What Are The 6 Main Indicators Of Water Quality


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Water quality is often described by different indicators such as temperature, dissolved oxygen, pH, total dissolved solids, conductivity, suspended sediment, nutrients, metals, hydrocarbons, and industrial chemicals. 

Water quality is one of the most important factors in aquatic ecosystems and ensuring water is safe for human use. Actions made on land have a major effect on what happens in water-based ecosystems which is why monitoring water quality levels is so important. 

In this article, we will look at the main indicators of water quality and the effects of irregular levels. 

The 6 Main Indicators Of Water Quality

The following indicators are important to determine water quality. 

  1. Temperature & Dissolved Oxygen (DO)
  2. Conventional Variables: pH, Total Dissolved Solids (TDS), Conductivity, & Suspended Sediment
  3. Nutrients
  4. Metals
  5. Hydrocarbons
  6. Industrial Chemicals 

1. Temperature & Dissolved Oxygen (DO)

Water temperature is one of the most important factors affecting water systems. Temperature can affect dissolved oxygen levels, chemical and biological processes, species compositions, water density and stratification, and life-stages in different marine organisms. 

For optimal health in aquatic organisms, the temperature must be within their optimal range. Anything outside this range can have detrimental effects on aquatic life; increasing stress levels and often causing death. The reproductive stage (spawning & embryo development) in fish is the most temperature-sensitive period. Temperature also affects ammonia levels in the water, the rate of photosynthesis, metabolic rates in aquatic organisms, and how sensitive aquatic organisms are to pollution. 

Due to changes in external environmental conditions, water temperatures fluctuate throughout the day, and between seasons. The temperature in freshwater systems is heated by the sun, and although other water inputs such as precipitation, groundwater, and surface runoff can affect water temperatures, heat is either lost or gained via condensation and evaporation. 

The temperature of water affects the amount of dissolved oxygen (DO) water can hold. As water temperatures increase, the amount of DO in water decreases. DO is the amount of oxygen dissolved in water, which can also fluctuate both daily and seasonally. 

DO comes from both the atmosphere and photosynthesis from aquatic plants, which is depleted via chemical oxidation and respiration from aquatic organisms (including microorganisms), mostly through decomposition of organic material and plant biomass. The solubility of oxygen in water has an optimal pressure of 1 atm (atmospheric pressure) that ranges from ~15 mg/L at 0ºC to 8 mg/L at 30ºC. 

Large fluctuations in DO can damage environmental ecosystems that are influenced by stream-flow runoff, precipitation, and temperature changes. Fish and other aquatic animals and plants need DO to survive. Some organisms can adapt to changes, however, the majority are not able to. DO can also affect the solubility and availability of nutrients in the water. 

2. Conventional Variables: pH, Total Dissolved Solids (TDS), Conductivity, & Suspended Sediment

Conventional variables are indicators that are measured to understand aquatic environments, including drainage basins, local environmental conditions, and daily and seasonal changes. 

pH (potential of hydrogen) is a measurement of hydrogen ion concentration with a scale of 0-14 where 7 is neutral, >7 is alkaline, and <7 is acidic. The pH in most natural water environments is between 6.0 and 8.5. pH values below 4.5 and higher than 9.5 are considered lethal to aquatic organisms, and pH values less extreme can interfere with reproduction and other essential biological processes. 

Metals, salts, and organic compounds can be affected by pH. In highly acidic water, some minerals dissolve in water, releasing metals and other chemical substances. pH levels can change from different water inputs such as runoff from land, groundwater, and even water draining from forest areas where weak organic acids and organic matter can alter pH levels. 

The concentration of total dissolved solids (TDS) is the measurement of dissolved material in a solution. TDS includes solutes (sodium, calcium, magnesium, chloride, and bicarbonate) that remain as a solid residue after water evaporation in a solution/sample.

The main sources of TDS are:

  • Natural weathering
  • Mining
  • Industrial waste
  • Agriculture
  • Sewage 

High levels of TDS diminish water quality, making it unsuitable for drinking and irrigation. Generally, freshwater has a TDS level between 0 and 1,000 mg/L. This is dependent on regional geology, climate and weathering processes, and other geographical characteristics that affect sources of DO and transport to water systems. 

Conductivity is the measurement of conducting an electrical current that is expressed in units of µS/cm (microsiemens per centimeter). In rivers and lakes that have an outflow, the conductivity usually ranges from 10 to 1,000 µS/cm. 

In water, the higher the ion concentration, the more current can be conducted. Conductivity is dependent on the number of electrical ion charges, ion mobility, and temperature. 

Conductivity ValueWater Body Type
Freshwater<600 µS/cm
Brackish600-6000 µS/cm
Saline>6000 µS/cm

Suspended sediment is the mass of sediment transported by a fluid like water, measured in mg/L. Particles are transported via flowing water and settle when water flow is reduced. Most suspended sediments consist of silt and clay.

