A Methylmercury Budget for San Francisco Bay

Donald Yee, San Francisco Estuary Institute

Mercury Conceptual Model

System is complicated, simplified by single box model

Slow response(decades)

MeHg matters most (to biota)

Methylmercury Conceptual Model

Need to track MeHg MeHg <1% of totHg Poor MeHg:totHg

correlation

Differences from Hg 1 Box Model Methylation &

demethylation Potentially rapid (days-

months)

Sed-water

exchange

Meth

Demeth

Demeth

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 won’t/can’t do– Identify “hot” spot impacts (1 box)– Predict long term fate (no Hg linkage)

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

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/dRMP measured watersheds (extrapolated)

+Wetlands (upper range estimate) 2.0 g/dVolume x (incoming - outgoing) concentrations

+POTWs (16 largest, ~95% discharge) 0.8 g/dFlow x concentration

= 17.6 g/d total

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

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)

Modeled Processes

• Degradation– Sediment: Marvin-DiPasquale demethylation

rates = 0.083/d (decay)• Assume demethylating zone (70% of mixed layer)

– Water: Krabbenhoft Petaluma water half life~7 days (0.10/d decay)

• Benthic flux– In daily resuspension & de/sorption

Large uncertainties some parameters– Some have small ~no effect

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

Base Case Run• Mass (inventory) vs daily

flux/degrade/produce• Water Mass

– Net sediment to water exchange, ext load =

Degradation>, GG outflow, >> bio-uptake,volatilization

• Total (Water+Sediment)– Production ~balances

degradation >> all other processes

* Flux box measurement similar: ~.014 kg/d (Choe et al)

Mass in Water 0.236 kg

Ext. Load 0.018 kg/d

Sed to Water* 0.021 kg/d

Water Degrade 0.024 kg/d

GG Outflow 0.014 kg/d

Bio-uptake <0.001 kg/d

Volatilize <0.001 kg/d

Mass in Sediment

30.8 kg

Methylate 1.82 kg/d

Sed Degrade 1.79 kg/d

Sed to Water 0.021 kg/d

Burial 0.007 kg/d

Hot &@%$! 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

Parameter Sensitivity

Scenario Mass S Mass W

Base Case 30.8 kg 0.236 kg

Load /3 30.7 0.191

Load x3 31.0 0.370

WaterDegrade /3 30.9 0.317

WaterDegrade x3

30.6 0.134

SedDegrade /3 88.8 0.556

SedDegrade x3 10.4 0.123

Methylate /3 10.3 0.123

Methylate x3 92.0 0.574

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

Management Strategy – Dr. EvilAcquire $1 MillionOption A- Control Methylation:• Sterilize the Bay (thermonuclear device)Option B- Control Demethylation:• Equip sharks w/ UV lasers to

photodemethylate

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 > $1 million)

Acknowledgements

Too many to list…

“If I have seen further it is by standing on ye shoulders of Giants”

– Sir Isaac Newton

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

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

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

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)

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 (growth~0.3/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 (<1% of phyto?)

– Expect less (short term) cycling than algae, “irreversible” net loss by incorporation into higher trophic levels