Teacher Notes

Water Quality Index (WQI) Field Trip

Student Laboratory Kit

Materials Included In Kit

Dissolved Oxygen TesTabs®, 60
Nitrate TesTabs, 15
Phosphate TesTabs, 15
Bulbs, rubber, 5
Dissolved Oxygen Color Comparison Charts, 5
Dissolved oxygen test vials, 30
Lactose broth/indicator tubes, 15
Nitrate Color Comparison Charts, 5
pH test strips, 1–12, 2 vials
Phosphate Color Comparison Charts, 5
Pipets, sampling, sterile, disposable, 15
Polyethylene storage bottles, wide-mouth, 500-mL, 15
Secchi disk
Water sample tubes, 15

Additional Materials Required

Aluminum foil
Barometer or access to the Internet to obtain barometric pressure
Beakers, borosilicate glass, 250-mL
Bottles, 2-L, 2
Bottles or flasks, sterile, 50- or 100-mL
Graduated cylinder, 100-mL
Hot plates or Bunsen burners (for boiling/drying “total solid” samples) or drying oven
Thermometer, alcohol-filled or digital
Water dipper/sampling stick

Prelab Preparation

Field Trip

  1. Divide class into numbered groups of two (maximum 15 groups).
  2. Students need to label their wide-mouth storage bottles with their names and group number.
  3. Prepare one or more liquid waste bottles using two liter bottles. These will be used to capture all of the liquid waste generated from the tests.
  4. Prepare one or more solid waste containers for disposal of plastic wrappers and TesTab foil wrappers.
  5. Obtain the barometric pressure as close to the time of testing as possible.
  6. The test for temperature change is best done by sampling two sections of the river or lake approximately one mile apart. Prior to the field trip determine an appropriate location to obtain a second temperature reading approximately one mile from the location of the field trip.
  7. Prepare 50 mL to 100 mL sterile bottles or sterile test tubes for fecal coliform water sampling.

Safety Precautions

Nitrate, Phosphate, Dissolved Oxygen TesTab® precautions: TesTabs contain chemicals which may irritate skin or be harmful if swallowed. The TesTab reagents used in this kit were designed with safety in mind. The single-use, foil-packaged TesTabs are easy to dispense. Store TesTabs in a cool, dry place and only open when ready to use the tablet. A single tablet, either alone or reacted with a sample, is a low health hazard; however TesTabs should not be ingested. Wear chemical splash goggles and chemical-resistant gloves.

Fecal Coliform testing precautions: Field testing should be carried out under the supervision of a teacher or other qualified adult. By the very nature of coliform testing, the potential exists for the culturing of pathogenic microorganisms to hazardous levels. Students should never be permitted to open tubes after the tubes have been inoculated. Follow all normal safety precautions and guidelines.


Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Nitrate, Phosphate, Dissolved Oxygen TesTab® disposal: In the field, reacted samples may be poured into a waste container for later disposal. Dispose of reacted samples down the drain with plenty of water according to Flinn Disposal Method #26b. Fecal Coliform testing tubes should be disposed of by: (1) Autoclaving and discarding with regular solid waste or (2) filling a tub or sink with a mixture of half cup commercial chlorine bleach to one gallon of water, fully immersing tubes and uncapping them while submerged, and soaking them for one to two hours. Rinse sink or tub contents down the drain with excess water. Wash hands thoroughly.

Lab Hints

  • This kit is designed for 30 students working in pairs (15 groups maximum). All 15 groups will complete the following tests: Dissolved Oxygen, BOD, Phosphates, Nitrates, pH. Rotate the equipment that will be shared by different groups as follows:

    Odd groups: Phosphates → DO → pH → Nitrates
    Even groups: Nitrates → pH → Phosphates → DO

  • The first five groups that finish should obtain turbidity readings.
  • Three groups should measure the water temperature.
  • All groups need to fill their 500 mL total solid sample bottles and prepare their fecal coliform sample tubes.
  • TesTabs are a vendor product of the LaMotte Company. SDSs are available through the manufacturer website.
  • The rubber bulbs should be shared among the student groups.