During periods of increased water flow, for example from rainfall, the concentration of suspended sediment usually increases. Increased suspended sediment levels decrease light penetration in water and allows water to absorb more heat energy, increasing water temperatures. High concentrations of suspended sediment can move plants, invertebrates, and other aquatic life that live in stream beds. Increased concentrations can also affect food sources, decreasing aquatic fish populations. 

3. Nutrients

Nutrients are essential for the growth and survival of living organisms. Nitrogen and phosphorus are extremely important in aquatic ecosystems in addition to other elements like iron, magnesium, and copper. 

In aquatic systems, nutrients are found in different chemical forms: organic and inorganic particulates, and dissolved organic and dissolved inorganic particulates. 

Phosphorus is released from minerals during weathering events where some inorganic materials in soils can bind and prevent transporting it. 

Sewage, agricultural fertilizers, and animal waste are all artificial sources of nutrients. Elevated concentrations of nutrients usually come from the direct discharge of wastewater systems or runoff, which can limit primary productivity and enhance algae growth, which leads to eutrophication.

Eutrophication is a natural process, commonly occurring in freshwater ecosystems, however, it can also be human-induced (artificial) degrading water quality and threatening species survival. As algae (and plants) excessively grow, less sunlight is penetrated through the water, preventing photosynthesis and producing toxins. When the plants and algae finally die and decay, a decrease in DO concentrations can affect the diversity of aquatic organisms and reduce human use of the water. 


4. Metals

Copper, manganese, and zinc, are imperative for biochemical forms that maintain life, but in high concentrations, these can become toxic if ingested by humans and animals, or humans that consume animals exposed to high concentrations. 

Metal toxicity and bioavailability depend on the form they occur and their oxidation state; dissolved metals are more toxic and bioavailable than metals that have been absorbed by sediments or bound with other molecules. The oxidation state, bioavailability, toxicity, and solubility are influenced by other water indicators such as pH and DO. 

The weathering of rocks and soil like erosion and sedimentation introduce metals into aquatic ecosystems where the chemical characteristics of the water will determine how metals are introduced into the sediment. Metals can also appear in water unnaturally from wastewater treatments, industrial waste, sewage, contaminated soils, and mining operations. 

When metals accumulate in fish, they can be passed onto humans during consumption. Mercury is especially susceptible to bioaccumulation, causing major risks to human health. Japan’s 1968 Minamata Bay disaster is a good example. Industrial waste containing mercury was dumped, affecting thousands of people that ingested local fish and shellfish that had bioaccumulated the mercury in their tissues. Many people died, some suffered from convulsions and paralysis, and pregnant women gave birth to poisoned babies with severe deformities like blindness, deafness, and gnarled limbs. 

Metals in water

5. Hydrocarbons

Hydrocarbons are organic chemical compounds containing only carbon and hydrogen. 

Polycyclic aromatic hydrocarbons (PAHs) are complex compounds that originate from fossil fuels, organic matter combustion, and chemical and biological transformation of organic molecules. They are known to cause cancer and are toxic to aquatic organisms when found in water. 

Hydrocarbons need to be regulated and controlled in water systems for the safety of human health and aquatic species. Petroleum hydrocarbons are a major pollutant that is often discharged into coastal waters. Bottom sediments act as potential hydrocarbon reservoirs, posing risks to both aquatic animals and humans due to bioaccumulation. 


6. Industrial Chemicals 

Industrial chemicals can be introduced from industrial waste. Industrial chemicals such as PCBs (polychlorinated biphenyls) threaten aquatic ecosystems and people that frequently eat contaminated fish. 

PCBs are known to have negative effects on the immune, nervous, reproductive, and endocrine systems of living creatures. Because PCBs can resist biological, chemical, and thermal degradation, it is extremely difficult to break them down in the water systems. 

Diotoxins and furans are toxic organochlorine compounds found in the air, water, sediments, animals, and foods. They come from burning waste, iron and steel production, and the combustion of fossil fuels. We should be concerned when they are present in water due to their ability to accumulate in body fat and bioaccumulate in fish, making their way to the top of the food chain (to humans). 

Industrial chemicals

Summing Up The Main Indicators Of Water Quality

Water quality is important in terms of aquatic organisms and safe human usage, which is why testing the main indicators of water quality is paramount. Temperature, dissolved oxygen, pH, total dissolved solids, conductivity, suspended sediment, nutrients, metals, hydrocarbons, and industrial chemicals are the main indicators that can determine water quality. 

If you have any questions regarding water quality or the water testing kits we have to offer, please feel free to reach out to our world-class team at Atlas Scientific, we are always happy to help!

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