G-069, in K.A. Fields and G.B. Wickramanayake (Chairs), Remediation of Chlorinated and Recalcitrant Compounds—2010.Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds (Monterey, CA; May 2010). ISBN 978-0-9819730-2-9, Battelle Memorial Institute, Columbus, OH, www.battelle.org/chlorcon.

Attenuation of Naturally Occurring Arsenic atPetroleum Hydrocarbon–Impacted Sites

Richard A. Brown (dick.brown@erm.com) and Katrina E. Patterson

(katrina.patterson@erm.com) (ERM, Ewing, New Jersey, USA) Mitchell D. Zimmerman (ERM, Austin, Texas, USA)

G. Todd Ririe (BP, La Palma, California, USA) ABSTRACT: In January 2006, the United States Environmental Protection Agency (USEPA) lowered the maximum contaminant level (MCL) for dissolved arsenic in groundwater from 0.050 mg/L to 0.010 mg/L due to long-term chronic health effects of low concentrations of arsenic in drinking water. This has heightened public and regula-tory awareness of dissolved arsenic in groundwater.

Arsenic occurrence at petroleum-impacted sites can be summarized by five basic principles that govern the fate and transport of arsenic in shallow aquifers impacted by petroleum hydrocarbons. These are:

1. If arsenic is not present in the site mineralogy, or if arsenic has not been emplaced due to human activity, petroleum impacts will not cause arsenic impacts to groundwater. Arsenic is not a major contaminant in petroleum hydrocarbons;

2. For sites that have naturally occurring arsenic-bearing minerals, sorbed arsenic phases, or aged anthropogenic arsenic sources, there is a stable arsenic geochemis-try present that determines the ambient (background) level of dissolved arsenic in groundwater. If the background level of arsenic naturally exceeds the new MCL, then the MCL is unachievable as an attenuation or remediation goal;

3. The introduction of petroleum hydrocarbons (or other degradable organics) may cause a perturbation to the existing geochemistry, resulting in the mobilization of existing naturally occurring arsenic at concentrations above the ambient level;

4. The perturbation of the ambient arsenic geochemistry (and related arsenic mobili-zation) will persist until the soluble hydrocarbons are attenuated; and

5. Once the hydrocarbons are attenuated, the arsenic will revert to its pre-existing stable geochemistry, which may be above or below the arsenic drinking water MCL of 0.010 mg/L, it depends on the background geochemistry and background arsenic concentrations.

Proper management of a petroleum-impacted site at which arsenic has become mobi-lized requires an understanding of the site-specific ambient conditions and how petroleum impacts affect arsenic chemistry and mobility in the subsurface. This understanding can be refined by developing a site-specific conceptual model incorporating background and site data to guide further investigation and remedial actions concerning arsenic. INTRODUCTION

In January 2006, the United States Environmental Protection Agency (USEPA) low-ered the maximum contaminant level (MCL) for dissolved arsenic in groundwater from 0.050 mg/L to 0.010 mg/L due to long-term chronic health effects of low concentrations

of arsenic in drinking water. This has heightened public and regulatory awareness of dissolved arsenic in groundwater.

While petroleum hydrocarbons, themselves, are not a source of arsenic, naturally-occurring arsenic may be mobilized into shallow groundwater by inputs of biodegradable organic carbon, including petroleum hydrocarbons. “Naturally-occurring arsenic” refers to arsenic that is present in the solid phase prior to impacts by degradable organic carbon, in-cluding petroleum hydrocarbons. Arsenic may be present as specific minerals, as an amor-phous phase, or adsorbed onto iron oxyhydroxides and other soil constituents, either as a natural trace metal in native rocks and soils or from human activity such as agriculture or waste disposal. Hydrocarbons can mobilize arsenic by creating reduced conditions.

When a petroleum release occurs, the more soluble hydrocarbon fractions can dis-solve into groundwater, stimulating biological activity. Bacteria degrade the dissolved hydrocarbons and consume the available terminal electron acceptors (TEAs), creating reduced groundwater environments. The redox level attained is a function of the TEA availability and the amount of hydrocarbon released. Once the redox conditions are at or below the Eh for iron reduction, ferric oxides in the soils are reduced to the more soluble ferrous form. Because most soil arsenic is associated with ferric oxides, arsenic will also be released and mobilized into groundwater. Dissolution of ferric oxides not only releases arsenic to the groundwater, but also decreases the future adsorption sites for arsenic. Arsenic is also reduced from As+5 to the more soluble As+3, which is present as the arsenite anion (AsO3

-3), and further increases mobility. When the petroleum hydrocarbons are attenuated, the natural attenuation of arsenic

will occur as the aquifer is restored to its original aerobic conditions. Arsenite is re-oxidized to the less soluble arsenate. Reduced iron is reoxidized and re-precipitates on the soil particles as an oxyhydroxide. These iron oxyhydroxides adsorb and bind arsenate. Over time, the adsorbed arsenate can mineralize and become even more stable. The natural attenuation of arsenic is coupled to the attenuation of hydrocarbon plumes.