Teacher Tips

  • Because two of the tests require incubation, (fecal coliform 3 days, BOD 5 days), the actual field trip should be scheduled for a Friday. If this is not possible, some of the testing will need to be done on the weekend, or samples will need to be collected on the weekend.
  • Many local sewage plants provide tours and/or will provide fecal coliform and 5-day BOD testing of water samples at no cost to the school. It is a great idea to combine the water testing field trip with a stop at your local sewage plant. The “Tour de-Sewer” will be a memory your students never forget. Having the sewage plant test fecal coliform and 5-day BOD samples can give some flexibility on when the trip needs to take place.
  • The testing materials provided in this kit should provide results that fall within the range of typical water testing values. If the water being tested produces results outside the range of the equipment provided, other methods of testing may be used. Consult the Flinn Scientific Catalog or books noted in the reference section for other testing alternatives.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Planning and carrying out investigations
Analyzing and interpreting data

Disciplinary Core Ideas

MS-LS2.A: Interdependent Relationships in Ecosystems
MS-LS2.C: Ecosystem Dynamics, Functioning, and Resilience
MS-ESS3.C: Human Impacts on Earth Systems
HS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems
HS-LS2.C: Ecosystem Dynamics, Functioning, and Resilience
HS-ESS3.C: Human Impacts on Earth Systems

Crosscutting Concepts

Stability and change

Performance Expectations

MS-LS1-5: Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms.

Sample Data

WQI Calculation Chart


Dissolved Oxygen

DO measured with TesTabs = 8 ppm
Barometric pressure = 684 mm Hg therefore the correction factor = 0.90
8 ppm measured x 0.90 = 7.2 ppm corrected DO measured
Water temperature = 20 °C
% saturation by connecting 20 °C with the corrected DO of 7.2 = 80% saturation

Fecal Coliform

15 tubes tested: 12 positive, 3 negative

5-Day BOD

5 day DO measured with TesTabs = 4 ppm
Barometric pressure = 699 mm Hg therefore the correction factor = 0.92
4.0 ppm measured x 0.92 = 3.7 ppm corrected 5-day DO measured
Original corrected DO = 7.2 ppm – 3.7 ppm = 3.5 ppm


Location 1 = 20.2 °C
Location 2 = 10.2 °C
ΔTemp = 10.0 °C

Total Solids

Mass of clean, dry beaker = 150.22 g
Mass of beaker with dry residue = 152.15 g
Mass of residue = 152.15 g – 150.22 g = 1.93 g of total solids residue
Total volume of water added (mL)
90.1 mL + 85.3 mL + 99.0 mL + 100.0 mL + 94.3 mL = 469.0 mL total water evaporated
Total Solids Calculation: (1.93 grams / 469 mL) x (1000 mg / 1 g) × (1000 mL / 1 L) = 4,120 ppm

Answers to Questions

Many questions do not have a single right answer. Student answers will vary. Suggested answers, and answers to questions with absolute answers are listed.

  1. Which test is the most difficult? Why?

    Student answers will vary.

  2. Which test has the greatest chance of having a large experimental error? Explain?

    Student answers will vary. If all 15 fecal coliform tubes give positive test results, this may mean that the actual fecal colony count is much higher than the 400 colony testing limit.

  3. Which test is weighted the most heavily when determining the Q-value of the water? Why do you think so much importance is placed on this test result?

    Dissolved Oxygen is the most heavily weighted at 17% of the WQI. Dissolved oxygen is necessary for much of the aquatic life in a river or lake. DO is also a good indicator of other factors in the stream such as BOD.

  4. Which test is weighted the least heavily when determining the Q-value of the water? Why do you think so little importance is placed on this test result?

    Total Solids is valued as the least important at only 7% of the WQI. Many living things need some total solids in the water to function. The solids may actually be harmful, harmless or even beneficial.

  5. What was the biggest difference in the data when you compared the data from the stationary lake to the moving water? (Make sure to explain how you quantified “the biggest difference.”) If a lake and river were not tested, which test do you predict would show the biggest difference?

    The biggest differences would probably be in DO and turbidity levels, both are higher in a moving stream. Oxygen is dissolved as water tumbles. The energy of moving water allows small particles to remain suspended while a quiet lake allows suspended particles to settle out.

  6. Are biotic (living) or abiotic (non-living) factors the main influence on the quality of the water tested? Describe the evidence for your answer.

