Lessons: Water

Teaming Up for Clean Water

A WATER QUALITY STUDY

Respect Rule: Look, Listen, Learn, and Leave Alone (until instructed).

Overview

To understand the importance of water, look beyond its liquid, invisible nature, and test its inherent properties, its essential qualities. Water quality scientists rely on five parameters to give them a snapshot of water’s nature. These parameters are temperature, dissolved oxygen, pH, turbidity, and conductivity. Measuring these parameters in the laboratory and understanding their importance to stream organisms will help students prepare for field work, and future study of stream ecology.

Background

Water is taken for granted. For a liquid that is critical to survival, and which is taken from the tap, from bottles flavored, caffeinated, mineralized or carbonated, very little is known about the true nature of water. Is water just a clear liquid essential for survival? What is in water, and what makes it healthy for aquatic animals?

Protecting the water quality of streams in California is the responsibility of the State Water Resources Control Board, or Water Board. This state agency has a program called the Clean Water Team which encourages citizens to protect their local waterways. During this study of water quality, students will be part of the Clean Water Team. The fact sheets used by students for research are developed by the Water Board, and available on their website at:
www.waterboards.ca.gov/nps/cwtguidance.html#10
(Scroll down to Section 3, and view the fact sheets for each parameter.)

This lesson focuses on five measurements of water quality often taken by aquatic biologists when assessing stream health. These parameters, temperature, dissolved oxygen, pH, turbidity, and conductivity, are critical to the survival of aquatic organisms. They are not the most important parameters for the quality of drinking water. However, they are readily measured and are a valuable prelude to more detailed laboratory experimentation in a high school, college, or commercial chemistry lab.

Temperature
Water temperature is a measure of the kinetic energy of water molecules. It is very important to aquatic organisms. As it not only affects metabolic rates, it alters environmental conditions important to survival (see dissolved oxygen discussion below). For example, cold-blooded animals, like fish, will double their metabolic rates for each 18°F increase in temperature and be reduced by one-half for each 18°F decrease in temperature.

Temperature will vary naturally primarily with the energy of sunlight, water flow, and depth of water. On dammed streams, like the Mokelumne River and Stanislaus River, temperature can be dependent on the regulation of flow from dams and the depth at which water was removed from the upstream reservoir. Water released from the bottom will be colder than water spilled from the surface.

A stream’s temperature can inadvertently be changed in numerous ways. Removing riparian vegetation for flood control can reduce shade provided by overhanging limbs. Channelizing a stream for flood control can alter stream flow, reduce the depth of pools or reduce groundwater recharge. Water diversions and dams can also affect temperature. Soil erosion can increase temperatures since soil particles absorb heat.

An accurate picture of stream temperature and its effect on stream health is a difficult monitoring task unless there are automated, computerized recorders that take ongoing, regular temperature measurements. However, temperature is an important measurement to monitor when sampling a creek because of its influence on oxygen solubility and its importance to aquatic organisms.

Dissolved Oxygen
Dissolved oxygen (often referred to as “D.O.”) is the amount of oxygen gas dissolved in water. Some species like trout and certain stoneflies require high amounts of oxygen. These organisms are usually found in high gradient, cold and/or fast moving waters where oxygen enters the stream as water tumbles over boulders or stones. Since higher concentrations of oxygen gas can dissolve in water that is cold compared to warm, temperature is an important factor to consider when sampling dissolved oxygen or evaluating the effect of low dissolved oxygen. Another natural condition, altitude, affects dissolved oxygen concentrations. At higher altitudes, water holds less oxygen. At sea level and low temperature, the oxygen saturated in freshwater is in concentration of approximately 14 mg/l. Dissolved oxygen is measured in milligrams per liter (mg/l) or parts per million. (A liter of fresh water is 1000 grams.) In streams where trout like to live, dissolved oxygen levels should not generally fall below 6 to 8 mg/l. In streams where bass or other warm water fish live, concentrations should not fall below 5 to 6 mg/l.

Chemistry classes can discuss important chemical principles such as the Ideal Gas Law and Henry’s Law. Remember that the Ideal Gas Law states that the volume of a gas is inversely proportional to pressure, and directly proportional to temperature. And Henry’s Law states that the concentration of a gas dissolved in a solution is directly proportional to the partial pressure of the gas above the solution.

Reducing dissolved oxygen can kill sensitive species, reduce the growth of organisms, or prevent egg hatching. D.O. may vary diurnally with temperature change, and algal photosynthesis and respiration. D.O. may be reduced by human activities or pollutants. Several factors that may reduce the D.O. are removal of riparian vegetation that shades the creek, increased nutrients and subsequent algal blooms, and increased sediment. Sediment may enter a stream if construction, logging, dirt roadways, or other soil erosion factors cause sediment loaded water to run off the watershed and into a stream. Once in the stream, sediment captures heat, temperatures can thus rise, causing oxygen levels to decline.

