Performance Task:
Dissolved Oxygen Monitoring

James Good
5 Rousseau Street
Winslow, Maine 04901
Tel. (207)-872-8521
jrgood@mint.net
jrgood51@hotmail.com

Acknowledgement: Permission to use the "Sewer Lice" exercise was made available through Flinn Scientific Inc.  Equipment and information was donated from the various sources, Kennebec Sewer District, Waterville, Maine, YSI Inc., Yellow Springs, Ohio, S.D. Warren/Sappi, Skowhegan, Maine, and Acheron Labs Newport, Maine.

Abstract:  As a group of science students you are concerned about water pollution.  You have joined the Messalonskee Lake Association.  Your primary task is to measure and record dissolved oxygen (D.O.) data, at an assigned water body.  You may also be required to take other supporting measurements and observations, as assigned.  Your data will be compiled with similar work in surrounding communities.  The data will be evaluated at the Maine State Department of Environmental Protection (MeDEP), to determine any changes in the water quality, in your area of the state.

Grade Level Range: 7 - 8

Time Required: 1 - 2 weeks or as long as you want to do the study.

Groups of Students: 3 - 4 students per group, for the field study.

Learning Result Indicators: Scientific Report ? English Language Arts (H1), Science & Technology (B3, K6, M4, and M7).  Calibration and use of dissolved oxygen meter ? Science and Technology (J1).  Graphs and data tables ? Science and Technology (L4 and L6), Math (K2).

Materials:  YSI dissolved oxygen meter model 54A, dissolved oxygen probe model 5739 / 5740, thermometer °C, barometric pressure, and field data sheets.

Lesson 1: Introduce the "Sewer Lice" exercise from Flinn Scientific, Inc.  This demonstration should help to support discussion around water pollution, the scientific method, the importance of careful observation, and the idea that gases are soluble in water.  Discuss the sources, effects and possible solutions, to stopping and reducing pollution, due to sewage, thermal effluent, chemicals, and bacteria.
Lesson 2: Allow the students to observe the dissolved oxygen (D.O.) meter and D.O. probe.  Place the students into groups of 3 or 4, and have them read the abbreviated owner manual on how to calibrate the meter.  Allow the students to tell you how to calibrate the meter, and have a few students actually calibrate the meter.  The barometric pressure for this area is available through an automated system at the LaFleur Airport in Waterville, by calling 877-0519.  Demonstrate how warm and cold water effects the amount of dissolved oxygen in water.  Also demonstrate how shaking a container of water effects the amount of D.O. in water.  Ask the students how this action applies to naturally occurring events around the movement of water i.e. water falls, fast versus slow moving water, wind, and rain. Ask the students how CO2 is put into drinks and why soda will go flat if left open and is warmed.
Lesson 3: Determine how groups of students will obtain D.O. readings at a water body, based on your schoolís schedule.  Add any additional data you might want to include in the study.  For example: pH, transparency, turbidity or clarity, and the presence of flora and fauna.  Students can also make a site map and should always take the D.O. reading at the same point from day to day.  Also note the time and weather conditions at which D.O. readings were taken.  Other observation that students can make concerning the water quality are signs of algae blooms, clumps of stringy algae called filamentous, odor, pollen, oily sheen, dead fish, and foam caused by soaps or detergents.
    Students should plot their D.O. and temperature readings against time.  Ask the students if any apparent changes in D.O. or temperature have taken place over time.  Discuss with the students why or why not these changes have occurred.  Also have the students compile a data table.
    The students should write a scientific report that includes the following:  1) ideas that reflect an understanding that the amount of D.O. in water is dependent on temperature and barometric pressure 2) analyze the data and include the graph and data table 3) that temperature, barometric pressure and dissolved oxygen change overtime depending on conditions in the environment 4) include the effects that various types of pollution might have on the amount of D.O. present in water, based on classroom discussions and this field study.

 Abbreviated YSI Model 54A Operating Instructions

1. Preparing the Instrument
A. Place the instrument in the intended operating position vertical, tilted, or on its back.  Otherwise readjustment may be necessary if the instrument operating position is changed.
B. Switch the instrument to off and adjust meter pointer to Zero with the screw in the center of the meter panel, if necessary.
C. Switch to RED LINE and adjust the RED LINE knob until the meter needle aligns with the red mark at the 31°C position.
D. Switch to Zero and adjust to zero with zero control knob.
E. Attach the prepared probe to the PROBE connector of the instrument and adjust the retaining ring finger tight.