    Most streams and lakes are affected more by biotic factors, humans being on the top of the list. Many human activities can contribute abiotic influences such as fertilizer. In this case an interesting discussions can be had classifying an influence as biotic or abiotic.

  7. Choose one water quality parameter tested. What could be done to the environment to improve the Q-value of this parameter?

    Dissolved oxygen levels are often elevated in lakes with artificial fountains and in rivers with artificial rapids. Decreasing fertilizer use can reduce phosphates and nitrate contamination.

  8. Which parameter tested should be of the greatest concern to the public? Why?

    Fecal coliform is a solid answer. The presence of fecal coliform bacteria can indicate the presence of other pathogenic bacteria.

  9. Which parameter tested should be of the least concern to the public? Why?

    Turbidity is a possible answer. Turbidity can just be a result of high stream flows keeping bottom sediment in suspension.


Special thanks to Bill Grosser, Glenbard South High School, Glen Ellyn, IL, for compiling the information in this test kit.

Bryan, Burbank, Ballanger. Rivers Curriculum Guide, Chemistry; Dale Seymour Publications, Parsippany, NJ. 1995.

Mitchell, M. K.: Stapp, W. B. Field Manual for Water Quality Monitoring; 9th ed. Thomson-Shore, Dexter, MI. 1995.

Student Pages

Water Quality Index (WQI) Field Trip


The Flinn water quality test kit is designed to allow the quality of a lake or river to be determined using a tool called the “Water Quality Index.” The Water Quality Index (WQI) is an environmental quality “score” based on a combination of nine different chemical and biological tests of water. The WQI, which is rated on a scale from 0 (Very Bad) to 100 (Excellent), allows comparison of data from different rivers or lakes and/or the same river or lake over an extended period of time. Most of the tests are done in the field. Some of the tests are completed in the lab with samples taken from the lake or river.


  • Concentrations of dissolved ions and molecules in water
  • Parts per million
  • Biotic and abiotic influences on aquatic systems
  • Equilibrium
  • Water testing


The Nine WQI Tests

Dissolved Oxygen
Dissolved oxygen is the amount of gaseous oxygen, O2, dissolved in a body of water. Oxygen enters into the water by aeration, diffusion from air and as a byproduct of photosynthesis. Oxygen levels change throughout the day as a result of photosynthesis usually peaking in late afternoon. Water temperature has a large impact on the amount of oxygen that can dissolve. Gases dissolve more easily in cooler water than warmer water. In the summer, extremely warm water temperatures can result in very low oxygen levels. High flow rates and/or turbulence can increase oxygen levels due to aeration. Slow moving or stagnant water usually has very low oxygen levels.

Human-Caused Changes in Dissolved Oxygen: Organic waste is anything that was once part of a living plant or animal. Organic waste can come from sewage, urban and agricultural runoff and discharge from food processing plants, meat packing plants, dairies or other industrial sources. Organic waste provides a food source for aerobic bacteria, which then consume the oxygen dissolved in the water. Fertilizer runoff also negatively effects the amount of dissolved oxygen by stimulating the growth of algae and other aquatic plants. When these plants die they are consumed by aerobic bacteria that again deplete the amount of available oxygen. Building dams can decrease stream flows resulting in less aeration and lower dissolved oxygen levels.

Changes in Aquatic Life: The concentration of dissolved oxygen is one of the most important indicators of the overall health of a body of water. When bodies of water contain a large amount of oxygen, the quality of the water is generally considered as being good. When the amount of oxygen in the water is very low, the pollution level of the water is most likely very high. Water with consistently high levels of dissolved oxygen (6 ppm or more) typically support the most diverse biological communities. Water with consistently low dissolved oxygen levels (below 3 ppm) is extremely stressful to aquatic organisms and may be virtually void of aquatic life or may harbor only a few species adapted to such conditions. Dissolved oxygen levels below 2 ppm will not support fish life.

The water quality index measures oxygen as “percent saturation.” Percent saturation refers to how close the water is to holding its maximum amount of a given temperature. Rivers that have oxygen saturation levels between 90% and 110% are considered healthy. Water less than 90% saturated may contain large amounts of oxygen-demanding organic material. Water with over 110% saturation can result from excessive turbulence.

