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SALTS AND SALINIZATION OF SURFACE WATERS
Salts are made up of the combination of the chemicals: Sodium (Na+), Potassium (K+), Calcium (Ca++), Magnesium (Mg++), Chloride (Cl-), Sulphate (SO4--), and Bicarbonate (HCO3-). Ions that are positively charged are called cations and those that are negatively charged are called anions. The cations (Ca++, Mg++, Na+, K+) and anions (Cl-, SO4--, HCO3-) are collectively known as major ions. Concentrations of these major ions are basic descriptors of water quality on which many criteria for water use are based (such as drinking water, agriculture and industrial use).Total Dissolved Solids (TDS) is a measure of the total amount of major ions (plus Silica - SiO2) in water. TDS are naturally highly variable in surface waters and there is no global reference value that can be used to assess a contamination level. In coastal areas, river chemistry can reflect marine salt deposited in rain water. Lake chemistry depends primarily on the hydrological balance of the lake. Lakes with outlets tend to reflect the chemistry of their tributaries, while lakes without outlets have waters that may be saline (TDS > 3 g L-1 to a maximum of 300 g L-1) due to evaporation (Dead Sea, Lake Assal in Djibouti, Kara Bogaz in Turkmenistan).
Calcium, Magnesium and Potassium
Calcium, the most common cation found in surface waters, is mainly a function of geology especially when carbonate or gypsum deposits are present.Concentrations of magnesium are not strongly influenced by anthropogenic activities and therefore magnesium is not used as an indicator of pollution stress.
In comparison with other continents, European rivers have the highest concentration levels of Ca++. These continental differences are not caused by anthropogenic impacts but by geological influences. At the global scale, natural Ca++ levels range from 0.06 to 210 mg L-1 in streams (<100 km2) and from 2 to 50 mg L-1 in major rivers (>100 000 km2); see ref. 4.
Potassium-bearing minerals, mostly feldspar and mica, are abundant but poorly soluble. The natural potassium concentrations in rivers are very low (< 5 mg L-1). Even though potassium is affected by fertilizer use, it never reaches levels of concern for water quality. The highest K+ concentrations in these selected European watersheds are found downstream from major mining districts (potash and salt mines) on the Rhine, Weser and Elbe rivers, where concentrations may exceed the 12 mg L-1 WHO guideline for drinking water.
Sodium and Chloride
In most waters sodium and chloride are tightly linked. They both originate from natural weathering of rock and from atmospheric transport of oceanic inputs and from a wide variety of anthropogenic sources. The WHO drinking water guideline for Cl- is 200 mg L-1.The anthropogenic sources of sodium and chloride are so pervasive that concentrations of sodium and chloride have risen by a factor of 10 to 20 in many rivers.
Since 1889 there has been a five-fold increase in Cl- concentrations at the water intake (Ivry) for the City of Paris. The background concentration is estimated to be about 5 mg L-1 and originates from marine aerosols. The increases in Cl- concentrations result from anthropogenic activities occurring upstream from this station. Present Cl- concentrations are well below the WHO standard for drinking water (200 mg L-1).
The Rhine River suffers from two major salt sources -- the Alsace potash mines and the Lorraine salt mines, both located in France. The brine from these sites is discharged to the Rhine downstream of Basel and to the Mosel River, respectively. The Alsace source (15 000 tonnes NaCl/day) represents 30 % of the Cl- flux measured at Lobith at the German/Netherlands border. Other contributions are mostly urban and industrial from the Ruhr area. Since the opening of potash mines, 100 years ago, Cl- levels and fluxes have increased by a factor of 15 to 20. The WHO standard for drinking water has been exceeded, as well as the guideline for greenhouse watering, a very important activity in the Netherlands.
In the Nile basin, chloride concentrations are lowest in Lake Victoria tributaries. Most of the Cl- originates from atmospheric fallout and hydrothermal activity. Cl- concentration increases at Khartoum from evaporation in the Sudd swamps of southern Sudan. As the Nile proceeds northwards across the desert, evaporation continues to dominate, raising the Cl- concentration to 15 mg L-1. Nile water remains in Lake Nasser for approximately two years; as the flow continues to the delta region, municipal and industrial wastes along with irrigation practices, raise the concentration significantly. The estimated five-fold decrease in water discharge to the Mediterranean Sea (Ref. 16) since construction of the High Aswan Dam has led to a lack of dilution capacity that was formerly present in the Nile.
Chloride in the Krishna River, India originates from atmospheric fallout and domestic sources, concentrated by evapotranspiration. In upstream stations to the west, relatively high Cl- levels are noted, most probably resulting from marine aerosols driven inland by westerly winds.
Many African waters are characterized by very low Cl- concentrations such as the Senegal, Niger, Zaire and Chari rivers. Evaporation and pollution effects are responsible for higher Cl- concentrations at the mouths of the Nile and Orange rivers. In industrialized parts of the world chlorine products are some of the most common chemicals used in a wide range of industrial processes and for treatment of drinking water.
In arid and semi-arid areas of the world evapotranspiration leads to an increase in the salt content (salinization) of surface waters and to an increase in the sodium and calcium concentrations. The ratio of sodium to calcium is a key descriptor in water for irrigation.
Water quality is often affected by salinization, particularly in surface waters, because of evaporation and the deposition of salts on the ground surface. Some rivers flow through arid regions when their source lies in wetter parts of upper basins (Colorado, Rio Grande, Orange, Nile, Indus, Murray). About 50 percent of arid land is located in 'endorheic' regions whence there is no flow to the ocean.. In these regions, rivers flow into lakes such as the Caspian, Aral, Chad, Great Salt, Eyre, and Titicaca, which have no outlets.
Dissolved salt content is regulated by the weathering of a few key minerals (halite and gypsum , carbonates and silicates, in decreasing order of solubility); therefore, Total Dissolved Solids (TDS) and ionic contents are linked to rock types. Soluble minerals are not found in metamorphic rock shields nor in volcanic rocks where silicates are more weathered. Hence waters tend to be low in TDS. TDS can, however, increase where hydrothermal groundwater inputs occur.
Sulphate
The sulphate ion (SO4--) is highly variable in surface waters where it is linked to sulphur-bearing minerals. Sulphate has greatly increased in some North American rivers (such as St Lawrence, Mississippi) over the last 100 years resulting largely as a result of increased industrial and agricultural activities (Ref. 10 , 11 and 15). When sulphur-bearing minerals are more abundant as in the Great Plains shales, SO4-- levels may exceed the 400 mg L-1 WHO guideline for drinking water.Development in the Ob River basin of Siberia has not affected sulphate concentrations (10 mg L-1) over the period for which records are available . The upper Ob basin is no longer pristine but SO4-- concentration changes at the mouth are not significant enough to detect any sulphate pollution.
In comparison, even though natural background levels of SO4-- in the Volga river are among the highest found for large rivers, SO4-- has increased from 50 to 60 mg L-1 since the 1950's. This is due to large-scale human activities, including mining, and oil exploration.
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