NATURALLY OCCURRING ARSENIC

One of the fundamental principles of arsenic mobilization and attenuation at hydro-carbon-impacted sites is that arsenic has to be present in the soil prior to the release of the hydrocarbons.

As shown in Table 1, crude oils and therefore, petroleum products, are not a source of arsenic. Arsenic can, however, be present at a site due to either natural site mineralogy or geochemistry, or due to anthropogenic activity.

TABLE 1. Summary of arsenic concentration in 26 crude oils.

Arsenic is naturally found in many soils. It may be present as specific minerals or it may be present as an adsorbed phase on metal (primarily iron) oxyhydroxides and other clay minerals. There are over 500 naturally occurring arsenic minerals. Naturally occur-ring arsenic is frequently associated with volcanic deposits and sulfidic minerals (e.g., pyrite [FeS2]). Over time, arsenic minerals may weather, redistributing arsenic in the soil matrix as a stable, adsorbed phase on ubiquitous metal (iron) oxyhydroxides. Geochemi-cal processes such as oxidation and reduction, pH shifts, precipitation, and adsorption result in arsenic redistribution in soils.

There are broad areas of the United States where arsenic in groundwater already ex-ceeds the old MCL (50 g/L) due to the naturally occurring mineralogy. The southwest-ern and the upper midwest US have natural dissolved arsenic concentrations greater than either the current or previous MCL due to naturally occurring arsenic minerals.

Arsenic also has many industrial uses. It is used in agricultural applications for animals and crops, and in lawn care. Arsenic is also used for wood treating, as a flame retardant in plastics, in semiconductors, and as a rat poison. Arsenic can be found as an impurity in mining and mineral processing sites. It is also found as a constituent of municipal landfills and leachate.

Industrial and agricultural uses of arsenic can result in both point source and non-point source contamination. Of greatest interest are non-point sources of arsenic. Typi-cally, these uses involve application of industrial chemicals (e.g., pesticides) over wide areas resulting in diffuse, low-level arsenic contamination. Nonpoint source arsenic has the greatest potential to overlap with areas of petroleum impact.

PRINCIPLES OF ARSENIC MOBILITY

The mobility of arsenic is controlled by redox conditions (Eh), by the pH and by the presence of metal oxyhydroxides that can adsorb and bind arsenic. With petroleum im-pacted sites, the aquifers most commonly encountered will, for the most part, be shallow and in contact with the atmosphere. Therefore, the most common background redox condition will be an aerobic environment in which arsenic will be present as the oxidized, less mobile, As+5. The ambient groundwater concentration of the arsenic will be con-trolled by pH and the soil mineral content (i.e. iron oxyhydroxides). As+5, present as the arsenate anion (AsO4

-3), is more soluble at low pH (< 4) and high pH (>8). This is in contrast to natural groundwater pH values typically ranging between 4 and 8. Arsenate is also strongly adsorbed to iron oxyhydroxides, which are fairly ubiquitous.

An important part of understanding the mobility of naturally-occurring arsenic at pe-troleum impacted sites is having a good characterization of the ambient arsenic geochem-istry and of the hydrogeology of the site. Site characterization should determine the ambient, background level of dissolved arsenic. The dissolved arsenic level at petroleum impacted sites, even after attenuation, cannot be lower than background. If the back-ground level of arsenic naturally exceeds the new MCL, then the MCL is unachievable as an attenuation or remediation goal. The ambient dissolved arsenic concentrations are a function of the site mineralogy, hydrogeology and redox conditions.

Figure 1 (Boulding and Ginn, 2004) superimposes the redox conditions of ground-water on an Eh-pH diagram of arsenic. The diagram identifies the thermodynamically stable arsenic species for a given range of Eh and pH. Under oxidizing conditions (high Eh), arsenates are more stable. As shown in Figure 1, aquifers that are in contact with the

FIGURE 1. Arsenic speciation in groundwater regimes.

atmosphere (unconfined conditions) will be mostly aerobic, and arsenic will be predomi-nately in the pentavalent (As+5; arsenate) valence state. The solubility of arsenic under aerobic conditions is determined by the pH and mineralogy, particularly the presence of iron oxy hydroxides (FeO(OH)).

