What determines the quality of water?
A useful definition is that water is of sufficient quality if it is fit for its intended purpose – ie what may be fine to water the garden vegetables may be totally unfit for drinking.
When we look at a water sample or a water body we form an immediate impression of its quality by examining its appearance. But appearances can often be deceiving and water quality is best assessed by PHYSICAL, BIOLOGICAL and CHEMICAL testing.
Types of water tests
Physical tests look at things such as water turbidity (this affects light penetration through the water column) while biological tests count the number of bacteria present, the amount of algae, the number of invertebrates present, the presence of fish species etc.
The overall health of a water sample can also be checked by running a series of chemical tests that determine things such as:
• Dissolved oxygen content
• Nitrogen content
• Phosphorus content
• Conductivity (a measure of the total number of ions present in the water)
• Chlorophyll content (photosynthetic pigment indicative of algal growth)
• Heavy metal content (e.g. Pb, Hg, Cd etc – all are toxic metals)
Chemical testing of water gives a rapid and very useful indication of water quality. Biological testing is usually more time consuming but generally gives the best indication as to the overall health of a water body.
Nutrients and Algal Blooms
The elements nitrogen (N) and phosphorus (P) play an extremely important role in water quality as they are nutrients that stimulate the growth of algae and plants.
An algal “bloom” refers to an excessive growth of these species and is indicative of a severely stressed aquatic environment. The reason for this is simple. When the nutrients become exhausted the algal growth crashes and a large carbon load is released, severely depleting dissolved oxygen in the water and leading to the subsequent death of fish and other aquatic life.
Nutrients such as N and P enter our waterways via natural means, including the weathering of rocks and the decomposition of organic matter. Human activities such as the use of detergents and fertilizers, and the release of organic wastes, sewage effluents and industrial discharges lead to what are described as “anthropogenic” inputs (ie man- made).
In countries with poor sanitation, direct discharge of sewage into rivers may occur leading to greatly elevated levels of N and P. (n.b. raw sewage contains ~ 10 mg/L P and 50 mg/L N).
How is a nutrient like phosphorus (P) quantified?
Bio-available phosphorus is the soluble inorganic phosphate in a water sample that is available for direct use by growing organisms. But this is just one form of phosphorus - P can be also bound up in either the organic matter (organic phosphate) or in insoluble metal phosphates that are often associated with clay minerals that are also found in the water. While the soluble P is only part of the total P content in a water sample, it is the easiest part to determine experimentally.
A pristine water sample should have a very low level of both soluble and total P but a contaminated water sample may have elevated levels of this element – and by just looking at the sample it is not possible to tell how much P is present. Analytical Chemistry allows us to accurately measure how much of something is present in a sample.
An EPA website describes P as “a nutrient essential for growth that can play a key role in stimulating aquatic growth in lakes and streams". Water quality guideline values are recommended as “0.1 mg/L for freshwater ecosystem protection (to prevent excessive plant growth) and 0.05 mg/L for irrigation purposes (to prevent fouling of irrigation pipes)”.
A concentration reported as 1 mg/L is sometimes referred to as 1 ppm (part-per-million). This means that there is 1 mg of P in 1 litre of water (whose mass would be 1000 g or 106 mg), hence this solution would be 1 ppm in P. Parts-per-billion expresses a concentration that is 1000 times more dilute, ie there are 1000 ppb in 1 ppm.
A concentration of 1 ppm or lower is generally described as a “trace” amount, while levels at the low ppb and below would be described as “ultra-trace”.
Chemists are fortunate in having a large range of techniques at their disposal to analyze samples to see what they contain and modern instruments can detect substances down at ppb levels. A statement such as “this water sample is contaminated with cyanide” needs to be backed up by careful measurements using sensitive instruments to prove the point. Analytical chemistry is consequently a very important branch of chemistry, particularly in fields such as environmental and forensic science.
One relatively simple method for chemical analysis is colorimetry. By converting the analyte into a coloured substance using a special colour reagent, the concentration of that analyte can be determined by passing light of a specific wavelength through the solution and measuring how much of the light is transmitted. You can find out more about this important technique here.