Fecal Coliform
Fecal coliform bacteria naturally occur in the digestive tracts of warm-blooded animals and are found in the feces produced by these animals. Fecal coliform bacteria are not pathogenic but their presence typically coincides with numerous potentially pathogenic bacteria, viruses and parasites that can cause diseases and illnesses. If coliforms are found to be present, it is assumed that potential pathogens are present.

Human-Caused Changes in Fecal Coliform: Animal wastes, agricultural runoff and untreated sewage effluent are all likely sources of fecal coliforms in lakes and streams. Untreated sewage effluent may originate from illegal discharge sites, or frequently, it is the result of heavy storm runoff overwhelming wastewater treatment plants that handle combined storm and sanitary input. Coliform standards vary between cities, states and regions of the country. Typical coliform density standards appear in Table 1.

Water contains both hydrogen (H+) ions and hydroxide (OH) ions. The pH test measures the hydrogen ion concentration in water. pH is measured on a scale that ranges from 0 to 14. Pure water contains equal numbers of H+ and OH ions, has a pH of 7, and is considered neutral. When water has more H+ ions than OH ions, it is considered acidic and will have a pH less than 7. If the water contains excess OH ions, the water is considered basic and has a pH greater than 7.

Human-Caused Changes in pH: In the United States the pH of natural water is usually between 6.5 and 8.5. Automobiles, coal-fired power plants and industrial emissions can add nitrogen oxides (NOx) and sulfur dioxides (SO2) to the air. These emissions are converted to nitric acid and sulfuric acid in the atmosphere. The acids combine with moisture in the atmosphere and fall to earth as acid rain or acid snow which can lower the pH of rivers and lakes. The hardest hit areas of acid rain and snow are generally located downwind of urban and industrial areas. Because living organisms are able to survive within only a small pH range, a change in pH may kill off native organisms and in some cases facilitate the growth of foreign organisms. In many areas of the United States, the types of rocks and minerals in the lakes and streams help to regulate the acid levels in the water. If limestone is present, the basic limestone neutralizes the effect of acid rain or snow.

Changes in Aquatic Life: At extremely high or low pH values (>9.6 or <4.5) the water becomes unsuitable for most organisms. serious problems occur in lakes with a pH below 5. Iimmature stages of aquatic insects and young fish are extremely sensitive to pH values below 5 (see Figure 1).
Water that is very acidic can cause heavy metals, such as copper and aluminum, to be released into the water. Heavy metals can accumulate on the gills of fish or cause deformities in young fish, reducing their chance of survival. Acid rain has been responsible for thousands of lakes in Eastern Canada, Northeastern United States, Sweden and Finland becoming acidified.

Biochemical Oxygen Demand (5-Day BOD)
When organic matter decomposes, it is digested by aerobic bacteria. Biochemical oxygen demand (BOD) is a measure of how much dissolved oxygen (DO) is used by these microorganisms in the aerobic oxidation of organic matter. BOD is calculated by measuring the level of DO in a river or lake, and then testing a second sample 5 days later. The amount of oxygen that gets consumed is the 5-day BOD.

High BOD values can result from excessive plant growth. the input of nutrients, such as nitrates, phosphates or organic waste into a river stimulates plant growth. more plant growth leads to more plant decay. Decaying plants are digested by aerobic bacteria. Nutrients can be a prime contributor to high biochemical oxygen demand in the rivers and lakes. Waters with a high level of plant growth and decay often have dissolved oxygen levels that fall below 90 percent.

Human-Caused Changes in BOD Sources of Organic Matter: When organic waste is released from identifiable points of discharge into lakes and rivers they are called point sources. Point sources include pulp and paper mills, meat packing plants, food processing industries and wastewater treatment plants. There are also non-point pollution sources that are more difficult to identify. these include:
  • Urban runoff of rain and melting snow that gets into illegal sanitary sewer connections, pet wastes, nutrients from fertilizers, leaves, grass clippings and paper from residential areas.
  • Agricultural runoff that includes nutrients like nitrogen and phosphates.
  • Fecal material from animal feedlots, which is carried into lakes and rivers.

