PREDICTING THE FATE OF PERSISTENT ORGANIC POLLUTANTS USING A BIOGEOCHEMICAL MODEL

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PREDICTING THE FATE OF PERSISTENT ORGANIC POLLUTANTS

USING A BIOGEOCHEMICAL MODEL


Created by


Susan Libes, Professor

Marine Science Department

Coastal Carolina University

Conway, South Carolina


For her Environmental Chemistry class after attending the
1996 NSF/DUE/UFE
-
sponsored

Summer Practicum on Environmental Problem Solving hosted by the New York Great Lakes Research Consortium


Background

There are many valuable research and management uses for a mass balance
-
based model of the
fate and transport of

toxic chemicals in aquatic systems. Most of these uses depend on the ability
of the model to deterministically and quantitatively relate the concentrations of toxic compounds
in water, sediments and biota to the source inputs. This type of model can then
be used to predict
the responses of a particular aquatic system to various alternative regulatory and remedial action
scenarios. The LAKE TOX spreadsheet model can be used to compare the steady
-
state responses
of a lake to a toxic chemical under many scena
rios. This model includes many important
biogeochemical processes, e.g., gaseous transport across the air
-
water interface, scavenging by
particles, pelagic sedimentation of these particles, bioturbation, first
-
order degradation reactions,
and uptake by aq
uatic organisms. The model makes an excellent tool for investigating the
sensitivity of toxic chemical concentrations in the water column and active sediments to the
various environmental conditions and processes that govern the fate and transport of toxic

chemicals in large lakes, like the Great Lakes, which are similar to marine systems. This model
was developed for the Great Lakes because of the long
-
standing and numerous pollution
problems that these large systems suffer. The model could theoretically b
e applied to the oceans
if we had a better understanding of how to model water circulation.


If you are interested in seeing the mathematics behind the modeling in this exercise, refer to the
documents by T.C. Young and J.V. DePinto in your workshop note
book and CD.


Project

The object of this exercise is to evaluate how a large lake’s morphometric, hydraulic and
sedimentary characteristics affect its response to loading of a hydrophobic organic chemical. We
will examine PCBs that are persistent bioaccumu
lative toxins (PBT) and hormonally active
agents (HAA). Loading is simulated as either an areal (watershed, non
-
point source) or
atmospheric input or as a point
-
source input such as from a tributary or outfall pipe. Your task is
to use the model to make th
e runs listed below for each of the Great Lakes: Superior, Erie,
Michigan and Ontario. Table 1 gives a comparison of the morphometric and hydrologic
properties of each lake. Table 2 gives the data needed to define the solids dynamics of each lake
that incl
udes the sediments as well as sinking and suspended particles. In addition to these lake
-
specific data, there are several additional model parameters (mostly chemical specific data) that
we shall assume are the same for all four lakes. They are given in Ta
ble 3. All of these data have
already been typed into each lake’s spreadsheet model so you have one spreadsheet file for each
Great Lake. Before you start modeling, open all of the spreadsheets (ONTARTOX, SUPERTOX,
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ERIETOX and MICHTOX) and acclimate yourse
lf to the model by answering the following
questions. Copies of the spreadsheets are on the CD provided by the instructor.


1. For Lake Ontario, look at the worksheet entitled “LakeTOX Mass Bal” to answer the
following questions.


What are the input pathw
ays of PCBs to the lake?




What are the output pathways from the lake?




Is the difference between inputs and outputs significant, i.e., is the lake in steady state?




2. Look at the worksheet entitled “WC and Sed Results” to answer the following que
stions:


In what two species/forms in the water column are PCBs found? ___________,
____________


In what two species/forms in the sediments are PCBs found? ____________,
____________


3. The model computes the concentrations of the species/forms usin
g a competitive complexing
approach (speciation). In this application, the equilibrium constants are referred to as “partition
coefficients”. For any one of the lakes, look at the worksheet entitled “Input Data” to answer the
following questions.


