The Key Indicators and Parameters of Water Quality

Indicators and Parameters of Water Quality

Water quality refers to the suitability of water for different uses according to its physical, chemical, biological, and organoleptic (taste-related) properties. Being familiar with key indicators and parameters of water quality is important as they directly impact human consumption and health, industrial and domestic use, and the natural environment.

The following are some of the key indicators and parameters of water quality that are frequently used by regulatory organizations such as the European Commission and the United States Environmental Protection Agency (EPA).

  • Dissolved Oxygen (DO)
  • Bioindicators
  • Nitrates and Nitrites 
  • Turbidity and Total Suspended Solids (TSS)
  • pH Scale
  • Water Temperature
  • Hardness of Water
  • Fecal Indicator Bacteria
  • Flow of the Water
  • Total Dissolved Solids (TDS)
  • Total Organic Carbon (TOC)
  • Polycyclic Aromatic Hydrocarbons (PAH)
  • Pesticides
  • Lead
  • Iron
  • Radionuclides

1. Dissolved Oxygen (DO)

Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in a liter of water at a specific temperature and salinity. DO is an indirect measure of organic pollution in streams, rivers, and lakes. Low dissolved oxygen concentrations give water a bad taste to most users.

Low DO content is an indicator of water degradation and stagnation, often termed eutrophication. This usually results from excessive organic material contamination and its decay by bacteria which consumes oxygen. According to the EPA, DO levels under 3 mg/L are of concern, and waters with levels below 1 mg/L are considered hypoxic and usually devoid of life.

DO is measured by most multi-parameter field and lab water-quality meters that automatically account for temperature and salinity, and use the membrane electrode method.

2. Bioindicators

Bioindicators refer to living organisms such as plants, planktons, animals, and microbes that depend on a given aquatic environment for survival. These organisms are highly sensitive to environmental changes and can be used as indirect markers of water quality as well as indicators of water pollution.

A study on Bioindicators as a natural indicator of environmental pollution from the Gujarat Forensic Sciences University in India found plankton to be excellent biomarkers for assessing water quality as well as good indicators of water pollution as they respond rapidly to changes in the surrounding environment.

Assessing the water quality of a site with bioindicators is not something that is routinely done, as it involves manual collection and counting of various indicator species and then using this data to estimate water quality based on predetermined biotic indices.

3. Nitrates and Nitrites

Nitrates and nitrates are naturally occurring forms of nitrogen that are formed when nitrogen is combined with oxygen or ozone. The sources of these nitrates in the environment include wastewater treatment plants, farmland fertilizer, animal manure runoff, septic systems, and industrial discharge.

Excess nitrate and nitrate contaminants in water can cause dramatic increases in aquatic plant growth such as algae blooms and can accelerate eutrophication. As a result, other parameters of water quality such as dissolved oxygen (excess nitrates can cause hypoxia) and temperature are altered. Additionally, excess nitrates are toxic to humans when ingested. These images show what algae blooms look like both up close and from satellite imagery.

Algae Blooms Seen from Space and Ground Level

Levels of nitrates in water can be measured similarly to DO, using meters that use a probe with an electrode sensor that measures nitrate activity in the water and convert this activity into millivolts that can then be used to estimate the concentration of nitrates. The calorimetric cadmium reduction method can also be used to estimate nitrate levels. This method relies on the reduction of nitrates into nitrites when coming in contact with cadmium, and subsequent reaction between nitrites with another reagent to form a red color whose intensity is proportional to the original amount of nitrate.

The US EPA sets a maximum contaminant level (MCL) in water for nitrates at 10 mg/L and nitrites at 1 mg/L.

4. Turbidity and Total Suspended Solids (TSS)

Turbidity refers to the cloudiness of water and is a measure of the ability of light to pass through it (Degree of clarity). Turbidity is determined by the total suspended solids (TSS) in water such as organic material, clay, silt, and other particulates.

High turbidity is aesthetically unappealing and can increase the cost of water treatment. Particulate matter can provide a hiding place for harmful microorganisms and shield them from disinfection processes and can absorb heavy metals and other harmful chemicals.

Turbidity is measured using a turbidimeter that shines a beam of light in the direction of the water sample and then determines via a photodetector how much light is either transmitted or scattered. For example, if the concentration of TSS is high more light will be scattered and the intensity of transmitted light will be less. Turbidity is reported as Nephelometric Turbidity Units (NTUs).

The US EPA sets acceptable turbidity levels at less than or equal to 1 Nephelometric Turbidity Unit (NTU) for water systems that use conventional or direct filtration systems. This picture shows three water samples from the Washington State Department of Transportation with turbidity levels of 5, 50, and 500 NTU.

