A mass balance model for the fate of PAHs in the San Francisco Estuary

Ben K. GreenfieldJay A. Davis

San Francisco Estuary Institute

Presented at the Calfed Science Conference, January, 2003 

Source: U.S.G.S./Center For Land Use Interpretation

Population Growth, Bay Area - 1860 to 2000Data Source: MTC and ABAG, 2002

0

1,000,000

2,000,000

3,000,000

4,000,000

5,000,000

6,000,000

7,000,000

1860

1870

1880

1890

1900

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

(Each Color is a Bay Area County)

Mass balance models• Understand compound fate• Identify research priorities

(e.g. chemical properties)• Synthesize available data• Estimate loading rate

Simple mass balance model• Mackay et al. (1994)• Equilibrium partitioning based on

chemical properties• Single pool of interacting water and

sediment• Daily rate constants• Spreadsheet format

Combined External Loads

Volatilization

Outflow

DissolvedPCB

Sorbed PAH

Dissolved PAH

Sorbed PAH

Burial

Water

Active

Sediment

Layer

Buried

Sediment

Dissolved PAH

Particulate PAH

DegradationDegradation

Degradation DiffusionDeposition

AndResuspension

• 2 ring – naphthalene

• 3 ring – phenanthrene

• 4 ring – fluoranthene and • benz(a)anthracene

• 5 and 6 ring – e.g. benzo(b)fluoranthene Fluoranthene Benz[a]anthracene

Year

0 1 2 3 4 5

PA

H in

Bay

Pe

rce

nt

Ori

gin

al M

ass

50

100

Naphthalene Phenanthrene Fluoranthene Benz(a)anthraceneBenzo(b)fluoranthene

0

Turnover Rate

Year

0 5 10 15 20

Per

cent

Orig

inal

Mas

s

0

25

50

75

100Phenanthrene

Fluoranthene

Benzo(b)fluoranthene

PCB 118

PCB 118

PAHs

Loss Pathways of PAH Mass in EstuaryAssuming No Load (One Year Simulation)

Compound

N F B(a)a B(b)f

Pro

port

ion

of T

otal

0.0

0.2

0.4

0.6

0.8

1.0

Volatilization Outflow Degradation Mass Remaining

Compound

N P F B(a)a B(b)f Da B(a)p

Deg

rada

tion

Rat

e (d

-1)

0.0001

0.001

0.01

0.1

1

10

Vary by PAH Compound

Wat

er S

ide

MT

C

Air

Sid

e M

TC

Hen

ry's

Law

Co

nst

ant

Ko

w

Deg

rad

atio

n W

ater

Deg

rad

atio

n S

edim

ent

Pro

port

ion

of T

otal

0

20

40

60

80

100

Uncertainty for Benzo(b)fluoranthene

0

20

40

60

80

100

Sensitivity to Different Chemical Parameters

Wat

er S

ide

MT

C

Air

Sid

e M

TC

Hen

ry's

Law

Co

nst

ant

Ko

w

Deg

rad

atio

n W

ater

Deg

rad

atio

n S

edim

ent

Losses

DegradationVolatilizationOutflowBurial

Inputs?

Loading Point Source Air Deposition Rivers Storm Drains

Trend?

Inputs?

Loading?

Trend?

Losses

Degradation?VolatilizationOutflowBurial

To

tal P

AH

s (µ

g/k

g)

Sediment Trends

Year

1990 1992 1994 1996 1998 2000 20020

200

400

600

800

1000

1200

1400

1600

1800

2000

To

tal P

AH

s (

g/k

g li

pid

)

Bivalve Trends

Year

1993 1994 1995 1996 1997 1998 1999 2000 20010

1000

2000

3000

4000

5000

6000

1990

1980

From Pereira, W.E., et al. 1999 Marine Chemistry

Dated Sediment Core Chemistry

Inputs?

Loading?

Trend?

Losses

Degradation?VolatilizationOutflowBurial

Loading rate (kg/yr)

Degradation rate (d-1)

300 < 0

2000 0

10,000 0.0002

20,000 0.00035

460,000 0.01

Lit

era

ture

Est

imat

es

Result summaryLoss rates (half life)• 2 ring PAH - 3 week• 4 ring PAH - 1 year• 5 ring PAH - 6 years• Degradation rate uncertainty causes

considerable model uncertainty• Obtained upper bound on degradation rate and

lower bound on loading rate

Significance of findings• Much more rapid expected response to

management changes than PCBs• Future priorities:

Local degradation rates Dr. Michael Montgomery (NRL) - experimental

data on Bay sediments Local sediment-water partitioning

Acknowledgments

• Don Yee

• Jon Leatherbarrow

• Sarah Lowe

• Cristina Grosso

• Patricia Chambers