sefs 9_flury et al

The contrasting effect of sediment methane gas voids on diffusive fluxes of soluble and sparingly soluble substances

Sabine Flury ,Ronnie N. Glud, Katrin Premke, Daniel F.

McGinnis

flury@igb-berlin.de

SEFS 9, Geneva

2015

Gas accumulation in sediments and sediment water interface

A common phenomenon

Methane O2 (photosynthetic biofilms)

http://myweb.fsu.edu/mhuettel/Projects/ONR_Gas.html

http://inhabitat.com/wp-content/blogs.dir/1/files/2013/01/green-algae-537x358.jpeg

marine freshwater

Gas tube bubbles

(Anderson and Martinez) 2015)

Gas distribution in sediments

(Cheng et al 2014) (Cheng et al 2014)

CH4 gas distribution in a lake

Gas profiles in marine sedimentsDeep gas: CH4 Surface gas: O2

Potential free gas accumulation from 0 cm to deep layers!

Enhanced pore water exchange due to ebullition

True at high bubble rates (e.g. seep sites) and in permeable sediments

http://inhabitat.com/wp-content/blogs.dir/1/files/2013/01/green-algae-537x358.jpeg

Pore water mixing

Pore water exchange

Photo – A. Maeck

In permeable and semi-permeable sediments:

Pore water flux: 3-21 x increase at high bubble rates

With 60-70 L m-2 d-1

(e.g. at seep sites)

(Cheng et al 2014)

How strong is the pore water exchange with bubble rates generally found in Lakes and

reservoirs?

E.g. Bubble rates in Lake Wohlen and Saar: 7-17 L m-2 d-1 (DelSontro et al 2010, Maeck et al 2013)

Experimental Set-UpSediment:- Muddy (Lake Müggel, Germany)- Sandy mud (Odense River, Denmark)

Tracer: - Br- (added as KBr) - Rhodamine WT

Gas production: - stimulated with Yeast Extract

Incubation Temperature:- 27 oC, dark- 20 oC, dark

Water mixing:- Eheim Aquarium pump (5L min-1)inactive sed.

live or .

Results

Tracer fluxes and Dapp reduced by 24% ! ̴

(Flury et al – in revision ES&T)

Rhodamine Bromide

Flux calculation in sediment: Fick’s first law

(Flury et al – in revision ES&T)

D in water: 10-9 m2 s-1

𝐽 i=𝐷i ∙∂𝐶i

∂ 𝑧Ji = fluxCi = concentration of diss. sp.Z = depthDi = Diffusivity of species i

In sediments:

Di = function of Tortuosity θ

Molecular diffusion:

Effect of gas voids on pore water and gas flow

(Flury et al – in revision ES&T)

D in water: 10-9 m2 s-1 D in gas: 10-5 m2 s-1

Effect of gas voids on pore water and gas diffusivities in

SOIL

𝐷us=𝐷w ∙𝛼73

𝜑2

Millington and Quirk model for water unsaturated porous media (Millington & Quirk 1961)

Dus = Diffusivity in unsaturated porous mediaDw = Diffusivity in free water or gas= volumetric water or gas content = total porosity

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

RWT sq

Br r

Br sq

RWT sq

Br r

Br sq

Dus

,s:D

s,s

Water content (vol water/vol sediment)

0.0 0.2 0.4 0.6 0.8 1.0Gas content (vol gas/vol sediment)

For soluble species:

- If gas cont ↑ then D ↓

- If water cont ↑ then D ↑

SandMud

Effect of gas voids on pore water diffusivities in SEDIMENT

(Flury et al – in revision ES&T)

Stan

dard

ized

Diffu

sivity

(-)

𝐷us=𝐷w ∙𝛼73

𝜑2

Diffu

sivity

of g

as (m

2 s-1

)Effect of gas voids on gas diffusivities in SEDIMENT

Small amounts of gas can increase Dgas substantially!

(Flury et al – in revision ES&T)

𝐷us=𝐷w ∙𝛼73

𝜑2

Implications of gas voids on:Pore water and gas fluxes - overestimation of pore water fluxes

- large underestimation of gas fluxes

Ecosystem functioning: - ↓ transport of diss. nutrients (↓internal nutrient)

- ↑ transport of e.g. CH4 to oxidizing zones - ↑ transport of O2 into sediment

http://myweb.fsu.edu/mhuettel/Projects/ONR_Gas.html

http://inhabitat.com/wp-content/blogs.dir/1/files/2013/01/green-algae-537x358.jpeg

Gas tube bubbles

marine freshwater

Thank you! Claudia Theel (IGB)

Workshop SDU

Anni Glud (SDU)

Morten Larsen (SDU)Jael Brüning (IGB)

Amanda Cheng (IGB)