Changes in Aquatic Life: In rivers with high BOD levels, much of the available dissolved oxygen is consumed by aerobic bacteria leaving little oxygen for other aquatic organisms. High BOD may cause organisms that are more tolerant of low dissolved oxygen levels, such as carp, midge larvae and sewage worms, to become more numerous. Organisms that are intolerant of low oxygen levels, such as caddis fly larvae, mayfly nymphs and stonefly nymphs, may not survive. when high bod levels exist, aquatic ecosystems can change from supporting a high diversity of organisms to supporting only a low diversity of population-tolerant organisms.

The temperature of water is very important because it influences the amount of oxygen that can be dissolved in the water, the rate of photosynthesis by algae and larger aquatic plants, the metabolic rate of organisms and the sensitivity of organisms to toxic waste, parasites and diseases.

Human-Caused Temperature Changes: Thermal pollution results when excess heat energy released into a river or lake causes an unnatural increase in the water temperature. Typical industries that release significant amounts of heat energy include nuclear and coal-fired power plants that use the cool water from rivers or lakes to condense steam. people can also cause temperature changes by cutting down trees that shade the lakes and rivers. Soil erosion can indirectly cause increases in water temperature. Suspended particles from erosion can cause the water to become cloudy, which increases the amount of sunlight absorbed by the water.

Changes in Aquatic Life: Both the rate of photosynthesis and plant growth increase as the water temperature rises. Increased plant growth results in greater decomposition by aerobic bacteria and decreases oxygen levels in the water. The metabolic rate of fish and other organisms increases as the water temperatures increase. The result of higher metabolic rates in living organisms when oxygen levels are dropping is a bad combination for many species. Warm water also speeds up the life cycles of aquatic insects. This change in life cycle negatively affects animals that feed on insects.

Most aquatic organisms have adapted to survive within a range of temperatures. Altering the temperature of a lake or river can cause some species to die off completely. As the temperature of a river increases, cool water species will be replaced by warm water species.

Phosphorus is a vital element of life and is usually found naturally in waste as phosphate ions (PO43–). Phosphorus is needed for plant growth. The small amount of phosphorus present in healthy streams and lakes usually limits plant growth to healthy levels. Phosphate levels greater than 0.1 ppm may lead to an overgrowth of aquatic plants.

Human-Caused Phosphate Contributions: Phosphate can originate from fertilizers, sewage, animal and plant residue and from wastewater from industries and laundry detergents. Phosphorous in the form of phosphate ions can also increase as a result of soil erosion or draining swamps or marshes.

Changes in Aquatic Life: Excessive levels of phosphates can result in overgrowth of algae (also known as algae blooms), which in turn will cause the algae to die at a high rate and undergo decomposition by aerobic bacteria. This decomposition process depletes oxygen from the water and can result in fish kills and or the replacement of a diverse aquatic population with only species that can survive in low oxygen levels.

Lakes that have high phosphate levels undergo a process called eutrophication. Eutrophication is the process that promotes plant growth and subsequent decay. There are two types of eutrophication: cultural eutrophication and natural eutrophication. Cultural eutrophication is water pollution caused by excessive amounts of phosphates introduced by human activities. The rapid growth and die-off of plants causes lakes to “fill-in” and age rapidly. In contrast, natural eutrophication, which is the process where lakes gradually age and become more productive, requires thousands of years to come to completion.

Unpolluted water generally has an overall nitrate level less than 4 ppm. If the concentration of nitrates is greater than 10 ppm, water may be unfit to drink. High levels of nitrates in drinking water can lead to a condition known as methemoglobinemia or “blue baby disease.” Blue baby disease occurs when high levels of nitrates enter the digestive tract of infants. In some infants, bacteria convert excess nitrates into nitrite. Nitrites react with hemoglobin and decrease the amount of oxygen in the baby’s bloodstream. As the oxygen level is depleted, the infant gradually suffers from a lack of oxygen, which causes the skin of the infant to turn a blue color. Blue baby disease can become fatal if not treated. Pregnant women and infants should not drink water that contains nitrate levels greater than 20 ppm.

Human-Caused Nitrate Contributions: Nitrate (NO3) ions accumulate in water systems from decaying vegetation, the atmosphere, fertilizer used in agriculture, animal excrement and sewage. Sewage can enter waterways from inadequately treated wastewater at sewage treatment plants, illegal sewer connections and poorly functioning septic systems. Sewage is the main source of nitrates added by humans to rivers. Other sources of nitrates are fertilizers and runoff from cattle feedlots, dairies and barnyards. Commercial farms where large numbers of animals are concentrated in a small area can produce large amounts of waste that is rich in ammonia and nitrates. If not properly treated, these wastes can add significant amounts of nitrates to streams and lakes. Fertilizers used in agriculture and lawn care may contain up to 20% nitrogen in the form of nitrates.

