General Groundwater Chemistry

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General Groundwater Chemistry
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Groundwater

• Occurs in the saturated zone below the
  groundwater table

(Vadose Zone)
(Phreatic Zone)
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Groundwater

• Some groundwaters are isolated from direct
  atmospheric contact by an aquitard.

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Groundwater

• Groundwaters are recharged by surface
  precipitation either locally (unconfined
  aquifers) or remotely (confined aquifers)

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Groundwater

• Time in the groundwater system may be hours,
  days, weeks, months, years, decades, centuries,
  or millennia.
• Dependent upon
• rock/sediment properties such porosity and
  permeability
• Depth and distance of transport
• During the time in the groundwater system, the
  water can react with the rock/sediment.

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Groundwater
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Groundwater
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Groundwater
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Groundwater
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Groundwater

• Controls on water chemistry in the unsaturated
  (vadose) zone
• Ingorganic reactions
• Gas dissolution and redistribution
• CO2 H2O ? H2CO3
• H2CO3 ? HCO3- H
• HCO3- ? CO32- H
• Weak acid strong base reactions
• CaCO3 H ? Ca2 HCO3-
• Albite ? Kaolinite
• Cation exchange
• Precipitation-dissolution of gypsum
• CaSO42H2O ? SO42- Ca2 2H2O
• Sulfide oxidation
• 4FeS2 15O2 14H2O ? 4Fe(OH)3 16H 8SO42-

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Groundwater

• Controls on water chemistry in the unsaturated
  (vadose) zone
• Organic reactions
• Dissolution of organic matter at the ground
  surface
• Complexation of Fe and Al
• Sorption of organic-metal compounds
• Oxidation of organic compounds
• CH2O O2 ? CO2 H2O energy

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Groundwater

• Controls on Chemistry in the saturated (phreatic)
  zone
• Weak acid strong base reactions
• Carbonate minerals H ? cations HCO3-
• Silicate minerals H ? cations H4SiO4
• Aluminosilicates H ? cations H4SiO4 clays
  and iron oxide/hydroxides
• Dissolution of soluble salts
• Halite NaCl ? Na Cl-
• Anhydrite CaSO4 ? SO42- Ca2
• Gypsum CaSO42H2O ? SO42- Ca2 2H2O
• Sylvite KCl ? K Cl-
• Redox reactions
• Cation exchange reactions

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Groundwater
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Groundwater

• Salt dissolution in western Oklahoma

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Groundwater

• Controls on Chemistry in the saturated zone
• Controls on Redox reactions
• Oxygen content of the recharge water
• Pass through organic rich soil or fractures
• Distribution and reactivity of organic matter and
  other potential reductants is the aquifer
• organic matter in rock/sediment
• Presence of hydrocarbons
• Distribution of potential redox buffers in the
  aquifer
• Particularly MnO2, Fe(OH)3, and Fe2O3
• Circulation rate of the groundwater
• Longer residence time, more chances to work down
  redox ladder
• Isolation from direct atmospheric input
• Time in contact with sediment/rock
• Function of depth and distance from recharge area
  as well as rock/sediment porosity and
  permeability
• Types of sediment and/or rock the water passes
  through

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Water Chemistry and Rock Type
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Groundwater Chemical Evolution Example
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Groundwater Chemical Evolution Example
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Weathering and Water Chemistry

• Classic case study II Igneous rock weathering
  and Spring water composition
• Garrels and McKenzie (1967) reviewed in Drever
  (1997)
• Ephemeral and perennial springs in the Sierra
  Nevada
• Basic Assumptions
• Water compositions result from attack by carbonic
  acid on silicate minerals
• Plagioclase feldspar is the sole source of Na
  and Ca2 because it is abundant and readily
  weathered.
• No calcite present.
• Dissolved silica is derived primarily from
  weathering of plagioclase with minor Fe-Mg
  minerals

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Weathering and Water Chemistry

• The general model
• Snow melt is primary water source
• Need to remove dissolved components from snow

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Weathering and Water Chemistry

