Where should groundwater samples be collected relative to the water table?

Groundwater samples are ideally collected at the soil/water interface to ensure that the sample accurately represents the conditions present in the aquifer. This location is critical because it allows for the collection of water that is in direct contact with the sediment and soil matrix, where interactions with potential contaminants occur.

When sampling at the soil/water interface, the sample reflects the true groundwater quality, as it is less likely to be influenced by surface runoff or other external factors that may alter water quality if samples were taken from the surface or from deeper within the aquifer. Additionally, sampling at this interface ensures that the water being tested has not been subjected to disturbance that can occur while attempting to sample from the top of the water table or deeper within the aquifer.

This method is standard practice for environmental assessments related to USTs, as it provides a more comprehensive understanding of groundwater contamination risks in the vicinity of such storage tanks.