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Basic MeHg Mass Budget Don Yee CFWG July 2008 Meeting.

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Presentation on theme: "Basic MeHg Mass Budget Don Yee CFWG July 2008 Meeting."— Presentation transcript:

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 ( 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 m 2 ), 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 ng/L (mean ~0.42ng/L) Discharge e9 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 ) Would otherwise need complex C & S mass balance/speciation & porewater redox model Assume ½ of mixed sediment layer methylates

13 Biouptake “Loss” Phytoplankton? Cloern productivity ~210gC/m 2 y Hammerschmidt MeHg 0.5ng/g ww =5ng/g dw LakeMichMassBal phyto MeHg = 30 ppb dw C→CH 2 O, 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 g/m 3 (Suisun lowest, Central highest, mostly anchovies) mean ~0.17g/m 3 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·m 3 /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 T 0 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 Mass in Water0.235kg Ext. Load0.010kg/d Sed to Water*0.028kg/d GG Outflow0.014kg/d W Degrade0.024kg/d Volatilize1.24E-07kg/d Biouptake1.30E-04kg/d Mass in Sed18.02kg Methylate1.513kg/d Sed Degrade1.476kg/d Sed to Water0.028kg/d Burial0.009kg/d

20 Deep Mix Case Run Mixed depth to 15cm Methylation zone still set to ½ mixed depth Equilibrium reached quickly, ~10-20% diff from T 0 Change only in sediment mass

21 Parameter Sensitivity ScenarioMass SMass W Base Case17.8 kg0.235 kg 3x Mix Layer305%101% Load /3100%89% Load x3.3101%139% WaterDegrade x0.3101%137% WaterDegrade x 399%57% SedDegrade /4383%337% SedDegrade x2.542%52% Methylate x0.334%45% Methylate x3336%297% LogKd %83%

22 2 Parameter Changes ScenarioMass SMass W Base Case17.8 kg0.235 kg Meth x0.3, SedDeg x2.514%28% Meth x3, SedDeg /41285%1092% Load x3, SedDeg /4385%378% Meth x3, SedDeg x3101% Meth x0.3, SedDeg x0.3129%125% 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 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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