This lesson is designed to give you an overview of the different kinds of field and laboratory methods people use. The important idea to learn here is that each method has advantages and disadvantages. As you would expect, inexpensive and easy-to-use methods are usually not as accurate, while highly accurate methods often cost a lot. It is important to understand this tradeoff so that you can select a method that fits the goals of your water quality project. Details on how to use some of these methods will be given in a field seminar in the spring.
The three major types of methods are field test kits, calibrated instruments, and laboratory analysis.
Field test kits measure water quality in the field, and require very little training or equipment. The exact methods vary, but most involve adding tablets to a 5- to 10-milliliter water sample. The tablet contains a chemical that reacts with the water sample, causing it to change color after a short period of time. The concentration of the chemical you are measuring is then shown by the intensity of the color. For example, in the GREEN test kit dissolved oxygen is measured by adding two tablets to a sample. If the sample remains clear, there is very little dissolved oxygen. If it turns pinkish/orange dissolved oxygen is about 4 mg/l, and if it turns deep orange dissolved oxygen is above 8 mg/l.
These test kits are great teaching tools, but are too imprecise and inaccurate for detailed scientific studies. For example, the GREEN test kit can tell you if dissolved oxygen is above 8 mg/l, but cannot tell you if it is 8, 9, or 10 mg/l. Given that the State water quality standard for dissolved oxygen in Class AA waters is 9.5 mg/l, there is no way to use the kit to tell if a Class AA stream is meeting water quality standards. It's also difficult to get repeatable results, since different people will see different colors depending on their eyesight and the amount of light.
Field test kits are most often used for educational monitoring and as a quick way to identify gross water quality problems. They are not appropriate for studies designed to measure changes in water quality or to check if a water body is meeting water quality standards.
Calibrated instruments are portable battery-powered instruments with a probe that can be dropped into a stream to get a digital water quality reading. These instruments are relatively easy to use, and are moderately expensive. Once purchased they can be used over and over, and in the long run are cheaper than test kits if you are measuring a lot of samples. These types of instruments are available for only a few water quality parameters, including dissolved oxygen, pH, temperature, and conductivity.
These instruments are accurate if they are calibrated frequently in the laboratory. Calibration is done by checking the meter against a sample with a known concentration of what you are measuring (this kind of sample is known as a standard solution ). If the instrument reading is in error, the instrument is adjusted to match the correct value. Calibration should be done using a range of standard solution concentrations, to ensure that the instrument reads both low and high concentrations correctly.
A big advantage to many calibrated instruments is that they can be used directly in the stream or river, thus avoiding errors that occur when you handle samples. For instance, when you collect a dissolved oxygen sample you have to be very careful not to trap extra air bubbles in the sample bottle. With a dissolved oxygen probe you don't have this worry - you can simply drop the probe into the river and get a direct reading of dissolved oxygen.
Calibrated instruments are used in almost all scientific water quality studies to get field measurements of dissolved oxygen, pH, temperature, and conductivity. They are sufficiently accurate to measure changes in water quality and to identify if waters are meeting water quality standards. The main drawback is that good field instruments are not available for most chemical parameters, including coliform bacteria, nitrates, phosphates, metals, pesticides, herbicides, solvents, and petroleum byproducts. In scientific studies these must be measured in the laboratory.
There are many laboratory methods, but the one thing they all have in common is that they require lots of specialized training and equipment. Laboratory analysis is needed when you are studying water quality parameters that cannot be measured with field instruments, and when you need a high level of accuracy.
Most water quality scientists do not do their own laboratory analysis, and have to send their samples to special laboratories. A few methods such as titration for dissolved oxygen can be done in conventional high school laboratories. However, techniques for measuring metals and toxic chemicals use expensive equipment available only in specialized private, government, and university laboratories. Examples of this kind of technology include gas chromatography for measuring organic chemicals (solvents, petroleum products), and atomic absorption for measuring metals (copper, lead, iron).
For the field specialist the important thing to know about these methods is how the sample should be handled. Laboratory methods all have very specific ways in which the sample must be treated. Many require that the sample be put on ice and analyzed within a few hours. Some require that a chemical fixative be added to the sample immediately after it is collected. Others need special sampling bottles made from a specific type of material. In all cases it is important to avoid accidental contamination from your hands or other sources. Laboratories usually supply clean sample bottles, and will tell the field specialist how to handle the samples.
1. What kinds of studies use field test kits?
2. What method would you use to see if the Chehalis River is meeting water quality standards for dissolved oxygen?
3. Calibrated instruments often provide accurate measurements. Why would you ever use the more expensive laboratory methods?
4. What does the field water quality scientist need to know if he/she is using a laboratory to analyze water quality samples?
This water quality course material created by Rob Schanz. Send comments to Rob Schanz
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