The primary forms of inorganic arsenic in both oxidizing and reducing groundwater are oxyanions. Oxyanions of arsenic readily sorb to solid phase metal oxyhydroxides such as goethite. (Wilkin, 2003) Adsorption of arsenic at mineral surfaces occurs as a result of a set of chemical reactions generally referred to as sorption.

The most important reactive surface phases for arsenic attenuation in many soils and subsurface systems are cationic metal surfaces, including iron, aluminum, and calcium mineral phases. Arsenic sorption has been demonstrated for a wide range of minerals common to soils and sediments with iron oxides and sulfides playing a dominant role in oxidizing and reducing environments

IMPACT OF PETROLEUM HYDROCARBONS ON ARSENIC MOBILITY

When petroleum hydrocarbons are released to groundwater, there is a progression from aerobic to anaerobic conditions with an associated reduction in the redox conditions of the groundwater system. The progression is, in decreasing order of redox potential, aerobic respiration, followed in sequence by nitrate reduction, manganese reduction, iron reduction, sulfate reduction, and finally, methanogenesis. Typically, the most reducing conditions are in the source area and the least reducing conditions (i.e., aerobic condi-tions) are at the plume boundary. The relative reaction rates and levels of microbial

Aerobic ZoneO2 > 1.5-2.0

Aerobic ZoneO2 > 1.5-2.0

Arsenic Reduction

activity under each of these different metabolic environments are controlled by the avail-ability of the TEAs, the types and concentrations of organic substrate(s) that can be util-ized by the bacteria, and specific type and population of the microbial community. This redox progression results in a loss of organic carbon and depletion of various electron acceptors from the aquifer system as well as a progression in the types and metabolic activity of the indigenous bacteria. Figure 2 shows that the relative areas of metabolic activity vary in the direction of groundwater flow. The most reduced conditions are found in the source area. The aquifer conditions become less reducing in the direction of groundwater flow. Aerobic conditions generally bound the plume in both directions.

FIGURE 2. Conceptual model of biodegradation of apetroleum-hydrocarbon plume.

If microbial activity is high and there is sufficient dissolved hydrocarbon, the aquifer environment will progress rapidly through these different anaerobic metabolic conditions. Once the microbial conditions reach iron reduction or below, arsenic will be reduced and mobilized.

ATTENUATION OF HYDROCARBONS AND ARSENIC

Migration of the dissolved hydrocarbons and the resulting microbial activity creates overlapping hydrocarbon and arsenic plumes. As pictured in Figure 3, the hydrocarbon impact reduced the redox. Arsenic is initially mobilized by the change in redox. The hydrocarbons attenuate due to biological activity. The arsenic plume commonly extends beyond the hydrocarbon plume, with arsenic remaining above background concentrations until aquifer redox conditions return to aerobic. This downgradient portion of the plume is a transition zone where dissolved arsenic concentrations decrease as the aquifer be-comes more oxidizing, the arsenic is readsorbed and immobilized.

The combined plume goes through three stages over time —an initial phase of plume expansion, a period of plume stability where the footprint is static, and a final stage in which the plume retreats toward the petroleum source area. Plume expansion occurs until

AmbientArsenic

HydrocarbonPlume

Transition Zone

AmbientArsenic

AmbientArsenic

HydrocarbonPlume

Transition Zone

AmbientArsenic

Groundwater flow

Arsenic

Hydrocarbons

Redox

the dissolution of hydrocarbons is balanced by their degradation and removal. When there are no longer sufficient hydrocarbons present to maintain the plume, the plume begins to retreat. As the plume retreats, redox conditions gradually revert to ambient conditions. Once the hydrocarbons are attenuated, the aquifer becomes aerobic, and the arsenic reverts back to the existing ambient (background) conditions

When the petroleum hydrocarbons are attenuated, natural attenuation of arsenic will occur as the aquifer is restored to aerobic conditions. Arsenite is reoxidized to the less soluble arsenate. Reduced iron is reoxidized and re-precipitates on the soil particles as an oxyhydroxide. These iron oxyhydroxides adsorb and bind arsenate. Over time, the ad-sorbed arsenate can mineralize and become even more stable.

