Page 126 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 113

3. What is the natural origin of dissolved sodium, potassium, magnesium, and sulphate in
unpolluted groundwater?

4. Explain why K fixation affects the availability of K for plant uptake .

5. At which pH is aluminium least soluble? What could be the cause of detectable amounts
of Al being present in water?

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6. The Cl concentration in precipitation is measured as 4.7 mg l and the annual
precipitation is 800 mm.
-
a. Calculate the local evapotranspiration rate if the Cl concentration in groundwater is
-1
15 mg l .
b. What are the assumptions in the above calculation?
7. What is the difference between alkalinity and TIC? Both alkalinity and TIC are usually
-1
-1
-
reported as mg l or mmol l of HCO . Why?
3
8. Calcite (CaCO ) in soil and sediment dissolves as a result of acids that are present in
3
the percolating water. These acids (e.g. carbonic acid (CO ) and nitric acid (HNO ))
2 3
may be naturally present or anthropogenic in origin. The supply of additional acids of
anthropogenic origin and its consequences for soil and water is called acidification.
a. Give the reaction equation for the dissolution of calcite by carbonic acid (CO ).
2
b. Give the reaction equation for the dissolution of calcite by nitric acid (HNO ).
3
c. Will acidification lead to a decrease in pH in soils containing calcite?
2+
-
The table below lists the analysis results for Ca and HCO concentrations in shallow
3
groundwater (< 10 m below the surface) in a sandy, unconfined aquifer below fields of
grass.
Sample no. Ca 2+ HCO 3 -
(mg l ) (mg l )
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-1
GW1999-Gr03 61 182
GW1999-Gr06 204 488
GW2000-Gr02 98 234
GW2001-Gr05 105 236
GW2001-Gr08 305 753
GW2002-Gr11 124 298
2+
-
d. What is the average Ca :HCO molar ratio for the above samples?
3
e. What can you conclude about the contribution of acidification to calcite dissolution
in the area?
9. Give the half reaction equation for the reduction of sulphate to sulphide. What will
happen to the sulphide ion in reduced environments?

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Soil and Water.indd 125