Measuring dissolved oxygen requires a titration called the Winkler Method. A dissolved oxygen kit, including sample vials and reagents, can be purchased from chemical supply companies such as LaMotte or Hach. Kits are also available locally through the Upper Mokelumne River Watershed Council at the Central Sierra Resource Conservation and Development office in Jackson.

For chemistry classes, the titration equations (below) will be of interest. More simply put, the titration uses several reactions to produce iodine in equal concentration to the original dissolved oxygen. The iodine can be observed in the reaction vessels, and is noticeably darker in samples with high dissolved oxygen. The iodine sample is then titrated with a sulfate compound, altering the iodine to form a charged iodine which is colorless. The reaction has a very clear endpoint, and is a good example of the value of titrations in chemical analyses.

Here are the equations. O2 is reduced by Mn2+ at high pH. Divalent maganous hydroxide, a brown precipitate, is formed.

4e- + 4H+ + O2 —> 2H2O
2Mn2+ + 4OH- —> 2MnO2(s) + 4H+ + 4e-
                                                         
2Mn2+ + 4OH- + O2 —> 2MnO2(s) + 2H2O

Upon addition of iodine and H2SO4 to create an acid environment, MnO2 oxidizes I-,

4e- + 2 MnO2(s) + 8H+ —> 2Mn2+ + 4H2O
4I- —> 2I2 + 4e-
                                                         
MnO2(s) + 4H+ + 2I- —> I2 + Mn2+ + 2H2O

to form a yellow brown solution.

The I2 is then titrated with thiosulfate, S2O32-, to form I- and tetrathionate, S4O62-.

2e- + I2 —> 2I-
2S2O32- —> S4O62- + 2e-
                                                         
2S2O32- + I2 —> S4O62- + 2I-

The endpoint is enhanced visually by the addition of a blue starch indicator.

pH
pH (the power of hydrogen) is a measure of the strength of the hydrogen ion in water. In familiar terms, pH tells how acidic or basic the water is. It is defined as the negative log of the hydrogen ion concentration. The more acidic the water is the lower the pH. For each whole number increase (e.g. from 1 to 2) the hydrogen ion concentration decreases tenfold and the water becomes less acidic. The range of measurements is 0 (high concentration of positively charged hydrogen ions, very acidic) to 14 (high concentration of negatively charged hydroxide ions, very basic). When both types of ions are in equal concentrations the pH is 7 or neutral.

Alkalinity is a slightly different measurement than pH. Alkalinity tells how well water can withstand a change in pH if an acid is added.

Most natural aquatic systems have a pH range between 4 and 9. The pH of the outside environment will affect cellular reactions inside aquatic organisms. Most freshwater organisms live within the range of 6.5 to 8.5. However, some fish such as carp and catfish can tolerate higher pHs.

Natural environmental factors affect the pH of water. Streams carve their way through rocks of different types, such as granite and limestone. The chemical nature of these rocks will determine the minerals that are leached from the rocks into the stream. Areas rich in carbonate minerals such as limestone will help buffer a stream’s pH. Granite soil contains few buffering agents and streams running through granite areas are more susceptible to acidic pollution such as smog.

Tree leaves dropping into a stream can alter pH. Pine needles and oak leaves are acidic, while maple leaves are more basic. Waters with high algal blooms can show a diurnal change in pH. When algae grow and reproduce they use carbon dioxide. This loss of CO2 causes the pH to increase and the water to become more basic. pH will increase at the height of photosynthesis when temperatures are warmest and decline at night when algae are respiring and using CO2. These algal changes in pH are most problematic in standing waters or pools with high algal growth.

Turbidity
Turbidity is a measure of the amount of suspended particles in water. Sediment is an important component, but algae and organic matter may also make the water less clear, or turbid.

Streams erode and transport sediment downstream. Views of the Mokelumne River canyon from Highway 49 give Sierra residents an idea of how water can carve away soil and move rocks. This natural process is accelerated in stormy weather, thus local streams appear muddier after storms. Sediment can be suspended in the water during high flows and then settle out in low flows. The natural process of transporting sediment obviously varies depending on the river system. A large river, like the Sacramento River, flowing through a long, large flat valley, naturally carries more sediment than a river like the American River, where much of the upstream sediment is trapped behind Folsom Dam. The difference in these two river systems can be seen in the colors of the rivers as they merge in Sacramento at Discovery Park.

While sediment transport is a natural process, one of the greatest pollutants in California is sediment. Erosion from inappropriate land practices can add too much sediment to a stream. Logging on steep slopes in the riparian corridor can cause erosion and sedimentation in streams. Overgrazing in the riparian corridor or failure of appropriate barriers on large construction sites may also cause high sediment load.