• Ephemeral Springs
• Sourced by CO2-charged rainwater
• The following weathering reactions accounted for
  virtually all of the dissolved constituents
• Plagioclase to kaolinite (by far the most
  important)
• 80 of rock-derived constituents
• Biotite to kaolinite
• K-feldspar to kaolinite
• No significant dissolution of quartz

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Weathering and Water Chemistry

• Perennial Springs
• Sourced by deeper, older waters in that have
  equilibrated with the rocks
• Higher concentrations of ions and dissolved
  silica
• Lower SiO2Na ratio than ephemeral springs
• The following weathering reactions accounted for
  virtually all of the dissolved constituents
• Plagioclase to kaolinite
• Biotite to kaolinite
• Plagioclase to smectite (to account for lack of
  silica)
• Dissolution of calcite (needed to balance
  reactions)
• Source of Ca2 and HCO3-

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Carbonate Groundwater Chemistry

• Carbonate-rock aquifers include some of the most
  important and prolific aquifers for groundwater
  supply
• Edwards aquifer central Texas
• Floridian aquifer Florida and southern Georgia
• These aquifers are unique in that they exhibit a
  high solubility of the aquifer framework
• Controls the water chemistry
• Dissolution controls the landscape - karst

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Carbonate Groundwater Chemistry
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Carbonate Groundwater Chemistry

• Karst aquifers
• Recharge occurs by
• direct transfer from the surface to the saturated
  zone
• Slow infiltration through the vadose zone
• Chemistry differences reflect complex
  interactions between
• Recharge
• Storage
• Flow

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Carbonate Groundwater Chemistry

• Flow varies from
• Intergranular flow
• Tubulent flow through large open conduits

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West-Central Kentucky Example

• Mammoth Cave
• Developed in Mississippian Girkin and St.
  Genevieve limestones
• Most of the cave in the vadose zone
• Over 300 miles long
• Overlain by units containing perched aquifers or
  confining beds
• Because of short flow path and residence times,
  water flowing from springs in the Haney Fm. Are
  undersaturated with respect to calcite

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West-Central Kentucky Example
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West-Central Kentucky Example

• Where water reaches the dry cave passages,
  sulfate minerals (incl. gypsum) are precipitated
  on the cave ceilings.
• Vadose water enters the cave from the side of
  karst valleys
• Along fractures undersaturated w/calcite
• After passing through soil oversaturated
  w/calcite
• Drip waters form stalactites and stalagmites
• Groundwater that continues through the conduit
  system remains undersaturated as it exits the
  cave as base level springs

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Carbonate Aquifers

• Similar to broad models of flow (Shuster and
  White, 1972) developed for PA carbonate aquifers.
• Diffuse flow system
• Relatively constant hardness through year
• Generally at or near saturation for calcite,
  undersaturated in summer
• Exhibits constant year-round temperature
• Conduit flow system
• Large seasonal variation in hardness
• Undersaturated with calcite
• Exhibits strong seasonal variation of temperature
• Both systems exhibit a strong seasonal variation
  in pCO2

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Deep Carbonate Aquifer Example

• Deeper aquifers may have a significantly
  different water chemistry from shallow aquifers
• Central Kentucky karst region (Scanlon, 1989)
• Three groundwater types
• Ca-Mg-HCO3- Type
• Most common
• Greatest amount of seasonal variation
• Shortest residence time of the three types
• Na-HCO3- Type
• Results from ion exchange with interbedded shales
  
• Na-Cl Type
• Also developed due to ion exchange with shales

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Deep Carbonate Aquifer Example
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Deep Carbonate Aquifer Example

• As the water exited the aquifer at springs, the
  water re-equilibrated with the atmosphere
• Degassing of CO2
• As water makes its way downstream it continues to
  lose CO2
• Water in stream becomes saturated with calcite
• No precipitation until stream flows over a
  waterfall 1km from spring
• Agitation and turbulence causes rapid degassing
  of CO2 raising saturation index above critical
  point for precip.
• Travertine precipitates on everything in contact
  with water
• Rocks, twigs, leaves, etc.