Title: Sensitivity analysis on the biochemical reactions that affect mercury fish concentrations.
1Sensitivity analysis on the biochemical
reactions that affect mercury fish concentrations.
A modeling approach
Representation of mercury cycle in E-MCM
Parthavi Pathak
2Table of contents
- Introduction
- Objective
- Approach
- Sensitivity Analysis
- Outlook
3Mercury Cycle in Sediment
According to current version of the model
- Sulfate reducing bacteria are the major
biological contributors of methyl mercury
formation in sediments.
4- 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
5Objective
- 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.
6Approach
- 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
7Effect 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) -
8- 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
9Methyl Switch
This input is a switch which selects which HgII
complexes are available for methylation
10Methylating Efficiency of microbes
- Methylating efficiency of microbes, per unit of
carbon flux and unit available HgII
concentration. (g MeHg/g TOC labile )
11Half 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
12Depth 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.
13Bacterial 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
14De-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
-
15- Hg Rate Constant Input Values in E-MCM for
methylation - De-methyl Switch
- De-methylating Efficiency of microbes
16De-methyl Switch
- This input is a switch which selects which HgII
complexes are available for de methylation.
17Demethylating 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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19Summary
- 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.