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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.

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groundwater analysis
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

pH

Alkalinity

TDS

Conductivity

Total Hardness

Silicon

Carbonic Acid

Major Constituents (> 5 mg/L)
other reported results
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

Carbonate

Fluoride

Bromide

Nitrate/Nitrite

Carbon Dioxide

Oxygen

Boron

Strontium

Minor Constituents (0.01 to 10 mg/L)
trace constituents 100 m g l
Li 3

Be 4

Al 13

Sc 21

Ti 22

Cr 24

Mn 25

Co 27

Ni 28

Cu 29

Ru 44

Ag 47

Cd 48

In 49

Sn 50

Sb 51

Cs 55

Ba 56

La 57

Ce 58

Trace Constituents ( < 100 mg/L)
  • Zn 30
  • Ga 31
  • Ge 32
  • As 33
  • Se 34
  • Rb 37
  • Yt 39
  • Zr 40
  • Nb 41
  • Mo 42
  • W 74
  • Pt 78
  • Au 79
  • Tl 81
  • Pb 82
  • Bi 83
  • Ra 88
  • Th 90
  • U 92
  • Iodide
  • Phosphate
organic constituents
Humic acid

Fulvic Acid

Carbohydrates

Amino Acids

Tannins

Lignins

Hydrocarbons

Acetate

Propionate

Organic Constituents

Typically concentrations are very low

routine analysis
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

mg/L

Sulphate 59

Bicarbonate 1315

Chloride 45

Fluoride 0.25

Nitrite/Nitrate <0.1

Total Alkalinity 1078

TDS 1321

Balance* 1.01

Example Routine Analysis

*The balance is the ratio of anions to cations in meq/L

charge balance
mg/L meq/L

Calcium <1 <0.05

Magnesium <1 <0.08

Sodium 550 23.9

Potassium 3.5 0.09

Iron 8.7 0.31

Total 24.4

Molarity 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

Charge Balance
errors in analysis
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
Specialized Analysis
  • Additional specialized measurements are sometimes required for specific projects including:
    • 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
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
collins diagram
Collins Diagram

120

Na++K+

Mg2+

100

Ca2+

Cl-

80

SO42-

HCO3-+CO32-

Concentration (meq/L)

60

40

20

0

Sample 1

Sample 3

Sample 2

stiff diagram

Na++K+

Ca2+

Mg2+

Cl-

HCO32-

SO42-

Stiff Diagram

meq/L

80

70

60

50

40

30

20

10

0

10

20

30

40

50

Cations Anions

piper diagram
Piper Diagram

SO4 + Cl

Ca + Mg

Groundwater

Facies

SO4

Mg

Cations

Anions

HCO3 + CO3

Na + K

Ca

Na + K

HCO3 + CO3

Cl

plotting on a piper diagram
Plotting on a Piper Diagram

Ca + Mg

SO4 + Cl

SO4

Mg

HCO3 + CO3

Na + K

Ca

Na + K

HCO3 + CO3

Cl

groundwater facies

Calcium-Magnesium

Chloride-Sulphate

Chloride-Sulpahte-Bicarbonate

Calcium-Sodium

Sodium-Calcium

Bicarbonate-Chloride-Sulphate

Bicarbonate

Sodium-Potassium

Groundwater Facies

Cations

Anions

classification

Ca + Mg

SO4 + Cl

SO4

Mg

HCO3 + CO3

Na + K

Ca

Na + K

HCO3 + CO3

Cl

Classification

Grouping of waters on the Piper Diagram

suggests a common

composition and origin.

Red:

Ca-Mg-SO4

Yellow:

Ca-Mg-Na-Cl-SO4

facies mapping
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 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