Changes in Aquatic Life: Surface water that is high in nitrates can lead to an overgrowth of algae and other organisms, which will foul the water. This overgrowth of algae is known as an algae bloom. Algae blooms deplete the water of oxygen and may even create “dead zones” where fish will no longer live. Excessive nitrates can also contribute to cultural eutrophication. Eutrophication is the process that promotes plant growth and subsequent decay. The rapid growth and die-off of plants causes lakes to “fill-in” and age rapidly. Excessive plant decomposition can also reduce oxygen levels, leading to replacement of a diverse aquatic population with low-oxygen tolerant species.

Turbidity is a measure of the clarity of a lake or river. Water with a high turbidity will appear very cloudy whereas water with a low turbidity will be very clear. High turbidity is a result of suspended particles in the water. The suspended particles may be organic (living) or inorganic. High turbidity can result in the color of water changing from clear, to white, red, brown or green. Natural contributors to high turbidity levels include abundant bottom-feeder fish (such as carp) which stir up bottom sediments, algae growth and soil erosion.

Human-Caused Turbidity Contributions: Human-caused activities that result in decreased oxygen levels can give rise to in overpopulations of fish, such as catfish or carp, tolerant of low oxygen levels. These bottom feeders stir up bottom sediments. Farming and urban development can also contribute to soil erosion, which significantly increases the turbidity of rivers and lakes during high flow periods.

Changes in Aquatic Life: The ability of a body of water to support a diversity of aquatic organisms decreases at higher levels of turbidity. High turbidity levels can cause more heat energy to be absorbed from the Sun, resulting in warmer water with lower oxygen levels. High turbidity can also reduce the depth that sunlight penetrates the water, resulting in a decrease in the rate of photosynthesis. A decrease in the rate of photosynthesis results in a decrease in dissolved oxygen. Oxygen depletion, warmer water and less light make it impossible for some aquatic life to survive.

Total Solids
Total solids are a measure of all of the dissolved and undissolved material in a sample of water. This is different than turbidity, which only measures suspended materials. Dissolved inorganic materials can include calcium, bicarbonate, nitrates, phosphates, iron, sulfur and other ions found in a water body. Suspended materials may include silt and mud, algae and even small living organisms. High concentrations of total solids affect the taste of drinking water and can cause a laxative effect in humans. Drinking water should not exceed 0.5 g of total solids per liter (500 ppm).

Human-Caused Sources of Total Solids: Many sources can affect the natural balance of total solids in lakes and rivers. One example is runoff from urban areas. Runoff can carry salt from streets in the wintertime, fertilizers from lawns in the summer and silt and mud from construction erosion. Another source of total solids are wastewater treatment plants. Sewage plants can contribute phosphorus, nitrogen and organic matter to rivers and lakes.

Changes in Aquatic Life: A constant level of some of dissolved ions is essential for the survival of aquatic life. The amount of total solids determines the flow of water in and out of an organism’s cells. Also many dissolved ions, such as nitrogen, phosphorus and sulfur are building blocks for the molecules of life (e.g., proteins, DNA). Levels of total solids that are either too high or too low can be detrimental to aquatic life. High total solids can also increase the turbidity if they are predominantly suspended solids. High turbidity levels can negatively affect the temperature, dissolved oxygen levels and diversity of species in a lake or river.

Experiment Overview

The “Water Quality Index” (WQI) is a tool that makes it possible to easily compare water quality at locations across the country. The WQI was originally developed in 1970 by the National Sanitation Foundation and consists of nine different tests or indicators: Dissolved Oxygen (DO), Fecal Coliform, pH, 5-day Biochemical Oxygen Demand (BOD), Temperature Change, Total Phosphates, Total Nitrates, Turbidity and Total Solids.