Three water samples showing different NTU turbidity levels

5. pH Scale

The pH of water measures the degree of its acidity or basicity (alkalinity) and is expressed on a scale of 0 to 14. Water with a pH of 7 is neutral. pH less than 7 is acidic, with 0 the most acidic. More than 7 is basic or alkaline, with 14 the most basic. It is technically possible for pH to be higher or lower than the 0-14 range, but this is rare.

pH itself has no direct impact on the quality or safety of water, but it affects the way water interacts with its environment and is used as an indicator parameter. Excessively high or low pH (less than 4 or greater than 11) is detrimental to the use of water as it can alter its taste (low pH water takes on a sour or metallic taste, and high pH a bitter or baking soda taste), the effectiveness of its chlorine disinfection process (high pH greater than 8 makes disinfection ineffective), and increase the solubility of toxic compounds in water such as ammonia, aluminum, and cyanide, making them more toxic.

pH can be measured using either optical or potentiometric methods, according to the U.S. Geological Survey's (USGS) Water Science School.

Optical methods of estimating pH are the most appropriate for regular consumers. Optical methods use pH-sensitive strips (litmus strips) that contain indicator compounds. When a sample of water is applied to these test strips, they will change color. The pH value then can be roughly estimated by comparing the color of the test strip against a provided color scale. This picture shows a typical pH test strip and color chart.

pH Testing Strips

Potentiometric methods are used by researchers in the field and scientists in the lab. Potentiometric devices, such as a portable pH meter for water or lab-based devices, use the electrical potential differences of pH-sensitive hydrogen, metal, or glass electrodes placed in control and test samples to detect the pH of liquids.

6. Water Temperature

Water temperature is another indirect indicator that influences many other parameters of water quality such as pH, specific electrical conductivity, and dissolved oxygen concentration.

Water temperature influences the palatability, viscosity, solubility, and odor of water. It affects the disinfection and chlorination processes, biological oxygen demand (BOD), and the degree of toxicity of compounds such as cyanide and ammonia in water.

Water temperature can be easily measured with a standard thermometer of the same time that might be used to test for drinking hot water, or a multi-parameter field water-quality meter.

8. Fecal Indicator Bacteria

Fecal indicator bacteria, according to the US Geological Survey, are used to determine the sanitary quality of water for recreational, industrial, agricultural, and water supply purposes.

Fecal indicator bacteria are organisms that naturally inhabit the gastrointestinal tracts of humans and animals. The most common fecal indicator bacteria include coliforms and fecal streptococci/enterococci. These organisms are released into the environment with feces. Fortunately, they are only adapted for growth in the gastrointestinal tract and die within several hours of being exposed to environmental stressors such as sunlight, temperature, other competitive and predatory organisms, and toxic disinfectant and industrial chemicals.

The USGS and EPA assume the “death rate” of fecal indicator bacteria to be the same as the death rates of other harmful pathogens. Therefore, if a high amount of fecal indicator bacteria is found in a body of water, it is assumed that other harmful pathogens are likely to be present as well.

Fecal indicator bacteria are tested by growing or “culturing” them from water samples in microbiological laboratories using special growth mediums and conditions. This picture shows coliforms cultured on agar media.

Coliforms cultured on agar media in laboratory

9. Flow of the Water

Streamflow refers to the volume of water that moves past a specific point during a defined interval of time. Water that has high flow rates dilutes pollutants to a greater degree. High flow rates also increase the dissolved oxygen (DO) content of water by increasing turbulence and therefore mixing in more air.

The flow of water can be measured by estimating the number of cubic feet of water (width of the body of water in feet times depth of the body of water in feet) moving down the stream in one second (velocity in feet per second). The flow of water is expressed in cubic feet per second.

10. Total Dissolved Solids (TDS)

Total dissolved solids (TDS) describes the overall concentration of inorganic salts and organic contaminants that is present in solution in water. The main constituents that determine TDS make their way into water from natural sources, wastewater, or urban and agricultural run-off. The main TDS components found in water include sodium, potassium, chloride, magnesium, calcium, carbonate, nitrate, sulfate, and bicarbonate.

In bottled water and well maintained municipal water systems the TDS level often indicates the presence of beneficial ions and minerals such as sodium, potassium, calcium, and magnesium rather than harmful toxins and heavy metals.

TDS has effects on the hardness and organoleptic properties of water. The main organoleptic property that TDS affects is taste. According to a 1966 article in the Journal of Applied Psychology, the palatability of drinking water in relation to levels of TDS was found to be as follows.

