Environment and development
in coastal regions and in small islands

Coastal region and small island papers 19


  Water quality

A clear, blue sea does not
necessarily indicate clean
water, South Friar’s Bay,
St Kitts, 2002.

Runoff from coastal development and
discharges from boats are among the potential
sources of pollution in coastal waters, Cane
Garden Bay, Tortola, British Virgin Islands, 1990.


The condition or quality of coastal waters is very important for health and safety reasons and also for visual impact. Disease-carrying bacteria and viruses (or pathogens) associated with human and animal wastes pose threats to humans by contaminating seafood, drinking water and swimming areas. Eating seafood and even swimming can result in hepatitis, gastrointestinal disorders, and infections. There are several sources of bacterial contamination in coastal waters, e.g. leaking septic tanks, poorly maintained sewage treatment plants, discharges from boats, and runoff from the land during heavy rains and storms.

Water quality also depends on the level of nutrients. These are dissolved organic and inorganic substances that organisms need to live. The most important nutrients of concern in coastal waters are nitrates and phosphates. In excessive quantities these can cause the rapid growth of marine plants, and result in algal blooms. Sewage discharges, and household and commercial waste that is carried to the sea by storm runoff, add excess nutrients to coastal waters. Detergents and fertilizers supply high quantities of nutrients to streams and rivers and ultimately the marine environment.

The visual quality of the water is also important; a beach environment is much more attractive when the water is clear and one can see the sea bottom. However, even clear water may sometimes be polluted. Rivers and streams often carry a heavy load of sediment (soil particles) to the sea, and in many countries, the nearshore waters may turn a brown colour after heavy rainfall.

Activity 8.1


Measuring water quality

What to measure


There are a number of simple indicators which can be used to measure water quality. These are:

  • Faecal coliform bacteria: naturally present in the human digestive tract, but rare or
    absent in unpolluted water;
  • Dissolved oxygen: needed by all aquatic organisms for respiration and their survival;
  • Biochemical oxygen demand: a measure of the quantity of dissolved oxygen used by bacteria as they break down organic wastes in the water;
  • Nitrate: a nutrient needed by all aquatic plants and animals to build protein;
  • Phosphate: also a nutrient, and needed for plant and animal growth;
  • pH: a measure of the acidic or alkaline properties of the water;
  • Temperature;
  • Turbidity: a measure of the amount of suspended matter and plankton in the water.

How to measure

Measuring water quality at Old
Point Regional Mangrove Park
in San Andres.

There are many sophisticated field and laboratory methods to measure water quality, and there are also simple kits that can be purchased which measure quantitatively the various indicators described above. One such kit referred to in Annex 1 is designed for testing salt and brackish waters for coliform bacteria, salinity, dissolved oxygen, biochemical oxygen demand, nitrate, phosphate, pH and turbidity. The kit comes with all reagents and components to test 10 water samples together with complete instructions, colour charts and safety information. Similar kits are also available for freshwater. Since the kits vary with different manufacturers, no attempt is made here to describe the step by step instructions – rather the reader is referred to the detailed instructions that come with the kit. These kits are designed for schools and citizen monitoring groups and are very easy to use.

Collecting the water sample properly is very important to ensure that correct results are obtained. Collect the water sample in a sterile, wide mouthed jar or container (approximately 1 litre) that has a cap. If possible, boil the sample container and cap for several minutes to sterilize it and avoid touching the inside of the container or the cap with your hands. The container should be filled completely with your water sample and capped to prevent the loss of dissolved gases. Test each sample as soon as possible within one hour of collection. When possible, perform the dissolved oxygen and biochemical oxygen demand procedures at the monitoring site immediately after collecting the water sample.

The collection procedure is as follows:

  • Remove the cap of the sampling container.
  • Wear protective gloves and rinse the bottle 2–3 times with the seawater.
  • Hold the container near the bottom and plunge it (opening downward) below the water surface.
  • Turn the submerged container into the current or waves and away from you.
  • Allow the water to flow into the container for 30 seconds.
  • Cap the full container while it is still submerged; remove it from the sea immediately.


  When to measure

The kits only have a limited supply of tests; however, there are some indicators such as temperature and turbidity which do not require specific reagents or chemicals and can be measured as many times as desired. It is important to design the monitoring programme based on the number of tests/kits available, e.g. if one kit only has enough materials for 10 phosphate tests, and two samples are measured each time, then this will allow five tests over the monitoring period. When measuring water samples, it is advisable to collect two sets of water samples and duplicate each test. This way more students can be involved and sample duplication also provides for added reliability of the results.

