A Methylmercury Budget for San Francisco Bay

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A Methylmercury Budget for San Francisco Bay

• Donald Yee, San Francisco Estuary Institute

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Mercury Conceptual Model

• System is complicated, simplified by single box
  model
• Slow response
• (decades)
• MeHg matters most (to biota)

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Methylmercury Conceptual Model

• Need to track MeHg
• MeHg lt1 of totHg
• Poor MeHgtotHg correlation
• Differences from Hg 1 Box Model
• Methylation demethylation
• Potentially rapid (days- months)

Demeth
Sed-water exchange
Demeth
Meth
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WWMMBD?

• What Would the MeHg Mass Budget Do?
• Synthesize- do Bay data make sense given
• Loading, production, degradation, sed-water
  exchange, and other processes?
• Quantitative conceptual model of MeHg
• ID key factors for MeHg fate
• Feasibility/needs of refined model(s)
• E.g. temporal spatial detail
• What it wont/cant do
• Identify hot spot impacts (1 box)
• Predict long term fate (no Hg linkage)

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MeHg 1 Box Model

• Adapted from PCB 1 box model
• One water compartment
• One sediment compartment (10cm mixed layer)
• Daily time step
• Annually uniform (no seasonality)
• Constant uniform mixing
• Equilibrium partitioning
• Simplifications worked for PCBs, PBDEs

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External Loads (Imports)

• Direct atmospheric (wet) deposition 0.1 g/d
• Area x literature rain MeHg x local rainfall
• Delta (Mallard Island) discharge 9.8 g/d
• Flow x concentration (Region 5 MeHg TMDL)
• Local watersheds 4.9 g/d
• RMP measured watersheds (extrapolated)
• Wetlands (upper range estimate) 2.0 g/d
• Volume x (incoming - outgoing) concentrations
• POTWs (16 largest, 95 discharge) 0.8 g/d
• Flow x concentration
• 17.6 g/d total

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Internal Load- MeHg Production

• Function of multiple factors-
• Would need complex C S Hg model
• Next best- lab incubation production rates?
• Marvin-DiPasquale et al anaerobic incubations
• Assume portion of sediment layer methylates
• Methylating zone in fraction (30) of sediment

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Loss Processes

• Bio-uptake export from Bay 0.13 g/d
• Small fish biomass (CDFG) x concentration (RMP)
• 1-Box Model Losses
• Volatilization
• Air/water partitioning (Lindqvist Rodhe 1985)
• Outflow (through Golden Gate)
• Tidal mixing (Connolly), assume ocean MeHg 0
• Burial
• Fuller sedimentation 0.88cm/yr (9 of mixed
  layer)

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Modeled Processes

• Degradation
• Sediment Marvin-DiPasquale demethylation rates
  0.083/d (decay)
• Assume demethylating zone (70 of mixed layer)
• Water Krabbenhoft Petaluma water half life7
  days (0.10/d decay)
• Benthic flux
• In daily resuspension de/sorption
• Large uncertainties some parameters
• Some have small no effect

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Base Case Run

• Base case averaged
• initial concentrations (from RMP monitoring)
• loading/process parameter values
• Quick steady state, within 5 of T0
• Sediment mass
• Water mass lower

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Base Case Run

• Mass (inventory) vs daily flux/degrade/produce
• Water Mass
• Net sediment to water exchange, ext load
• Degradationgt, GG outflow, gtgt bio-uptake,volatiliza
  tion
• Total (WaterSediment)
• Production balances degradation gtgt all other
  processes
• Flux box measurement similar .014 kg/d (Choe
  et al)

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Hot _at_! Model Responds Fast!?

• Seasonal de/meth rates (winter -30)
• month response!
• Yes, but
• Model oversimplifies (mixing, equilibrium)
• Processes vary on microscale (e.g.
  de/methylation)
• Still a good order of magnitude tool

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Parameter Sensitivity
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WDMMBD?