  • Excellent: Less than 300 mg/L
  • Good: 300-600 mg/L
  • Fair: 600-900 mg/L
  • Poor: 900-1200 mg/L
  • Unacceptable: Greater than 1200mg/L

Taste preferences are subjective, and differences are particularly notable in different regions of the world, with Central and Eastern Europeans often preferring the very high TDS waters found there, versus the lower TDS levels preferred in North America and Northern Europe. Iceberg water and rainwater have low TDS levels and a light taste because they are pure precipitation from the sky that has had little or no contact with the surface where minerals or pollutants would otherwise be picked up.

The World Health Organization’s background document for the development of WHO Guidelines for Drinking-water Quality suggests that water with a TDS concentration of less than 1000 mg/L is acceptable to most users.

High TDS of water can affect the hardness of the water if calcium or magnesium in particular are major components. Hardness greater than 500 mg/L can lead to scaling and corrosion of water pipes, heaters, boilers, and household appliances.

There have not been any health harms causally linked to excess TDS in water. Epidemiological studies from Australia, Canada, and even the former Soviet Union have attempted to determine the effects of TDS on health, but their results have been varied and have contradicted each other, and have failed to account for another confounding which may have affected their results.

It is more useful to consider the specific components of the TDS of any given water. High calcium or magnesium levels are often a healthy supplement. High arsenic levels are dangerous. High sodium levels are a valuable electrolyte in moderation, but can be unhealthy if someone already has a high sodium intake in their diet.

Measuring TDS in water is most commonly done by determining specific conductivity with a handheld conductivity probe that detects the presence of ions in water. Water by itself does not conduct electricity, but when minerals and other elements are dissolved, it will conduct electricity and can be measured. These measurements are then converted into TDS values using specific factors that vary with a specific type of water and temperature. This video from Svalbarði shows TDS being measured via a handheld meter that utilizes the conductivity method.

TDS in water is also measured via gravimetric or weight-based methods. Masurements are done by heating the water until it evaporates and weighing the remaining residue. This and laboratory measurements of individual TDS constituents are less commonly used than handheld TDS meters but are more precise.

No standard is established for TDS as it is considered a Secondary Drinking Water Standard that does not pose a direct health risk. The US EPA still recommends treatment of water systems when TDS concentrations exceed 500 mg/L as elevated TDS concentrations may indicate increased levels of ions such as aluminum, copper, lead, arsenic, and nitrates which may pose a health risk.

11. Total Organic Carbon (TOC)

Total Organic Carbon (TOC) is a measure of the total amount of carbon from plant, animal, or synthetic carbon-containing compounds found in water. These substances can be either dissolved in water or exist in water as undissolved, suspended materials.

TOCs are indirect markets for organic matter. Excessive organic matter found in water can lead to a decrease in dissolved oxygen (DO) and increased production of disinfection by-products (DBP). Both DO and DBP are important indicators for water quality, and as such monitoring TOC can be a useful adjunct indicator.

Typical TOC levels in various types of water are as follows.

  • Bog: 10-60 ppm, average of 33 ppm
  • Marsh: 10-60 ppm, average of 17 ppm
  • Eutrophic lake: 12 ppm
  • River: 1-10ppm, average 7 ppm
  • Groundwater: 700 ppb
  • Wastewater: 500-1000 ppm
  • Drinking water: 100 ppb-10 ppm
  • Purified water: 1-500 ppb
  • Ultrapurified water: 0.1-10 ppb

Typical Total Organic Carbon (TOC) content of different water types

Total Organic Carbon (TOC) is measured in laboratories by oxidizing the various organic compounds present in water to forms that can be quantified.

12. Polycyclic Aromatic Hydrocarbons (PAH)

Polycyclic Aromatic Hydrocarbons (PAHs) are a heterogeneous group of compounds that all contain two or more fused aromatic rings of carbon and hydrogen atoms. They occur in hydrocarbons including oil, gasoline, and coal. They are released into the environment when coal, oil, gas, wood, garbage, or tobacco are burned, according to the US Centers for Disease Control (CDC).

PAH are not highly soluble and are not usually found in drinking water in notable concentrations. They do not exert “quality altering” effects on water. Their ingestion of water over long periods has been linked with cancers of the lung, skin, esophagus, colon, pancreas, bladder, and breast.

The US EPA has different standards for individual PAHs. Some maximum contaminant levels (MCL) include the following.

