Science of Water : Concepts and Applications

Environmental Biomonitoring, Sampling, and Testing 261
Example 8.3
Problem:
Using the data provided below, determine the BOD 5 :
BOD 5 of Seed Material 90 mg/L
Dilution no. 1 mL of seed material 3 mL
mL of sample 100 mL
Start DO 7.6 mg/L
Final DO 2.7 mg/L
BOD (Seeded)
90 mg/L 3 mL
Seed Correction
0.90 mg/L
300 mL
5
[(7.6 mg/L2.7 mg/L) 0.90] 300
12
mg/L
100 mL
Temperature Measurement
As mentioned, an ideal water supply should have, at all times, an almost constant temperature or
one with minimum variation. Knowing the temperature of the water supply is important because
the rates of biological and chemical processes depend on it.
Temperature affects the oxygen content of the water (oxygen levels become lower as temperature
increases); the rate of photosynthesis by aquatic plants; the metabolic rates of aquatic organisms;
and the sensitivity of organisms to toxic wastes, parasites, and diseases.
Causes of temperature change include weather, removal of shading streambank vegetation,
impoundments (a body of water confi ned by a barrier, such as a dam), and discharge of cooling
water, urban stormwater, and groundwater infl ows to the stream.
Sampling and Equipment Considerations
Temperature, for example, in a stream, varies with width and depth, and the temperature of wellsunned
portions of a stream can be signifi cantly higher than the shaded portion of the water on a
sunny day. In a small stream, the temperature will be relatively constant as long as the stream is uniformly
in sun or shade. In a large stream, temperature can vary considerably with width and depth,
regardless of shade. If safe to do so, temperature measurements should be collected at varying
depths and across the surface of the stream to obtain vertical and horizontal temperature profi les.
This can be done at each site at least once to determine the necessity of collecting a profi le during
each sampling visit. Temperature should be measured at the same place every time.
Temperature is measured in the stream with a thermometer or a meter. Alcohol-fi lled thermometers
are preferred over mercury-fi lled ones ones because they are less hazardous if broken.
Armored thermometers for fi eld use can withstand more abuse than unprotected glass thermometers
and are worth the additional expense. Meters for other tests [such as pH (acidity) or DO] also
measure temperature and can be used instead of a thermometer.
Hardness Measurement
As mentioned, hardness refers primarily to the amount of calcium and magnesium in water. Calcium
and magnesium enter water mainly by the leaching of rocks. Calcium is an important component