Which of the following is NOT a source from which field samples can be obtained?

The reason pipelines are not considered a source for obtaining field samples in the context of aggregate base testing is that they are typically used for transporting liquids, gases, or sometimes very fine particulate matter, rather than solid aggregate materials like crushed stone or gravel. Field samples are generally taken from physical sources that contain solid aggregates, which can be effectively sampled from transport units, conveyor belts, and stockpiles.

Transport units refer to vehicles or containers that are used to carry aggregates to a construction site, making them a direct source for sampling. Conveyor belts are systems used in the movement of aggregates on site, and they provide a continuous flow of material from which samples can easily be taken. Stockpiles, which are accumulations of aggregate materials, also serve as a primary location for sampling since they contain the finished or partially processed materials ready for use.

In contrast, while pipelines might transport aggregates in slurry form or similar mixtures, they do not typically provide the solid materials needed for a representative field sample. Thus, pipelines fundamentally differ from the other options, establishing them as an unsuitable source for sampling in the context of aggregate base testing.