A Mechanistic Model Analysis of Mercury in the Yolo

Bypass and the Sacramento/San Joaquin River Delta

Reed Harris, David Hutchinson, Matt Gove, Don Beals

Reed Harris Environmental Ltd.

Carol DiGiorgio, Nicky Prabhjot, Ali Abrishamchi, Jamie Anderson, En-Ching Hsu, Kevin He, Kijin Nam, David Bosworth, Hari

Rajbhandari, Ming-Yen Tu

California Department of Water Resources

Mark Stephenson, Wes Heim

Moss Landing Marine Laboratory

Gary Gill,

Pacific Northwest Laboratory

Randall Hunt, Paul Work, David Schoellhamer

US Geological Survey

September 26, 2019

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• Assist California Department of Water Resources to meet requirements for the open water portion of Delta/Bypass methylmercury TMDL.

• Better understand factors controlling MeHg cycling and sources in particular.

• Investigate effects on MeHg of operational changes in water management and flood conveyance in the Yolo Bypass. Develop best management practices.

Mercury Modeling Objectives

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Model Study Approach

1. Develop necessary modeling tools

2. Simulate existing conditions• One model for Yolo Bypass• One model for Delta• Manual calibration followed by parameter estimation

software (PEST)

3. Pass results from Bypass model to Delta model

4. Examine model sensitivity to different inputs(using parameter estimation software called PEST)

5. Simulate future scenarios

6. Examine uncertainty (using PEST) 3

Using two mechanistic models:

1 – An existing model of mercury cycling for Yolo Bypass (D-MCM)

2 – Added mercury to an existing DWR model of hydrodynamics and water quality (DSM2)

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Features of Both Models

1. Mass balance, process-based

2. Time dependent

3. Simulate inorganic mercury and methylmercury

4. Include dissolved and particulate mercury forms in water column and surface sediments

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Sediment transport

Hydrodynamics

Mercury in water and sediments

Major Components of Modeling Framework

Yolo Bypass Delta

TUFLOW

TUFLOW +experimental studies

D-MCM + experimental studies

DSM2

DSM2

Not includedMercury in vegetation

DSM2D-MCM

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Group of cellsWetland Cell

One water compartment

Four sediment compartments

Yolo Bypass model: D-MCM

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Yolo Bypass model grid

• 47 cells• Each cell has a specific land use

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Potential Vegetation Effects on MeHg

1. Reduced light (affects photochemical reactions)

2. Increased atmospheric deposition

3. Increased flux of Hg(0) to atmosphere

4. Supply of organic matter and mercury

5. Mercury uptake via roots, changes in hydrology.

6. In-plant methylation?

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DSM2

• Developed by California Department of Water Resources

• Previously simulated hydrodynamics (1D) and water quality.

• For this project:• Added mercury• Added sediment bed

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• Correctly simulate whether areas are sources or sinks for total mercury and MeHg.

• Predict whether operational changes will cause MeHg supply to increase or decrease

• Yolo Bypass: Unlikely to capture day to day variability in such a dynamic system

• Using PEST software to help assess uncertainty and estimate confidence in model results.

Model uncertainty: What is realistic to expect?

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Yolo Bypass Model Results(Simulations from 1997-2012)

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Fremont Weir is the primary source of water and solids

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Fremont Weir is the biggest estimated external source of MeHg overall….. ….but other sources are also important….importance depends on hydrology (e.g. Fremont Weir does not always flow)

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* Fraction of inflowing load trapped ((inflow - outflow)/inflow)

No trapping

Export = 2X input

Nothing escapes…

Yolo Bypass estimated to be a source of MeHg, sink for solids and THg

Sou

rce

Sin

k

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Yolo Bypass is a source of methylmercury in simulations….

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Yolo Bypass is a source of methylmercury in simulations….

Model Foe et al. (2008)

Water year 2006 comparison:

Outflows Outflows

Inflows Inflows

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Example of simulated and observed suspended solids (Cell receiving inflow from Putah Creek)

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Examples of simulated and observed MeHg in Yolo Bypass waters

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Simulated and Field-Estimated MeHg concentrations in surface sediments in Yolo Bypass

• Vegetated cells tend to have higher predicted MeHg concentrations• Field data are limited

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Pasture Vegetated Core

Pasture Devegetated Core

Study used cores to investigate effect of vegetation on methylmercury concentrations

Mark Stephenson, Wes Heim, Gary Gill, Carol DiGiorgio, Dave Bosworth

Experimental studies have shown a vegetation effect on methylmercury

22Cores with vegetation had higher MeHg concentrations

Stephenson et al.

Cores with above-ground vegetation had more methylmercury…

USGS studies also showed vegetation to increase MeHg in sediments.

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Delta DSM2 Model Results(Simulations from 2000-2006)

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Examples of DSM2 Simulated and Observed MeHg in water 2000-2006

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Example of simulated MeHg in Delta waters

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Data Assembly

Model Calibration/Validation

Sensitivity Analysis

Scenario Testing

Yolo Bypass Delta

Not started Complete

Status of Tasks

Legend

Uncertainty Analysis

Model Development(except effects of vegetation)

Data Assembly

Model Calibration/Validation

Sensitivity Analysis

Scenario Testing

Uncertainty Analysis

Model Development(nearly completed)

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What model inputs have the most influence on model results?(Sensitivity analysis)

Simulations are being carried to examine influence of…

• Tributary loads of solids, inorganic mercury, methylmercury

• Atmospheric loads of inorganic mercury

• MeHg production in Yolo Bypass and the Delta

• Vegetation

• Hydrology

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Time

Base Case

Remediation Scenario

Pre

dic

ted

Me

rcu

ry c

on

cen

trat

ion

Model certainty: Are two model simulations meaningfully different?

How certain are these confidence limits??- Need to estimate uncertainty associated with inputs…

Using PEST software to help estimate confidence limits and uncertainty.

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Summary

• Mechanistic mercury model framework developed for Yolo Bypass and Delta

• Simulations of existing conditions nearly complete

• System is very dynamic, especially Yolo Bypass.

• Vegetation appears to play important role in Yolo Bypass

• Sensitivity, uncertainty, and scenario analyses ongoing or about to be carried out30