Freshman Engineering Clinic

Dr. Joseph Orlins, P.E.

Civil and Environmental Engineering

Fall 2000

Water Quality in Lakes

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Background

Water quality in rivers, lakes, and the oceans is affected by the concentrations of dissolved gasses in the water. For example, if there is too little dissolved oxygen in a river, then aquatic life such as fish can die, and the river may have odor problems. At the other end of the spectrum, too much dissolved nitrogen can also be harmful to organisms such as fish. Thus, there is a range of dissolved gas concentrations that define an acceptable level of water quality. In general, dissolved oxygen concentrations in the range of 6 to 12 mg/L are adequate for fish to survive.

The amount of gas that can be “in solution” (dissolved in the water) is a function of temperature and the composition of the water (i.e., what impurities are present). At higher temperatures, less gas can remain in solution. Another way of stating this is that the “saturation concentration” decreases with increasing temperature. This is one reason that local lakes suffer “fish kills” during the summer: as the temperature increases, the maximum amount of dissolved oxygen that can be in the water decreases. In addition, naturally occurring bacteria and other organisms in the water and at the bottom of the lake use up oxygen. If the remaining dissolved gas concentration falls too low, fish may die.

In addition, gas transfer is used in many water treatment processes. The transfer of oxygen to liquid systems is particularly important in oxidation of iron and manganese in water treatment (i.e. for drinking water), and in the biological treatment of wastewaters (i.e. at sewage treatment plants).

How do these gasses get IN to the water (or out of it)?

The basic processes are simple: In natural systems such as rivers, lakes, and oceans, atmospheric gasses (which are mainly nitrogen and oxygen) dissolve into the water across the water surface. Waterfalls, breaking waves, and air bubbles entrained in the flow all contribute to the gas transfer. In engineered systems (such as water and wastewater treatment), gasses are introduced to the water by “aerators,” which are mechanical devices such as mixers, fountains, diffusers, and spargers (bubblers).

The direction and rate of the gas transfer depends on the amount of gas already dissolved in the water, the limiting (or saturation) concentration, the surface area for gas exchange, and the turbulence of the system.

In this lab, students will be exposed to the fundamentals of water quality in natural water bodies by taking measurements of a few water quality parameters in Rowan Pond.

Water Quality Parameters in Lakes

We have already seen that dissolved oxygen concentration is an important water quality parameter. Other chemical and physical measures of importance are the water temperature itself, the clarity, the pH (acidity), amount of dissolved minerals, and algae concentration.

In today’s lab, we will look at three indicators of water quality in Rowan Pond: water clarity, temperature, and dissolved oxygen concentration.

Each of these parameters can vary over the seasons and from place to place in a given lake. The temperature and dissolved oxygen concentration may also vary with depth, depending on the size of the lake and the time of year.

Water Clarity

One method of measuring water clarity is with a simple device called a “secchi disk.” A secchi disk is a flat plate, painted contrasting colors, which is lowered in to the water. The depth at which the disk is no longer visible is called the ‘secchi depth,’ and is a measure of the water clarity. In very clean, clear lakes (such as Crater Lake in Oregon and Lake Superior), the secchi depth can be as high as 120 feet; these lakes are considered “oligotrophic.” In “eutrophic” lakes (polluted with too much algae and other living matter), the secchi depth may be less than a foot.

Dissolved Oxygen

Dissolved oxygen (DO) concentration can be measured in lakes and streams using a portable DO meter. We will use the meters from the Environmental Engineering lab for our measurements of Rowan Pond. The meters indicate the concentration of dissolved oxygen in mg/L, as well as in units of “percent of saturation.” In addition, they measure the water temperature.

There are three different types of DO meters that will be used in this lab:

YSI Model 58: A basic field unit for measuring DO concentration and/or temperature, depending on which parameter is selected via a panel-mounted switch. The switch can be set to read dissolved oxygen concentration in mg/L or percent saturation, but not both at the same time.

YSI Model 5100: An advanced bench top (laboratory) instrument, which displays DO concentration in both mg/L and percent saturation simultaneously, along with temperature and local barometric pressure.

YSI Model 600XL Sonde: An advanced field probe, which can measure DO concentration, pH, conductivity, temperature, and depth. The probe can be connected to a laptop computer for interactive use, or connected to a data logger for unattended operation.

Temperature

The dissolved oxygen meters also measure temperature; so we can record both DO concentration and temperature from the same instrument.

