We usually encounter ground water under you swimming pool in the spring, after heavy rain falls, if you have an underwater river, low lying lands, or if the ground is saturated from a long term pool leak. Before a new liner can be installed the ground water has to dry up. We have seen ground water running in as fast as water comes out the end of a garden hose. This is not all that common but we see enough of it to make mention of it.
There are a few options on what we can do if the groundwater does not stop. It is best to consult a professional for the options available if this is a problem you are experiencing.