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