Hg Process Study Options

RMP CFWG

September 14, 2007

RMP Mercury Management Questions

1. Where is mercury entering the food web?

2. Which sources, processes, and pathways contribute disproportionately to food web accumulation?

3. Can we do anything about these high-leverage processes, sources, and pathways?

4. What effects can be expected from management actions?

5. Will total mercury reductions result in reduced food web accumulation?

Which sources, processes, and pathways contribute disproportionately to food web accumulation?

Sources- is all Hg ultimately bioavailable (for methylation, uptake)?

Processes- what (co)factors affect transport, transformation, uptake (what spatial and temporal scales of interest)?

Bioaccumulation- biggest jumps at lowest trophic levels (primary producer/consumer)

Can we do anything about these high-leverage sources, processes, and pathways?

Sources- Reduce tot Hg? Selected “reactive” souces? MeHg inputs?

Processes- Adjust (co)factors (aeration, flushing, nutrient loading)

Bioaccumulation- choose species, habitat design? (likely least flexibility?)

How to prioritize?

MeHg mass “budget” as tool to ID gaps– Large uncertainties, not very quantitative– Illustrating sensitivity to uncertainties

Large factors, large uncertainties first– Portion of Hg convertible to MeHg– MeHg sources- in situ production, sed-water

exchange, tributary loads,– Loss pathways- degradation, export

What Might We Try

Sources- is all Hg ultimately bioavailable (for methylation, uptake)?

Is availability more important than total load? Uptake- sufficiently established that MeHg

concentration is most directly correlated to biota?– Slotton pelagic fish and water column MeHg– Petaluma high marsh sediment and inverts

MeHg in all sources (needed for MeHg mass budget) “Bioavailability” for methylation

– Timescale of relevance- season/year/decade?

Hg Sources Availability

Load Availability

Stormwater high med?

Air Dep low high

Watershed sediments high low?

Hotspots med/low? med?

Buried sediments high med?

Dredging low med?

Effluent low med/low?

Hg Source Availability Approaches

Reactive Hg measures (sequential extractions, SnCl2 reductions)

– + straightforward, feasible– - how relevant to field conditions, microbes?

Incubation experiments with various sources– + uses microbes, can try to mirror field WQ measures– - different strains, differences from field, difficulty in

maintaining, interpreting, timescale >season hard Combination (correlational)

– Reactive Hg measures + incubation kinetics = correlation to MeHg (Marvin-DiPasquale approach)

Hg Source Availability Approaches

Hg CM (conceptual model)- Field incubation experiments with lower Hg sediment sources– + mirrors possible mgmt option (capping)– - preventing cap loss, mixing, controlling for

temporal variation of various factors

Point source near-field transects– + real world effect– - might be other cofactors (TOC, hydrology)

Hg Transport Factors

Sed water exchange large uncertainty Non turbulent exchange- bioturbation, diffusion

(Kuwabara, Gill flux boxes)– + feasible– - Likely site specific, but constrains bounds?

Turbulent exchange- resuspension, advective– - ?? Methods?? porewater measurements + assumptions ??– Also likely site specific (hydrology, sediment quality, porewater

MeHg)

Hg Transformation Factors

Axis of methylation C (N?) + SO4 + Hg (– O2) SF Bay, plenty of Hg (bioavailable?), SO4

Hg CM priority- Characterize Bay Sediment Profiles of salinity, DO, TOC, SO4, S2, MeHg

Organic matter supply limiting methylation?– Measure & correlate w/ MeHg– Incubations with more bacterial food

+ isolate limiting factor - relevance to field, ? usability as control option (nutrient controls?)

Hg Transformation Factors

O2 limiting factor? Measure DO/ORP & correlate w/ MeHg

– Already component of RMP S&T– - ORP a crude measure of “net” redox balance

Incubations with more/less O2

– + outcome likely predictable– - usability as control option- only applicable on small

scales- ?? Undesired consequences ?? Control options ID/test already part of LFR/UCSC prop

13 grant

Hg Transformation Factors

Hg CM suggests testing water column MeHg production via incubations– + might be needed to close bounds on MeHg mass

balance– ?? Is this necessary or high priority (few Bay surface

waters anoxic, little stratification like in lakes) ?

Hg Uptake Factors

Need to follow trophic link back to right starting matrix Field approaches, correlations

– + field conditions– - dynamic, episodic (time scale), confounding factors

Incubations– + control of factors– - relevance to field

Time scale of interest– Do we care about daily scale variation- week+?

Hg Uptake Factors

Low trophic resident biota integrators– + directly relevant to food web exposure– - life cycle, growth dilution, depuration confounding

SPMDs as integrator surrogates?– + more stable predictable response– - still need to correlate back to biotic uptake

Hg Integrated Measures

Hg stable isotopes- assumes distinct source signatures and predictable process fractionation– + not subject to incubation artifacts, may be able to

eliminate/establish significance of some sources, processes.

– - isotope signature highly source specific, but not knowable until actually measured. Differences might be too small to track across a mix of sources

Hg Integrated Measures

Other tracers?

Priorities?

Find & characterize methylation hotspots? Control methylation hotspots? Characterize range (random?) of methylation

zones? Control range of methylation zones? Deduce contribution of individual sources (to

methylation/accumulation)?(assign % of effort?)