Hydrogeochemistry
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“Geochemistry of Natural Waters”
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No wastewater, water resources
Study chemistry of rivers, lakes, ground
water, oceans etc.
Questions considered:
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Why do different waters have different
chemical compositions?
What controls the compositions?
How do compositions vary with setting?
How do they vary with time?
Why consider these questions?
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Diagenesis
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Chemical (and physical) alteration of solid
material (low T and P)
Material: rocks, sediments, minerals, plants,
animals, bacteria
Often involves gas phases
Why consider these questions
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Hydrology and Hydrogeology
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Variations in chemical composition can be
used to understand (map) flow paths
Flow can alter chemistry
Diagenesis and hydrology are linked
Hydrologic cycle
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Free water distribution
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96% in oceans
3% in ice
1% in ground water
0.01% in streams and lakes
0.001% in atmosphere
The hydrologic cycle – this figure is for water
How would dissolved mass be included in this?
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Reservoirs = location of water (e.g. lake,
ocean, river etc.)
Flux = motion of water between reservoirs
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Units = mass per time (per area)
Hydrologic cycle = closed loop of the flux
of material
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Flux = can be motion of any material (e.g.
water, solutes, students in room)
Flux only applicable if cycle is steady state
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Not changing through time
Input = output
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For systems in steady state – can consider
the “Residence Time”
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Average time that material is in reservoir
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Definition
T= A/J
Where:
A = abundance (not concentration of material
J = flux of material (into or out of)
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System not in steady state called transient
Best defined by “Response time”
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The amount of time for mass to change to
certain value
Typically doubling or halving.
Sometimes considered “e-folding time”
Amount of time for exponentially growing quantity
to increase by a factor of e.
 Exponential decay = time to decrease by a factor
of 1/e
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Quantification of hydrologic
cycle – a box model
A more complicated (complete?)
box model
A natural system:
Suwannnee River
What are the values water and mass for each box?
Abundance in reservoir
What are values for arrows?
Fluxes
Quick discussion of chemical
changes in hydrologic cycle
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Rain
Streams
GW
Meteoric vs non-meteoric water
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Chemical (and Isotopic) composition of
water
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Natural water always in contact with soluble
material – air, sediments, rocks, organic
matter
Consequence – no natural water is “pure”
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Importance
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Dissolution of gases (e.g., CO2)
Dissolution of solid phases –porosity
Precipitation of solid phases –cements
Coupled with hydrologic cycle - controls
flux of material
Rain water chemistry
Na+ concentrations
Cl- concentrations
•What might be the most
likely source for Na and Cl?
•How could you test to see
if this hypothesis is true?
•What are implications if
this is true, e.g. what and
where are other sources?
Relative concentrations, Rainfall
Pollution – H2SO4
Gypsum dust
Excess Ca, Mg, less Na
and K from oceans
SO4 matches pH – H2SO4
SO4 matches Ca
SO4 marine influence –
dimethyl sulfide
Temporal variations
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During storm
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Rain starts salty, becomes fresher during
strom
O and H isotopes also change during storm
Snow melt initially saltier & lower pH
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change in melting temperature
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Rainfall not the only mechanism to deposit
material from atmosphere to land surface
Aerosol – suspension of fine solid or liquid in gas
(e.g. atmosphere)
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Examples – smoke, haze over oceans, air pollution,
smog
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Dry deposition – aerosols
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Occult deposition
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Dissolution of gases and aerosols by
vegetation and wet surfaces
Sedimentation of large aerosols by gravity
More general term - Dry deposition plus
deposition from fog
Dry and Occult deposition difficult to
measure
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Atmospheric deposition of material called
“Throughfall”
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Sum of solutes from precipitation, occult
deposition, and dry deposition
A working definition
Data Available
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National Atmospheric Deposition Program
napd.swsl.uiuc.edu
Compositional changes resulting from
throughfall – NE US
Open box –
throughfall
composition
Closed box –
incident
precipitation
composition
Hydrology/hydrogeology
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Atmospheric deposition leads to surface
and ground water
Variety of processes alter/move this
water:
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Evaporation
Transporation (vegetative induced
evaporation
Evapotranspiration
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Movement across/through land surface
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Overland flow – heavy flow on land surface
Interflow – flow through soil zone
Percolate into ground water
Conceptualizaton of water flow
Throughfall
Important to consider how
each of these flow paths
alter chemical compositions
of water
Examples of changing chemistry
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Plants
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Soil/minerals
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Provide solutes, neutralize acidity, extract N
and P species
Dissolve providing solutes
Evaporation
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Increase overall solute concentrations
Elevated concentrations lead to precipitation
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Salts/cements
Stream Hydrology
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Baseflow
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Augmentations of baseflow
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ground water source to streams
Allow streams to flow even in droughts
Interflow, overland flow, direct precipitation
Result in flooding
Chemical variations in time
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caused by variations in compositions of
sources
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Bank storage
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Flooding causes hydraulic head of stream to
be greater than hydraulic head of ground
water
Baseflow direction reversed
Water flows from stream to ground water
Hyporheic flow
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Exchange of water with stream bed and
stagnant areas of stream
Nutrient spiraling – chemical changes in
composition because changing reservoir
Stream compositions
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Generally little change downstream
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Changes usually biologically mediated
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Short residence time in stream
Little contact with solids
Nutrients (N, P, Si) uptake and release
(Nutrient spiraling)
Pollutants
Chemistry changes with discharge
Chemistry changes with exchange of GW
and SW
Ground water
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Unconfined example
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Porosity – fraction of total solid that is void
Porosity filled w/ water or water + gas
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Vadose zone – zone with gas plus water
(unsaturated – can be confusing term)
Phreatic zone – all water (saturated zone)
Water table – separates vadose and phreatic
zone
Ground water flow
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Flow through rocks controlled by
permeability
Water flows from high areas to low areas
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Head gradients
Water table mimics land topography
Flow rate depends on gradient and
permeability
Confined aquifers
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Regions with (semi) impermeable rocks
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Confining unit
Confined aquifers have upper boundary in
contact with confining unit
Water above confining unit is perched
Level water will rise is pieziometric surface
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Hydrostatic head
Effects of confined aquifers
Perched aquifers,
springs, water
table mimic
topography
GW withdrawal
lowers head
Other types of water
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Meteoric water – rain, surface, ground
water
Water buried with sediments in lakes and
oceans
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Formation waters
Pore waters
Interstitial water/fluids
Typically old – greatly altered in composition
Other water sources
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Dehydration of hydrated mineral phases
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Water from origin of earth – mantle water
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Clays, amphiboles, zeolites
Metamorphic water
Juvenile water
Both small volumetrically; important
geological concequences
Concentrations/Units
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Need common way to describe dissolved
components
Many ways to do this:
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Solutes: mass (e.g. g, kg) or moles
Solvent: amount of solvent or solution
Geology usually reported by mass – units
of analyses
Chemical calculations always by moles
Concentration terminology
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Total dissolved solids (TDS) – mass of
solid remaining after evaporation of water
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Bicarbonate converted to carbonate
Units of mass (e.g. g, kg, etc.)
