Biological Controls on Water Chemistry - November 21, 2012
1. Biological controls on water
chemistry
or
how microbes may one
day control the earth
Richard Behr, Certified Maine Geologist
November 20, 2012
2. The “Take away”
• Chemical composition of ground water often results
from microbial activity
• Human activities often enhance the microbial
community’s influence
• Today’s examples are but a few that illustrate the human
component
• Microbial activity often inferred from water quality
data
4. Microbes are the catalysts
• Microbial mediated redox processes control solubility,
speciation, mobility, toxicity, bioavailability of many
elements:
– Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg …..
• Things were different in the 1960s and 70s….
• Microbial degradation of organic carbon is often the
driving force
• Organic carbon is both an energy and carbon source
• One microbe’s waste is another’s resource
5. Let’s begin with the basic
requirements for microbes
• Where do they obtain energy for cell growth and
reproduction?
• Source of carbon?
• What can they breath?
6. Terminology/Classification
• Metabolism, “denotes the complex series of energy-utilizing
chemical reactions carried out by the cell” - Two general types….
• Catabolism – extracting energy from organic compounds by breaking
them down into component parts…thereby releasing energy
• Anabolism – using energy to build organic compounds by fitting the parts
together.
To obtain energy from a substrate, the microbes remove
electrons and transfer them to other chemicals (so-called
terminal electron acceptors)
• Respiration - the use of inorganic chemicals as
terminal electron acceptors (e.g., oxygen, iron,
manganese or sulfate)
7. Terminology/Classification (cont.)
• Nutrition –
• Heterotrophs – organisms that use organic carbon as
energy and carbon source – humans too
• Lithotrophs – use inorganic carbon, such as CO2 or
HCO3
- as carbon source and an external source of
energy
• Chemolithotrophs – energy from oxidizing reduced inorganic
chemicals…such as iron
• Photolithotrophs – obtain energy from light
8. Terminology/Classification (cont.)
• Aerobes – use oxygen as electron acceptor
• Obligate aerobe – can only use oxygen
…. That’s what we are doing
• Anaerobes – respire using something other than oxygen
as a terminal electron acceptor
• Obligate anaerobes – grow only in the absence of
oxygen
• Falcultative anaerobes – use oxygen when available but
may use other alternate electron acceptors or
fermentation in absence of oxygen
9. Microbes alter geochemistry largely through
oxidation (degradation) of organic carbon
• It’s really all about producing energy
• transferring electrons from a reduced species, often an
organic carbon molecule, to an oxidized substrate, an
electron acceptor (e.g., oxygen)
• Energy released depends on the electron acceptor
• It begins with aerobic respiration
10. Aerobic respiration
or where it all begins
• Many microbes respire (or breathe) using oxygen in
ground water
• Under natural conditions the mass of organic carbon
often does not exhaust the dissolved oxygen
– Organic mass and flow paths
• But lots of human activities are capable of
overwhelming the natural system
11. Lots of human activities can
overwhelm the natural system
18. Some background
• Monitoring wells installed to provide a means
to sample groundwater
• Groundwater sampling methods
• Characterize groundwater quality
30. Unlined Construction and Demolition Debris Landfill
Groundwater Quality at MW-2
0
2
4
6
8
10
January-93
O
ctober-95
July-98
April-01
January-04
O
ctober-06
TOCandDO(mg/L)
Total Organic Carbon
Dissolved Oxygen
So, what happens after the oxygen disappears?
31. Transition from
Aerobic to Anaerobic respiration
• After oxygen disappears, degradation continues….
– lots of microbes continue to degrade the organic carbon
• Microbes use a series of so-called alternate terminal
electron acceptors
– There is an order to their use
• The order dictated by energy released (and/or
thermodynamics and kinetics)
• Energy released/available depends on the electron
acceptor
32. Nitrate reduction - NO3
- ⇄ N2 or NO2
Manganese reduction - MnO2 ⇄ Mn2+
(insoluble species ⇄ soluble species)
Iron reduction - Fe2O3 ⇄ Fe2+
(insoluble species ⇄ soluble and
insoluble species)
Sulfate reduction - SO4
2- ⇄ H2S
Carbon dioxide reduction - CO2 or CH2O ⇄ CH4
(Methanogenesis)
Alternate terminal electron acceptors
The order after oxygen is depleted
35. GRAVEL PIT RECLAMATION
• Affected land must be restored: Establish
a vegetative layer to reduce erosion
• 3 Acre portion of a working gravel pit
• Reclaimed with a manufactured topsoil
rather than natural topsoil
• Monitor groundwater
• Evaluate groundwater impacts
• Establish acceptable level and duration of impact,
if any
36. MANUFACTURED TOPSOIL
WHAT IS IT?
Yds/Acre Lb Nitrogen/Yd
• Short Paper Fiber 1200 2.2
• Municipal WW Sludge 120 56
• Sand 1200 0
• Nitrogen primarily organic
• Metals, organic carbon and other nutrients
• Final thickness: 12 - 15 inches
37.
38. Portion of the gravel pit reclaimed with a
manufactured topsoil
43. Full Circle
• The reduced iron and manganese represents
an energy source, if conditions are right
– In the presence of oxygen, some microbes obtain
energy from the oxidation of iron and manganese
Fe 2+ (aq) Fe3+ (s) + e-
50. Components of Gasoline
B – Benzene
T – Toluene
E – Ethyl Benzene
X - Xylene
Approximate Plume
Boundary
51. 1
10
100
1000
10000
0 200 400 600 800 1000 1200
Distance from Source (feet)
TotalBTEX(ug/L)
BTEX
Pete’s Garage – North Fryeburg
The microbes are responsible for the natural attenuation of the petroleum
components…..otherwise the plume would be significantly larger
57. • Typical lake cross-section after stratification
• Experimental acidification is nothing more
than a large titration
• If one estimates volume of lake, pH and alkalinity
• Should be able to estimate the amount of acid needed
to reduce pH
58. Lake 223 – 1976 to 1978
• pH progressively lowered during 3 year period
– pH in 1976 reduced to 7.0
pH in 1977 reduced to 6.2
pH in 1978 reduced to 5.9
• But there was a big surprise….
– Original experimental design expected to focus on
biological changes (e.g., fish, algae and macro-invertebrates)
– Majority of buffering capacity from watershed?
• Added more acid than calculated to reduce pH
• Why?
59. Microbial activity increased
• The lake sediments were already home to lots of
microbes, including… anaerobic bacteria
capable of reducing the added sulfate and
nitrate
• Sulfate and nitrate “fueled” an unexpected
increase in sulfate and nitrate reduction
• Reduced the effectiveness of acidification by
more than 60%
60. Micro-organisms provide internal lake
buffering system
• Sulfate reduction
– CH2O + H2SO4
2- H2S + HCO3
-
– Sulfate reduction consumes hydrogen ions and
produces alkalinity
• Nitrate reduction (aka denitrification)
Also produces alkalinity and consumes hydrogen ions
