Transcript Chemical Analyses

Unit 08b : Advanced Hydrogeology
Groundwater Chemistry
Groundwater Analysis
• It is not feasible (or sensible) to attempt to
measure all possible constituents present in
groundwaters.
• A “routine” analysis involves measuring a
standard set of constituents.
• The “routine” analysis forms the basis for
assessing the suitability of the water for
human consumption or various agricultural
and industrial uses.
Major Constituents (> 5 mg/L)
• Calcium
• Magnesium
• Sodium
• Bicarbonate
• Chloride
• Sulphate
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pH
Alkalinity
TDS
Conductivity
Total Hardness
• Silicon
• Carbonic Acid
Other Reported Results
• pH is routinely recorded that measures the hydrogen
ion concentration (-log[H+]).
• Conductivity (electrical) is routinely measured in
mS/m (TDS  0.66 Ec).
• TDS is the total dissolved solids measured as the
residue on evaporation.
• Total alkalinity (expressed as mg/L CaCO3)
measures the acid-neutralizing capacity of a water
sample. It is an aggregate property, primarily due to
the carbonate, bicarbonate, and hydroxide content.
• Total hardness (expressed as mg/L CaCO3) is
another aggregate property of a water sample.
Hardness measures the concentration of multivalent
cations, particularly calcium and magnesium.
Minor Constituents (0.01 to 10 mg/L)
• Potassium
• Iron
• Ammonium
• Carbon Dioxide
• Oxygen
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•
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•
• Boron
• Strontium
Carbonate
Fluoride
Bromide
Nitrate/Nitrite
Trace Constituents ( < 100 mg/L)
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Li
Be
Al
Sc
Ti
Cr
Mn
Co
Ni
Cu
3
4
13
21
22
24
25
27
28
29
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Zn
Ga
Ge
As
Se
Rb
Yt
Zr
Nb
Mo
30
31
32
33
34
37
39
40
41
42
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Ru
Ag
Cd
In
Sn
Sb
Cs
Ba
La
Ce
44
47
48
49
50
51
55
56
57
58
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W
Pt
Au
Tl
Pb
Bi
Ra
Th
U
74
78
79
81
82
83
88
90
92
• Iodide
• Phosphate
Organic Constituents
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Humic acid
Fulvic Acid
Carbohydrates
Amino Acids
Tannins
Lignins
Hydrocarbons
• Acetate
• Propionate
Typically concentrations are very low
Routine Analysis
• A “routine” analysis usually includes all
the major constituents (except carbonic
acid).
• All the minor constituents (except B and
Sr that tend to be regarded as trace
consituents) are also to be expected.
Example Routine Analysis
mg/L
• Calcium
<1
• Magnesium
<1
• Sodium
550
• Potassium
3.5
• Iron
8.7
• Total Hardness 8
_______________________
• pH
7.7
• Conductivity
23 mS/m
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Sulphate
Bicarbonate
Chloride
Fluoride
Nitrite/Nitrate
Total Alkalinity
TDS
• Balance*
mg/L
59
1315
45
0.25
<0.1
1078
1321
1.01
*The balance is the ratio of anions to cations in meq/L
Charge Balance
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•
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mg/L
Calcium
<1
Magnesium <1
Sodium
550
Potassium 3.5
Iron
8.7
Total
• Molarity
meq/L
<0.05
<0.08
23.9
0.09
0.31
24.4
0.0477 M
mg/L meq/L
• Sulphate
59
1.22
• Bicarbonate 1315
21.6
• Chloride
45
1.27
• Fluoride
0.25
0.01
• Nitrite/Nitrate <0.1 >0.01
• Total
24.1
• Cations/Anions
1.01
Errors in Analysis
• Checking the ion balance is good
practice to eliminate gross errors
• Errors mainly arise as a result of:
– failure to measure rapidly changing
parameters in the field (pH, alkalinity, etc)
– poor or no filtration (removal of suspended
solids)
– improper sample storage (pressure,
temperature, buffering, sealing)
Specialized Analysis
• Additional specialized measurements are
sometimes required for specific projects
including:
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trace metals
nutrients (mainly N and P species)
organics (mainly hydrocarbons, chlorinated hc’s)
stable isotopes (14C, 18O, 2H etc)
radionuclides (226Ra, 207Pb,208Po,231Th etc)
• Such analyses (listed above in order of
increasing cost) can be very expensive and
QA/QC is a significant concern.
Visualizing Chemical Data
• There are a large number of plots used to
visualize ion abundances:
– Pie Chart
– Collins Diagram (Bar Chart)
– Stiff Diagram
• Other plots are use to group waters and
interpret their origins:
– Piper Diagram
– Fence Diagram
Pie Charts
Na+K
Na+K
Ca
Ca
Mg
Cl
SO4
Mg
HCO3
Ca
Cl
Mg
SO4
Na+K
SO4
Cl
HCO3
HCO3
1
100
1000
10000 mg/L
Collins Diagram
120
Concentration (meq/L)
Na++K+
Mg2+
100
Ca2+
Cl-
80
SO42HCO3-+CO32-
60
40
20
0
Sample 1
Sample 2
Sample 3
Stiff Diagram
Na++K+
Ca2+
Mg2+
ClHCO32-
SO42-
80
70
60
50
40 30
Cations
20
10
0
10
20
30
40
50
Anions
meq/L
Piper Diagram
Mg
Groundwater
Facies
Cations
Ca
SO4
Anions
Na + K
HCO3 + CO3
Cl
Plotting on a Piper Diagram
SO4
Mg
Ca
Na + K
HCO3 + CO3
Cl
Groundwater Facies
Cations
Anions
Classification
Grouping of waters on
the Piper Diagram
suggests a common
composition and origin.
Red:
Ca-Mg-SO4
Yellow:
Mg
Ca-Mg-Na-Cl-SO4
Ca
SO4
Na + K
HCO3 + CO3
Cl
Facies Mapping
• Spatial mapping groundwater facies
based on classification on the Piper
Diagram can help in the visualization of
progressive changes in chemistry.
• A fence diagram is a convenient method
of viewing a 3D region as a series of
intersecting cross-sections.
Fence Diagram
Fence diagrams can be
used to show groundwater
facies in the same way that
lithofacies are displayed.
calcium-magnesium
calcium-sodium
sodium-calcium
sodium