As each test in this survey of water quality is completed, data will be entered into a class data table to obtain median values. Calibration graphs will then be used to convert the median data to Quality Values (Q-Values) on a scale from 0 to 100. The overall WQI for the river or lake is obtained by multiplying each of the nine Q-values by appropriate weighting factors, then adding the results.

Being able to determine a quantitative value to express the water quality allows comparison of data from different locations and/or from the same location over an extended period of time.


Aluminum foil
Dissolved oxygen BOD TesTabs®, 4
Nitrate TesTab
Phosphate TesTab
Barometer or access to the Internet to obtain barometric pressure
Beakers, borosilicate glass, 250-mL
Bottle or flask, sterile 50- to 100-mL
Bulb, rubber
Dissolved Oxygen Color Comparison Chart (shared with class)
Dissolved oxygen/BOD test vials. 2
Graduated cylinder, 100-mL
Hot plate or Bunsen burner (for boiling/drying “total solid” samples)
Lactose broth indicator tube
Nitrate Color Comparison Chart
pH test strip
Phosphate Color Comparison Chart (shared with class)
Pipet, sterile, disposable
Polyethylene bottle, wide-mouth, 500-mL
Secchi disk (to be shared with class)
Thermometer, alcohol-filled or digital
Water dipper/sampling stick
Water sample tube

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water after testing in the field or before leaving the laboratory.


Your teacher will assign each student a partner and a group number.

  1. Label a 500-mL wide mouth storage bottle with your name and group number. This will be used to bring a sample back to the lab for total solids testing.
  2. Label a lactose broth indicator tube with your name and group number. This will be used to test for the presence of fecal coliform bacteria.
  3. Prepare a sterile 50- or 100-mL collecting bottle as instructed by your teacher to use for fecal coliform testing.

Testing Procedures

Dissolved Oxygen Testing (Location: Field)

  1. Fill a dissolved oxygen test vial to overflowing with a water sample.
  2. Add two Dissolved Oxygen TesTabs to the test vial.
  3. Cap the vial and be sure that there are no air bubbles in the sample.
  4. Invert the vial and mix until the tablets have dissolved.
  5. Wait five minutes.
  6. Compare the color of the sample to the Dissolved Oxygen Color Chart. Record the value in ppm on the WQI Student Data Table.
  7. Dispose of the reacted sample according to the instructor and rinse the test vial twice.
  8. Use the barometric pressure to find the correction factor. Multiply the measured DO value by the correction factor to obtain the corrected DO level (see Figure 2). Record in the data table.
  9. Use the percent saturation conversion chart (see Figure 3) to find the percent saturation for your sample. Draw a line between water temperature and oxygen ppm. The point of intersection is % saturation. Record all values and calculation on the WQI Student Data Table.

Phosphate Testing (Location: Field)

  1. Fill the water sample tube to the 5-mL line with a water sample.
  2. Add one Phosphate TesTab to the tube.
  3. Cap the tube and mix until the tablet has dissolved.
  4. Wait five minutes.
  5. Compare the color of the sample to the Phosphate Color Comparison Chart. Record the value in ppm on the WQI Student Data Table.
  6. Dispose of the reacted sample according to the instructor and rinse the sample tube twice.

Nitrate Testing (Location: Field)

  1. Fill the water sample tube to the 5-mL line with a water sample.
  2. Add one Nitrate TesTab to the tube.
  3. Cap the tube and mix until the tablet has dissolved.
  4. Wait five minutes.
  5. Compare the color of the sample to the Nitrate Color Comparison Chart. Record the value in ppm on the WQI Student Data Table.
  6. Dispose of the reacted sample according to the instructor and rinse the sample tube twice.

pH Testing (Location: Field)

  1. Dip one pH test strip into sample water so that the chip is approximately half wet.
  2. Compare the color of the strip to the color on the pH vial. Record the pH value on the WQI Student Data Table.
  3. Dispose of the test strip properly.

Temperature Testing (Location: Field)

  1. Measure the temperature of the lake or river at the test location. Record in the WQI Student Data Table.
  2. Measure the temperature of the same lake or river approximately 1 mile away. Record in the WQI Student Data Table.
  3. Calculate the difference between the two readings. Record the temperature difference in the WQI Student Data Table.

