CE 326 Principles of Environmental Engineering
Water Quality Concepts II
Dr. S.K. Ong
General Characteristics of Groundwater and Surface Water
Groundwater
Surface Water
Typical Water Quality (common cations and anions)
Major (________ mg/L)
Secondary (____________ mg/L)
Cations
Sodium, Na+
Calcium, Ca2+
Magnesium, Mg2+
Cations
Iron, Fe2+ and Fe3+
Potassium, K+
Manganese, Mn2+
Anions
Bicarbonate, HCO3Sulfate, SO42Chloride, Cl-
Anions
Carbonate, CO32Nitrate, NO3Silica, SiO2 or Si4+
Fluoride, F-
pH
- is a measure of the hydrogen ion activity/concentration for dilute solution and is given by:
where [ ] is in terms of moles/liter
Water ionizes to form H+ and OH- and is given by the balanced equation
The product of the activity or concentration of H+ and OH- is a constant at a given temperature, i.e.,
[H+][OH-] = Kw = __________
Kw is called the ion product of water
Carbonate system
Bicarbonates (HCO3-), carbonates (CO32-) and carbon dioxide in water (H2CO3) are the most dominant
______________acids/bases system in natural waters.
Weak acid or bases are defined as acids or bases that do _________ dissociate completely.
In contrast, strong acids such as sulfuric acid, hydrochloric acid and nitric acid dissociates completely when
added to water, ex.
1
Let us examine the carbonates in water:
Sources:
- carbon dioxide in the atmosphere dissolve in water to form carbonic acid
- carbonate minerals such as limestone will contribute HCO3- and CO32-to natural waters
The different carbonate species are formed in natural water depending on the pH and can be expressed as
follows:
Ka1 = 10-6.3
Ka2 = 10-10.3
______________ – measure of the capacity of a water to neutralize strong acids (attributed to bases such as
HCO3-, CO32-, OH-)
______________ – measure of the capacity of a water to neutralize strong base (attributed to acids such as
HCO3-, H2CO3, H+)
Total alkalinity (moles/L) = _________________________________________________
Total alkalinity (mg/L as CaCO3) = _________________________________________
(units in mg/L as CaCO3)
Alkalinity is measured by titrating a known volume of water sample (usually 50 ml) with a
standard solution of a strong acid to about pH 4.5.
Alkalinity
in mg/L as CaCO3
= Normality of Acid x Volume of acid added x 50,000
Volume of water sample
__________________(TH) – is the sum of all polyvalent cations (expressed as mg/L as CaCO 3)
Example -
Ca2+, Mg2+,
Fe2+, Fe3+, Mn2+
But in most waters, Ca2+ and Mg2+ are dominant cations
Total Hardness (TH) commonly taken as equal to the sum of _________________ expressed as mg/L as
CaCO3
2
Carbonate Hardness (CH)
- temporary hardness, heating removes it
- equal to the total hardness or alkalinity whichever is less
Noncarbonate hardness (NCH)
- permanent hardness, cannot be removed by heating
- equal to total hardness in excess of alkalinity
- if alkalinity is equal to or greater than total hardness – no noncarbonate hardness is present
Classification of Degree of Hardness of Waters
Soft
______________ mg/L as CaCO3
Moderate Hard
______________
Hard
______________
Very Hard
______________
Example Problem
Your friend dug a well recently and sent the water sample for analysis. The water analysis is as follows:
Ca2+
Mg2+
CO2
pH
= 83 mg/L = _________________
= 27 mg/L = __________________
= 48 mg/L
= 9.43
HCO3- = 220 mg/L = _______________________
CO32- = 15 mg/L = _______________________
Questions
(i) Express the concentrations above in terms of mg/L as CaCO 3
(ii) What is the [H+] concentration in the water? _____________________________________
(iii) What is the [OH-] concentration in water? _____________________________________
(iv) What is the alkalinity of the water as mg/L as CaCO3?
(v) What is the total hardness in mg/L as CaCO3? Is the water hard or soft?
(vi) What is the carbonate hardness?
(vii) What is the noncarbonate hardness?
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