THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

• CHEMICAL PRINCIPLES
• CHAPTER 1c - Kehew (2001)

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LEARNING OBJECTIVES

• Review basic fundamentals of chemistry.
• Understand commonly used concentration units and
  unit conversions.
• Learn to calculate and understand the
  significance of water hardness and charge-balance
  error.
• Be introduced to some common ways of graphically
  displaying natural water compositions.

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COMPOSITION OF NATURAL WATERS

• In general, a relatively small number of
  inorganic constituents occur in substantial
  concentrations in most natural waters.
• Major constituents
• Ions bicarbonate (HCO3-), calcium (Ca2),
  chloride (Cl-), magnesium (Mg2), sodium (Na),
  and sulfate (SO42-).
• Neutral species silica (H4SiO40)
• There are numerous possible minor and trace
  constituents.

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WATER HARDNESS

• Hardness - an undesirable property of water
  resulting from the presence of primarily Ca2 and
  Mg2 ions.
• Hardness produces a scummy residue and scale on
  plumbing and sinks.
• Hardness is expressed as the equivalent weight of
  CaCO3 or mg L-1 as CaCO3.
• Total hardness 2.5(mg L-1 Ca2) 4.1(mg L-1
  Mg2)

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CHARGE-BALANCE ERROR - I

• Aqueous solutions must be electrically neutral.
  In other words, the sum of all negative charges
  must equal the sum of all positive charges.
• One check on the quality of a water analysis is
  the charge-balance error, calculated as follows

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CHARGE-BALANCE ERROR - II

• There is always some error in the measurement of
  cation and anion concentrations.
• Thus, we cannot expect a charge-balance error of
  zero for any analysis.
• The C.B.E. may be positive or negative, depending
  on whether cations or anions are more abundant.
• A reasonable limit for accepting an analysis as
  valid is 5.

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REASONS FOR C.B.E. VALUES GREATER THAN 5

• An important anion or cation was not included in
  the analysis.
• Sometimes this can point out the presence of a
  high concentration of an unusual anion or cation.
• A serious, systematic error has occurred in the
  analysis.
• One or more of the concentrations was recorded
  incorrectly.