Is the water brown and muddy?

Too much soil in the water makes it difficult for plants to grow and fish to breathe. Check the banks to see if soil is being washed directly into the stream.

Ammonia, cyanides and phenols

Ammonia, cyanides and phenols are considered together because, with copper and zinc, they are the most widespread and serious toxic water pollutants in industrialised countries. Ammonia and its compounds are ubiquitous constituents of industrial effluents because ammonia is a staple raw material in many branches of the chemical industry; it is, therefore, a common end-product of industrial processes as well as an important by-product of others, notably the production of coke and gas from coal, from power generation and from most processes involving- the heating or combustion of fuel. It is also a natural product of the metabolism of organic wastes in treatment plants and receiving waters. The toxicity of ammonia to fish is well documented, and although less is known of its effect on invertebrates it appears that levels of ammonia which are tolerable to fish present little danger to most invertebrates. In aqueous solution, ammonia forms an equilibrium between unionised ammonia, ammonium ion and hydroxide ions:

NH3 + H2O = NH4+ + OH-

Unionised ammonia is very toxic to most organisms, but ammonium ion is only moderately toxic. The toxicity of the solution therefore depends on the quantity of unionised ammonia. This in turn depends upon the pH and temperature of the water as pH and temperature rise, the proportion of unionised ammonia also rises. The effect of pH and temperature on ammonia toxicity is therefore considerable. In order to know whether a given level of total ammonia is likely to be toxic, it is necessary to use the pH and temperature values to calculate the corresponding level of free ammonia. As an example, the European Inland Fisheries Advisory Commission (EIFAC) recommends that unionised ammonia concentrations should not exceed 0.025 mg1-1. In a water of pH 8.5 at a temperature of 20° C, this corresponds to a total ammonia concentration of 0.22 mg1-1. In a cooler, acid water, however (pH 6.5, 5° C), a concentration of total ammonia of 63.3 mg1-1 would be acceptable, whereas at pH6.5 and 20'C, the maximum acceptable concentration of total ammonia would be 20 mg1-1.

KCN + H2O = H+ + K+ + HCN = CN- + H+

The dissociation, and consequently the toxicity of cyanide, is pH-dependent, low pH favouring the formation of undissociated HCN which is highly toxic.

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