Glossary and Definitions
  Biochemical oxygen demand (BOD5)  
Biochemical oxygen demand (BOD) refers to the amount of dissolved oxygen needed by microbes to break down organic material present in a given water sample at certain temperature over a specific time period. BOD is measured in a water sample during 5 days of incubation at 20℃, known as BOD5. BOD represents the amount of organic matter which can be decomposed by aquatic organisms. It also indirectly represents the degree of organic pollution of the water.
  Chlorophyll-a (Chl-a)  
There are many kinds of chlorophyll and the most common types are the bluish-green chlorophyll-a (Chl-a) and the yellowish-green chlorophyll-b (Chl-b). The compositions of Chl-a and Chl-b are very similar. Both of them are able to absorb sunlight, yet, they only differ in their internal structure and the wavelength of light they absorb. The ratio of Chl-a to Chl-b in land plants is approximately 3 to1. When the concentration of Chl-a is high in water, it represents the over production of algae and indicates indirectly the degree of eutrophication.
  Total alkalinity (TAlk)  
The Total alkalinity (TAlk) of water is a measurement to assess a water sample's ability to neutralize acid. The alkalinity in natural water is mostly caused by weak acid salts, especially HCO3 which is the main form of alkalinity. Other organic acids (e.g. humic acid) which are harder to decompose, would also form salts to increase alkalinity of the water. Weak acid salts such as salts of acetic acid, propionic acid and hydrosulphuric acid would be formed in polluted or anaerobic water. Weak acid salts that are combined with ammonia, hydroxyl ions compose the total alkalinity of a water sample.
  Total dissolved solids (TDS)  
Total dissolved solids (TDS) is the total amount of solids dissolved in the water, including soluble hydrogen carbonate ions, chloride salts, sulphates, calcium, magnesium, sodium, potassium, volatile solids and non-volatile solids. Its concentration will affect the taste of drinking water. A way to measure it is to filter the water sample (0.45μm), dry the solution at the temperature of 103℃ to105℃ and then weigh the remaining residuals.
  Ammonia nitrogen  
Organic nitrogenous compounds are mainly from the decomposition of feces and animal and plant remains. In the decomposition process, amino acids are formed first and then ammonia nitrogen, nitrite nitrogen and nitrate nitrogen form in sequence. Thus, if ammonia nitrogen exists in a body of water, it indicates the water has not been polluted for long.
  Enterococci group  
Enterococci and Coliform are parasites in the intestine.They are classified as Type IV Streptococcus, which are Facultative Anaerobic Bacteria. The type of Enterococci which commonly infects the human body, Fecal Enterococci, can cause more infections because this bacteria has a strong resistance to antibiotics. Coilform is a common pathogen, which causes infections of the Urinary Tract, Gastrointestinal Tract and Pelvic area because the Fecal Enterococci is closely related to Heart Tissues. Once Enterococci enters the blood, it is a very dangerous pathogen to the Cardiovascular System.
  Phosphate (PO43-)  
Phosphorous in the water mostly exists in the form of phosphate which is the essence of soil nutrients and animals and plants bioplasm. Phosphorous is an important nutrient for the maturation of plants. Excessive phosphorous existing in a body of water will cause mass reproduction of algae and death to other organisms. Moreover, the decay and decomposition of algae would exhaust the dissolved oxygen in the water and lead to eutrophication.Water phosphate is commonly expressed as milligrams of phosphate per liter of water (mg/L of PO43-).
Hard water is water that has metal cations such as calcium (Ca2+),magnesium (Mg2+) and iron(Fe2+) and bicarbonates and sulphates dissolved in it. When soap dissolves in water with high hardness, it forms an insoluble grayish white precipitate which impairs the efficacy of the soap. When hard water boils, it will form calcium and magnesium salt deposits known as boiler scale. Boiler scale will reduce the conduction of heat, influence the efficiency of boilers and impede water flow in pipes. Water hardness is commonly expressed as milligrams of calcium carbonate per liter of water (mg/L of CaCO3).
  Magnesium (Mg)  