In the s
ection labeled “PARTITIONING DATA”, find the two following parameters and
record their original values (CHECK THAT THESE NUMBERS ARE THE SAME AS THOSE
IN TABLES 1


3):


Org Carbon Partition Coefficient in the water column (K
ocw
): _______________


Org Car
bon Partition Coefficient in Sediments (K
ocs
): _________________



K
ocw
= Concentration of toxicant in particles/Concentration of toxicant in water

K
ocs

= Concentration of toxicant in sediment/Concentration of toxicant in pore water


In the section entitle
d “CHEMICAL PROPERTIES AND DYNAMICS DATA”, find and
record the value of the Decay rate of "dissolved" chemical in the water column: ____________


What is the value of the Decay rate of "dissolved" chemical in Sediments? ___________


What does this tell you

about the relative reactivity of PCBs? ______________

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Under the section entitled “SOLIDS DYNAMICS ”, find and record in the following
table the concentrations of the Suspended Solids in the water column for each lake.


Lake

Suspended Solids in Water Col
umn (g/m
3
)

Superior


Michigan


Ontario


Erie



4.
The model also computes the concentration of a toxicant in the tissues of organisms from each
level of a four
-
level food chain. This calculation is done on the worksheet entitled: “Bioaccum”.


What a
re the four trophic levels in the model food chain?




This bioaccumulation calculation also relies on a mass
-
balance approach. The following input
processes are included. Adsorption of the pollutant from the water to organisms is modeled using
a partitio
n coefficient called a
Bioconcentration Factor (BCF)
. This is defined as the toxicant
concentration in the organism/toxicant concentration in the water. The other input is through
consumption of food contaminated with the toxicant. The latter is only a con
cern for consumers
and not for primary producers.


Since the concentration of toxicant in a consumer is a complicated function of multiple input and
output routes (e.g., surface absorption, food intake, depuration, excretion), the bioconcentration
factors

for these organisms are referred to as
Bioaccumulation Factors (BAF)

and represent the
impact of all the processes taken together. (In the case of the phytoplankton the BCF is the
BAF.) Loss of the toxicant through excretion is output process included in
the model.


Because the concentration of a toxicant also depends on the mass of the organism, the model also
must include corrections for changes due to growth (e.g., pollutant concentrations are “diluted”
as fish grow, concentrated as they accumulate fat,

etc.). Submodels for fish bioenergetics are
used to estimate fish growth and can be found in the worksheets entitled: “smfish growth” and
“lrgfish growth”. We do not have time to explore the bioenergetics model in this exercise.


Look at the graph at the
bottom of the “Bioaccum” spreadsheet. What happens to the
toxicant concentration with increasing trophic level?




This is referred to as biomagnification. The degree of
biomagnification (BMF)

for each trophic
step is computed by taking the ratio of the B
AF’s for each of the two steps. For Lake Ontario:

What is the BMF for Phytoplankton
-
to
-
Zooplankton? _________

What is the BMF for Zooplankton
-
to
-
Small Fish? _________

What is the BMF for the Small Fish
-
to
-
Large Fish? _________


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Give a possible explanatio
n for why biomagnification occurs and why it varies from
trophic level to trophic level.






If a toxicant is reactive, will it have as significant a biomagnification effect? Explain
your answer.







Model Run 1: Exploring the Effect of Equal Areal

Loadings (Unequal Total Loadings)


1.

Run the model for each lake using an equal areal loading (relative to the lake surface area) of
20

g/m
2
-
yr of PCBs. To do this, remove any tributary and point source loading (make this
value = 0) and adjust the atmospheric deposition until the areal atmospheric loading is 20

g/m
2
-
yr (a rough dimensional analysis computation should help you estimate th
e
appropriate value to enter). On a physical level, it is easiest to achieve a constant areal
loading of pollutant from lake to lake using aerial deposition because the dry or wet fallout
would have the same pollutant content from lake to lake. Record the
resulting Total
Chemical Loading in kg/yr for each lake in the following table. This value represents the sum
of the delivery routes of pollutant: (1) river and point source loading (now set to 0), (2)
atmospheric dry and wet fallout, and (3) gas phase ads
orption (simple dissolution).


Lake

Total Loading (kg/yr)

Superior


Michigan


Ontario


Erie



FOR EACH CHART THAT FOLLOWS, BE SURE TO INCLUDE THE CHART AND THE
DATA USED TO CONSTRUCT IT IN YOUR REPORT. INCLUDE A DESCRIPTIVE TITLE
AND LABEL THE AXES (
INCLUDING UNITS)


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2.

Make a bar chart (as illustrated below) comparing the steady
-
state water column
concentrations of the particulate (ng/g) and dissolved (ng/L) species among the lakes.




3.

Make a similar chart comparing the total sediment concentrations

(ng/g) for each lake.