2. Air Calibration ? Fresh Water
A. Carefully remove the plastic cover and sponge from the end of the D.O. probe.   Shake all droplets of water off the membrane if present.  Gently dab membrane with kimwipe or kleenex, if necessary.  Then replace the plastic protective cover with the damp sponge positioned at the bottom of the plastic protective cover.
B. Wait 10 to 15 min for probe to polarize.  Again, probe should have itís protective plastic cover on, containing a damp sponge, located at the end of the probe.  Repolarize whenever the instrument has been OFF or the probe has been disconnected.
C. Switch to Zero and adjust to "0" on PPM scale.
D. Switch to TEMP and read on °C scale.
E. Use probe temperature and true local atmospheric pressure (or feet above sea level) to determine calibration values from Tables 1 and 2. The barometric pressure for this area is available through an automated system at the LaFleur Airport in Waterville, by calling 877-0519.
EXAMPLE: Probe temp = 21°C, Altitude = 1000 ft.
From Table 1 the calibration value for 21°C is 9.0 PPM.
From Table 2 the altitude factor for 1000 ft is about 0.96
The correct calibration value is 9.0 PPM x 0.96 factor = 8.64 PPM
F. Switch to 0-10 or 0-20 PPM dissolved oxygen (D.O.) range and adjust meter with CAL control knob until the meter reads the correct D.O. value.  Wait two minutes to verify calibration stability.  NOTE: It is desirable to calibrate probe in high humidity environment.  For this reason it is important to calibrate the probe with itís protective plastic cover and damp sponge in place, but not touching the probeís sensitive membrane.
G. Note: The determined D.O. value based on the temperature and pressure of step E. can also be determined from The Algorithym which combines temperature, salinity and pressure reading into one chart to look-up D.O. in mg/l, uM, or ml/l.
H. The meter is now ready for use.
I. Similar instructions and Tables 1 & 2 can be found in the ownerís manual and on the back of the D.O. meter.
 3.   Measurement
A. Place probe in a sample and stir.
B. Allow sufficient time for probe to stabilize to sample temperature and dissolve oxygen.
C. Read dissolved oxygen on appropriate range (0-10 or 0-20 PPM).
D. The instrument should be left on between measurements to avoid the necessity to repolarize the probe.
4.   General Care
A. Recharge batteries every 100 hours of operation.  Recharge 16-20 hours.  Replace batteries after 3 years.  Use Burgess CD-6 or equal.  For example Panasonic AA rechargeable sealed nickel-cadmium battery (P-50AA, 1.2V), do not use regular AA batteries.
B. Replace membrane every 2-4 weeks.  Probe should be stored in humid environment to prevent drying out.  Keep itís wet sponge and plastic shield in place when not in use.

Source:  Instructions taken from YSI Manual
Table 1 shows the amount of oxygen in PPM that is dissolved in air saturated fresh water at sea level (760 mmHg atmospheric pressure) as temperature varies from 0° to 45°C.

Table 1 ó Solubility of Oxygen in Fresh Water
Temperature °C
PPM Dissolved Oxygen
Temperature °C
PPM Dissolved Oxygen
0
14.6
23
8.7
1
14.2
24
8.5
2
13.9
25
8.4
3
13.5
26
8.2
4
13.2
27
8.1
5
12.8
28
7.9
6
12.5
29
7.8
7
12.2
30
7.7
8
11.9
31
7.5
9
11.6
32
7.4
10
11.3
33
7.3
11
11.1
34
7.2
12
10.8
35
7.1
13
10.6
36
7.0
14
10.4
37
6.8
15
10.2
38
6.7
16
9.9
39
6.6
17
9.7
40
6.5
18
9.5
41
6.4
19
9.3
42
6.3
20
9.2
43
6.2
21
9.0
44
6.1
22
8.8
45
6.0

Source: Derived from "Standard Methods for the Examination of Water and Wastewater."

 Table 2 shows the correction factors used to correct the calibration value for the effect of atmospheric pressure or altitude.  Find true atmospheric pressure in the left hand column and read across to the right hand column to determine the correction factor.  (Note that "true" atmospheric pressure is as read on a barometer.  Weather Bureau reporting of atmospheric pressure is corrected to sea level.)  The barometric pressure for this area is available through an automated system at the LaFleur Airport in Waterville, by calling 877-0519.  If atmospheric pressure is unknown, the local altitude may be substituted.  Select the altitude in the center column and read across to the right hand column for the correction factor.

Table 2 ó Correction for Atmospheric Pressure
Atmospheric Pressure
or
mmHg
Equivalent
=
Ft
Correction
Factor
775 540 1.02
760 0 1.00
745 542 0.98
730 1094 0.96
714 1688 0.94
699 2274 0.92
684 2864 0.90
669 3466 0.88
654 4082 0.86
638 4756 0.84
623 5403 0.82
608 6065 0.80
593 6744 0.78
578 7440 0.76
562 8204 0.74
547 8939 0.72
532 9694 0.70
517 10472 0.68
502 11273 0.66
Source: Derived from "Standard Methods for the Examination of Water and Wastewater."

