Unit 08b : Advanced Hydrogeology Groundwater Chemistry.
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Transcript of Unit 08b : Advanced Hydrogeology Groundwater Chemistry.
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
• 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 S/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
Trace Constituents ( < 100 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
• Zn30
• Ga31
• Ge32
• As33
• Se34
• Rb37
• Yt39
• Zr40
• Nb41
• Mo42
• 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
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 Analysismg/L
• Calcium <1• Magnesium <1• Sodium 550• Potassium 3.5• Iron 8.7• Total Hardness 8_______________________• pH 7.7• Conductivity 23 S/m
mg/L• Sulphate 59• Bicarbonate 1315• Chloride 45• Fluoride 0.25• Nitrite/Nitrate <0.1• Total Alkalinity 1078• TDS 1321
• Balance* 1.01
*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
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:– 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
1 100 1000 10000 mg/L
Collins Diagram
0
20
40
60
80
100
120
Sample 1 Sample 2 Sample 3
Co
nc
entr
ati
on
(m
eq/L
)
Na++K+
Mg2+
Ca2+
Cl-
SO42-
HCO3-+CO3
2-
80 70 60 50 40 30 20 10 0 10 20 30 40 50
Stiff DiagramNa++K+
Ca2+
Mg2+
Cl-
HCO32-
SO42-
meq/LCations Anions
Piper Diagram
Cations Anions
GroundwaterFacies
Ca
Mg
Na + K HCO3 + CO3 Cl
SO4C
a + MgS
O 4 +
Cl
HC
O3 +
CO
3
Na + K
Plotting on a Piper Diagram
Ca
Mg
Na + K HCO3 + CO3 Cl
SO4C
a + Mg
SO4 +
Cl
HC
O3 +
CO
3
Na + K
Groundwater FaciesC
alci
um-M
agne
sium
Sodi
um-P
otas
sium
Chloride-Sulphate
Bicarbonate
Bicarbonate-C
hloride-Sulphate
Chloride-Sulpahte-B
icarbonateC
alci
um-S
odiu
mSo
dium
-Cal
cium
Cations Anions
Classification
Ca
Mg
Na + K HCO3 + CO3 Cl
SO4
Ca + M
g
SO4 +
Cl
HC
O3 +
CO
3
Na + K
Grouping of waters on the Piper Diagramsuggests a commoncomposition and origin.Red: Ca-Mg-SO4
Yellow:Ca-Mg-Na-Cl-SO4
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-magnesiumcalcium-sodiumsodium-calciumsodium