Once in the stream, suspended sediment can affect stream health in several ways. First, sediment particles absorb heat and can increase temperature. Second, sediment settles onto the gravel and cobble of a stream bed. Sediment may smother fish eggs nestled in the gravel of a stream bed.

A rough estimate of turbidity can be determined in the field using a transparency tube. The student fills a long cylinder with stream water and then views a dark dot at the bottom of the tube. The clarity of the dot is compared to water standards of a known turbidity. This method could be compared to a laboratory measurement if a turbidity meter is available. These meters pass a light through the water sample and measure the amount of light absorbed by the sample.

Conductivity
Since streams naturally transport sediment, minerals will be dissolved in the stream. These dissolved minerals can be positively charged ions (cations) or negatively charged ions (anions). Conductivity is a measure of the ability of water to carry an electrical current. As in a liquid battery, conductivity increases with the amount of negatively charged ions present.

Conductivity will vary depending on the geology of the watershed. Streams following through sedimentary rocks like limestone will have higher conductivity than streams following through granite deposits. The most important anions in streams are generally bicarbonate (HCO3-), chloride (Cl-), and sulfate (SO42-).

Changes in conductivity can indicate a pollution source. If a sewage pipe ruptures, raw sewage would have a much higher conductivity than the stream water. Industrial or agricultural waste (e.g. animal waste, wastewater from food processing) would also have high conductivities.

Conductivity is measured using a small electrical probe. A drop in voltage between the two electrodes measures resistance, which is the reciprocal of conductivity. Conductivity is measured in “mho” because it is the inverse of “ohm,” the unit of resistance. The natural levels of conductivity in water fall in the one thousandths (mmhos) or one millionths of a mho (umhos) over a certain area (cm). Thus, the units observed in the field are in umhos/cm. Tap water ranges between 50 and 800 umhos/cm.

“If facts are the seeds that later
produce knowledge and wisdom,
then the emotions and the impressions
of the senses are the fertile soil in
which the seeds must grow.”

—Rachel Carson

Before-the-Field-Trip Activities

Activity 1: Water Quality Research
Time: One Class Period
Materials: Water Quality Fact Sheets, Water Quality Parameters Student Work­sheet and Answer Key

  1. Introduce the importance of water quality to stream organisms.
  2. Introduce the five parameters.
  3. Divide the class into five groups to research a specific parameter. Studying temperature will take the least time, so that topic would be suitable for students who need more time. The dissolved oxygen packet takes the most time.
  4. For early finishers, ask them to find the value or range of values that protect aquatic life.
  5. Pass out Water Quality Fact Sheets for background information.
  6. Have students read about the parameter and record what it is, how it is measured, importance to aquatic life, and what factors will change.
  7. Have a group spokesperson sell the importance of this parameter to other groups, while they record information on their worksheet.

Activity 2: Water Quality Measurements
Time: One Class Period
Materials: Water Quality Para­meters Stu­dent Worksheets, thermometers, pH probes, conductivity probes or meters, pH standard solutions (7 or 10), conductivity standard solution, Dissolved, Oxygen Kit, Turbidity Kit, Turbidity meter (if available), gloves, protective eyewear (Some of the above water quality instruments can be borrowed from the Upper Mokelumne River Water­shed Council.) Water Quality Monitoring Kit (STE Lending Library)

  1. Have students learn how to measure the water quality parameters. They will use the methods that are approved by the State Water Board for Clean Water Teams.
  2. Discuss care of equipment. Both the pH and conductivity probes should not be submerged more than a couple inches. The Winkler titration uses sulfuric acid; discuss risks of handling and disposing of a strong acid.
  3. Demonstrate the Winkler titration method. Discuss the chemical reactions. (See background information.) Discuss color changes and what that reflects about dissolved oxygen levels. Make sure students know the intervals on the syringe and can read the endpoint. Discuss the validity of recording an answer that is more accurate (e.g. +/- 0.1 ppm) than the method allows. Discuss what would happen if over titrated. Would the measurement be high or low? This would be a good time to introduce or reinforce types of errors in analysis.
  4. Demonstrate the use of the pH and conductivity probes. An important first step on both these methods is calibration using standard pH or standard conductivity. Discuss what would happen if the standards were contaminated with sample water, or if samples were taken before calibration.
  5. Demonstrate the use of the turbidity tube. Discuss the difference, and the likelihood of error in this measurement compared to other measurements.
  6. Set up a thermometer in a water bath. Review how to read a thermometer. Students may measure temperature as they complete another parameter.
  7. Divide the class into four groups or four stations to learn how to take measurements of dissolved oxygen, pH, conductivity and turbidity. The dissolved oxygen test will take the most time. Consider having two kits set up so groups may complete this station more quickly.
  8. When students have completed all tests, review their measurements. Did groups get similar results? If there are differences, what could account for them? Discuss the validity of averaging the measurements to get a more accurate measurement. Include a standard deviation in the final measurement. What does the standard deviation tell you about the accuracy of the work or the experimental conditions?