FIGURE 3. Change in hydrocarbons, arsenic, and redox with distance.

CASE STUDIES Four case studies illustrate the basic principles of arsenic mobilization and attenuation

discussed above.

1. An Operating Refinery—Arsenic mobilization associated with the presence of hydrocarbon LNAPL is present in an alluvial terrace sand aquifer. Correlations between iron and arsenic in both soil and groundwater indicate arsenic mobiliza-tion occurs with the loss of iron oxyhydroxide sorption sites due to changes in redox conditions. Concentrations of arsenic in groundwater downgradient of hydrocarbon impacts indicate that arsenic is not mobile under the ambient aerobic

conditions at this site. Once the hydrocarbons are attenuated, as the hydrocarbon plume migrates down gradient, aerobic conditions are re-established and the arsenic is re-oxidized and re-adsorbed onto the soil matrix when DO is observed to be ~ 1.5 to 2 mg/L.

2. A Former Refinery—The water bearing unit in a bluff underlying a former tank farm is impacted with hydrocarbon LNAPL and arsenic. The presence of iron oxyhydroxides is visually evident as orange and red staining of quartz grains in cored sediment from outside the hydrocarbon plume, while within the plume re-ducing conditions are evident by grey to black sandstone. Arsenic mobilization appears to be a result of changing redox conditions, leading to elevated arsenic in seepage water from the bluff. The arsenic concentrations correlate to dissolved iron.

3. A Former Exploration Reserve Pit —A former drill site reserve pit and gravel pad in northern Alaska received drilling waste, followed by closure and corrective action activities. Samples of surface water surrounding the pit before corrective action revealed evidence of potential hydrocarbon impacts and elevated dissolved arsenic concentrations. Later samples showed decreases in dissolved arsenic con-centrations as the geochemical parameters pH and dissolved iron returned to background aerobic conditions.

4. A Former Fuel Terminal—A former fuel terminal contains elevated hydro-carbon in soil and groundwater at various locations throughout the site. Ambient geochemical conditions are naturally reducing due to native organic carbon. Dis-solved arsenic has been measured throughout and upgradient of the site where groundwater conditions are reducing. Removal of hydrocarbon impacts does not decrease arsenic concentrations due to the ambient naturally occurring reduced conditions that exist at the site.

CONCLUSIONS

Five basic principles govern the fate and transport of arsenic in shallow aquifers im-pacted by petroleum hydrocarbons. These are:

1. If arsenic is not present in the site mineralogy, or if arsenic has not been emplaced

due to human activity (agriculture, wood treating, mining, etc.), petroleum im-pacts will not cause arsenic impacts to groundwater.

2. For sites that have naturally-occurring arsenic-bearing minerals, sorbed arsenic phases, or aged anthropogenic arsenic sources, there is a stable arsenic geochem-istry present that determines the ambient (background) level of dissolved arsenic in groundwater. The ambient dissolved arsenic level is controlled by complex geochemical interactions among Eh, pH and minerals able to adsorb, complex, or precipitate arsenic.

3. The introduction of petroleum hydrocarbons (or other degradable organics) may cause a perturbation to the existing geochemistry, resulting in the mobilization of arsenic at concentrations above the ambient level. Petroleum and other degradable organics lower the redox state to more reduced conditions. The primary mecha-nism for lowering the Eh is anaerobic biological activity.

4. The perturbation of the ambient arsenic geochemistry (and related arsenic mobili-zation) will persist until the soluble hydrocarbons are attenuated.

5. Once the hydrocarbons are attenuated, the arsenic will revert to its pre-existing stable geochemistry, which may be above or below the drinking water MCL for arsenic of 0.010 mg/L depending on the background geochemistry.

NOTEThis work is a combined effort of the American Petroleum Institute (API), The Petro-

leum Environmental Research Forum (PERF) and ERM. The API will be publishing a document, “API Arsenic Manual: Attenuation of Naturally Occurring Arsenic at Petro-leum Impacted Sites” in 2010.

