Page 109 - Soil and water contamination, 2nd edition

96 Soil and Water Contamination

source is sea salt (NaCl), either in seawater or in evaporite minerals, the Na concentration is
often strongly proportional to the chloride concentration.
In soils and closed surface water bodies (i.e. lakes without an outlet) enrichment of Na
in solution may occur through evaporation . This process of salt build-up (called salinisation )
occurs most intensively under natural conditions in arid and semi-arid regions. Salinisation
can be amplified on irrigated arable land when the irrigation water is reused. Other human
activities can also alter the Na concentrations in surface water and groundwater. The most
widespread dispersal of Na in the environment occurs through the application of road salt
for de-icing roads during winter periods. The release of municipal and industrial wastewater
generally causes elevated Na concentrations downstream of the discharges. Furthermore,
releases of mining wastewater and brines pumped or flowing from oil wells can have
noticeable regional effects on Na concentrations in surface water or groundwater. In coastal
areas, the pumping of water destined for drinking water or for maintaining groundwater
levels in low-lying polder regions may cause intrusion of salt or brackish groundwater into
freshwater aquifers.
The environmental reactivity of Na is limited. Sodium is readily soluble in water and
there are no important precipitation reactions that control the Na concentrations in natural
waters. Furthermore, it does not participate in redox reactions , since – as mentioned above –
all Na is in the 1+ oxidation state . Because Na is a cation, it participates in sorption reactions
at the surfaces of minerals and organic matter, especially of minerals with a large cation
exchange capacity such as clay minerals. However, the affinity of the mineral surfaces for Na,
a monovalent ion, is much less than for divalent ions. Accordingly, Na is quite mobile in soil
and water.

5.3 POTASSIUM
Like Na, potassium (K) is an alkali metal and occurs only in the 1+ oxidation state under
natural conditions. Potassium is an essential nutrient for both plants and animals: it is
needed for cell division and is involved in various enzyme reactions. Like Na, K plays an
important role in the regulation of water in the cells. It maintains the osmotic pressure
and so helps to prevent dehydration and excess fluid retention . In animals, K is also
responsible for the transmission of nerve impulses and for muscle contractions. Compared
to other nutrients (especially nitrogen and phosphorus ; see Chapter 6), K shows the largest
concentrations in living tissues.
The principal sources of K in soil and water are weathering of K-bearing minerals,
decomposition of organic detritus , and fertiliser application. Potassium is slightly less
common than sodium in igneous rocks but more abundant in sedimentary rocks . The
main K-containing minerals of silicate rocks are the K-bearing feldspars orthoclase and
microcline (KAlSi O ) and various micas (muscovite and biotite). The weathering rate of
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the K-feldspars is, however, low compared to other feldspars (Hem, 1989). In sedimentary
rocks , K is present in unaltered grains of K-feldspars and micas and in clay minerals ,
especially illite . The total K content in soils mostly ranges between 0.2 and 3.3 percent
(Scheffer and Schachtschabel, 1989). As demonstrated in Section 4.2.3, K exhibits a strong
tendency to be incorporated in the lattice structure of illite. Therefore, deposition of fine
sediments including illitic clay minerals is an important source of K in floodplain areas. The
major anthropogenic source of K is the application of fertilisers . Because K is an essential
nutrient, fertilisers contain a considerable amount of K. On arable land, potassium fertilisers
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are applied to up to 300 kg K ha depending on the crop type and the soil characteristics,
especially the contents of exchangeable K and clay.

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