Complex pollution problems expression

These units are more convenient for low concentrations because they avoid using the very low numbers that would be involved if the concentrations were to be expressed in mol dm water pollutants are often given in ppm units. Also, we may use these units to describe trace concentrations of metals when we are not sure of the exact nature of the species involved. For example, drinking water may be found to have a total aluminium content at a concentration of 3 ppm. The nature and concentration of the different aluminium compounds present in a sample of drinking water is dependent on pH and can be a very complex problem to solve the ions AP, AlOH " and Al(OH)2 are but a few species that may be found. Without exact knowledge of the species present, calculations of molar concentration cannot be made, but it is always possible to express concentration in terms of the total mass of metal present. 

Estimation of the safe yield is a complex problem that must take into account the climatic, geological and hydrological conditions. As such, the safe yield is likely to vary appreciably with time. Nonetheless, the recharge-discharge equation, the transmissivity of the aquifer, the potential sources of pollution and the number of wells in operation must all be given consideration if an answer is to be found. The safe yield, G, often is expressed as follows ... 

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