RECOMMENDATIONSProper management of a petroleum impacted site at which arsenic has become mobi-

lized requires development of a site specific conceptual model (SSCM). The SSCM should have four main elements:

1. The general site geology and hydrogeology of the groundwater bearing units

(GWBU) that have been or can be impacted by a petroleum release; 2. The ambient arsenic geochemistry within the impacted GWBU; 3. The petroleum distribution and microbial conditions (redox zones); and 4. A survey of potential receptors and exposure pathways for arsenic that has been

mobilized. A well-constructed SSCM has a number of uses including:

Determining the appropriate locations for long term monitoring; Determining the key parameters needed to monitor the effectiveness and status of natural attenuation at the site; Supporting the inclusion of a natural attenuation based approach in the remedia-tion strategy; Illustrating the processes of mobilization and attenuation of arsenic at a petroleum impacted site for discussing with regulators and stakeholders; and Assessing whether efforts beyond natural attenuation are necessary.

REFERENCES

Boulding, Russell and Ginn. 2004. Practical Handbook of Soil, Vadose Zone, and Ground-Water Contamination: Assessment, Prevention, and Remediation. CRC Press, Figure 3.3, pp. 98.

Magaw, R.I., S.J. McMillen, W.R. Gala, J.H. Trefry, and R.P. Trocine. 2001. Chapter 12: Risk evaluation of metals in crude oils. In Risk-Based Decision-Making for Assessing Petroleum Impacts at Exploration and Production Sites, ed. McMillen, S.J. et al., Department of Energy and the Petroleum Environmental Research Fund.

Wilkin, R.T., D. Wallschlaeger, and R.G. Ford. 2003. Speciation of arsenic in sulfidic waters. Geochemical Transactions. Vol. 4, pp. 1-7.

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es p

rese

nt?

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

9

Ars

enic

Sol

ubili

ty v

s. V

alen

ce

Tabl

e 2-

1: R

elat

ive

Solu

bilit

ies

of A

rsen

ite

and

Ars

enat

e Fi

nal C

once

ntra

tion

Cat

ion

Add

ed

Initi

al A

s C

onc.

A

rsen

ate

Ars

enite

Fe

rric

Iron

35

0 g/

L 6

g/L

40

g/L

Ferr

ic I

ron

300

g/L

6 g/

L 38

g/

L

Alu

min

um (A

lum

) 35

0 g/

L 74

g/

L 26

3 g/

L

Alu

min

um (A

lum

) 30

0 g/

L 30

g/

L 24

9 g/

L A

lum

inum

(A

lum

ina)

00

g/

L 4

g/L

00

g/L

Cal

cium

2

mg/

L 20

g/

L 60

g/

L

Tabl

e 2-

3: S

olub

ility

of M

etal

Ars

enat

es

Met

al C

atio

n C

ompo

und

Log

KSP

Al

AlA

sO4

15.8

Mg

Mg 3

(AsO

4) 2

19.7

Ca

Ca 3

(AsO

4) 2

18.2

Ba

Ba3(A

sO4) 2

13

Cr

CrA

sO4

20.1

Fe

FeA

sO4

20.2

Ni

Ni 3(

AsO

4) 2

25.5

Cu

Cu 3

(AsO

4) 2

35.1

2

Zn

Zn3(A

sO4) 2

27

Pb

Pb3(A

sO4) 2

35

.39

Mn

Mn 3

(AsO

4) 2

10.7

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

10A

bsor

ptio

n of

Ars

enat

e an

d A

rsen

ite o

n H

ydro

us

Ferr

ic O

xide

(from

Sm

edle

y an

d K

inni

burg

h, 2

002)

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

11N

atur

al A

ttenu

atio

n of

Ars

enic

•As

cond

ition

s in

gro

undw

ater

bec

ome

mor

e re

duci

ng, a

rsen

ic is

mob

ilize

d•A

ttenu

atio

n of

pet

role

um im

pact

s by

bi

odeg

rada

tion

allo

ws

ambi

ent c

ondi

tions

to

retu

rn•A

s am

bien

t con

ditio

ns re

turn

, red

ox s

tate

of

arse

nic

is re

vers

ible

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

12A

rsen

ic R

edox

ver

sus B

iode

grad

atio

n

4H

+ +

HA

sO4

-2+

2e

-H

3A

sO3

+ H

2O

(E

h0

= ~

+5

0)

3H

3A

sO3

+ 9

H+

+ 5

S-2

AsS

2-+

As 2

S3

+ 3

H2O

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

13Pe

trol

eum

Hyd

roca

rbon

Deg

rada

tion

Arse

nic

Mob

iliza

tion

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

14Th

e A

rsen

ic P

lum

e C

once

ptua

l Mod

el

•A p

lum

e of

ars

enic

mob

ilize

d by

pet

role

um

hydr

ocar

bon

impa

cts

will

mim

ic th

e hy

droc

arbo

n pl

ume

•Sta

ges

of a

rsen

ic m

obili

zatio

n ca

n be

rela

ted

to

the

hydr

ocar

bon

plum

e co

nditi

ons

(exp

andi

ng,

stab

le, o

r shr

inki

ng)

