Geochemical Processes of Lake Averno

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Geochemical Processes of Lake Averno
Kate Overmoe and Avery Cota
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Background

• Lake Averno is located in NW sector of the Campi
  Flegrei Volcanic District.
• 15km west of Naples, Italy.
• Maximum depth of lake is 33m.
• Circumference of lake is 3.2km.
• No natural outlet.

S. Caliro et al., 2008. Geochemical and
biochemical evidence of lake overturn and fish
kill at Lake Averno, Italy. J. Volcanol.
Geotherm. Res. 178 (305-316), 2.
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Lake Averno

• Lake Averno represents an outcropping of the
  water table that is feed directly by meteoric
  water.
• Agricultural and illegal disposal of materials
  contribute to lake pollution.
• Previous studies suggest that an active magma
  source may still contribute to the heat and
  deep fluid release into the Campi Flegrei
  District.

http//faculty.uml.edu/nelson_eby/Field20trip20p
hoto20essays/IGC202004/IGC202004.htm
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Fish Kill Event

• Fish kills recorded January 2002, February 2003,
  and February 2005.
• Likely to have occurred throughout history.
• Journal concentrates on two surveys conducted
  February and October 2005.
• Following fish kill event February 2005, the
  media linked the cause of the event to volcanic
  sources. These accusations triggered geochemical
  investigations of Lake Averno to identify
  conditions at which this overturn and fish kill
  event took place.
• The February 2005 survey was conducted 3 days
  following the event.

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Methods

• Surveyed vertical profiles of the unstratified
  water column (Feb.) and the stratified water
  column (Oct.).
• A multiparameter probe (Hydrolab multiprobe) was
  used to collect temperature, Eh and pH data.
• Water samples were collected by pumping water to
  surface by means of a manual vacuum pump, keeping
  the pressure difference within 0.2 atm.
• Partial pressure of gases (CO2, N2, O2, Ar, CH4,
  He, H2S) were collected in pre-evacuated vials
  and analyzed by a gas chromatograph equipped with
  Thermal Conductivity Detector (TCD).

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The Results

• February 11, 2005

• October 27,2005

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The Results
October 27,2005
Depth (m) T (C) pH SO4 HCO3 pO2 pCH4 pCO2 pH2S
1 20.72 8.82 178 368 135 0.3 1.8 0
3 20.55 8.84 179 362 4.8 0.4 3.8 0
6 19.99 8.83 177 380 67.8 0.2 1.5 0
9 12.4 7.56 166 416 0.5 51.7 8.9 0
12 9.53 7.57 164 401 0.9 51.7 8.4 0
15 9.12 7.59 164 400 0.6 52.2 8.2 0
18 9.07 7.53 171 430 0 113.8 29.3 2.68
21 9.07 7.54 164 402 0.2 125.1 9.1 0
24 9.06 7.55 162 427 0 131 9.3 0
27 9.06 7.56 163 422 0 140 12.3 0.11
30 9.05 7.57 165 418 0 136.8 10 0.02
33 9.42 7.05 99 725 0 1026 42.2 4.56
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The Results
Sulfate reduction SO42- organic matter ? H2S
2 HCO3-
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The Results
Carbonate reduction CO2 4H2 ? CH4
2H2O Acetate fermentation CH3COO- H ? CH4
CO2
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The Results
Fishkill caused by seasonal overturn
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Our Results
Initial Conditions
Resulting Conditions
Phase SI logIAP log KT  
Anhydrite -2.14 -6.49 -4.35 CaSO4
Aragonite 1.01 -7.3 -8.31 CaCO3
Calcite 1.15 -7.3 -8.46 CaCO3
CH4(g) -7.72 -10.54 -2.82 CH4
CO2(g) -3.35 -4.77 -1.42 CO2
Dolomite 2.35 -14.64 -16.99 CaMg(CO3)2
Fluorite -0.35 -11 -10.65 CaF2
Gypsum -1.91 -6.49 -4.58 CaSO42H2O
H2(g) -25.64 -28.77 -3.13 H2
H2O(g) -1.62 0 1.62 H2O
Halite -5.15 -3.58 1.57 NaCl
N2(g) -4.35 -7.6 -3.25 N2
O2(g) -5.37 -8.23 -2.86 O2
Phase SI logIAP log KT  
Anhydrite -2.08 -6.43 -4.34 CaSO4
Aragonite -0.5 -8.74 -8.24 CaCO3
Calcite -0.34 -8.74 -8.4 CaCO3
CH4(g) -7.85 -10.54 -2.7 CH4
CO2(g) -2.11 -3.33 -1.22 CO2
Dolomite -0.89 -17.52 -16.63 CaMg(CO3)2
Fluorite -0.09 -10.94 -10.85 CaF2
Gypsum -1.83 -6.43 -4.6 CaSO42H2O
H2(g) -23 -26.06 -3.06 H2
H2O(g) -2.02 0 2.02 H2O
Halite -5.11 -3.57 1.54 NaCl
N2(g) -4.4 -7.6 -3.19 N2
O2(g) -5.5 -8.23 -2.73 O2

• Drop in Calcite to a nearly saturation level
• Water becomes more dense than the bottom layers
• Allowing turnover to occur due to seasonal
  differences

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Our Results
Initial Conditions Resulting
Conditions
Phase SI logIAP log KT  
Anhydrite -2.06 -6.4 -4.34 CaSO4
Aragonite -0.33 -8.58 -8.25 CaCO3
Calcite -0.17 -8.58 -8.41 CaCoO3
CH4(g) -4.17 -6.9 -2.72 CH4
CO2(g) -1.41 -2.67 -1.26 CO2
Dolomite -0.75 -17.46 -16.71 CaMg(CO3)2
Fluorite 0.24 -10.56 -10.81 CaF2
Gypsum -1.81 -6.4 -4.59 CaSO42H2O
H2(g) -22.1 -25.18 -3.08 H2
H2O(g) -1.94 0 1.94 H2O
H2S(g) -7.12 -7.93 -0.81 H2S
Halite -5.06 -3.52 1.54 NaCl
N2(g) -4.75 -7.96 -3.21 N2
NH3(g) -7.46 -5.36 2.1 NH3
O2(g) -44.57 -47.33 -2.76 O2
Sulfur 8.9 14.17 5.27 S
Phase SI logIAP log KT  
Anhydrite -2.08 -6.44 -4.36 CaSO4
Aragonite 0.4 -7.93 -8.34 CaCO3
Calcite 0.55 -7.93 -8.48 CaCO3
CH4(g) -4.04 -6.9 -2.86 CH4
CO2(g) -1.71 -3.18 -1.47 CO2
Dolomite 0.93 -16.16 -17.09 CaMg(CO3)2
Fluorite 0 -10.6 -10.6 CaF2
Gypsum -1.86 -6.44 -4.58 CaSO42H2O
H2(g) -23 -26.15 -3.15 H2
H2O(g) -1.51 0 1.51 H2O
H2S(g) -7.38 -8.38 -1 H2S
Halite -5.1 -3.52 1.58 NaCl
N2(g) -4.7 -7.96 -3.26 N2
NH3(g) -6.19 -4.42 1.77 NH3
O2(g) -37.19 -40.08 -2.89 O2
Sulfur 9.74 14.62 4.88 S

• Increase in Calcite to slightly oversaturated
• Water becomes less dense than surface waters
• Allowing a turnover to occur due to geothermic
  activity

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Questions?