The concentration of magnesium in natural bodies of water is second highest after the concentration of sodium. However, magnesium dominance is rarely seen in natural water. Fresh water is usually dominated by calcium, while in alkaline water sodium is the main element. In most water bodies, magnesium concentration falls between 1 mg/L to 40 mg/L. The calcium to magnesium ratio in natural water has a certain relationship. In water where the total dissolved solids (TSD) are less than 500mg/L, the ratio ranges from 4:1to 2:1. If the TSD increases further, the concentration of magnesium will exceed calcium by several times. Fresh water contains much more calcium than magnesium due to the abundant calcium in the earth’s crust. However, since the chloride and sulfate of magnesium has higher degree of hydrolysis than calcium does, in alkaline water, the magnesium content is higher than calcium.
  Total organic carbon (TOC)  
Total organic carbon means the total organic materials which can be oxidized in the water. In other words, TOC refers to the milligrams of carbon in the organic pollutants per liter of water. For water with low organic materials, measuring the amount of total organic carbon is a good way to measure the amount of organic materials. The measurement method for TOC is to oxidize the organic carbon to carbon dioxide and then use an infrared detector to measure the concentration of carbon dioxide to calculate the equivalent of carbon.
  Chromium (Cr)  
Chromium is an essential trace element for humans and other living beings. However, high concentrations of Chromium are toxic. Chromium has three valence states, divalent chromium, trivalent chromium and hexavalent chromium. Its toxicity is related to its valence states. Trivalent chromium is essential for the human body for carbohydrate metabolism. However, hexavalent chromium is extremely toxic to humans. It could make our skin rough and do damage to the liver. It is also carcinogenic and will accumulate in the human body. The chromium content in natural bodies of water is low and it is even lower in seawater.
  Coliform group  
Coilform group is a group of short rod-shaped bacteria, which normally parasitizes animal intestines. There are also many Coliform group bacteria found in feces. Most of these bacteria wii not cause disease, but can serve as an index for water polluted by feces. Coliform group bacteria are no-spore bacilli that can decompose Lactose and produce gas, which are aerobic and facultative; it shows Gram Negative under the observation of a microscope. Coliform cannot directly reproduce in water, and the feces of warm-blooded animals generally contain this kind of bacteria, therefore, if we can detect large number of Coliform in water, it indicates water recently polluted by human or animal feces. Coliform and other pathogenic bacteria all come from warm-blooded animals and Coliform’s survival time is longer than pathogenic bacteria’s.Therefore if Coliform bacteria cannot be detected in water, the probability that the water contains other pathogenic bacteria is lower. Coliform group is a common biological index to estimate water quality.
  Selenium (Se)  
Selenium usually exists in inorganic form. Selenium and its compounds in water vaporize into the air due to evapotranspiration. They follow convection currents to other areas and then condense into rain in the selenium cycle. Selenium is an essential trace element for living beings. However, selenium overdose would cause selenosis and the symptoms are similar to those of arsenic causes.
Temperature refers to how cold or hot water is. It is an important physical parameter to examine and assess the water quality. The variation of water temperature is mainly affected by the climate. However, waste water discharge also influences temperature. Temperature changes the physical characteristics of the water such as density, viscosity, vapor pressure and surface tension. It also influences the chemical characteristics including chemical reaction rates, gas solubility and so on. Biologically it affects the activity of microbes and the metabolic rate as well. Usually the water temperature could be measured by a calibrated thermometer.
  Carlson trophic state index  
THE E.P.A currently uses the Carlson trophic state index (CTSI) as the index of the degree of eutrophication. CTSI consists of the concentration of three items of water quality parameters: Transparency(SD), Chlorophyll-a(Chl-a), Total Phosphate(TP), which are calculated to form an index value, and determine the eutrophication level of reservoir water quality.
Carlson index(CTSI)= ﹝TSI(SD)+TSI(Chl)+TSI(TP)﹞
SDTransparence;Chl-a:Chlorophyll-a;TP:Total Phosphate;TSI Exponent calculation method
TSI(SD)=60-14.41 x ln(SD),The unit of SD(Transparence) is meter
TSI(TP)=14.42 x ln(TP)+4.15,The unit of total phosphate is μg/L
TSI(CHA)=9.81 x ln(CHA) +30.6,The unit of Chlorophyll-a is μg/L
Note: ln is Natural logarithm.
Carlson index(CTSI) Eutrophication level Marked color


