Which geophysical method is NOT used to identify and investigate ground water pollution?

The assertion that all of the mentioned geophysical methods are used to identify and investigate groundwater pollution is incorrect. Geomagnetics, electromagnetics, and electrical resistivity are indeed commonly utilized techniques in hydrogeological investigations, especially in detecting and mapping groundwater contamination.

Geomagnetics measures variations in the Earth's magnetic field. It is not typically used to directly assess groundwater pollution. Instead, it is more suited for geological mapping and can indicate the presence of certain types of geological formations but does not specifically relate to detecting contaminants in water.

Electromagnetics and electrical resistivity, on the other hand, are practical tools for groundwater pollution investigations. Electromagnetic methods can identify different subsurface materials based on their electrical conductivity, which may change due to contamination. Similarly, electrical resistivity techniques are designed to differentiate between contaminated and uncontaminated water based on their differing resistive properties.

Thus, the correct interpretation is that geomagnetics is the method that is not employed for identifying and investigating groundwater pollution, making the assertion about all methods being applicable incorrect.