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

15St

ages

of A

rsen

ic M

obili

zatio

n

•Exp

andi

ng p

lum

e•

Influ

x of

HC

>>

Atte

nuat

ion

•Ste

ady

Stat

e•

Influ

x of

HC

= A

ttenu

atio

n

•Shr

inki

ng p

lum

e•

Influ

x of

HC

<<

Atte

nuat

ion

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

16Ex

pand

ing

Plum

e

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

17St

eady

Sta

te

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

18Sh

rink

ing

Plum

e

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

19C

reat

ing

a Si

te-S

peci

fic M

odel

•App

ly g

ener

al m

odel

of a

rsen

ic m

obili

zatio

n w

ithin

a

hydr

ocar

bon

plum

e•U

nder

stan

d th

e am

bien

t con

ditio

ns (w

ill b

e th

e lim

it of

nat

ural

atte

nuat

ion)

•Und

erst

and

the

plum

e st

age

•Det

erm

ine

atte

nuat

ion

indi

cato

rs•U

nder

stan

d ris

k (m

obili

ty

rece

ptor

s)

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

20Si

te-S

peci

fic C

once

ptua

l Mod

el

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

21Ex

posu

re P

athw

ay A

naly

sis

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

22A

pply

ing

a Si

te-S

peci

fic M

odel

•Und

erst

and

the

ambi

ent c

ondi

tions

(will

be

the

limit

of n

atur

al a

ttenu

atio

n)•U

nder

stan

d th

e pl

ume

stag

e•D

eter

min

e at

tenu

atio

n in

dica

tors

•Und

erst

and

risk

(mob

ility

re

cept

ors)

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

23C

ase

His

tory

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

24Po

tent

iom

etri

c Su

rfac

e

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

25Be

nzen

e

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

26A

rsen

ic

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

27A

rsen

ic V

ersu

s TO

CAr

seni

c vs

. Tot

al O

rgan

ics

UTS

Gro

undw

ater

0.00

1

0.010.

1110

0.1

110

100

1000

Tota

l Org

anic

s in

Gro

undw

ater

(mg/

L)

Arsenic in Groundwater (mg/L)

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

28A

sses

smen

t Too

ls

Dat

a ga

ther

ing

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

29

Tabl

e 3-

1: K

ey G

roun

d W

ater

Geo

chem

ical

Par

amet

ers f

or A

sses

smen

t of N

atur

al

Att

enua

tion

of A

rsen

ic a

t Pet

role

um H

ydro

carb

on S

ites

Pa

ram

eter

A

ppro

ach

Met

hod

R

efer

ence

A

sses

smen

t

pH

Flow

-thro

ugh

cell

or

dow

n-ho

le

mea

sure

men

t; pH

pr

obe

Follo

w th

e pH

pro

be o

r m

ulti-

para

met

er p

robe

m

anuf

actu

rer’

s in

stru

ctio

ns

Mas

ter v

aria

ble

– af

fect

s ar

seni

c m

obili

ty,

part

icul

arly

in te

rms o

f su

rfac

e re

actio

n,

sorp

tion

Eh (O

RP)

Flow

-thro

ugh

cell

or

dow

n-ho

le

mea

sure

men

t; pr

obe

can

mea

sure

ORP

; m

easu

re re

dox

pair

co

ncen

trat

ions

for

reac

tion-

spec

ific

E0

Stan

dard

Met

hods

(A

PHA

, 199

2) 2

580B

O

RP p

rovi

des r

elat

ive

data

for a

sses

sing

redo

x co

nditi

ons a

nd c

an

calib

rate

dis

solv

ed

oxyg

en v

alue

s. If

mor

e re

actio

n/m

echa

nism

sp

ecifi

c re

dox

info

rmat

ion

is

nece

ssar

y, re

dox

pair

co

ncen

trat

ions

shou

ld

be a

sses

sed

(see

ars

enic

sp

ecia

tion

or T

EA)

Alk

alin

ity

Fiel

d tit

ratio

n or

co

lori

met

ric

kit,

such

as

Hac

h

Hac

h A

lkal

inity

test

kit;

C

hem

etri

cs; f

ield

tit

ratio

n (d

igita

l or u

se

Stan

dard

Met

hods

(A

PHA

, 199

2))