What will the measurements show








The measurements will show variation in the water quality indicators over a period of time. The accompanying box gives some ideas on interpreting what the indicators signify. It is not necessary to measure all the indicators described; a school group may wish to select just two or three.


Faecal coliform bacteria themselves are not harmful; however, they occur with intestinal pathogens (bacteria or viruses) that are dangerous to human health. Hence, their presence in water serves as a reliable indicator of sewage or faecal contamination. These organisms may enter waters through a number of routes, including inadequately treated sewage, stormwater drains, septic tanks, runoff from animal grazing land, animal processing plants and from wildlife living in and around water bodies.

Dissolved oxygen is an important indicator of water quality and is measured as percentage saturation. Much of the dissolved oxygen in water comes from the atmosphere. After dissolving at the surface, oxygen is distributed throughout the water column by currents and mixing. Algae and rooted aquatic plants also deliver oxygen to water through photosynthesis. Natural and human-induced changes to the aquatic environment can affect the availability of dissolved oxygen. For instance, cold water can hold more oxygen than warm water, and high levels of bacteria from sewage pollution can cause the percentage saturation to decrease.

Using a kit to measure water
quality, St Lucia, 2001.

Biochemical oxygen demand – in general, the higher the biochemical oxygen demand, the worse the quality of the water. Natural sources of organic matter include dead and decaying organisms. However, human activities can greatly increase the available organic matter through pollution from sewage, fertilizers or other types of organic wastes. The decomposition of organic wastes consumes the oxygen dissolved in the water – the same oxygen that is needed by fish and shellfish.

Nitrate – excess nitrate will cause increased plant growth and algal blooms, which may then out-compete with the native submerged aquatic vegetation. The excess algae and plants may smother the habitat used by the aquatic fauna and their decomposition can lead to oxygen depletion. Sources of nitrate in coastal waters include runoff containing animal wastes and fertilizers from agriculture, and the discharge of sewage or waste effluents.

Phosphate is a fundamental element in metabolic reactions. Sources and effects of excess phosphates are similar to those of nitrates. High levels may cause overgrowth of plants and increased bacterial activity and decreased dissolved oxygen levels.

pH – the pH scale ranges from 0–14, 0 is very acidic and 14 is very alkaline; freshwater usually has pH values between 6.5 and 8.2. Most organisms have adapted to life in water of a specific pH and may die if it changes even slightly. The pH level can be affected by industrial waste, agricultural runoff or drainage from unmanaged mining operations.

Temperature affects many physical, biological and chemical processes, e.g. the amount of oxygen that can be dissolved in water, the rate of photosynthesis of plants, metabolic rates of animals, and the sensitivity of organisms to toxic wastes, parasites and diseases. It is most often measured in degrees Celsius. Many factors affect water temperature. These include changes in air temperature, cloudiness and currents. Wastes discharged into water can also affect temperature if the effluent processing or treatment temperature is substantially different to the background water temperature. For example, discharges of water used for cooling in industrial processes can be considerably warmer than the water into which they are discharged.

Turbidity is often measured in arbitrary units called Jackson Turbidity Units (JTU). Suspended matter usually consists of organic debris, plankton and inorganic matter, e.g. clay, soil and rock particles. Turbidity should not be confused with colour, since darkly coloured water can still be clear, not turbid. High turbidity affects the aesthetic appeal of waters, and in the case of recreational areas may obscure hazards for swimmers and boaters. Its environmental effects include a reduction in light penetration which reduces plant growth, and in turn reduces the food source for invertebrates and fish. If turbidity is largely caused by organic particles, their microbial breakdown can lead to oxygen depletion.

One example might be to see how turbidity conditions vary between the rainy season and the dry season, e.g. the turbidity may be higher during the rainy season when storm runoff is high and excess organic and inorganic materials are carried into the sea. Such a case is shown in Figure 16. Rainfall records can be obtained from the local/national meteorological office.


Figure 16
Line graph showing turbidity and rainfall changes over time


It is important to realize that water quality measurements often show considerable variation, and tests need to be repeated to verify the results. Furthermore, if water quality problems such as high coliform bacteria readings are found at a local beach, the first step should be to contact the local environmental and health authorities.

Start     Chapter 9

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