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AGU FALL MEETING 13 December 2017

Improving our process understanding of methane emissions from a mid-latitude reservoir by combining eddy covariance monitoring

with spatial surveys

Sarah Waldo, Jake J. Beaulieu, and John T. Walker USEPA Office of Research and Development

National Risk Management Research Laboratory

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Anthropogenic Methane Budget

Best estimate of global CH4 from reservoirs is uncertain

The IPCC is actively working to establish a methodology for reservoir CH4 emissions

Rice Cultivation 36 Tg/yr

75 Tg/yr

Ruminants 89 Tg/yr

Landfills

Reservoirs ~18 Tg/yr

(12-30) Deemer et al (2016)

Biomass Burning and

biofuels 35 Tg/yr

Fossil Fuels 96 Tg/yr

Sources other than reservoirs from IPCC 5th Assessment

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Background: why and how reservoirs emit methane

Methane (CH4) is produced by methanogens in oxygen-poor, carbon-rich sediment

CH4 makes it to the atmosphere via

diffusion

ebullition

Rate/timing of CH4 emissions are affected by physical and biotic factors

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Acton Lake Study: site description

Cropland

Pasture/Hay

Forest

Developed

Water

Land cover data: http://www.mrl c.gov/nlcd2011. php

https://www.epa.gov/national-aquatic­resource-surveys/ecoregional-results-national­lakes-assessment-2012

Acton Lake site description:

Surface area: 2.4 km2

Maximum depth: 10 m

Watershed is 81% agriculturalland

Dam was constructed in 1956

Acton Lake Study: methods

Executed spatially balanced surveys:July 10th – 11th & Aug 31st – Sept 1st

Deployed inverted funnels overnight ( >12 hr)

Measured diffusive fluxes with a floating chamber + ultraportablegreenhouse gas analyzer

Eddy covariance flux towerdeployed at the NW corner of the lake

Monitoring fluxes of CH4, CO2, water vapor, and energy

Pseudo-continuous, 30-minute measurements from Feb 2017 ­present

flux tower location

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Acton Lake Study: research questions

1. How do magnitudes ofebullitive and diffusive emissions compare?

2. What temporal patterns do we see in CH4 fluxes?

3. What environmental drivers are associated with elevated emissions?

flux tower location

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Emission Pathway: Lake-wide surveys

Results from inverted funnel and chamber measurements of CH4 emissions

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Surprising Spatial Finding: Ebullition vs. Depth

Typical for areas in theriver-reservoir transition area near the inflow to have elevated ebullition rates

• more organic matter fromsedimentation

• deep sites

• suppress bubbling

• more oxidation in water column

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Emission Pathway: Lake-wide surveys

Ebullition Diffusion Ebullition was the main CH4 emission pathway:

• 82% of total lake-wide e missions on July 10th

• 94% of total lake-wide e missions on August 31st

July 10th July 10th Aug 31stAug 31st

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Eddy covariance observations: temporal patternsand relationship with environmental drivers

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400 m

200 m

flux tower location U-14

Temporal Trends: Seasonal

Seasonal trends (numbers reported as mean ± se):

Winter and early spring saw lowemissions. 1 Feb – 1 April:

14.1 ± 2.2 mg CH4 m-2 d-1

Maximum sustained emissions in the late summer. July 25 – Sept 1:

582 ± 31 mg CH4 m-2 d-1

Emissions back to winter baseline by late fall. Oct 25 – Nov 16:

19.3 ± 4.1 mg CH4 m-2 d-1

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Temporal Trends: Seasonal

The lake-wide CH4 emission results from the spatial surveys agreed relatively well with the eddycovariance results

Spatially, would have expected thelake-wide characterizations to be higher than the EC

survey results indicated that the NWportion of the lake had lower emissionsthan the rest of the lake

Temporally, looks like we may havemissed peak emissions

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Temporal Trends: Seasonal

Total hydrostatic pressure(pressure from water column +atmosphere) has a threshold effect

Temperature has a threshold effect as well

Air temperature needs to be above 8 °C for substantial methane emission to occur

Q10 analysis (DelSontro et al.,2016)

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Summary & Future Work

• Summary:

• Ebullition accounted for >80% of summertime CH4 emissions at Acton Lake

• Clear seasonality: summertime emissions were 40xhigher than wintertime

• Temperature and hydrostatic pressure exerted controlon CH4 emissions

• Future work:

• Gap-filling and identification of CH4 flux drivers using wavelet analysis

• Compare and contrast CH4 and CO2 fluxes • Use water temperature profile measurements to

investigate impact of hydrodynamics on air-water gasexchange

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Acknowledgements

• USEPA ORD Cincinnati & RTP

• Karen White • Kit Daniels • Megan Berberich • Ryan Daly • William Yelverton • Bill Mitchell

• Pegasus Technical Services

• Adam Balz • Adin Pemberton • Keith Bisbe • Paul Trygstad • Mia Varner

• Vanni Research Group at Miami University of Ohio

• Mike Vanni • Tanner Williamson • Tera Ratliff • Tom Radford

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Disclaimer The information in this presentation has been reviewed and approved for public dissemination in accordance with U.S.

Environmental Protection Agency (EPA). The views expressed in this presentation are those of the author(s) and do not necessarily

represent the views or policies of the Agency. Any mention of trade names or commercial products does not

constitute EPA endorsement or recommendation for use.

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Questions? Questions?