What's in a glass, a sink, a river full of water?... A refreshing drink... a cleansing wash... an invigorating swim... a home for plants, insects, fish, birds and mammals. It all depends on the water quality. We tend to think of water in terms of a particular purpose: is the quality of the water good enough for the use we want to make of it?
Water fit for our own use may be unfit for another. We may, for instance, trust the quality of lake water enough to swim in it, but not enough to drink it. Along the same lines, drinking water can be used for irrigation, but water used for irrigation may not meet drinking water standards. It is the quality of the water which determines its uses.
Scientists, on the other hand, are interested in other aspects of water quality. To them quality is determined by the kinds and amounts of substances dissolved and suspended in the water and what those substances do to inhabitants of the ecosystem. It is the concentrations of these substances that determine the water quality and its suitability for particular purposes. The water of even the healthiest rivers and lakes is not absolutely pure. All water (even if it is distilled) contains many naturally occurring substances - mainly bicarbonates, sulphates, sodium, chlorides, calcium, magnesium, and potassium (total dissolved solids). They reach the surface and groundwater from:
- soil, geologic formations and terrain in the catchment area (river basin)
- surrounding vegetation and wildlife
- precipitation and runoff from adjacent land
- biological, physical and chemical processes in the water
- human activities in the region
Water is naturally purified in large part by the routine actions of living organisms. Energy from sunlight drives the process of photosynthesis in aquatic plants, which produces oxygen to break down some of the organic material such as plant and animal waste. This decomposition produces the carbon dioxide, nutrients and other substances needed by plants and animals living in the water. The purification cycle continues when these plants and animals die and the bacteria decompose them, providing new generations of organisms with nourishment.
Wetlands also serve as ecological kidneys and can remove 20 to 60 percent of metals in water, trap and retain 80 to 90 percent of sediment from runoff, and eliminate 70 to 90 percent of the nitrogen in water. Riparian (streamside) forests also act as living filters that intercept sediments, absorb and store excess nutrients, and transform and remediate the effects of many water contaminants and pollutants carried in runoff from adjacent lands. Riparian areas can reduce the nitrogen concentration in storm water runoff by up to 90%, and can reduce phosphorus levels by as much as 50 percent.
Natural purification of water in liquid form depends on chemical absorption and adsorption by soil particles and organic matter, living organism uptake of nutrients, and living organism decomposition processes in soil and water environments. Human activities that compact soil, degrade soil structure in other ways, contaminate storm water with pollutants, or alter the composition of soil and water-based organisms, will reduce or retard the natural water purification process and cause accelerated movement of unfiltered water through the system and into our water supplies. This reinforces the idea that to the environment works together as an interlocking system; one part carefully working together with the next.