Environmental Engineering Reference
In-Depth Information
Electrical Conductivity (EC) - it is also known as “specific conductance”, and is a
measure of the ability of water to conduct an electric current. Measuring the
electrical resistance between 2 electrodes and comparing it with the resistance of a
standard solution of potassium chloride at 25 o C determines the EC. It is sensitive to
variations in TDS, mostly mineral salts. The degree to which salts dissociate into
ions, the amount of electrical charge on each ion, ion mobility and the temperature
of the water all have an influence on conductivity. There is a linear relationship
between TDS (in mg/l) and EC (in µS/cm) with a coefficient varying between 0.55
and 0.75 (Chapman 1998). This coefficient is specific for each type of water body,
but remains approximately constant, provided the ionic proportions of the water
body remain stable. The conductivity of most freshwaters ranges from 10 to 1000
µS/cm but may exceed 1000 µS/cm in polluted waters or those receiving large
quantities of land runoff (Chapman 1998). In addition to being a rough indicator of
mineral content, conductivity can be measured to establish a pollution zone e.g.
around an effluent discharge, or the extent of influence of runoff waters. It is
usually measured in-situ by a conductivity probe.
Alkalinity of water is a measure of its capacity to neutralize acids. In natural waters
the alkalinity is related to the concentrations of bicarbonate, carbonate and
hydroxide ions. Total alkalinity usually is expressed in terms of equivalent calcium
carbonate in mg/l.
Hardness of water is a characteristic, reflecting the concentrations of selected ions,
but the principl eones are Ca and Mg. Other bivalent and trivalent dissolved metal
ions such as Al, Fe, Mn and Zn, also contribute to total hardness. “Hard” water
contains excess concentrations of these metals, leading to specific properties, such
as scale formation and low foaming capacity, resulting in increased soap
consumption.
• Dissolved gases - the two most common gases present in water are carbon dioxide and
oxygen.
Carbon dioxide is one of the minor gases present in the atmosphere and is the end
product of both aerobic and anaerobic biological decomposition. Rainwater and
most surface water supplies contain small amounts of CO 2 (less than 5 mg/l) but
ground water may contain a significant amount resulting from the biological decay
of organic matter. The presence of CO 2 is important because it affects the pH of
water.
The oxygen content of natural waters varies with temperature, salinity, and
turbulence, the photosynthetic activity of algae and plants and atmospheric
pressure. The higher the temperature of water, the less oxygen it can hold. The
large fluctuations in daily oxygen levels (high during midday from algal
photosynthesis, low at night when both algae and microorganisms are respiring) are
stressful and sometimes deadly to fish and other aquatic life. At DO levels less than
80% saturation in drinking water, odor and taste problems occur. In freshwaters,
dissolved DO at sea level ranges from 15 mg/l at 0 0 C to 8 mg/l at 25 0 C. The
determination of DO concentration is a fundamental part of a water quality
assessment since oxygen is involved in, or influences, nearly all chemical and
biological processes in water bodies. The DO can be used to indicate the degree of
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