Activity 3: Water Quality Experiments
Time: One class period
Materials: Use the materials from Before-the-Field-Trip Activity 2 and variety of water samples

  1. Prior to lesson, consider asking students to collect water samples. These water samples will be used to evaluate different water quality parameters, particularly pH, conductivity, and turbidity. Dissolved oxygen will change over time and the reaction must be started in the field to a stage where oxygen is no longer in a volatile state. Possible sources include local creeks. If there is access to high elevation mountain streams, collect a sample to compare to local streams. Call the wastewater treatment plant and see if they will provide a sample of creek water downstream of their effluent. Consider sampling bottled waters and mineral waters.
  2. Prior to lesson, have available or ask students to bring in materials that would change pH, turbidity, or conductivity. Possibilities include leaf litter (let it steep in water on a windowsill), soil, compost, waste from pens of confined animals on campus, rain water, or rain water directly from a roof gutter.
  3. Set up possible water sources to be tested.
  4. Have students brainstorm specific hypotheses for various water sources and their parameters. For example, the pH of water with leaf litter will be lower (more acidic) than the same source water without leaf litter. Or, the conductivity of rain water will be lower than the conductivity of stream water.
  5. Have students design a data sheet for their experiments. Will they take multiple measurements of a parameter and average?
  6. Test and record results. Make sure pH and conductivity probes are calibrated by the first group of students using these probes.
  7. Discuss results.

Field Trip Activity

Activity: Sampling Design
Time: Half Day
Materials: Water Quality Parameters Student Worksheets as from Before-the-Field-Trip Activity 2, field clothes and appropriate shoes, equipment, pencils, clipboards

  1. Prior to the field trip, discuss possible sampling design options. One design is to monitor upstream and downstream of an activity that might alter water quality. For example, do they want to take water quality measurements upstream and downstream of a wastewater treatment facility? If so, contact the treatment operator and find out where there current monitoring sites are and schedule a visit. Another design is sampling one location over time. How would water quality change at a given site over the course of several hours. A third sampling design is to test specific inputs and the mainstream of a creek. Consider storm drain outflows and compare them to the main stream.
  2. Discuss safety precautions around flowing water.
  3. Discuss appropriate field trip behavior.
  4. At the field location, follow the water quality instructions and use the Water Quality Parameters Student Worksheets. Make sure students uniformly and accurately identify their locations.

After-the-Field-Trip Activity

Activity: Water Quality Field Report
Time: One class period
Materials: Data from field trip

  1. Discuss the results of the field trip: What did they learn about water quality?
  2. The students should write a quick field report. It should include the parts of a scientific report (per previous instruction) or based on accepted protocols—abstract, introduction, materials and methods, results, and discussion.
  3. Guide the reporting process as needed for students and assign completion for homework. Remind students that the discussion section is the appropriate place to discuss possible follow-up investigations.

Source

  • Clean Water Team (CWT) 2004. The Clean Water Team Guidance Compendium for Watershed Monitoring and Assessment, Version 2.0 Division of Water Quality, California State Water Resources Control Board (SWRCB), Sacramento CA.

Objectives

Students will:1. measure five parameters in water: temperature, dissolved oxygen, pH, turbidity, and conductivity; 2. design an experiment to evaluate these parameters in different water sources, or in different experimental conditions; 3. travel to a stream to monitor these parameters.

Grade Levels

7–12

Adult/Student Ratio

Normal class size

Location

Classroom wet laboratory (creeks must have safe public access). Water should be flowing but low enough for students to safely sample from streamside.

Skills

Analyzing, formulating hypotheses and questions, generalizing, graphing, predicting, researching, writing a report in scientific format

Key Words

Algal, Conductivity, Dissolved oxygen, Diurnal, pH, Temperature of water, Turbidity

Downloads [PDF]

Resources

For the Teacher

  • Clescerl, Lenore S. (Editor), Arnold E. Greenberg (Editor), Andrew D. Eaton (Editor), Standard Methods for Examination of Water and Wastewater, Hardcover, 20th edition, 1999, by American Public Health Association.
  • Wetzel, Robert G. and Gene E. Likens Springer, Limnological Analyses, 3rd edition, 2000.
  • Wetzel, Robert G., Limnology: Lake and River Ecosystems, 3rd edition, 2001, Academic Press.

    For the Student
    There are many citizen groups across the world that monitor stream health. Many of these groups have web sites. Several good places to visit on the Internet are:
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