Because changes in such basic water quality properties may occur so quickly, what should be determined in the field immediately after collecting the sample?

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

Answer: (C)

The important idea here is that some basic water quality properties can shift quickly once a sample is taken, so measuring the ones most prone to rapid change in the field helps keep the result accurate. Temperature and pH are both highly sensitive to handling and storage conditions. If the sample sits in a bag or bottle and warms or cools, the pH can drift because the carbonate system and CO2 exchange with air are temperature-dependent. You also need the actual sample temperature because pH readings are temperature dependent and most meters use that temperature to apply corrections or to interpret the reading correctly. Measuring both right away gives you the true in-sample values and allows proper interpretation later. Conductivity can change too, but the key properties that drift the most immediately and require in-field measurement are temperature and pH.