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

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

Answer: (C)

The key idea is that some basic water quality properties can change almost immediately after a sample is collected, so measuring those that are most sensitive right in the field ensures the reading reflects the water’s condition at collection. Temperature changes quickly once the sample is out of the body of water because it equilibrates with the surrounding environment, and many other measurements depend on it. pH is highly sensitive to dissolved CO2 and to gas exchange with air; as soon as the sample is exposed to air, pH can drift, so capturing it immediately in the field provides the true in-situ value. Conductivity can be read later with temperature corrections, but the most time-critical measurements are temperature and pH, which is why they’re determined right away.