Analysis and Speciation. JournalAWWA, Vol. 90, No. 3, pp 103 (1998).
6. Battelle, Quality Assurance Project Plan for Evaluation of Treatment Technologies for the Removal
of Arsenic from Drinking Water. Prepared for EPA's NRMRL, Cincinnati, OH.
7. Lowry, J.D., Arsenic Removal: Small System Treatment Using Ion Exchange, presented at the
1998 Annual AWWA WQTC - As Workshop, San Diego, CA, November 4,1998.
8. Edwards, M, Chemistry of Arsenic Removal During Coagulation and Fe-Mn Oxidation. Journal
AWWA, Vol. 86, No. 9, pp. 64 (1994).
9. Sorg, T.J., and Logsdon, G.S., Treatment Technology to Meet the Interim Primary Drinking Water
Regulations for Inorganics: Part 2. Journal AWWA, Vol. 70 , No. 7, pp 379 (1978).
10. McNeill, L.S., and Edwards, M., Soluble Arsenic Removal at Water Treatment Plants. Journal
AWWA, Vol. 87, No. 4, pp. 105 (1995)
11. Pierce, M.L., and Moore, C.B., Adsorption of As(lll) and As(V) on Amorphous Iron Hydroxide.
Water Res., 16:1247 (1982).
29
-------
6. Appendices
Appendix A. Northeast Site Data
Appendix B. Midwest Site Data
Appendix C. Southwest Site Data
30
-------
Appendix A: Northeast Site Data
31
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Appendix B: Midwest Site Data
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Appendix C: Southwest Site Data
United States
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Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
Please make all necessary changes on the below label,
detach or copy, and return to the address in the upper
left-hand corner.
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detach, or copy this cover, and return to the address in the
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PRESORTED STAND!
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