40≦CTSI≦50 Mesotrophic Green



Table2 Eutrophication standards for single parameter index
Class Total Phosphate(μg/L) Chlorophyll-a(μg/L) Transparence(m)




Mesotrophic 12~24 2.6~7.2 2~4





  Calcium (Ca)  
Calcium is a major positive ion in natural fresh water. It is widely found in many minerals. Calcium makes up 4.9% of the surface of the earth and 0.07% to 1.7% of the soil. Its concentration in rivers is 15 mg/L and in groundwater it ranges from 1 mg/L to less than 500 mg/L. A common form of calcium is calcite (calcium carbonate). If it coexists with magnesium, they form dolomite. The carbon dioxide from the soil organism decomposition dissolves in water and forms carbonic acid. When the water flows through lime, calcium carbonate is dissolved into calcium bicarbonate (Ca(HCO3)2) which is the main source of calcium ions in water.
Sulfate is a major ion found in water. It has the highest oxidation state of Sulfide. Sulfates exist in almost all kinds of natural water. Metal sulfides such as pyrites in igneous rocks and sedimentary rocks are oxidized to sulfate by dissolved oxygen through the weathering process. Sulfate has low toxicity. However, high concentrations would cause calcium deposits and indirectly poison the crops with sodium. Sulfate in the water would be reduced to hydrogen sulfide by microorganisms in anaerobic environments.
Turbidity refers to the degree light scatters as it passes through a body of water. Turbidity factors include clay, silt, small organisms, plankton or microbes. High turbidity will affect the appearance of water, impede light from passing through water and further influence the photosynthesis of aquatic plants. It can also influence the growth and reproduction of fish and even cause them to suffocate. Moreover, high turbidity interferes with water disinfection as well. The measurement of turbidity is based on the principle of light scattering. A nephelometer is the measurement instrument and the commonly used measurement unit for turbidity is Nephelometric Turbidity Unit, NTU.
  Dissolved oxygen (DO)  
Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water. It is an important index for assessing water quality. The oxygen dissolved in water could come from the air, natural or artificial aeration and photosynthesis of aquatic plants. If the water is polluted by organic matter, microbes in the water would consume the dissolved oxygen to break down that matter. Thus the DO level decreases or is even depleted.
Silicon in natural bodies of water mainly come from the hydrolysis of silicate and silicate minerals. When the amount of H4SiO4 released from weathering is high, SiO2 would be deposited. Crystalline and amorphous SiO2 are soluble and temperature changes its solubility. For example, the concentration is 6.0mg/L at 25℃ while 26 mg/L at 84℃. In natural water, SiO2 content is higher than that of quartz but is lower than amorphous SiO2. In most conditions, the concentration falls between 1 mg/L to 30 mg/L. In a few areas, the concentration can reach 100 mg/L. In some extreme conditions such as an intermittently hot spring, the SiO2 concentration could be as high as 762 mg/L to 804mg/L. In surface seawater, the concentration is lower than 1mg/L. SiO2 concentration is also low in lakes due to absorption by organisms. Generally speaking, silicon accumulates in seashells and bones.
  Silver (Ag)  
Silver has accumulative toxicity. It can cause algyria which causes permanent bluish-grey discoloration to the skin and eyes. It also kills or inhibits aquatic life.
  Zinc (Zn)  
Zinc is one of the four most commonly used domestic metals (iron, copper, lead and zinc) in modern society. It is usually plated on top of iron to prevent it from rusting. Zinc is an essential element for the human body. It has low toxicity to the body. However, it is poisonous to aquatic organisms. The lethal concentration is less than 0.1mg/L and that to roe is 0.4mg/L/
  Cadmium (Cd)  
Cadmium does not exist as a single element in the natural world. In natural water, the concentration of Cadmium is very low. Cadmium is an accumulative toxicant. Cadmium poisoning causes itai-itai disease and stimulates the respiratory tract. Long-term exposure to cadmium will cause Ansonia or yellow spots on gums. Cadmium compounds are not easily absorbed by the intestinal tract. However, they can be absorbed by the respiratory tract and accumulate in the liver or kidneys to harm health.
  Nitrite nitrogen  
Nitrogen existing in the water in the form of a nitrite compound is called Nitrite nitrogen. Nitrite nitrogen is formed in an aerobic environment. Nitrosomonas turns ammonia nitrogen into nitrite nitrogen. Since nitrite nitrogen is easily oxidized to nitrate nitrogen, when the dissolved oxygen is abundant in the water, nitrite nitrogen only exists in a short period of time.
