What type of sample should always be collected to determine the presence of coliform bacteria?

To determine the presence of coliform bacteria, a grab sample is the most appropriate choice. A grab sample is a single sample taken at a specific point in time, providing an immediate snapshot of the water quality. This is crucial for coliform testing because coliform bacteria can indicate the potential presence of harmful pathogens and can vary throughout time and space due to factors like environmental changes or contamination events.

In practice, collecting a grab sample allows for quick analysis and rapid response to potential water quality issues, which is key in ensuring public health, particularly in drinking water systems. The immediacy of a grab sample is vital for timely detection of contamination and subsequent action.

Other sampling methods, such as composite or continuous samples, may aggregate information over time, which might obscure shorter-term variations in coliform levels. Since coliform presence can fluctuate due to various environmental conditions, relying solely on these methods may lead to delays in identifying issues and responding appropriately.