16 Dissolved Mass in Groundwater Hydrochemistry. Introduction Water Chemistry: Origin of water Uses...

16Dissolved Mass in

Groundwater

Hydrochemistry

Introduction Introduction

Water Chemistry: Origin of water Uses of water Water quality (contamination)

Topics: 16.1 Dissolved constituents in groundwater 16.2 Types of water analyses 16.3 Water-quality standards 16.4 Examples of data collected in chemical

surveys 16.5 Working with chemical data Case studies :

Madinah Basalt aquifer, Taif Alluvium Aquifer

16.1 Dissolved constituents in groundwater

Rocks, minerals dissolve in water: ions Cations: positively charged ions

(e.g., Ca2+, K+)

Anions: negatively charged ions (HCO3-, Cl-)

Organic compounds, dissolve, form non-charged molecules

16.1 Dissolved constituents in groundwater

Examples of dissolution of minerals, liquids,

Halite dissolution: NaCl = Na+ + Cl-

Calcite dissolution: CaCO3 + H+ = Ca2+ + HCO3

-

TCE dissolution: TCE = TCEaq

Concentration Scales

1. Molar concentration: Number of moles of a species per liter of solution (mol/L)

mole: formula weight of a substance in grams

Example: one-liter solution containing 1.42 g of Na2SO4 has a molarity of (Na2SO4) of:

1.42/(2X22.99 + 32.06 + 4 x 16.00) = 0.010 M

2. Molal concentration

Number of moles per kilogram of solvent (mol/Kg)

Concentration Scales

3. Equivalent charge: number of equivalent charges of an ion per lietr of solution (units: eq/L, meq/L)

Equivalent charge = number of moles of an ion multiplied by the absolute value of the charge:

Example: 1 M Na+ equals 1 eq/L1 M Ca2+ equals 2 eq/L

4. Mass per unit massMass of a species or element per total mass of the system (ppm,

ppb, mg/kg, ug/kg)

5. Mass per unit volume (most common):Mass of a solute dissolved in a unit volume of solutionunits: mg/L, g/L)1 ppm = 1 mg/kg = 1 mg/L

Concentration Scales

Conversion between mg/L to molar concentration:

molarity = mg/L x 10-3

___________________________

formula weight Conversion between mg/L to meq/L:

Meq/L = mg/L___________________________

formula weight/ charge

Example 16.1:The concentration of SO42- in water is 85.0 mg/L. Express this

concentration as molarity and meq/L:

SOLUTION:

mol/L = 85x10-3 / (32.06 + 4 x 16.0) = 0.89 x 10-3

meq/L = 85 /(32.06 + 4 x 16.0)/2 = 1.77

16.2 Types of Water Analyses

Minerals

Organic solids

Organic liquids

Gases Oxygen carbon dioxide hydrogen sulfide, methane

Dissolve in groundwater

Dissolved Constituents in Groundwater Classified by Relative Abundance (Table 16.1)

Major ( 5

mg/L)

Minor

.01 - 10.0 mg/L

Trace (< 0.1 mg/L)

Bicarbonate Boron Aluminum Platinum Copper

Silicon Nitrate Arsenic Radium Gold

Calcium Carbonate Barium Rubidium Lead

Sodium Potassium Beryllium Silver Lithium

Chloride Fluoride Bromide Cobalt Manganese

Sulfate Strontium Cadmium Selenium Nickel

Magnesium Iron Cerium Thorium Phosphate

Carbonic Acid

Chromium Zinc Platinum

Tungsten Vanadium Uranium

Dissolved Constituents in Groundwater Classified by Relative Abundance (Table 16.1)

Organic Compounds

(shallow)

Organic Compounds (Deep)

Humic Acid Acetate

Fulvic acid Propionate

Carbohydrates

Amino acids

Tannis

Lignins

Hydrocarbons

Routine Water Analyses

Routine: measuring concentration of standard set of

most abundant constituents : Major constituents (except Silicon, Carbonic Acid)

Minor constituents (except Boron, strontium)

pH, TDS (mg/L), Conductance (microS/cm, micromhos/cm)

TDS: total quantity of solids when a water sample is

evaporated to dryness

SC: measure of sample’s ability to conduct electricity

Routine analyses: defines almost all dissolved mass, except

when water is highly contaminated

Example of a routine water analysis(Wadi Al-Arj, Taif, Summer 2002)

Parameter Mg/L Parameter Mg/L

pH (25 C) 7.3 Total Iron ND

TDS 1610 Bicarbonate 274Conductivity 2300 Chloride 340

Sodium 265 Sulfate 430Potassium 12 Nitrate 46Calcium 168 Phosphate 12Magnesium 38 Carbonate 134

Mercury ND Ammonia ND

Total Bacteria 1000 Arsenic 1

Specialized Analyses

Trace metals ( Mn, Cr, Cd, Pb, Zn)

Radioisotopes

Organic compounds

Nitrogen-containing species (NO3-, NH4+)