  • 0.0001 mg/L for benz(a)anthracene
  • 0.0002 mg/L for benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, chrysene
  • 0.0003 mg/L for dibenz(a,h)anthracene
  • 0.0004 mg/L  for indenol(1,2,3-c,d)pyrene

13. Pesticides

Agricultural pesticide chemicals such as organomercurials, chlorpyrifos, azinphos-methyl, atrazine, alachlor, diazinon, carbaryl, and fipronil used to control weeds, insects and other pests produce serious water pollution problems.

Depending on the particular chemical, possible health effects from overexposure to pesticides include cancer, reproductive, endocrine, or nervous system disorders, as well as acute toxicity if ingested in high concentrations.

The levels of several pesticides in drinking water are regulated and monitored by the US EPA, but not all pesticides are monitored. Levels of pesticides can also be difficult to determine because their levels vary widely from month to month, and from season to season.

Measurement of levels of pesticide contaminants is undertaken by local water regulatory bodies and their evaluations are made available to consumers through regular consumer water quality reports.

14. Lead

Lead is one of the most common heavy elements found on earth. Lead contamination of water occurs in small amounts from the dissolution of natural sources, but primarily via leaching from old household plumbing systems that contain lead, such as pipes, solder, fittings, or service connections.

Lead does not alter the organoleptic properties of water. According to the United States Center for Disease Control and Prevention (CDC), lead cannot be seen, tasted, or smelled in drinking water.

The best way to determine the amount of lead in drinking water is to refer to your annual water quality report or to have it tested in a qualified laboratory. Individuals can contact their state or local drinking water company to find a certified laboratory for analysis.

The US EPA has set the maximum contaminant level goal for lead in drinking water at zero as lead is a toxic metal that can be harmful to human health even at low exposure levels. This video shows what happened when Flint, Michigan’s municipal water was contaminated with lead.

15. Iron

Iron is found in nature and combines with sulfur- and oxygen-containing compounds to form sulfides, carbonates, hydroxides, and most commonly oxides. Iron in water can be measured as part of TDS or as an individual parameter.

Iron can affect the organoleptic properties of water. According to a study on the taste threshold concentrations of metals in drinking water published in the Journal of  American Water Works Association, a metallic taste was imparted to distilled and mineralized waters when concentrations of iron reached levels ranging from 0.04 mg/L to 3.4 mg/L. Concentrations of iron above the range of 0.05-0.1 mg/L can settle out of solution and alter the turbidity and color of water, imparting a cloudy or rusty appearance.

The US Environmental Protection Agency limits the amount of iron to less than or equal to 0.3 mg/L in municipal drinking water, as only a small percentage of the population will be able to taste iron in drinking water at concentrations below 0.3 mg/L.

A common method of measuring the amount of iron in drinking water is the phenanthroline spectrophotometric method. This method relies on the fact that the iron ion forms a stable intensely red-orange colored complex with phenanthroline, which can then be analyzed with spectrophotoscopy and levels of iron determined using a calibration curve.

16. Radionuclides

Radioactive materials, also called radionuclides, are substances that break down (decay) to emit beta, alpha, and gamma radiation, which has numerous detrimental effects on human health.

Radionuclides occur naturally in the environment and can be man-made. Both types of radionuclides can make their way into groundwater and surface waters and create radiation as they decay. Although radiation is harmful to hematopoietic, gastrointestinal, reproductive, and nervous systems, the degree of harm is related to the source of radioactivity, how much radiation you are exposed to (total dose), and how long you are exposed to the radiation.

One of the most infamous incidents of radionuclide contamination of water occurred with the 2011 meltdown of the Fukushima Daiichi nuclear power plant in Japan. This picture shows storage tanks for contaminated water at the Fukushima site, with 400 tons of groundwater seeping into the site each day.

Tanks to store water contaminated with radionuclides at Fukushima


Water quality parameters therefore commonly monitor the concentrations of alpha particles, beta particles, radium, and uranium. The US EPA Radionuclides Rule has four standards for radionuclides in drinking water, which ensures that the consumer is in contact with only very low doses of radiation every day.

The EPA Radionuclides Rule defines safe drinking water as containing up to or less than the following amounts.

  • 15 picocuries of alpha particles per liter of water (pCi/L)
  • 5 pCi/L of combined radium 226/228
  • 20 pCi/L of uranium
  • 4 millirem of beta/photon emitters per year (mrem/yr)

What is the Importance of Water Quality to Human Health?

The importance of water quality to human health is the assurance that people drinking and using the water will remain healthy. Poor quality of drinking water, domestic use water, and recreational water due to pollution can lead to human illness and disease.

