Page 29 - Soil and water contamination, 2nd edition

16 Soil and Water Contamination

Table 2.1 Units of concentration of substances in soil and water
Unit Description
mg l -1 milligrams per litre water (aqueous solution)
mg kg -1 milligrams per kilogram soil or H 2 O
ppm parts per million by weight
ppb parts per billion by weight
mmol l -1 millimoles per litre water
M molality, moles per kilogram H 2 O
mmol kg -1 millimoles per kilogram soil or H 2 O
N normality, equivalents* per litre water
meq l -1 milliequivalents* per litre water
meq/100 g milliequivalents* per 100 g of soil
-1
-1
* The official SI unit for equivalents is moles of charge (mol c ). For example, meq l is equal to mmol c l .
In this text, the unit equivalents are used.

the figures should be divided by the molar mass or molecular weight, which is the mass in
23
grams of 1 mol of molecules or atoms (1 mol = 6.022⋅10 molecules = Avogadro’s number ).
The molar mass can be calculated by summing the atomic weight s of the atoms that form
the molecule (the atomic weights are given in Appendix 1). For example, the molar mass
-
of nitrate (NO ) is 1 × 14 (atomic weight of N) + 3 × 16 (atomic weight of O) = 62. To
3
-1
-1
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convert the results from mmol l to meq l the concentration in mmol l is multiplied by
-1
-1
2+
the charge z of the ions. For example, 1 mmol l of Ca equals 2 meq l .
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-1
The units mg l (water) and mg kg are mostly used to report environmental
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-1
concentrations of chemicals. However, the molarity (mmol l ) or molality (mol kg ) is
preferable for the chemical evaluation of the concentrations in solutions, since a balanced
chemical equation gives stoichiometric information directly in terms of moles of reactants
-1
and products. The unit meq l is often used to check whether the charges of the cations and
anions balance each other (see Section 5.1).
Example 2.1 Conversion of concentration units
3-
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The PO concentration is 23 μg l . Calculate the concentration in meq l .
4
Solution
3-
3-
First, calculate the molar mass of PO : 1 mol PO is made up of 30.97 g of P and 4 ×
4 4
16.00 g of O (see Appendix I), making a total mass of 94.97 g mol -1
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-1
-4
-1
Thus, 23 μg l corresponds to 23/94.97 = 0.242 μmol l = 2.42 10 mmol l . To obtain
-1
-1
the concentration in meq l , multiply the concentration in mmol l by the charge of the
3-
3- -1
-4
3- -1
PO ion. Thus, 2.42 10 mmol PO l × 3 = 7.26 meq PO l
4 4 4
In addition to the concentration units listed above, a number of additional parameters
related to the concentrations of substances in water are relevant in environmental studies;
they include pH , total dissolved solids , electrical conductivity (EC ), and redox potential
(Eh). These parameters are further specified in Table 2.2. Other common parameters related
to soil and water, for example cation exchange capacity, alkalinity , biological oxygen demand ,

are covered in the chapters on solid phase constituents (Chapter 4) and major dissolved phase
constituents (Chapter 5).
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