Which lake(s) has the highest steady
-
state concentration in each medium (water and
sediment)?

Water: Sediment:



Which lake(s) has the lowest?


Water:

Sediment:


Explain why the lakes have different responses to the same loading. Hint: Look at the
differences in properties given in Tables 1 and 2.


Water:




Sediment:





Steady-State Water Column Concentrations
Run 1: 20

g/m
2
-yr
log K
OCW
= 5 and log Kocs = 4
0.16
0.18
0.20
0.22
0.24
Superior
Michigan
Ontario
Erie
PCB Conc (ng/L)
Dissolved
Particulate
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4.

Make

a bar chart showing bioaccumulation across the four trophic levels for each lake as
shown above.


Explain the general trends seen in the bioaccumulation chart within and between lakes.







Model Run 2: Exploring the Effect of Equal Total Loading


1.

Rep
eat Run #1 using a constant total PCB loading of 5000 kg/y. This loading models the
effects of point source inputs through either tributaries or outfall pipes. To do this, zero out
the Atmospheric Deposition, and enter 5000 kg/yr as the Tributary and Point

Source
Loading. Make a bar chart comparing the steady
-
state water column concentration for each
lake at this new loading level.


2.

Make a similar plot comparing the sediment concentration for each lake.


3.

Also make a bar chart showing bioaccumulation acro
ss the four trophic levels for each lake.


4.

Compare the results of Run #2 to Run #1 by answering the following questions. Hint: Make
sure to compare the relative size of the loadings between Runs 1 and 2 using common units.


What effect does this change in

loading have on the water and sediment concentrations?
Explain why this occurs.




Is the order of response among the lakes the same? If so, why? If not, why not?







Bioaccumulation of PCBs
Run 1: 20

g/m
2
-yr
log K
OCW
= 5 and log K
ocs
= 4
0
10
20
30
40
50
60
70
80
Level 1
Level 2
Level 3
Level 4
PCB Conc (ng/g)
Superior
Michigan
Ontario
Erie
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How does this change in loading affect the bioaccumulation trends within each la
ke and
between lakes?









Model Run 3: Exploring the Effects of Pollutant Type


1.

1. Using the input data from Run #1 for Lake Erie, change K
ocw

= 10
5

to 10
6

and K
ocs

= 10
4

to 10
5

(increased hydrophobicity) and then change K
ocw

to 10
4

and K
ocs

to 10
3

(decreased
hydrophobicity). Since a given toxicant has a unique K
ocw

and K
ocw
, changing these
parameters gives you a chance to see what the behavior of different toxicants would be like.
The higher the K
oc
, the more hydrophobic the chemical and hence the

less soluble in water it
is. Plot the total water column response for all three runs on a bar chart.

2.

Make a similar plot comparing the total sediment concentrations for each run.

3.

Make a bar chart showing bioaccumulation across the four trophic levels for
Lake Erie as


shown below.

Explain what happens to the partitioning of the toxicant between sediment and water as
the hydrophobicity of the toxicant is increased. Explain why this happens.







Effect of K
ocw
on Steady-State Arochlor 1254 Concentrations
in Lake Erie for Run 3: 20

g/m
2
-yr
0
10
20
30
40
50
60
70
80
4/3
5/4
6/5
log K
ocw
/log K
ocs
Concentration (ng/g or ng/L)
Level 1
Level 2
Level 3
Level 4
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Which chemicals (the most or the least hydrophob
ic) create the biggest sediment
pollution problems? Why?





Explain what should happen to the bioaccumulation of toxicants as the hydrophobicity of
the toxicant is increased. Did you observe this? If not, explain what needs to be changed
in the model to
produce more realistic bioaccumulation results.









If you have time remaining, feel free to explore the sensitivity of the model for
one

lake to
changes in one input or loading parameter at a time. Try
+
2X and
+
10X changes. BE SURE TO
SAVE THE F
ILES UNDER NEW NAMES BEFORE MAKING CHANGES.



REFERENCES


DePinto, J.V. 1996. Spreadsheet model for PCB mass balance in the Great Lakes. Great Lakes
Research Consortium Summer Practicum for Environmental Problem Solving.
NSF/DUE/UFE.


Rodgers, P.W., J.V. D
ePinto, W. Booty and T. Slawecki. 1987. LTI toxics model application:
PCB’s in lake Ontario


An exploratory application. Report to the IJC Task Force on
Chemical Loadings. 49 p.