Poor water quality due to contamination by heavy metals and chemicals such as arsenic, mercury, chromium, and lead cause chronic damage to organs including the liver, kidney, brain, endocrine, and reproductive organs. Contaminants such as arsenic, fluoride, nitrate, and PAHs can cause a variety of cancers that involve the gastrointestinal, urinary, and reproductive tracts among others.

Poor water quality due to contamination by biological organisms contributes heavily to the global burden of disease mainly in the form of  “Water-Related Diseases'' such as diarrhea, cholera, dysentery, and hepatitis A. According to the WHO, cholera alone affects 1.4 to 4 million people and accounts for 21,000 to 143,000 deaths globally every year. The WHO further reports that approximately 829 thousand people, of which 297 thousand are children aged under the age of 5 years, are estimated to die each year from exposure to water pollution-related diseases, chiefly infectious diarrhea.

How to Test Water Quality

Water quality is tested in laboratories or at home with simple water quality test kits.

  • Laboratory water testing is carried out by collecting samples in sterile containers and sending them to an accredited laboratory. Results can take days, are more expensive, are available for hundreds of parameters, and are the most accurate.
  • Home water testing is available via test strips, color disk kits, or handheld digital instruments. Fewer water parameters can be measured at home than in laboratories, such as pH, TDS, hardness, or the presence of chemical cleansers.
    • Test strips are strips of paper with squares that change color when dipped in water.
    • Color disk kits have plastic containers the water is placed in, with the water changing color and compared to a colored disk when a liquid or powder reagent is added.
    • Handheld digital meters are dipped in water and give a numerical readout of the level of the metric.

What is the Highest Quality Water in the World?

The highest quality water in the world for meeting human, industrial, and environmental needs is Icelandic public water. This is because of the following factors.

  • Its purity at the borehole requires no chemical, physical, or biological treatment.
  • It has a TDS of 75 mg/l which is enough to prevent industrial fouling, prevent corrosion of infrastructure, and provide a light mineral supplement to drinkers.
  • It has a slightly alkaline pH of 7.3 which means it is non-corrosive to infrastructure.
  • Iceland is a low population country with minimal sources of industrial and agricultural runoff and high-quality water treatment facilities.
  • Its quality is constantly monitored throughout the distribution system.
  • It is ranked number one for water quality in both the Environmental Performance Index (EPI) for human safety, and the OECD Better Life water quality index with 98% of people satisfied with the water quality.

There are other countries with similar very water high-quality as Iceland, but water in the Reykjavik region has a unique combination of factors that make it the best quality water. This picture shows some of the typically clean water of Iceland as in the Hvítá river.

Good Quality Water in Iceland

Svalbarði Polar Iceberg Water is different from other high-quality waters because of its iceberg source. Because it was preserved inside a glacier for thousands of years before modern pollution, when it emerges as an iceberg it has extreme chemical and physical purity. It is not quite as ideal for industrial purposes since it has the slightly acidic pH of clean precipitation from interacting with natural atmospheric gases when it originally fell as snow. But iceberg water has a light and airy taste that consumers regularly report is among their favorites.

What are some Misunderstandings and Myths about Water Quality Parameters?

Some misunderstandings and myths about water quality parameters are about primary and secondary water quality parameters. These misunderstandings are often related to pH, chlorine, and the concentration of total dissolved solids (TDS).

  • Myth 1 - pH is a direct indicator of water quality. pH is not a primary water parameter and is not in itself a measure of the quality of water. pH can affect how water interacts with its environment and how various dissolved substances behave in water. For example, at high pH values compounds such as ammonia, aluminum, and cyanide become soluble and more toxic. pH is a secondary water parameter that can be used as an indirect marker for water quality, but whether a drinking water leans alkaline or acidic is not in itself an indication of quality.
  • Myth 2 - Concentration of TDS is a measure of water quality. TDS, similar to pH, is a secondary water parameter. TDS tells us that there is a certain concentration of organic and inorganic material dissolved in water. But TDS alone provides no information on whether these substances are healthy minerals like calcium and magnesium, or toxic heavy metals like arsenic or lead. TDS is an indirect marker that can indicate if more testing is worthwhile.
  • Myth 3 - The presence of Chlorine is a marker of poor water quality and makes water unsafe to drink. So long as levels are moderate, the presence of chlorine residual in municipal drinking water is a marker of good water quality. Chlorine residual levels indicate that a sufficient amount of chlorine was added during initial treatment to the water to inactivate bacteria and viruses that cause diarrheal disease and that the water is protected from recontamination during storage.

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