Chloride refers to chlorine ions in water. The concentration of Chloride in natural fresh water is low. The main sources are seawater intrusion, salt bed exudation, and industrial wastewater. High chloride concentration is corrosive and can retard crop growth. Chlorine ions are essential to maintain cell permeability in the human body. Within normal concentrations, they do no harm to the body. However, high concentrations would affect patients with kidney disease. Chloride can be used as an important index to assess the influence of seawater intrusion caused by groundwater over-pumping in coastal areas.
  Potassium (K)  
Potassium content in natural bodies of water is far lower than that of sodium. Its concentration is usually only 4% to 10% of that of sodium. In some quartzite areas, the potassium content in natural water is near or exceeds sodium content but both of them are very low, only some single-digit mg/L respectively. In most fresh water with sodium concentration below 10mg/L, the potassium concentration is only 10% to 50% of the sodium concentration. In some alkaline water or hot springs, the potassium concentration multiplies by orders of magnitude. In brine, the potassium concentration is ten mg/L to hundreds of mg/L. Furthermore, potassium is a basic nutrient of plants and is absorbed by plants during the weathering process.
  Total Kjeldahl nitrogen (TKN)  
Total Kjeldahl nitrogen is the sum of ammonia nitrogen and organic nitrogenous compounds. The Kjeldahl method turns organic substances in the water into ammonium sulfate. The amount of nitrogen is measured after distilling and titrating.
  Nitrate nitrogen  
Nitrate nitrogen is the final product of nitrification in the nitrogen cycle. Thus, the amount of nitrate nitrogen can indicate the degree of pollution of the water. When there is too much nitrate nitrogen existing in the rivers, lakes or reservoirs, it could cause mass reproduction of algae and lead to eutrophication of water bodies.
  Total Phosphate  
Total phosphate is composed of orthophosphate, polyphosphate and organic phosphorous. Phosphorous in water mostly exists in the form of phosphate which is the essence of soil nutrients and animal and plant bioplasm. Phosphorous is an important nutrient for growing plants. Excessive phosphorous existing in bodies of water will cause mass reproduction of algae and death of other organisms. Moreover, the decay and decomposition of algae would deplete the dissolved oxygen in the water and lead to eutrophication.Water total phosphate s is commonly expressed as milligrams of phosphorus per liter of water (mg/L of P).
  Hexavalent chromium  
Chromium is an essential trace element for the human body. It has a specific role in carbohydrate metabolism and lipid metabolism. Trivalent chromium is beneficial to our body while hexavalent chromium is toxic. Contact with the hexavalent chromium may cause contact chromium dermatitis and eczema. Ingesting hexavalent chromium compounds may cause hyperplastic oral mucosa, edema which leads to crusting, nausea, severe abdominal pain and hepatomegaly. In some serious situations, it could cause circulation failure, loss of consciousness and even death.
  Dissolved oxygen saturation  
Dissolved oxygen saturation is calculated by dividing the measured dissolved oxygen by the saturated dissolved oxygen. It is expressed in percentage. Dissolved oxygen in the water mainly comes from the air. The saturation level does not reach the maximum amount which occurs in a natural balanced state. The saturated DO level lowers as the temperature increases. Moreover, atmospheric pressure and salinity also affect the saturation level. For example, 15℃ clean water with salinity of zero at one atmosphere of pressure, has a saturation level of 10mg/L while at 34℃ the saturation level is 7mg/L.
  Nitrogen Organic  
Most nitrogen originally occurs in the form of organic nitrogen (proteins) or ammonia. After a period of time, organic nitrogen turns into ammonia nitrogen. Afterwards, ammonia could be oxidized to nitrite and nitrate in aerobic environments. If most of the nitrogen in the water were organic nitrogen and ammonia nitrogen, it means the water was recently polluted and is very dangerous.
  Copper (Cu)  
Copper is an abundant metal. Fifty to eighty percent of copper in rivers adheres to suspended solids and is insoluble. Copper is essential to the human body. Its toxicity has no accumulation effect in the human body. However, copper overdose will cause damage to the liver and the central nervous system. For aquatic organisms, the concentration of copper close to 1.0mg/L will intoxicate fish.
  River pollution Index  
The E.P.A currently uses the “River Pollution Index” as the comprehensive index for assessing river water quality, calling it “RPI” for short. RPI consists of four water quality parameters, Dissolved Oxygen(DO), Biochemical Oxygen Demand(BOD5), Suspended Solids(SS) and Ammonia Nitrogen(NH3-N), which are calculated to index integral value, and which indicate the pollution level of river-water. The calculation and comparison baselines of RPI are shown below: Si: The points of the “i” item water parameter, water parameters including DO(mg/L), BOD5(mg/L), NH3-N(mg/L), SS(mg/L)

