STANDARD PRESSURES

A water quality index gives an adequate overall summary of water quality based on the pressures (indicators) derived from the driving forces, which in turn will determine the catchment status of the water and the related catchment impact.

The index is geared to establish the pollution status of the water, which is based on:

• Trophic status of the water and thus the potential for algal growth
• General organic and inorganic pollution
• Sewage in the water
• Mine pollution
• Erosion

The index is extremely useful for pinpointing polluted areas, tracking management strategies for solving pollution problems, establishing trends and seasonal variations (Cydna Laboratories, 1998).

• A water quality index method developed by Umgeni Water has been adapted for Cydna Laboratories (see Table ). The parameters / indicators were selected according to the uses of surface water.
• The Jukskei Forum developed an index based on the pressures as prescribe by DWAF for different water users.

Within Johannesburg the main uses of surface water are:

• Recreation: e.g. wind surfing, canoeing and swimming
• Environment: health of fish stock, survival of fauna and flora.
• Domestic: drinking, cooking, cleaning
• Industry : discharge of effluents

These pressures or (indicators) are being used to determine the state and impact of the driving forces. The pressures are being determined through sampling points across the two catchment areas. A colour code is then assigned to the sampling point according to the severity of the pressure.

Each driving force of water impacts on the environment whereas the pressures of these driving forces may have a cumulative effect. The list of pressures constituted by the driving forces follows. An increase in one of these pressures will determine the severity of the driving force.

DO is a very important indicator of water quality. Low DO values normally point to organic pollution. Natural water has the ability to stabilise pollutants that enter the environment, but if the natural self-purifying ability of the water is overpowered then the DO is depleted and anaerobic conditions set in which results in bad smells and fish mortalities. At the opposite end of the scale too much oxygen (supersaturation) can also cause bubble disease and mortality in fish. Supersaturation is often caused by prolific algae growths.

pH

- pH is an indication of whether a water sample is acid, alkaline or neutral:

- pH below 7,0 is acidic

- pH above 7,0 is alkalinic

- pH at exactly 7,0 is neutral

• Increases in pH increases the ammonia toxicity to aquatic life;
• Lowering of pH increases the cyanide toxicity;
• Acid water adversely affects tanneries, textile industries, steam generation and will speed up corrosion of water pipes.

Electrical conductivity

Conductivity is a convenient way of determining the total dissolved solids (TDS) of a water sample. By applying an empirical factor of 6,6 to the conductivity, expressed in mS/m (milli Siemens per meter), a reasonable estimate of the dissolved solids in the water is obtained, expressed as mg/l. High TDS content may lead to:

• Scaling occurring in pipes, which results in lower water supply flows or increased pumping costs;
• Premature replacement of geysers and household utensils;
• Sodium and Magnesium sulphate will have a laxative effect on consumers.

Turbidity

High turbidity values will shorten filter runs at water purification plants. Silting up of an impoundment will be speeded up considerably by high amounts of suspended solids.

Chemical Oxygen Demand (COD)

The COD method is used to determine the pollution level in a water system. High COD values are mainly caused by organic matter in the water, for example, a blocked sewer in the vicinity.

Phosphorous

Phosphorous is an important nutrient, but phosphate in an impoundment leads to algae blooms, which may cause:

• odours in drinking water by releasing 2-methyl-isoborneal into the water body;
• toxin production by the blue-green algae, Microcystis, also known as cyanogosin, which is toxic enough to kill cattle that ingest this poison; and
• clogged filters at the water treatment plant, thereby increasing operational costs.

The single most common source of phosphate in water is from blocked sewers that overflow into the stormwater systems.

Nitrogen

Ammonia in water is usually related to sewage being found in the system.

• Ammonia concentrations of between 0,4 and 2,0 mg/l are lethal to most fish species.
• Nitrates may cause methaemogloinaemia (blue baby) in infants who drink liquids with a nitrate concentration greater than ten (10 mg/l).

REFERENCES