Fiel

d al

kalin

ity

mea

sure

men

ts a

id in

ge

oche

mic

al fa

cies

id

entif

icat

ion

and

mea

sure

buf

feri

ng

capa

city

D

isso

lved

O

xyge

n (D

O)

Low

-flow

sam

plin

g or

dow

n-ho

le

mea

sure

men

t; ox

ygen

pro

bes

(pre

fera

bly

optic

al)

can

be u

sed;

fiel

d co

lori

met

ric

kits

can

be

mor

e ac

cura

te;

prop

er te

chni

que

criti

cal

Follo

w th

e D

O

prob

e/m

eter

m

anuf

actu

rer’

s in

stru

ctio

ns;

CH

EMet

rics

DO

test

ki

t; re

fer t

o St

anda

rd

Met

hods

(APH

A, 1

992)

45

00

Det

erm

ines

whe

ther

gr

ound

wat

er

cond

ition

s are

aer

obic

or

ana

erob

ic, w

hich

in

dica

tes

the

pote

ntia

l ab

iotic

and

bio

logi

cal

mec

hani

sms

for a

rsen

ic

fate

and

tran

spor

t

Com

petin

g Io

ns

Lo

w-fl

ow sa

mpl

ing:

sa

mpl

ed a

nd

pres

erve

d in

the

field

(r

efer

ence

met

hods

) to

ana

lyze

for P

O4,

Se

O3,

SiO

4, H

CO

3

Stan

dard

Met

hods

C

ompe

ting

ions

can

de

sorb

or d

ispl

ace

arse

nate

and

ars

enite

in

crea

sing

thei

r m

obili

ty. B

icar

bona

te

can

be p

rodu

ced

bilo

gica

lly

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

30

Iron

D

isso

lved

iron

can

be

mea

sure

d in

the

field

w

ith c

olor

imet

ric

kits

; sam

ples

can

be

colle

cted

for

Fe2+

/Fe3

+ spe

cies

or

tota

l dis

solv

ed ir

on

(Fe T

can

be

used

as

an a

ppro

xim

atio

n of

Fe

2+ fo

r man

y Eh

/pH

co

nditi

ons)

Stan

dard

Met

hods

(A

PHA

, 199

2) 3

500-

Fe

B; A

STM

D 1

068-

77,

Iron

in W

ater

, Tes

t M

etho

d A

; CH

EMet

rics

or

HA

CH

kits

(814

6)

Car

e m

ust b

e ta

ken

with

sam

ples

col

lect

ed

for F

e2+/

Fe3+

to

pres

erve

spec

iatio

n; th

e pr

esen

ce o

f iro

n (a

nd it

s sp

ecia

tion)

indi

cate

s cu

rren

t red

ox c

ondi

tion

of G

WBU

, as

wel

l as

atte

nuat

ion

capa

city

for

sequ

estr

atio

n of

di

ssol

ved

arse

nic

Ars

enic

Sp

ecia

tion

Low

-flow

sam

plin

g;

sam

pled

and

pr

eser

ved

in th

e fie

ld

(ref

eren

ce m

etho

ds)

to a

naly

ze fo

r tot

al

arse

nic

(As T

), A

s3+

and

As5

+

EPA

Met

hod

1632

A;

Stan

dard

Met

hod

(APH

A, 1

992)

350

0-A

s B

or C

(Hac

h M

etho

d 80

13);

tota

l ars

enic

by

SW-8

46 6

020B

; see

fu

rthe

r dis

cuss

ion

of

met

hods

in U

SEPA

, 20

07b

Pres

erva

tion

of a

rsen

ic

spec

iatio

n re

quir

es

spec

ial s

ampl

ing

met

hod;

var

ious

sa

mpl

ing

and

field

pr

eser

vatio

n m

etho

ds

are

avai

labl

e; a

rsen

ic

spec

iatio

n pr

ovid

es

info

rmat

ion

spec

ific

to

redo

x po

tent

ial f

or

arse

nic

as it

rela

tes

to

mob

ility

Tabl

e 3-

1: K

ey G

roun

d W

ater

Geo

chem

ical

Par

amet

ers f

or A

sses

smen

t of N

atur

al

Att

enua

tion

of A

rsen

ic a

t Pet

role

um H

ydro

carb

on S

ites

Pa

ram

eter

A

ppro

ach

Met

hod

R

efer

ence

A

sses

smen

t

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

31

Tabl

e 3-

2: K

ey M

icro

biol

ogic

al P

aram

eter

s fo

r Ass

essm

ent o

f Nat

ural

Att

enua

tion

of

Ars

enic

at P

etro

leum

Hyd

roca

rbon

Sit

es

Para

met

er

App

roac

h M

etho

d

Ref

eren

ce

Ass

essm

ent

Alte

rnat

e Te

rmin

al

Elec

tron

Acc

epto

rs

(TEA

)

