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Methane Bellows Hypothesis and its Link to Biogeochemical Cycling Processes at the Mesotidal Freshwater - Seawater Interface of the

Columbia River Estuary (A Discussion)

• Part I. Source of River’s Methane Super-Saturation: Groundwater Input• Part II. Significant Methane Contribution to Estuary: Intertidal Bellows Phenomenon• Part III. Other Estuary Impacts on Water Quality: Neap / Spring Nutrient Records

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Part I: Willamette River @ Corvallis

• Willamette always supersaturated in methane (range: 25-150x, average: 56x)

• At a given time, degree of super-saturation can differ significantly over a short distance (CD-WP: ~100nM)

• Evidence for groundwater source: most enhanced in low flow summer period, but also other low flow periods

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• CD-WP difference correlates inversely with river flow• amount added in ~2 km river stretch can

double the WP concentration• Groundwater source of Si and N (but not P)

also evident in same river stretch• NOTABLY: nutrient addition evident during

high not low river flow periods!

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Conclude• Nutrients introduced by seepage from rain saturated soils (seasonally driven process)• Methane ascribed to discharge from Willamette aquifer discharge (a more persistent

year round process)

Question: Why is there a strong correlation between DSi and DN but not DP?

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Part II: Cathlamet Bay Tidal Outwash (RM17 Site) – Conservative vs Non-conservative Mixing

• T – S (left, bottom) display conservative, two-endmember mixing under both spring and neap tidal forcing and low flow, summer conditions

• Si – S (right, bottom) trends similar, indicating minimal biogeochemical modification (i.e., loss due to diatom production) at this land-to-sea margin

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Methane

Tidal Bellows Effect

No Yes

sulfate

Salinity

None Weak Strong Strong

Methane Bellows Effect?• BAT-to-estuary transect

during an ebb tide on Fall 2008 cruise provided evidence for prospect

• Monitoring sites over ebb cycle on May 2009 cruise confirmed phenomenon and showed it is confined to river & upper portion of the estuary

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RM17 Monitoring• evaluates tidal

outwash from Cathlamet Bay

• Methane Bellows Effect evident in high flow late spring (top) and low flow late summer (bottom) periods sampled during 2009 CMOP campaigns

• ‘excess’ CH4 (observed – predicted) persistent in intermediate salinity bottom water during low flow late summer period

Question: If methane source restricted to freshwater, how is latter observation explained?

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RM17 Monitoring• May 2010 – high

flow periodspring neap

• methane bellows effect apparent during neap (right) BUT NOT spring (left) tidal outwash• expressed both in fresh surface & intermediate salinity deep waters (similar to observation

from May 2009 – 10,600 m3/s)

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RM17 Monitoring• August 2010 –

low flow period

• methane bellows effect apparent during neap (left) AND spring (left) tidal outwash• ‘excess’ CH4 (observed – predicted) persistent in intermediate salinity bottom waters

during both neap and spring tidal forcing (similar to September 2009 – 2970 m3/s)

Questions: How is methane introduced to the outwash water? Is hydrostatically driven groundwater discharge an important consideration?

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• Non-conservative mixing evident for ammonium under neap and spring tidal forcing: apparent excess indicates a significant estuarine source

• Conservative mixing for nitrate and phosphate evident under spring BUT NOT neap tidal forcing. Both of these nutrients show depletion, NOT an excess

Question: What is the biogeochemical explanation for the apparent depletion of each?

Part III – Nutrients

RM17 Monitoring• August 2010 – low

flow period

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‘excess’ type analysis applied to nutrient data

RM17 Monitoring• August 2010 –

low flow period

Denitrification would account for nitrate depletion• but depletion does not appear associated with tidal outwash• Question: where else might denitrification occur? within estuarine bottom waters, ETM?

Metal oxide dynamics might account for phosphate depletion• Questions: What is the process? …redox cycling? …flocculation during estuarine mixing?

neap spring

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Overall Summary• Groundwater source evident for methane, nitrate, silicate but not phosphate in

Willamette River. Methane source ascribed to discharge of Willamette aquifer, most highly expressed in low flow, summer period. N and Si source is seasonal rainfall-driven seepage from soil. Finer details of hydrology remain to be studied.

• Methane tidal bellow confirmed phenomenon in upper Columbia River Estuary. Buildup of ‘excess’ methane from tidal bellow apparent in stratified bottom waters of the CRE during low river flow and neap tidal forcing conditions. Physics of this process remain unclear. Groundwater discharge driven by tidally controlled hydrostatic forces now suspected.

• Silicate behaves conservatively in estuarine mixing during spring and neap tidal forcing, even under low river flow conditions. Significant estuarine source of ammonium with concentration buildup in saline bottom waters that appears independent of spring / neap tidal forcing. Nitrate and phosphate display conservative mixing behavior under spring but not neap tidal forcing. Both nutrients lost during neap tidal conditions: suspected causes are denitrification (N) and metal oxide redox cycling / flocculation (P). Both biogeochemical processes may be linked to ETM dynamics.

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an extra

compare with Methane Record - Slide 9