Page 104 - Soil and water contamination, 2nd edition

5

Major dissolved phase constituents

5.1 INTRODUCTION

The major dissolved phase constituents include those substances that are abundant as
2+
2+
dissolved ions in natural waters, namely calcium (Ca ), magnesium (Mg ), sodium
2+
+
+
-
2+
(Na ), potassium (K ), iron (Fe ), manganese (Mn ), carbonate species (H CO , HCO ,
3
2
3
2-
2-
-
and CO ), chloride (Cl ), and sulphate (SO ). Together with dissolved silica (SiO ),
3 4 2
these ions generally account for most of the total dissolved solids (TDS) in subsurface
3+
and surface waters. Aluminium (Al ) may also occur as a dissolved ion, but only under
exceptionally acid conditions. Since they occur in natural waters (see Section 1.2.2) these
major dissolved constituents are usually not considered as pollutants. Nevertheless, they may
cause contamination of soil and water, because they are often released into the environment
together with other polluting substances. Consequently, the major dissolved constituents
may be an important indicator of pollution when they occur in abnormal concentrations, so
knowledge of their behaviour and patterns may help when interpreting the occurrence and
dispersal of other, more harmful, pollutants.
Rainwater contains very few dissolved ions, most of which originate from atmospheric
gases (e.g. CO , SO , and HCl) and from aerosols originating from volcanic emissions and
2 2
biochemical emissions from soil and water. Dissolution of these gases and aerosols enhances
the concentrations of Na, carbonate species, SO , and Cl, and results in a lowering of the
4
pH of rainwater . Moreover, salt spray from seas and oceans brings substantial amounts of Na
and Cl into the atmosphere. These substances may also reach the Earth’s surface independent
of rainfall, by dry deposition of particulate aerosols and adsorption of atmospheric gases
(see also Section 6.2.3). As soon as rainwater infiltrates into soil, its composition is affected
by exchange processes between water and soil material through dissolution , sorption , plant
uptake , and concentration due to evapotranspiration . In general, the main source of ions
in groundwater and river water is the weathering and dissolution of rocks and minerals.
Some minerals, such as carbonates (e.g. chalk, limestone , dolomite , and marl) and evaporite
minerals (e.g. rock salt or halite, gypsum) dissolve readily in water; others, such as silicates
(e.g. quartz , feldspars , clay minerals ) dissolve much more slowly. Therefore, the composition
of water reflects both the geological setting in the drainage basin and the residence time .
Table 5.1 gives an overview of the main sources and typical concentration ranges of the
major dissolved constituents in unpolluted rainwater and fresh water.
Another factor in addition to the geology of the drainage basin that governs the water
composition is the total amount of annual precipitation, since it determines the degree of
dilution. Figure 5.1 shows this effect for typical water composition as function of mean
annual runoff from US drainage basins with different geology (Walling, 1980). This
figure illustrates that the TDS concentrations decrease sharply with mean annual runoff
in catchments dominated by granite, sandstone , shale, schist, and gneiss bedrock. This
indicates that the weathering rates of these rocks are in the order of the residence time of
the water within the catchment . In catchments dominated by sand and gravel, or volcanic

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