Low

-flow

sa

mpl

ing;

al

tern

ate

TEA

in

clud

e Fe

3+,

SO42-

, NO

3- , an

d C

O2,

mea

sure

d by

col

lect

ing

and

pres

ervi

ng

sam

ples

ac

cord

ing

to

appr

opri

ate

met

hod;

CO

2, or

oth

er g

ases

, sh

ould

be

sam

pled

by

gas

stri

ppin

g m

etho

d fo

r la

bora

tory

an

alys

is.

Met

hods

dep

end

on

anal

yte

– m

etal

s by

SW

-846

602

0B,

anio

ns b

y EP

A 3

00;

nitr

ate

by S

tand

ard

Met

hods

(APH

A,

1992

) 450

0-N

O3 D

(H

ach

Met

hod

8324

) or

EPA

353

.2/3

53.3

; su

lfate

by

Hac

h M

etho

d 80

51; C

O2

by C

HEM

etri

cs

Met

hod

4500

Inve

stig

ate

alte

rnat

e TE

A a

s app

ropr

iate

for

aqui

fer m

iner

alog

y an

d am

bien

t gro

und

wat

er

cond

ition

s; TE

A

conc

entr

atio

ns p

rovi

de

info

rmat

ion

on re

dox

cond

ition

s, de

grad

atio

n of

hyd

roca

rbon

, and

at

tenu

atio

n ca

paci

ty o

f th

e aq

uife

r.

Tota

l Org

anic

C

arbo

n Lo

w-fl

ow

sam

plin

g;

colle

ct sa

mpl

e fo

r lab

orat

ory

anal

ysis

.

SW-8

46 9

060

Tota

l org

anic

car

bon

indi

cate

s pr

esen

ce o

f en

ergy

sour

ce fo

r m

icro

bial

pro

cess

es.

Mol

ecul

ar

Hyd

roge

n, H

2 Lo

w-fl

ow

sam

plin

g;

head

spac

e eq

uilib

rium

by

“bub

ble-

stri

ppin

g”

met

hod.

Cha

pelle

, et a

l., 1

995,

19

97; W

eide

mei

er,

1998

Alth

ough

diff

icul

t to

colle

ct, u

sefu

l in

dete

rmin

ing

spec

ific

redo

x st

ate

and

prim

ary

TEA

.

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

32Si

te-S

peci

fic C

once

ptua

l Mod

el

Del

iver

ing

sust

aina

ble

solu

tions

in a

mor

e co

mpe

titiv

e w

orld

W.O

./Ini

t./D

ate,

33M

anag

ing

Ars

enic

at P

etro

leum

-impa

cted

Site

s

•Pe

trol

eum

hyd

roca

rbon

s pe

rtur

b th

e ex

istin

g ge

oche

mis

try,

mob

ilizi

ng a

rsen

ic if

it is

alre

ady

pres

ent (

as

a so

il m

iner

al o

r fro

m a

nthr

opog

enic

sou

rces

). •

This

per

turb

atio

n of

the

ambi

ent a

rsen

ic g

eoch

emis

try

pers

ists

unt

il th

e so

lubl

e hy

droc

arbo

ns a

re a

ttenu

ated

and

th

e am

bien

t red

ox c

ondi

tion

is re

stor

ed.

•O

nce

the

hydr

ocar

bons

are

atte

nuat

ed a

nd th

e re

dox

cond

ition

is re

stor

ed, t

he a

rsen

ic w

ill re

vert

to it

s pr

e-ex

istin

g st

able

geo

chem

istr

y, w

hich

may

be

abov

e th

e M

CL.

•A

site

-spe

cific

con

cept

ual m

odel

incl

udes

ass

essm

ent o

f th

e am

bien

t con

ditio

ns a

nd s

tate

of a

rsen

ic to

det

erm

ine

the

effe

ct o

f pet

role

um h

ydro

carb

on a

nd th

e po

tent

ial f

or

natu

ral a

ttenu

atio

n of

mob

ile a

rsen

ic.