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Salinity – similar to TDS except quantities
of Br and I replaced with Cl
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Salinity reported as ratio of electrical
conductivity to standard
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Operational definition
Cl titration includes Br and I
Originally “Copenhagen seawater”
Now KCl standard
Ratio so dimensionless (commonly ppt,
‰, PSU, nothing)
Chlorinity
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Determined by titration with AgNO3
Definition
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Total number of grams of major
components in seawater:
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Mass (g) of Ag necessary to precipitate Cl, Br,
and I in 328.5233 g of seawater
gT = 1.81578*Cl(‰)
S(‰) = 1.80655*Cl(‰)
Water salinity
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Fresh water
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Brackish
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Potable, generally < 1000 mg/L TDS
Non-potable, but < seawater
Seawater, salinity 34 to 37‰
Saline water/brine > seawater salinity
Other measures of TDS
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Refractive index
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Amount of refraction of light passing through
water
Linearly related to concentrations of salts
Conductivity/resistivity
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Current carried by solution is proportional to
dissolved ions
Conductivity
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Inverse of resistance
Units of Siemens/cm
1 Siemen = 1 Amp/volt = 1/Ohm = 1 Mho
Conductance is T dependent
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Typically normalized to 25º C
Called Specific Conductivity
Reporting units
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Need to report how much dissolved
material (solute) in water, two ways:
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Moles
Mass
Need to report how much water (solvent)
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Volume of water, typically solution amount
Mass of water, typically solvent amount
Molar units
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Number of molecules (atoms, ions etc) in
one liter of solution
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Most common – easy to measure solution
volumes
Units are M, mM, µM (big M)
Example
Na2SO4 = 2Na+ + SO421 mole sodium sulfate makes 2
moles Na and 1 mole SO4
Molal Units
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Number of molecules (atoms, ions etc) in
one kg of solvent
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Abbreviation: m or mm or mm (little m)
More difficult to measure weight of solvent,
not used so often
Difficult to determine amount of solvent in
natural waters with dissolved components
Why use molar units?
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Reaction stoichiometry is written in terms
of moles, not mass
CaCO3 = Ca2+ + CO32100 g
=
1 mole
40 g
=
1 mole
60 g
=
1 mole
Simple to convert between
mass (easily measured) and moles
Mass – Mole conversion easy
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Based on Avagadro’s number = 6.022 x
1023
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1 Mole is Avogadro’s number of stuff
Defined by number of atoms in 12 g of
12C
Example
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Nitrate a pollution of concern
Commonly measured as mass
Reported as mass of N in NO3
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N is the element of concern
If not specified, concentrations could be
very different
Moles of NO3 and N are identical
Alternative – Weight units
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Mass per unit volume
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For example: g/L or mg/L
If very dilute solution
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Mass per unit volume about the same as mass
per mass
1L water ~ 1000 g, variable with T, P and X
Alternative – charge units
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Equivalents – molar number of charges
per volume
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eq/L or meq/L
Used to plot piper diagrams
Used to calculate electrical neutrality of
solutions
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Calculation:
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Moles (or millimoles) of ion times its charge
Na2SO4 = 2Na+ + SO421 mole of Na = 1 eq Na solution
1 mole of SO4 = 2 eq SO4 solution
Although different number of moles,
solution is still electrically neutral
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Example
Charge Balance
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Electrical neutrality provides good check
on analytical error
Charge Balance Error – CBE
CBE =
SmcZc - SmaZa
SmcZc + SmaZa
Possible causes of errors
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Significant component not measured
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Commonly alkalinity – can be estimated by
charge balance
Analytical error
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+5% difference OK – acceptable
+ 3% good
0% probably impossible
Piper Diagrams
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Two triangular diagrams
Projected on quadralinear diagram
Very useful figure for comparing
concentrations of water
Santa Fe water chemistry
Construction
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Convert concentrations to meq/L
Use major cations and anions
concentrations
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Cations = Ca, Mg, Na + K
Anions = SO4, Cl, HCO3 + CO3 (or alkalinity)
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Calculate %’s of each element on ternary
diagram
For example Ca is:
[Ca]
[Ca] + [Mg] + [Na + K]
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*100
Plot %’s on ternary diagrams
Project each % onto diamond diagrams
Composition:
Ca = 22.3%
Mg = 13.7%
Na+K = 64%
Alkalinity = 31.3%
Sulfate = 54.5%
Chloride = 14.2%