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MeHg Mass Budget Evaluation in San Francisco Bay: A Modeling Approach

This project aims to model and synthesize the state of knowledge to evaluate MeHg fate in SF Bay. Key factors affecting MeHg, loading estimates, and refining models will be assessed. The study uses existing data on atmospheric deposition, water discharges, and wetland outputs to estimate MeHg loads. The MeHg production and biouptake processes, as well as other relevant factors, will be analyzed. The project seeks to identify major weaknesses and alternative approaches in understanding MeHg dynamics in the bay.

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MeHg Mass Budget Evaluation in San Francisco Bay: A Modeling Approach

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  1. Basic MeHg Mass Budget Don Yee CFWG July 2008 Meeting

  2. Problem Statement • MeHg very small % of totHg • Poor MeHg:totHg correlation in SF Bay • MeHg bioaccumulative form • Do Bay MeHg data make sense given… • Loading estimates • Production, degradation rates • Sediment-water exchange?

  3. Objective(s) • First order attempt to model MeHg • Synthesize state of knowledge • Identify key factors affecting MeHg fate • Evaluate need to better estimate MeHg load • Feasibility/needs of refined model(s) • Desired input • ID major weaknesses, alternative approaches and assumptions

  4. Approach • Synthesizing existing data: • Ambient Bay data from RMP S&T • Loading data from local (RMP SPL) studies where possible, literature where not • MeHg production/degradation rates from local studies where possible • Simple simulation of sediment-water processes using 1-box model

  5. 1 Box Model • Adapted from PCB 1 box (2 box?) model • One water box • One sediment box (5cm mixed layer) • Daily time step • Annually uniform (no seasonality) • Daily uniform mixing (to 5cm in sediment) • Equilibrium partitioning • Simplifications ~work for POPs • ? Will it work for MeHg ?

  6. External Loads (Imports) • Direct atmospheric (wet) deposition • Water Discharges from • Delta • Local watersheds • Wetlands • POTWs

  7. Atmospheric (wet) Deposition • 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

  8. Discharges from… • Delta (SWRCB Region 5) Hg TMDL • Flow weighted avg concentration x mean annual discharge (4.7g/d) • Local watersheds • SIMPLE Model urban totHg flux, assume constant %MeHg = 2.7g/d • %MeHg from lit median →1.1g/d • from local watershed Hayward Z4LA→ 4.1g/d

  9. Discharges from… • Wetlands • Audobon est. 40k acres wetland (1.6e8 m2), assume 0.3m overlying water every day • ~50% water particulate settles -1.2g/d • ebb tide dissolved conc ~2.5x flood tide (max 5x Petaluma) +3.2g/d • = net 2g/d discharge to Bay • Stephenson et al showed net import and export different events for single marsh • May be difficult to refine net load

  10. 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)

  11. Loads (Imports) • 0.10g/d atmospheric (wet) deposition • 4.7g/d Delta • 2.7g/d Local watersheds • 2.0g/d Wetlands • 0.79g/d POTWs ~10.4g/d total MeHg load (3.78kg/y) • Other “loads” • MeHg production = internal source • Biouptake = “export” from water/sed exch.

  12. MeHg Production • Unlike totHg & others, MeHg created in situ • Complex (non-linear) function of multiple factors- • C (not all C available), S(generally not limiting in estuary), Hg (poor regression for SF Bay) • Current best guess from range of production rates in lab incubation? • Marvin-DiPasquale et al 0.11ng/g·d (geomean of San Pablo, range 0.03-1.04) • Would otherwise need complex C & S mass balance/speciation & porewater redox model • Assume ½ of mixed sediment layer methylates

  13. Biouptake “Loss” • Phytoplankton? • Cloern 2002-2004 productivity ~210gC/m2y • Hammerschmidt MeHg 0.5ng/g ww =5ng/g dw • LakeMichMassBal phyto MeHg = 30 ppb dw • C→CH2O, geomean MeHg 12ng/g • = 19.5g/d MeHg into phytoplankton • Phytoplankton rapid turnover (µ~0.3/d?), reversible “loss” from water/sed pools, loss estimate probably too high

  14. Biouptake “Loss” • 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

  15. Other Processes • Modeled (dependent on MeHg conc) • Volatilization • Outflow (through Golden Gate) • De/sorption • Sedimentation • Benthic flux • Degradation

  16. Modeled Processes • Volatilization- • Henry’s Law const for MeHgCl = 0.05 Pa·m3/mol (Lindqvist & Rodhe 1985) • Outflow (through Golden Gate) • Tidal mixing from Connelly, assume ocean MeHg ~MDL, min of Bay, or 0 • De/sorption • Bay water particulate vs dissolved log Kd=4.1 (could instead use porewater (Choe et al 2004) mean log Kd=4.66)

  17. Modeled Processes • Sedimentation • Fuller burial rate (0.88cm/y) • Could be modeled as erosion • Benthic flux • Captured in daily resuspension and de/sorption? • Degradation • Marvin-DiPasquale San Pablo Bay geomean sed demethylation rates = 0.083/d (first order decay) • Krabbenhoft Petaluma wetland water half life~7 days (0.10/d decay) • Large uncertainties some parameters, but ~no effect

  18. Base Case Run • Averaged initial concentrations, parameters • Equilibrium reached quickly, ~10-20% diff from T0 • Sed mass up • Water mass down • (adjust Kd? Relative degradation rates?)

  19. Base Case Run • Mass (inventory) vs daily flux/degrade/produce • Water Mass • Net sediment to water exchange, ext load = Degradation>, GG outflow, >> biouptake,volatilization • Total (Water+Sed) • Production ~balances degradation >> all other processes *from 1box model, Choe et al flux box ~14g/day

  20. Deep Mix Case Run • Mixed depth to 15cm • Methylation zone still set to ½ mixed depth • Equilibrium reached quickly, ~10-20% diff from T0 • Change only in sediment mass

  21. Parameter Sensitivity

  22. 2 Parameter Changes • S&W very sensitive to methylation rate, S deg rate • W Moderately sensitive to load, W deg rate • Kd has small effect (particulate, dissolved offset?)

  23. Summary • Base case w/ average inputs near steady state • Close to “right” on Baywide scale? • Offsetting errors? • High degradation/methylation rates dominate • Rapid turnover, week-month scale • Quick response of ambient MeHg? • External Loads (to water) only small/moderate effect (even @ 0.3-3x base estimate) • Water & sed linked by Kd and SSC given equilibrium/ steady state assumptions

  24. Next Steps • Explore other parameter combos? • E.g. high meth + high sed deg look like base case, other mixes of parameters • Egregiously bad assumptions? • E.g. meth in only ½ of “well mixed” sed layer? • Equilib/SS model usable only for sensitivity test? • No benthic flux external load? • Seeking WG input • This budget (small scope, want to address major factors, not a full redesign) • Next generation model? (utility, feasibility)

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