Geol 230
GW/SW interaction
Week 9 Lecture
Anoxia
Reading: Ch. 11, Jones and Mulholland, pp. 259-279 by Baker, Dahm, and Valett
I) Introduction:
A) General assumptions:
- Surface water is oxygenated
- Surface water gains oxygen from photosynthesis
- Surface water loses water via respiration
- < 0.5 mg/l = anoxic
- Anoxia occurs in slow-moving surface water with fine, organic-rich sediment
- When oxygen is depleted: Other electron-receptor reactions take place
- Thermodynamic and geochemical constraints control the reactions that occur:
Gibbs free energy:
negative = a favorable reaction
Gives off energy: enthalpy < 0, entropy > 0
See table 1 from J&M, p. 261: Energy of redox reactions
B) Order of reactions for reductive processes: (given from most to least
energetically favored- in order of free energy change)
aerobic respiration > denitrification > Mn (IV) reduction > Fe III reduction
> sulfate reduction > methanogenesis > fermentation
- Fermentation must occur with metal and sulfur reduction (even though it is less
favored) because fermentation supplies hydrogen gas and acetate that
serve as electron donors.
- When fermentation rates are low, other reactions may be limited by lack of
electron donors.
- When fermentation rates are high, metal reduction reactions may out-compete
other reactions (sulfate reducing and methanogenic).
- When metal reduction (with fermentation) is done, sulfate reduction takes over.
- Sulfur reduction also uses (requires) some of the organic products of
fermentation at electron donors, but can only use certain kinds of organic
products (more selective than metal reduction).
- Methanogenesis: the most extreme reduction reaction. Occurs by “acute
fermentation” or reduction of CO2.
C) Spatial distribution:
- Reactions are segregated spatially in sediments:
See Figure 2 from J&M, p. 263
- Waste products build up from each step: are sometimes significant
volumetrically/ ecologically.
- Reduced forms may be oxidized later when hyporheic water re-contacts surface
water.
See Fig. 1 from J&M, p. 262
II) Measuring anaerobic processes:
A) Measure D.O.:
- Standard membrane technique is only effective to > 1 mg/l
- Winkler titration can be effective at lower D.O. concentrations
B) Calculate PCO2 (and other gases):
- Is done indirectly using measure pH, alkalinity, or TDC values. This method
makes some assumptions: carbon is from inorganic sources.
- Measure CO2 content in headspace (gas evolved from a sample)
- Measure methane in headspace
- Measure H2 in headspace: a byproduct of fermentation
C) Measure dissolved species:
Ex:
Fe+3 (oxidized) vs. Fe+2 (reduced)
SO4-2 (oxidized ) vs. S-2, H2S, HS- (reduced)
CO2 (oxidized) vs. CH4 (reduced)
D) Measure metabolism:
- Increases in CO2, methane indicate anaerobic respiration
- Other changes from oxidized to reduced forms (of species listed above) may
also indicate anaerobic respiration.
- Must use “killed controls” to separate biotic from abiotic processes
- Hydrogen uptake varies with anaerobic process. Therefore: hydrogen
consumption may provide evidence about the dominant electron receptor
process.
- Tagged radio isotopes: have been used to trace reactions in other systems (deep
sea, etc.). Should work in hyporheic systems, too.
III) Controls on anoxia:
- 2 major factors:
1) Residence time (contact time with sediment)
- Longer residence time = more oxygen consumption
See Fig. 5 from J&M (p. 271)
See Fig. 4 from J&M (p. 2710
2) Respiration rate
See Fig. 3 from J&M, p. 267 (What the heck is this?)
- Other controls:
3) Substrate type, grain size:
- Silt, clay, high organic composition favor anoxia
- Relates to hydraulic conductivity:
K α ___1____
anoxia
- Grain size may be the key factor here: increased surface area = more
reactive sites.
- Sometimes: (anoxic) respirtation is limited by the presence of organic
matter.
4) Seasonal/ diel patterns
- Anoxia may be more extensive at night: less photosysnthesis
IV) Temporal/ diel patterns:
A) Diel changes:
- Yes, they exist!
- Ex: Dissolved Mn was higher in surface water at night
- May be related to O2 availability and/or changing hyporheic flux at night
My note: Changing flux seems likely!
B) Seasonal changes:
- Occur in nearby aquifer/sediments
- Are related to D.O. concentrations:
- Higher D.O. in winter = lower dissolved metal concentrations
- Lower D.O. in summer (from increased respiration, lower saturation) =
higher dissolved metal concentrations.
See Garrels and Christ overhead: Solubility of Mn, Fe in different Eh/ pH
environments
- This brings up another important issue: relationship between Eh, pH
- Subsurface (hyporheic) environement: tends to be low pH, low O2
- Note: low O2 is not the same as low Eh, but they are related
- This has LARGE implications for contaminant mobility, GW studies !!!!
C) Other long-term trends:
- May be related to organic carbon, other nutrients in the system:
- One scenario:
Low nutrients = low DOC production (less algal productivity) = less O2
consumed = higher (than normal) D.O. levels = fewer metals RETAINED
in subsurface
V) Rates of anaerobic metabolism (reactions)
- Are largely unknown/ unmeasured
- Organisms capable of metal reduction have only recently been captured/cultured
(1987!!)
See Figure 6 from J&M, p. 274
- Trends:
- Baseflow tends to produce more
anoxia
- High hydraulic conductivity tends to
produce less anoxia
- Anoxia is related to methane and iron
production (in solution)
- Other studies: Have looked at
methanogenesis, sulfate reduction. (see
Table III, IV)
My comment: Not many trends here!!
- Comments in the book: “visual cues abound, although studies are rare”
ex: iron stain changes color
- My comment: I’ve seen it, too!
VI) Conclusion
- We need more studies on scale, extent, processes in stream systems