Pure natural water is formed mainly by evaporation from seas and lakes. As warm water vapor rises it meets cooler air and condenses into water droplets (just like hot water vapor rising from a bath and condensing into droplets on the cool bathroom walls). The water droplets begin to dissolve a whole range of substances in the atmosphere, such as gases, airborne dust particles and salt from sea spray. When this water eventually falls to earth as rain it acquires other substances from rock weathering and by drainage through fields and woods. Unfortunately, man's misuse of the waterways to dispose of industrial wastes and sewage also allows other, often undesirable, substances to affect the composition of the water.

Many atmospheric gases dissolve in water to some extent and carbon dioxide is one of the most important. It dissolves readily to form carbonic acid, giving a weakly acidic solution. This chemical reaction has important consequences, not only for the eventual chemical content of water, but also for other chemical and biological reactions that occur in the aquatic environment. The naturally acidic nature of rain allows it to bring other, less reactive, substances into solution. (The pH of naturally formed rainwater is about 5.6 but, as we shall see, local atmospheric and landscape variations can alter it significantly before it reaches our tap.) The affinity between carbon dioxide and water has many far-reaching effects on water quality, playing a major role in plant and animal respiration and pH buffering.

Sea spray and local dust particles can have a major influence on the composition of rain. Coastal areas may have quite high levels of dissolved sea salt, with substantially higher levels of sodium and chloride ions than inland areas. Likewise, dusty land surfaces can affect rain. For instance, rain from chalky areas near the coast often have higher than normal levels of calcium caused by the chalky soils in these areas. Indeed, during long dry spells the amount of calcium is often sufficient to neutralize the naturally acidic rain, leading, unusually, to slightly alkaline  rain!

As we are all aware, atmospheric pollution can have a major effect on the composition of rainwater, typically caused by heavy metals such as lead, zinc or chromium from industrial processes, although such effects are more likely to be local, not widespread. Fossil-fuel burning is a more global cause of atmospheric pollution, the two most important gases being sulphur dioxide (mainly from coal burning) and nitrous oxides from vehicle exhausts and oil-burning power stations. These gases can react with rain to form strongly acidic solutions, giving acid rain. Some parts of the world, including the UK have recorded pH values as low as 2.1, due to acid rain.

Rain picks up significant amounts of nitrogen, sulphur and sodium in the atmosphere. At this stage it is likely to be very soft. So we see that rain - which we like to think of as pure water - is already a complex chemical mixture before it hits the earth.

Most of the inorganic substances in tap water come from rock weathering. As the nature of the local rock varies, so does the content of the local water. The degree and rate of rock weathering depends on several factors. There are broadly speaking two types of rock of interest -igneous and sedimentary. (Metamorphic rock is not mentioned, as it weathers in much the same way as igneous rock.)

Igneous rocks have been formed by volcanic action and are common in mountainous regions. They are extremely hard and dissolve slowly, helped by the slight acidity of naturally occurring rain. Water draining from igneous rock will contain most of the common metals, such as magnesium, calcium, sodium and potassium, but usually only in small amounts. The degree or amount of rock weathering that occurs has two important effects on the character of the drainage water.

Water hardness is determined mainly by the amount of dissolved magnesium and calcium ions there are. Soft water is described as such because of the small amounts of dissolved "water hardness" forming ions. When these ions are dissolved they tend to neutralize the acidity of rain. If only small amounts of rock are dissolved (as with igneous rocks), the drainage water tends to still be acidic -although the local water board may well adjust this at the water purification plant.

Sedimentary rocks, on the other hand, are built from a jumble of rocks and some organic debris. Chalk or limestone are common examples. These rocks usually dissolve fairly easily so drainage waters contain high concentrations of the major ions, such as calcium and magnesium. Again, the slight acidity of rain is important in this process. Most of these types of rock have an abundance of calcium carbonate which, being alkaline, raises the pH of the water. The water draining from such rocks is usually neutral or alkaline. The high level of dissolved ions thus makes the water 'hard'.

The action of rain on natural rock and the subsequent weathering plays a major role in determining the over­all content or chemistry of the water. Rock weathering is an important factor in determining both the pH (acidity /alkalinity) and 'hardness' of water.

Water dissolves many more substances as it drains from land to river or reservoir, the number and type of substances depending on the land usage. In an agricultural area there will be run-off from fields. Often, water draining from such areas is high in nitrates and phosphates from fertilizers and may contain man-made pesticides and herbicides. Intensive stock-rearing units may yield 'slurry' which penetrates the streams and ground waters. Water treatment plants often release treated sewage into streams and rivers. A typical effluent will contain high levels of ammonia, nitrate and phosphate. Stream flora usually oxidize most of the ammonia, but significant amounts of nitrate and phosphate will remain. Industry still releases many wastes into fresh waters, the nature and amount of which depend on the industry. Unbelievably, we are still pumping toxic substances such as cyanide, oil, heavy metals and formaldehyde into our waters

The final cocktail we call water is complex and, as we have seen, varies with atmospheric and local land conditions. In general, water from our tap will contain:

Major ions such as sodium, potassium, magnesium, calcium, sulphates, chlorides and hydrogen carbonate. These are usually present in quantities of at least 1 mg/l. All of these ions play a major role in our chemistry and that of the water. They help determine both pH and water hardness.

Key nutrient ions, such as phosphates and nitrate. The concentrations of these are generally lower than those of the major ions and play a major role in algae growth.

Dissolved gases, which include oxygen and carbon dioxide.

Trace ions, such as copper, iron zinc, fluoride, cobalt and molybdenum, which are important in minute amounts, but can be toxic to fish at higher levels

The important message is that tap water is not a simple substance. What we call water is really a weak solution of several dozen dissolved substances, some beneficial to us, some not. (Water Boards everywhere devote a lot of effort to ensuring that water is fit for human consumption.) When we talk about water chemistry or water quality, what we are really talking about is the variety and quantities of these various substances, the reactions between them and their combined effect on our health and well being.

So water is variable.  We have seen that tap water is complex.  Whether the water coming out of the tap is of poor or good quality depends largely on your geographical location and your municipal treatment facility.

Consider a mountain stream. Often such streams near their source can be crystal-clear but devoid of life. Further downstream the water can be green and often muddy - yet such waters are usually highly productive and teeming with all forms of aquatic life. So, in natural conditions the clearest water is not always best for aquatic life. However, it is possible to have good quality water that is also clear.

Water is the most common substance on earth, covering 70% of the planet's surface. Plants and animals are mostly water - our brain is 85% water! Because of this abundance we often take it for granted and overlook what a remarkable substance it is. Water is a major part of all biological systems and there is almost no activity of a living organism that can take place without it.