Introduction

Occasionally, scientists perform water quality measurements just to learn about how mass and energy are transported and transformed in aquatic environments. Most of the time, however, these measurements are conducted to prevent problems from occurring (routine monitoring), assess how well management strategies are working, and identify problems that have appeared.

An interesting aspect of this water quality measurement issue is that a problem may clearly exist while the cause of the problem is not known. The measurements are then used to try to solve a puzzle: what has happened in the water to lead to the problem?
 

For example, you may find dead fish floating all over a lake or piled on the shore. What has killed the fish? A disease? A toxic substance? Lack of oxygen? When an event such as this occurs, scientists and state officials immediately begin a battery of tests to identify the cause of the fish kill. The tests can include measurements of dissolved oxygen, organic content of the water, bacteria and viruses, nutrient content, sediment content, and toxic substances to name just a few. A fish kill is just one type of problem that can occur. In the following sections, we will examine a number of water quality variables that are routinely tested and the types of problems and issues with which they are concerned.

Dissolved Oxygen
 

Most organisms, including aquatic organisms, require oxygen to live. Oxygen that occurs in water is called "dissolved oxygen" (DO). Although chemical tests can (and are) used to measure the DO content of water, the most common method is a DO meter such as the one at right. The DO sensor, attached to the cord, creates an electric current. The greater the DO content, the greater will be the size of the current. The meter measures the size of the current, converts it to meaningful units, and displays it to the user. The units that we use for DO are "milligrams of oxygen per liter of water", or mg/l. You will also see these numbers reported as "parts per million", or ppm, since a liter of water weighs roughly 1 million milligrams. The meter at right is reading 7.13 mg/l  and showing a temperature of 23.9 C.  Water can only hold so much oxygen. It is then said to be "saturated." Most meters have a switch that allows you to display percent saturation as well. A reading of 0% saturation corresponds to 0 mg/l. A reading of 100% saturation means that the water is at maximum holding capacity. The mg/l value at 100% saturation varies with water temperature: the warmer the water, the lower will be the saturation value. At most times of the year, fresh water is saturated at about 8 mg/l.  Typically, the oxygen content of water must be greater than a certain minimum for organisms to be healthy (this minimum varies from organism to organism).

The DO content of a body of water can vary both seasonally and daily (diurnally). Perhaps the most well known regional example of this is the 7000 square mile hypoxic zone located between Louisiana and Texas. During the winter, DO concentrations are high enough to support life. During summer, however, they drop below the minimum required by most organisms. Although this is partly due to the circulation patterns of the Gulf of Mexico, the occurrence of the "hypoxic" (low oxygen) zone is largely due to the huge nutrient loads from the rivers in the region (more on nutrients later).

The DO concentrations can also fluctuate in a body of water during a 24 hour period. The graph to the right is typical of many catfish ponds and other water bodies in the region. Single celled plants, called phytoplankton, produce oxygen during the day due to photosynthesis. All organisms in the water, including the phytoplankton, consume oxygen at night. As a result we see this characteristic curve that can extend above saturation in sunlight (I've measured DO's of 200% saturation during the afternoon) and drop to near zero by dawn. The size of the oxygen swings depend upon many factors. A body of water having too great a concentration of organic molecules and/or nutrients can lead to huge swings and can cause major stress to organisms and to environmental quality in general.

Biochemical Oxygen Demand (BOD)
 

Organic material is material that has been manufactured (synthesized) by a living creature. Most organic molecules are made up primarily of carbon and oxygen, although many other types of atoms can be included as well.  Only plants and certain bacteria can create organic molecules from inorganic constituents. We most commonly see this in photosynthesis. The top reaction at right can be written:  CO2 +  H2O --> CH2O + O2 where CH2O is a "model" carbohydrate (actual carbohydrate molecules are much larger but occur, more or less, as multiples of the model). When an organic molecule (such as our model carbohydrate) is consumed by

aerobic (oxygen breathing) bacteria, the reaction is reversed:

 CH₂O + O₂ --> CO₂ +  H₂O

When the organic molecule is digested by the bacteria, oxygen is used and inorganic molecules (CO₂ and  H₂O in this case) are produced. So, what does this have to do with environmental measurements and tests?
 

Say there was a town that was dumping raw sewage directly into a body of water (this was common in the U.S. 30 years ago), or a paper mill pouring waste into a river (this still happens occasionally), or a swine waste lagoon that overflowed its banks (this happened in North Carolina just last year). You would expect that all of the organic material going into the water would stimulate a large bacteria population to begin digesting it. This in turn could lead to oxygen depletion of the water. How quickly would the waste result in loss of oxygen and how much easily digested material was added? Well, there's a test for that. It's called biochemical oxygen demand (BOD).

We use the DO meter from section 1 to measure the tendency of bacteria in a water sample to "use up" oxygen. We take a sample of water and measure the DO concentration. Say it's 8 mg/l. We then hold the sample at 68 F for a period of time (most commonly 5 days; BOD5) and then measure the DO again. Say the second measurement, after 5 days, is 2 mg/l. The difference is 8 - 2 = 6 mg/l. If the sample was undiluted (all of the sample water from the source being measured), the the BOD5 was 6 mg/l.

Recall that saturated water has a DO in the neighborhood of 8 mg/l. Now look at the table to the right. How in the world were these BOD's measured? Answer: using dilution. BOD bottles are usually about 300 ml in volume. Say we took 30 ml of the sample to be measured and mixed it with 270 ml of buffered distilled water (BOD = 0 mg/l; buffered to prevent the pH from changing too much). The total sample would then be 10% original sample (30/300 = 0.10). We would call this a "1 in 10" dilution. Now, let's say that our initial DO measurement was 7.5 mg/l and our measurement after 5 days was 2.25 mg/l. The difference is 5.25 mg/l (7.5 - 2.25). We then account for the dilution by dividing by the fraction that was sample to be measured: BOD5 = 5.25 / 0.10 = 52.5 mg/l . For water having huge BOD's, dilutions of 1:100 (1 %) are commonly used.

Typical BOD5's Swine wastewater 400 mg/l Paper mill wastes 300 mg/l Domestic wastewater 200 mg/l River in poor condition > 20 mg/l River in good condition < 5 mg/l Drinking water 0 mg/l

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