Stewart, D.J. 1996. Spreadsheet model for fish bioenergetics. Great Lakes Re
search Consortium
Summer Practicum for Environmental Problem Solving. NSF/DUE/UFE.


Task Force on Chemical Loadings. 1988. Report on Modeling the Loading
-
Concentration
Relationship for Critical Pollutants in the Great Lakes. Report to the Great Lakes Water

Quality Board, International Joint Commission, Windsor, ON. 275 p.


Young, T.C. 1996. Spreadsheet model for bioaccumulation in an aquatic food chain. Great Lakes
Research Consortium Summer Practicum for Environmental Problem Solving.
NSF/DUE/UFE.



**This

exercise was modified by J.M. Haynes (8/02). Dr. Libes’ exercise (1997) was derived
from the GLRC/NSF Summer Practicum exercise created by J.V. DePinto (1996).**

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Table 1. Comparison of Lake Morphometric and Hydrologic Properties


Property

Lake

Superio
r

Michigan

Ontario

Erie

Volume, V (m
3
)

1.10 x 10
13

4.72 x 10
12

1.67 x 10
12

4.90 x 10
11

Surface Area, SA (m
2
) (V/z)

8.30 x 10
10

5.78 x 10
10

1.95 x 10
10

2.50 x 10
10

Mean depth, z (m)

132.5

81.7

85.6

19.6

SA/V Ratio (m
-
1
)

0.0075

0.012

0.012

0.051

Hydraul
ic Outflow, Q (m
3
/yr)

6.40 x 10
10

4.64 x 10
10

2.00 x 10
11

1.80 x 10
11

Hydraulic Retention Time, Tw (yr)

(residence time of water) (V/Q)

171.9

101.7

8.4

2.7

Overflow Rate, z/Tw (m/yr)

0.77

0.80

10.26

7.20


Table 2. Comparison of Lake Solids Dynamics


Pr
operty

Lake

Superior

Michigan

Ontario

Erie

Water Column Suspended Solids, S
w

(g/m
3
)

0.5

1.0

0.65

5.7

Sediment Bulk Density, S
s

(g/m
3
)

(particle density x porosity)


2.40 x 10
5


2.40 x 10
5


2.40 x 10
5


2.40 x 10
5

Gross Water Column Settling Velocity, v
s
(m/yr) (for suspended solids from sediment
trap data)

500

500

500

500

Gross Resuspension Velocity, vr (m/yr)

6.32 x 10
-
4

1.70 x 10
-
3

5.54 x 10
-
4

4.72 x 10
-
3

Sediment Burial Velocity, vb (m/yr)

4.10 x 10
-
4

3.83 x 10
-
4

8.00 x 10
-
4

7.20 x 10
-
3

Net Water
column Solids Deposition Rate,
vn (m/yr)

197

92

295

302

Net Water Column Deposition Flux, Fn
(g/m
2
-
yr)

98.4

90.4

192.0

1721.5

Depth of upper mixed sediment layer, zs
(m) (depth of bioturbated layer)

0.05

0.05

0.05

0.05

Solids Residence Time in Sediments
, Tss
(yr)

122

133

62.5

7.0

Solids Residence Time in Water Column,
Tsw (yr)

0.67

.09

0.29

0.065














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Table 3. Input Data Parameters Common to all Lakes


Property/Parameter

Value

Units

Gas film transfer rate, K
a

1.58 x 10
5

m/yr

Sediment
-
water d
iffusion rate, K
f

3.65

m/yr

Water column decay rate**, K
dw

0

yr
-
1

Sediment decay rate**, K
ds

0

yr
-
1

Arochlor 1254 PCB molecular weight, M
w

3.26 x 10
11

ng/mole

*Organic carbon partition coeff. (water), K
ocw

1 x 10
5

L/kg

*Organic carbon partition coeff.

(seds), K
ocw

1 x 10
4

L/kg

Organic carbon fraction of S
W
, f
ocw

0.1


Organic carbon fraction of S
S
, f
ocs

0.04


Atmospheric PCB gas phase conc., C
a

0.5

ng/m
3

Surface water temperature, T

12

o
C

**first
-
order process for all chemical and biological degrad
ation processes


*These partition coefficients are essentially equilibrium constants defined as:


K
ocw
= Concentration of toxicant in particles/Concentration of toxicant in water

K
ocs

= Concentration of toxicant in sediment/Concentration of toxicant in por
e water