Water Quality/Item Non-Polluted Lightly-Polluted Moderately-Polluted Severely-Polluted
Dissolved Oxygen(DO)mg/L





Biochemical Oxygen Demand(BOD5) (BOD5)mg/L





Suspended Solids(SS)mg/L





Ammonia Nitrogen(NH3-N)mg/L





Point Scores 1 3 6 10
Pollution Index Integral Value (S) S≦2.0 2.0<S≦3.0 3.1≦S≦6.0 S>6.0

Remark: According to the resolution of “River pollution index (RPI) standard and calculation revision” meeting on May 30, 2013, RPI classification standard is adjusted with reference to monitoring report decimal rules published by Environmental Analysis Laboratory, starting from 2013.

River pollution Index picture
  Arsenic (As)  
Arsenic has four valence states, pentad, trivalence, null valence and trivalent negative. Arsenic in water commonly exists in the form of compounds. In shallow water where the dissolved oxygen and pH measures are higher, arsenic exists in the form of pentavalent arsenate. Contrarily in deep water where the dissolved oxygen and pH measures are lower, arsenic exists in the form of trivalent arsenate or arsenic sulfide. Arsenide is extremely toxic. Its long-term harm to the human body include blackfoot disease, cancers (skin cancer, lung cancer, bladder cancer), heart disease, diabetes and hypertension.
  Suspended solids (SS)  
Suspended solids refer to the organic or the inorganic particles which have not settled due to the motion of the water. Those particles include colloid, dispersion and floc. Suspended solids will impede light passing through the water. Its impact on aquatic organisms is similar to that of turbidity. The suspended solids deposited in the riverbed would block the water flow. If they are deposited in a reservoir, the storage capacity would be impaired.
Mercury is a first order pollutant in the world. It is one of the widespread heavy metal toxicants in water. Mercury pollutants come into the water in three forms, metal, inorganic mercury compound and organic mercury compound. Inorganic mercury could be turned into organic mercury by microbes in the water, and its toxicity increases. Mercury is an accumulative toxic substance. It does great harm to human health. Inorganic mercury and organic mercury mainly damage the central nervous system and the kidneys.
  Chemical oxygen demand (COD)  
Chemical oxygen demand (COD) is commonly used to represent the amount of organic materials in the water which can be chemically oxidized. Under acidic conditions, with potassium dichromate as the oxidizing agent, the water is heated and organic compounds can be fully oxidized to carbon dioxide and water. The amount of potassium dichromate consumed could be used to calculate the amount of oxygen required which is namely COD. COD is commonly used to represent the degree of pollution in industrial wastewater.
  Sodium (Na)  
Sodium in natural bodies of water comes from the weathering process of igneous rocks and evaporated minerals. The amount of sodium varies greatly in different conditions. In most rivers, the concentration of sodium falls between single-digit mg/L to double-digit mg/L. However, the concentration in salt water can reach up to more than 100000 mg/L. In high salt concentration water, sodium is the dominant positive ion. In seawater, sodium accounts for 81% of the total positive ion weight. Sodium in water with a concentration below 1000 mg/L exists in a free state while in higher salt concentration water there may have other ions and complex compounds such like NaCO-, NaHCO3 and NaSO4-. Moreover, almost all of the negative ions can form ion-pairs with sodium in seawater.
