What are ways to track underground water?
Study of local geology, geophysics methods, chemical tracers
Most of the groundwater is hosted in specific geological structures called aquifers which are composed by a permeable layer of rock (such as sand or fractured limestone) overlying an impermeable one (such as clay).
The study of the local geology is a powerful tool to understand where an aquifer can be.
Several geophysical methods are also available; the most common are based on changes in electric conductivity of the soil due to the presence of water.
In some specific situations it is possible to track the water flow injecting some chemical (and harmless) tracers that can be then collected from the springs fed by the water identifying possible underground paths and estimating the velocity of the flow based on the arrival time of the tracers at the different points.