Does the water smell like rotten eggs?

If so, it might mean that sewage (waste water from toilets and sinks) is being dumped into the water.

Pollution

The nature of effluents

Water pollution is most commonly associated with discharge of effluents from sewers as sewage treatment plants, drains and factories. Outfalls of this kind are known as 'point-source discharges'. Most cases of accidental, negligent or illegal discharge are also from point sources. The concentration of pollutant in the receiving water is initially high decreasing as the distance from the point of discharge increases. The effects of the pollution are therefore frequently easy to observe. Some of the more serious forms pollution arise, however, from 'diffuse' sources, that is the pollutant does not enter the water from a single point. For example, in agricultural areas, surface water runoff and groundwater infiltration onto lakes and rivers can introduce plant nutrients (from fertilisers) and pesticides in substantial quantities to water bodies. The effects of pollution from diffuse sources can be serious, but are often less immediately obvious than those from point sources as there is no adjacent unpolluted area with which comparisons may be made. Many pollutants also enter water through fallout from the atmosphere. Historically, control and prevention of water pollution have concentrated on point sources as these are more obvious, easily identifiable and in theory easier to regulate at the point of origin. As awareness has increased of the significance of diffuse sources of pollution, control strategies have been under development but are based more on the application of good practices designed to reduce pollutant impact rather then on regulation of specific sources of input.

Most effluents are complex mixtures of a large number of different harmful agents. These include toxic substances of many kinds, extreme levels of suspended solids, and dissolved and particulate putrescible organic matter. In addition, many effluents are hot, of extreme pH value, and normally contain high levels of dissolved salts.

Most effluents also vary in their strength and composition, on a seasonal diurnal or even hourly basis. Most sewage treatment plants report regular diurnal peaks and thoughts in their output according to patterns of water use. Sometimes storm-water drains are connected to the sewerage system, so the strength of the sewage effluent will vary with rainfall. Alterations in the strength and composition of sewage also influence the efficiency of the sewage treatment process, so that dilution of the influent does not necessarily cause an improvement in the quality of the effluent. In industrial plants, variations in the quality of the raw materials, or changes in specification of the finished product, will require changes in the operating conditions of the plant and lead to changes in the composition of the effluent. Many industrial processes are 'batch' rather than continuous processes, so that some effluent discharges will be intermittent rather than continuous.

Organic pollution

The discharge of excessive quantities of organic matter is undoubtedly the oldest, and even today the most widespread form of water pollution.

The major sources of organic pollution are sewage and domestic waste; agriculture (especially runoff from inadequately stored animal waste and silage); various forms of food processing and manufacture; and numerous industries involving the processing of natural materials such as textile and paper manufacture. Most organic waste waters contain a high proportion of suspended matter, and in part their effects on the receiving water are similar to those of other forms of suspended solid. However, the most important consequences of organic pollution can be traced to its effect on the dissolved oxygen concentration in the water and sediments. In unpolluted water, the relatively small amount of dead organic matter is readily assimilated by the fauna and flora. Some is consumed by detritivorous animals and incorporated into their biomass. The remainder is decomposed by bacteria and fungi, which are themselves consumed by organisms at higher trophic levels. The activity of microorganisms results in the breakdown of complex organic molecules to simple, inorganic substances, such as phosphate and nitrate, carbon dioxide and water. During these metabolic processes, oxygen is consumed. However, where the organic load is light, the oxygen removed from the water is readily replaced by photosynthesis and be re-aeration from the atmosphere.

Where the input of organic material exceeds the capacity of the system tm assimilate it, a number of changes take place. How far the sequence of changes proceeds depends upon the severity of the organic load and the physical characteristics of the receiving water. Initially, the enhanced level of organic matter will stimulate increased activity of the aerobic decomposer organisms. When their rate of oxygen consumption exceeds the rate of re-aeration of the water, the dissolved oxygen concentration in the water will fall. This alone may be sufficient, as argued earlier, to eliminate some species, which may or may not be replaced by others with less rigorous demands for oxygen. If the drop in oxygen concentration is very severe, the aerobic decomposers themselves will no longer be able to function, and anaerobic organisms will become predominant.

>>>next