Which of the following water tests requires a chemical preservative to be added when the sample is taken from the distribution system?

Bacteriological testing is crucial for assessing the presence of harmful microorganisms in water supplies. When collecting samples for bacteriological analysis, it is essential to add a chemical preservative immediately to preserve the water sample. This is because bacteria can multiply or die off during transit to the lab, potentially skewing the results. Preservatives help maintain the sample's integrity until it can be analyzed, ensuring that the test accurately reflects the water quality at the time of sampling.

In contrast, tests for total chlorine residual, alkalinity, and pH typically do not require a chemical preservative. These parameters are often measured shortly after collection, and changes in concentration during transport and handling are less critical to the accuracy of the results compared to bacteriological analysis. Consequently, adding a preservative is specifically important for ensuring accurate bacteriological test results.