Turbidity Testing (Location: Field)

  1. Lower the Secchi disk until it can’t be seen. Note the depth when it first disappears. Record this value in the WQI Student Data Table.
  2. Slowly raise the disk until it first appears. Note the depth when the disk first reappears. Record this value in the WQI Student Data Table.
  3. Average the two depth readings. Record this value in the WQI Student Data Table. Note: It is important that the disk travels vertically up and down through the water and is not “swung out” by the river current.

Fecal Coliform Testing (Location: Field/Lab 3 days later)

  1. Uncap the sterile collecting bottle, immerse it mouth-first several inches below the water surface, and then turn the bottle mouth into the current. Remove and recap the filled bottle.
  2. Unwrap a sterile pipet and, using sterile technique, transfer a 1-mL sample of water from the collecting bottle to your labeled lactose broth tube.
  3. Label the lactose broth tube with the name of the collection site, site characteristics and the date and time. Incubate the tubes for 72 hours at room temperature. Check the tubes every 24 hours during the incubation period and observe and record any color changes. (Samples may also be incubated for 48 hours at 35 °C.)
  4. Any tubes that turn yellow or greenish-yellow after the incubation period should be considered positive for coliform bacteria. Calculate the MPN (most probable number) for your class using the following equation.

    Most Probable Number (MPN) calculation:

    A sample calculation for 10 positive out of 15 total tubes, with a 1-mL sample in each is shown:
    Note: A limitation of the MPN procedure is that coliform concentrations higher than ≈ 400 colonies/100 mL should cause all 15 tubes to turn. In the event that all 15 tubes give positive coliform test results, the MPN should be recorded as >400 colonies/100 mL.

Total Solids (Location: Field sample, lab testing)


  1. Obtain a water sample from below the surface and away from the bank or shore. If the sample contains any large floating particles, obtain a new sample. Transfer the sample water to a labeled wide mouth storage bottle. Store the bottle for later analysis in the lab.


  1. Measure and record the mass of a 250-mL beaker in the WQI Student Data Table. Make sure the beaker is clean and dry and at room temperature.
  2. Swirl the sample bottle gently, then pour some of the water into a 100-mL graduated cylinder. Measure the exact volume of water, then add the water to the beaker. Record the volume of water added in the first blank for Total Volume of Water Added in the WQI Student Data Table. Heat the beaker until most of the water has boiled or evaporated. Before it boils dry add some more water (step 4).
  3. Swirl the sample bottle gently again and repeat step 3. Measure and record the exact volume of water each time you add more to the beaker. Record all volumes in the WQI Student Data Table.
  4. When the beaker is almost dry, reduce the heat so that the residue does not splatter or burn.
  5. Stop heating when the residue is dry, not burned.
  6. Allow the beaker to cool to room temperature. The residue in the beaker contains the “total solids” from the water sample. Record the combined mass of the beaker and any residue in the WQI Student Data Table.
  7. Calculate the ppm of total solids using the following formula:

BOD Testing (Location: Field sample, lab testing)


  1. Rinse a dissolved oxygen test vial with river or lake water 2–3 times.
  2. Fill the dissolved oxygen test vial with water by submerging the bottle in the river or lake.
  3. Tilt the vial slightly and screw on the cap. If any air bubbles are trapped, discard the sample and start over.
  4. Wrap the test vial in aluminum foil so that it is “light tight.”


  1. Place the test vial in a light-proof drawer at approximately 20 °C for five days.
  2. After five days, add two Distilled Oxygen TesTabs to determine the ppm of oxygen remaining in the sample.
  3. Use the Dissolved Oxygen Color Comparison Chart to determine the dissolved oxygen level in ppm. Record this value in the WQI Student Data Table.
  4. Use the barometric pressure to calculate the corrected DO level (see Figure 3). Record this value in the WQI Student Data Table.
  5. Calculate the 5-day BOD using the following formula:

    Original corrected DO (ppm) – 5-day corrected DO (ppm) = 5-day BOD

  6. Consult your instructor for appropriate disposal procedures for all tests.

Calculations and Analysis

  1. Obtain the Q-value for each test using the Water Quality Index Q-Value Charts. Record all Q-values on the WQI calculation data table.
  2. After all Q-values have been determined, calculate the WQI for the water sample on the WQI Student Calculation table.
  3. Answer the questions on the Critical Thinking WQI Questions Worksheet.

Student Worksheet PDF


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