• What Did the MeHg Mass Budget Do?
• Did Bay data make sense?
• Base case near starting state- near right
  Baywide?
• Non-unique solution (e.g. offsetting errors?)
• Feasibility/needs of refined model(s)
• 1 box driven by steady state/equilibrium
• Basis for more detailed model?
• Much higher data needs
• Key factors affecting MeHg fate
• External loads have small/medium effect
• Very sensitive to de/methylation rates

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Management Strategy Dr. Evil

• Acquire 1 Million
• Option A- Control Methylation
• Sterilize the Bay (thermonuclear device)
• Option B- Control Demethylation
• Equip sharks w/ UV lasers to photodemethylate

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Management Strategy -RMP

• Option C- RMP Mercury Strategy
• Where biota affected (food web entry)
• ID disproportionate (high leverage) pathways
• ID intervention opportunities
• IF strategy finds locations where critical
  pathways (e.g. de/methylation) may be acted on
• THEN act (e.g.holding ponds, aeration, dredging,
  nutrient reductions, etc)
• Monitor model management effectiveness
  adaptive management
• (Unfortunately likely gt 1 million)

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Acknowledgements

• Too many to list
• If I have seen further it is by standing on ye
  shoulders of Giants
• Sir Isaac Newton

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Atmospheric (Wet) Deposition

• No local data
• RMP MDN station only measured totHg
• Literature rainfall MeHg (avg 0.11 ng/L)
• Watras Bloom (1989 Olympic Penins. WA 0.15ng/L)
• Risch et al (2001-2003 Indiana, 0.06ng/L)
• St Louis et al (1995, ELA area, 0.05ng/L)
• Mason et al (1997, Still Pond, MD, HgT x MeHg
  avg 0.04ng/L)
• x Local annual precipitation (0.45m/y)
• 0.10 g/d deposition Baywide

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Discharges from

• Delta (SWRCB Region 5)
• Flow weighted avg concentration x mean annual
  discharge 4.7g/d in Hg TMDL
• Revised to w/ later data
• Local watersheds
• Extrapolate w/ SIMPLE Model (modeling mine
  urban non-urban areas)
• Local MeHg data, extrapolated to Bay area (3.6
  g/d)
• Local Hg data x MeHg, extrapolated to Bay area
  (6.2 g/d)
• Use average of above 4.9g/d

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Discharges from

• Wetlands
• Wetland Goals est. 40k acres wetland (1.6e8 m2),
  assume 0.3m overlying water every day
• Petaluma marsh extrapolation
• 50 water particulate settles -1.2g/d
• ebb tide dissolved conc 2.5x flood tide (max 5x
  at Petaluma) 3.2g/d
• net 2g/d load to Bay
• USACE Hamilton AAF leaching assumptions
• 0.8/d of net production 4.0g/d load
• Stephenson et al showed net import and export
  different events for Suisun Marsh
• May be difficult to refine net load

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Discharges from

• POTWs
• Annual mean conc x discharge for 16 largest
  plants (loads for each plant calculated then
  summed) 0.79g/d
• Conc range 0.04-1.3ng/L (mean 0.42ng/L)
• Discharge 14-165e9 L/y (sum 2.15e9g/d 95 of
  discharge volume)

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Bio-uptake Loss

• Phytoplankton?
• Cloern 2002-2004 productivity 210gC/m2y
• Hammerschmidt MeHg 0.5ng/g ww 5ng/g dw
• LakeMichMassBal algal MeHg 30 ppb dw
• C?CH2O, geomean MeHg 12ng/g
• 19.5g/d MeHg into phytoplankton?
• Phytoplankton rapid turnover (growth0.3/d?),
  reversible loss from water/sed pools, loss
  estimate probably too high
• Small fish?
• Slater (CDFG, IEP) young of year pelagic fish
  est. 0.01-0.25g/m3 (Suisun lowest, Central
  highest, mostly anchovies) mean 0.17g/m3 ww
  biomass
• RMP anchovy Hg 0.049µg/g ww 0.13g/day MeHg into
  fish biomass (lt1 of phyto?)
• Expect less (short term) cycling than algae,
  irreversible net loss by incorporation into
  higher trophic levels