The variation of temperature and dissolved oxygen concentration as a function of depth is shown for a deep lake for three different seasons below (reproduced from Water Quality: Characteristics, Modeling, Modification, by Tchobanoglous & Schroeder, 1987).

Field Measurement of Water Quality Parameters

Apparatus: Dissolved oxygen meters and probes; laptop computers; secchi disk; tape measures.

Procedures:

Water Clarity:

  1. Lower the secchi disk from the edge of the platform on the south side of Rowan Pond.
  2. Record the depth at which you can no longer see the disk, to the nearest tenth of a foot.
  3. Lower the secchi disk all the way to the bottom, and record the depth of the pond at your measurement location.
  4. Repeat steps 2 and 3 a number of times (so that each member of your group takes at least one set of readings)
  5. Average the results from each of your measurements.

Dissolved Oxygen & Temperature:

  1. Lower the DO probe into the water.
  2. Keep the probe tip immersed just below the water surface for about a minute.
  3. Record the DO concentration (in both mg/L and % of Saturation), and the water temperature.
  4. Lower the probe 0.5 feet; allow the reading to stabilize for about a minute.
  5. Again, record the DO concentration and water temperature.
  6. Repeat steps 4 and 5, taking readings of DO and Temperature at 0.5-foot depth intervals, from the surface to 0.5 feet from the bottom. DO NOT lower the probe so that it touches the bottom!

Results:

Record and report your results in a tabular form as shown below. Use this format in your lab book. Feel free to add additional information that you deem important. Sketches of your experimental setup are always useful and informative. Don’t be shy about using more than one or two pages of your lab book – you purchased it to be used, not saved!

Class: / Freshman Clinic I / Section:
Experiment: / Water Quality - CEE / Date:
Group Members:
Purpose:
Water Clarity:
Equipment used: (e.g., size of secchi disk)
Procedure:
Trial / Secchi Depth (feet) / Water Depth (feet)
1
2
3
4
5
Average:
Temperature & Dissolved Oxygen Concentration:
Equipment Used: (e.g., type of DO probe & meter, serial numbers, etc.)
Procedure:
Depth (ft) / Temp (C) / DO Concentration
(mg/L) / (% Saturation)
0.0
0.5
1.0
…
5.0
5.5

WHAT TO HAND IN:

TODAY: Turn in one set of carbon copies from your lab notebook, showing your raw data and your preliminary analysis. This should include:

  • Tables, like those shown on the example above
  • Plots of DO concentration and Temperature as a function of depth

NEXT WEEK: Work together in the same group as you worked with today to complete the following homework assignment.

CEE Water Quality Module Homework Assignment:

Remember: Use the Rowan Engineering Homework Format!

Measurements of water quality were taken in a shallow lake at two different times. The data are shown below.

Depth (ft) / Season 1 / Season 2
Temp (C) / DO (mg/L) / Temp (C) / DO (mg/L)
0.0 / 25.0 / 7.0 / 15.0 / 10.0
1.0 / 24.7 / 7.5 / 14.0 / 9.8
2.0 / 24.4 / 7.5 / 13.8 / 9.8
3.0 / 24.1 / 7.4 / 13.6 / 9.7
4.0 / 19.3 / 4.8 / 13.5 / 9.7
5.0 / 18.8 / 4.0 / 13.5 / 9.8
6.0 / 18.6 / 3.0 / 13.5 / 9.7
7.0 / 18.5 / 1.5 / 13.4 / 9.6
8.0 / 18.4 / 0.8 / 13.4 / 9.7
9.0 / 18.3 / 0.3 / 13.4 / 9.7
10.0 / 18.2 / 0.2 / 13.3 / 9.6
11.0 / 18.1 / 0.1 / 13.3 / 9.5
12.0 / 18.0 / 0.1 / 13.3 / 9.4

Answer the following questions:

  1. Plot the temperature and dissolved oxygen concentration for each season as a function of depth. Set up your graphs so that depth is plotted vertically downward, as shown in the figure on page 3.
  2. When were each of the profiles measured (e.g. winter, spring, summer, fall)?
  3. What were the objectives of this laboratory?
  4. Why is gas transfer important for water quality?
  5. What processes of wastewater treatment include some form of gas transfer?
  6. What are the factors affecting gas transfer?
  7. Do you think Rowan Pond is “healthy”? Why or why not?
  8. How could this lab module be improved?

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