Which factor should be determined in the field as a basic water quality property that may change quickly and should be checked immediately after sampling?

• A. Temperature
• B. pH
• C. Temperature and pH
• D. Conductivity

Answer: (C)

Measuring water quality in the field focuses on properties that can shift quickly once a sample is removed from its source. Temperature and pH are two of these highly sensitive properties. Temperature can change rapidly due to exposure to ambient conditions, affecting many other water characteristics such as dissolved gas solubility and reaction rates. pH can swing quickly because gases like CO2 exchange with the atmosphere and because the carbonate system in the water is temperature-dependent; even short delays or exposure can alter the reading. Because both can drift in the short time after sampling, they should be determined in the field at the moment of collection to reflect the true in-situ state. Conductivity and other properties may be measured later with less immediate drift or after proper temperature compensation, so they aren’t as time-critical for on-site determination.