Factors Affecting Measurement of D.O. Using a Membrane Electrode

Solubility
When the partial pressure of oxygen in water equals the partial pressure in air the water is considered saturated with oxygen.  In this saturated state the amount of oxygen in the water is called the oxygen solubility.  Oxygen solubility varies with

The maximum amount of oxygen water can hold when at equilibrium is considered 100% saturated.

Temperature Effects on Solubility
The solubility of all gases decreases as temperature increases.  For example observe a pot of water being heated on a stove.  Before the water boils you can observe bubbles on the side of the pot forming.  This is gases that can no longer stay dissolved at higher temperature.  Gases can not be dissolved in boiling water (not even water vapor).  In this case we are talking about the concentration, mg/L of oxygen, not percent saturation.

Salinity Effects on Solubility
Freshwater can hold more oxygen than saltwater.  The dissolved salt forces dissolved gas out of water thereby lowering the solubility of water.  YSI sensors do not measure mg/L, per se, they measure partial pressure of oxygen.  So a known relationship between salinity and dissolved oxygen concentration allows for a correction for salinity.  See Appendix 1 for this correction table.  You wonít need this table unless your measuring saltwater.

 Dissolved Oxygen Measurement

Overview
Why Measure Dissolved Oxygen?
Water is necessary to all life on earth.  So it is very important to maintain the quality of water.  Most aquatic life depends upon oxygen dissolved in water for respiration, just as we breathe oxygen in the atmosphere containing oxygen.  If the quality of water is threaten by pollution that can reduce the ability of water to hold enough dissolved oxygen (D.O.) to sustain aquatic life.  Most species of fish need at least 4 mg/L of D.O. in water in order to survive.  Trout require at least 7 mg/L and catfish can survive down to 2 mg/L.  The quality of water in lakes and streams is rated according to what species of fish the water will support.  Think of an aquarium.  Why it is important to aerate the water?  Also temperature is important.  A "coldwater habitat" has D.O.s of at least 7 mg/L.  A "marginal warmwater habitat" has only 2 mg/L.

Diurnal cycle (daytime cycle)
Most us know that plants make oxygen, but this is true only during the daytime.  During the evening plants such as algae, phytoplankton, and aquatic plants respire, taking oxygen from the surrounding water.
    In lakes and streams the D.O. peaks in the afternoon, and falls at night with a low just before sunrise.  To measure D.O. accurately, one must sample in the daylight hours just before sunrise.  The diurnal cycle is changes the most when there is a lot of plant life (eutrophic).
    If  the growth of plant life such as algae is encouraged by pollution (e.g., nitrates, and phosphates in fertilizers and detergents) algae blooms may result.  At night this over abundance of algae respire until the D.O. is near zero and some fish may die of asphyxiation.  Can you name lakes in Central Maine that have algae blooms?

Microorganisms and Macroinvertebrates
Microorganisms, some invertebrates, and macroinvertebrates, in addition to fish, consume oxygen as they metabolize organic material.  Organic material is naturally supplied to the aquatic ecosystem by the decaying process of dead plants, animals, and microorganisms.
    Wastewater that is untreated or poorly treated can over load an ecosystem with organic material (waste).  This organic matter is food for microorganism which reproduce in large numbers.  While metabolizing the food the microorganisms consume oxygen.  If the discharge of waste is from a wastewater treatment plant the D.O. drop can stretch downstream for some distance.  If the D.O. drop is low enough fish die.  When the organic material is decomposed the D.O. will rise to normal concentration, but the damage had been done.
Biochemical Oxygen Demand (BOD)
    BOD is an indirect measure of organic waste material present in water.  In the USA municipal and industrial wastewater treatment plants are regulated by The National Pollution Discharge Elimination System (NPDES).  Permits are granted by state environmental protection agencies (EPA) that are based on the results of BOD testing and reporting.
    The BOD test method is described in Standard Methods for the Examination of Water and Wastewater.  The test follows the steps below.
    A sample is diluted to varying strengths and each BOD bottleís D.O. is measured at 20°C.  The diluted samples are sealed in airtight BOD bottles and incubated at 20°C in the dark for five days.  The bottles are removed after 5 days and each bottleís D.O. is measured again.  The before and after D.O. measurements and dilutions determine the sampleís BOD.
    NPDES permits require the monitoring and reporting of the following most common wastewater discharges parameters.

Stormwater discharge
Stormwater discharge is categorized as non-point source (NPS) pollution.  NPS pollution degrades water quality more than point source pollution in streams, rivers, and lakes.  The major contributor is agriculture (fertilizer, and animal waste).  Others are construction sites, logging, and mining operations, and urban runoff.  Runoff can contain many forms of chemicals and silt that harm streams, rivers, and lakes.  The USA Environmental Protection Agency (USEPA) is beginning to control NSP pollution through the NPDES permit system.  Some stormwater permits require the following parameters to be sampled and recorded.
This curiculum project was funded by the Colby Partnership for Science Education, the Howard Hughes Medical Institute,and the Bell AtlanticFoundation.