What should a water sample from a well be clearly marked with?

A water sample from a well should be clearly marked with the date and time it was taken. This information is crucial for several reasons. First, water quality can change over time due to various factors such as environmental conditions, contamination, and seasonal variations. By noting the date and time, it allows for a more accurate assessment of the sample's relevance and context. This is particularly important for monitoring purposes, compliance with regulations, and ensuring that proper assessments are made in relation to changing conditions or potential contamination events.

The other options, while relevant to various aspects of well data collection, do not carry the same level of importance in terms of ensuring the validity and interpretability of a water sample. The type of pump used, location of the contractor, and depth of the well can provide useful information, but they do not directly impact the assessment or understanding of the water’s quality and safety at the time the sample was collected.