Environmental isotopes

Gases

Specialized analyses done for:

groundwater contamination problems

Water-quality assessment

Research

Regulatory issues

16.3 Water Quality Standards

Designed to protect public health by requiring that

contaminants or naturally occurring constituents in water

be less than certain limits

Microorganisms

Disinfection and disinfection byproducts

Inorganic chemicals

Organic chemicals

radionuclides

16.3 water Quality Standards

Primary drinking water standards:

Secondary drinking water standards:

MCL: maximum contaminant level: highest level of a

contaminant that is allowed in drinking water

(enforceable standards)

MCLG: maximum contaminant level goal: the level of a

contaminant in drinking water below which there is no

known or expected health risk (targets)

Non-enforceable standardEPA Web page

16.5 working with chemical data

Presenting results of chemical analyses:

Abundance or relative abundance

1. Collins Bar diagram

2. Stiff pattern diagram

3. Pie diagram

4. Piper diagram

Abundance and patterns of change

Graphical/illustrative type diagrams

Statistics

Wadi Al-Arj Samples  

Well samples

Runoff samples

Average Average

TDS 2148 810

Conductivity (μS/cm 2843 1244

pH 6.5 6.48

T (oC) 24.2 24.3

Sodium 363.1 182

Potassium 18.9 14.4

Calcium 126.2 67.0

Magnesium 74.5 31.2

Sulfate 451.9 191.4

Chloride 584 177  

Nitrate 85.9 60.6  

Bicarbonate 251.1 266.5  

Pb 0.3 0.02  

Iron 1.6 0.3  

Zn 0.5 0.3  

Mercury (ppb) 12 ND  

Co (ppb) 0.01 0.01  

As (ppb) 1.1 0.2  Coliform Bacteria (per 100 ml) 33 46  

BOD 2.8 5 

Piper diagram

Procedure for plotting piper (trilinear)

1. Convert concentrations from mg/L (ppm) to equivalents

2. Normalize, cations an anions each separately add to 100

(combining Na + K)

3. Plot proportions on triangles

4. Transfer data from triangles to quadrilateral by drawing

straight lines In cation triangle, line is parallel to Mg axis

In anion triangle, line is parallel to SO4 axis

5. Intersection of the two lines is the location of point on diamond

plot

Piper diagram, EXAMPLE

Ca 40 HCO3 2.8 meq/l

Mg 15 SO4 234

Na 120 Cl 45

K 20Ca 1.996 HCO3 2.8

Mg 1.234 SO4 4.87

Na 5.22 Cl 1.27

K 0.511

equivalents

Ca HCO3

Mg SO4

Na + K Cl

proportions

100 100

Runoff

Groundwater

Madinah Basaltic Aquifer

80 60 40 20 20 40 60 80

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

Ca Na+K HCO3 Cl

Mg SO4

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A A

A

A A

A

AA

A

A A

A

A

A

A

AA

A

AA

A

A

A

A

A

A

A

A

A

A

A

A

A

A A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

AA

A

A

A

A

AA

A

80 60 40 20 20 40 60 80

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

Ca Na+K HCO 3 C l

M g SO 4

I I

I

II

I

II

I

II

I

CC

C

CC

C

C C

C

CC

C

CC

C

CC

C

CC

C

HH

H

HH

H

H H

H

HH

H

LegendLegend

I U pstream -2003C U pstream -1978H D ow nstream -1978

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

Ca Na+K HCO3 Cl

Mg SO4

Piper Plot

H

H

H

HH

H

H H

H

H

H

H

L

L

L

LL

L

L

L

L

LL

L

LL

L

LL

L

LL

L

LL

L

LL

L

LL

L

L L

L

L

L

L

II

I

II

I

I I

I

II

I

KK

K

B B

B

JJ

J

JJ

J

K K

K

BB

B

JJ

J

JJ

J

J J

J

J J

J

J

J

J

J J

J

J J

J

J J

J

JJ

J

JJ

J

KK

K

J J

J

JJ

J

JJ

J

J

J

J

J J

J

JJ

J

J J

J

JJ

J

JJ

J

J

J

J

J J

J

J

J

J

JJ

J

JJ

J

EE

E

CC

C

CC

C

CC

C

CC

C

CC

C

CC

C

LegendLegend

E DefaultJ Dry season-groundwaterK Dry season-RunoffL Wet season-groundwaterB Wet season-RunoffI Upstream-2003C Upstream-1978H Downstream-1978

Pie chart

Stiff diagram

1 1 2 2 3 3 4 4 5 (meq/l) 5

Na Cl

Ca HCO3

Mg SO4

wet-ro-001

LegendLegend

2.2 2.2 4.4 4.4 6.6 6.6 8.8 8.8 11 (meq/l) 11

Na Cl

Ca HCO3

Mg SO4

dry-gw-020

LegendLegend

Stiff diagram

Stiff diagramexample

Bar diagram(frequency)

Na+k

Mg

HCO3

0

1

1.5

2

3

SO4

Cl

Ca

cation anion

Meq/l