Sensitivity analysis on the biochemical reactions that affect mercury fish concentrations.

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Sensitivity analysis on the biochemical
reactions that affect mercury fish concentrations.
A modeling approach
Representation of mercury cycle in E-MCM
Parthavi Pathak
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Table of contents

• Introduction
• Objective
• Approach
• Sensitivity Analysis
• Outlook

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Mercury Cycle in Sediment
According to current version of the model

• Sulfate reducing bacteria are the major
  biological contributors of methyl mercury
  formation in sediments.

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• SRB mediate the formation of sulfide as a result
  of respiration processes that require sulfate
  (SO42-) as a terminal electron acceptor Loss of
  methylmercury from pore waters occurs as a result
  of demethylation
• Studies conclude that inhibition of MeHg
  formation occurs in high sulfate environment.
• Process involved are Methylation and
  De-methylation

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Objective

• To make use of E-MCM model (prepared by Tetra
  Tech Inc) for Sunday lake.
• To evaluate most important biochemical factors
  affecting fish mercury concentrations.
• To determine which of the kinetic parameters of
  the biochemical reactions most significantly
  affect mercury transformation.

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Approach

• All analysis has been done using generic data
  set.
• Picked mercury in fish as target for keeping it
  as dependent variable.
• Bacterial methylation
• Bacterial methylation occurs in active zone of
  sulfate reducers.
• It depends on quantity of accessible HgII in pore
  water or surface water, and the activity of
  methylating microbes.
• Methylation entails supply of carbon.
• To estimate activity of SRB carbon decomposition
  rate and limiting factor (carbon/sulfate) must be
  known.
• The overall decomposition rate has two
  components decomposition of DOC and POC

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Effect of sulfate on methylation reaction

• M Cdecomp Efficiency Sulfate Effect
  HgIIavail Area Porosity
• where
• M methylation rate, ug HgII day-1
• Cdecomp g carbon decomposed per day per m2
  sediment (for the sediment layer being
  modeled)
• Sulfate Effect 1 ((X SO4supply )
  (SO4supply KSO4,supply)-1) (dimensionless)
• X Factor to relate the efficiency with which
  microbes methylate
• SO4supply Sulfate concentration (eq L-1)
• KSO4supply Half saturation constant for sulfate
  effect (eq L-1)

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• Efficiency methylating efficiency of microbes
• HgIIavail concentration of dissolved HgII in
  water or pore water which is available for
  methylation (ug HgII m-3)
• Area sediment area (m2)
• Porosity porosity of sediments
• Hg Rate Constant Input Values in E-MCM for
  methylation
• Methyl Switch
• Methylating Efficiency of microbes
• Half saturation constant
• Maximum Sulfate effect
• System specific Hg Rate Constant Input Values
• Depth of methylation zone in sediments
• Base temperature at which methylation rate is
  measured
• Henrys law constant for MeHg

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Methyl Switch
This input is a switch which selects which HgII
complexes are available for methylation
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Methylating Efficiency of microbes

• Methylating efficiency of microbes, per unit of
  carbon flux and unit available HgII
  concentration. (g MeHg/g TOC labile )

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Half Saturation Constant for sulfate
Concentration (KS04)

• This value is used in the estimation of
  microbial methylation rates. Set KSO4 to zero to
  remove any sulfate effect on methylation. Lower
  Limit gt0 Upper Limit NA

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Depth of methylation zone in sediments

• Methylation begins in the sediments at the
  anaerobic threshold and only occurs where the
  sediments are assumed to be saturated. Lower
  Limit gt0 Upper Limit NA
• The model gives no change in graphs by changing
  the depth values.

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Bacterial de-methylation

• A bacterial process powered by methyl mercury
  availability and activity of de-methylating
  microbes.
• The product of bacterial de-methylation is
  assumed to be elemental mercury
• Similar to methylation, carbon decomposition rate
  and limiting factor must be known

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De-methylation Reaction

• D Cdecomp Efficiencyd MeHgavail Area
  Porosity
• where
• D De-methylation rate, ug HgII day-1
• Cdecomp g carbon decomposed per day per m2
  sediment (for the sediment layer being
  modeled)
• Efficiency De-methylating efficiency of
  microbes
• MeHgavail concentration of dissolved MeHg
  in water or pore water which is available
  for de-methylation (ug HgII m-3)
• Area sediment area (m2)
• Porosity porosity of sediments

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• Hg Rate Constant Input Values in E-MCM for
  methylation
• De-methyl Switch
• De-methylating Efficiency of microbes

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De-methyl Switch

• This input is a switch which selects which HgII
  complexes are available for de methylation.

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Demethylating efficiency of microbes

• Demethylating efficiency of microbes, per unit of
  carbon flux and unit available MeHg
  concentration.
• Demethylation does not depend on temperature in
  this version of the model.
• Lower Limit gt0 Upper Limit NA (g ElemHg/g TOC
  lab )

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Summary

• Kinetic parameters, representing principal
  mechanisms for mercury speciation and transport,
  are major targets for the sensitivity analysis.
• The efficiency of microbes is the most sensitive
  variable.
• Methyl or de-methyl switch What mercury
  complexes are we missing?
• Future work
• Use the model for Sunday lake calibrated data.
• Identify the most dependent kinetic variable for
  fish mercury concentration.