  Transparency (SD)  
Transparency refers to the degree of how much light can pass through the water. It is one of the index used to assess eutrophication in a reservoir. The way to measure it is to lower a Secchi disk into the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. Therefore it is also known as the Secchi transparency.
Salinity refers to the grams of salt dissolved per kilogram of water. It is usually expressed in parts per thousand. Ocean salinity directly reflects the ocean’s physical characteristics such as density, specific heat, sound and light. It also has a huge impact on the synthesis of algae and the distribution, growth and reproduction of marine life. Thus salinity is basic information indicating the physical characteristics of the ocean. It can be measured by an electro-conductivity method.
  Manganese( Mn)  
Manganese is a component of rocks and soils. It usually coexists with iron.
  Lead (Pb)  
Lead has accumulative and metabolic toxicity. In natural bodies of water, lead exists in different forms. Lead compounds are generally hard to dissolve and are easily adsorbed and deposited. They mainly comes from improper wastewater discharge and storage of batteries by the manufacturing industry. High lead concentrations serverely pollute the water. Granular lead pollutants from the combustion of lead compounds in petroleum also pollute the air. The pollutants finally drop to the ground or are brought into water bodies by rain, increasing the lead content of water.
  Hydrogen Ion Concentration Index (pH)  
pH refers to the negative logarithm of the concentration of hydrogen ions in the water. Natural water is generally neutral or weakly alkaline. When water is polluted by waster water, pH can vary substantially. pH affects the growth of living beings, substance deposits and dissolution and treatment of water and wastewater.
  Electrical conductivity (EC)  
Electrical conductivity (EC) is a measurement of water’s ability to conduct electricity. EC is related to water temperature and the total concentration, mobility, valence and relative concentration of ions. Generally speaking, higher EC means more electrolytes in the water. Since most salts can be ionized, EC can also represent the amount of total dissolved solids. Water with high EC is unsuitable for irrigation. Thus EC is an important criterion for irrigation water quality. EC is the reciprocal of electrical resistivity which is measured as the electric current that passes through a 1-centimeter long liquid column with a cross-sectional area of one square cm. Its unit is mho/cm. If the EC value is very low, it can also be expressed in the unit of mmho/cm or μmho/cm.
  Iron (Fe)  
Iron and manganese in the water will deepen the color and increase the turbidity which reduce light transmittance and thus influence the photosynthesis of aquatic plants. They also cause odors in the water and affect the taste of drinking water. Iron is also an essential element for the human body. Many proteins such as hemoglobin and cytochrome contain iron.