Basic MeHg Mass Budget

Don Yee

CFWG July 2008 Meeting

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?

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

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

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 ?

External Loads (Imports)

• Direct atmospheric (wet) deposition

• Water Discharges from• Delta• Local watersheds• Wetlands• POTWs

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

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

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

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)

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.

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

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

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

Other Processes

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

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)

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

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?)

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 Water 0.235 kg

Ext. Load 0.010 kg/d

Sed to Water* 0.028 kg/d

GG Outflow 0.014 kg/d

W Degrade 0.024 kg/d

Volatilize 1.24E-07 kg/d

Biouptake 1.30E-04 kg/d

Mass in Sed 18.02 kg

Methylate 1.513 kg/d

Sed Degrade 1.476 kg/d

Sed to Water 0.028 kg/d

Burial 0.009 kg/d

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

Parameter Sensitivity

Scenario Mass S Mass W

Base Case 17.8 kg 0.235 kg

3x Mix Layer 305% 101%

Load /3 100% 89%

Load x3.3 101% 139%

WaterDegrade x0.3 101% 137%

WaterDegrade x 3 99% 57%

SedDegrade /4 383% 337%

SedDegrade x2.5 42% 52%

Methylate x0.3 34% 45%

Methylate x3 336% 297%

LogKd 4.66 100% 83%

2 Parameter Changes

Scenario Mass S Mass W

Base Case 17.8 kg 0.235 kg

Meth x0.3, SedDeg x2.5 14% 28%

Meth x3, SedDeg /4 1285% 1092%

Load x3, SedDeg /4 385% 378%

Meth x3, SedDeg x3 101% 101%

Meth x0.3, SedDeg x0.3 129% 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?)

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

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)