I am a geologist, but not an expert in unconsolidated sediments.

My analysis of the chance of connection to your foundation is solely based on the apparent presence of an adjacent aquifer of sufficient extent to keep a groundwater pool full of water for extended periods. If they put gravel outside your concrete pours, but THAT permeable gravel was surrounded by impermeable clay, then your sump pump would pull out all of that water in a few hours after the rains ceased.

Your foundation might not be hydraulically connected to the pond, but may be connected to a separate aquifer. (However, I speculate that a connection to your forest pool is more likely.)

Glacial tills are notoriously heterogeneous, since they are not initially deposited in moving water like most sediments. However, most glacial tills are subsequently re-worked by more typical surface factors after the glacier recedes. This process tends to form layers of sand and gravel that make excellent aquifers.

As to your dye question, it is easy to observe dye when you are drawing directly from the dyed source. For example, if you dyed a pond red, and someone stuck a 3/4" hose into the pond and pumped water into their fish tanks, they would immediately notice the color.

When you are drawing groundwater, you would be first drawing from all of the connected pores BEFORE you started drawing from your dyed forest pool! However, there are dyes that were created for that situation. Unfortunately, it has been over 30 years since I have pulled dyes to monitor groundwater wells that were supposed to be pulling pollutants from contaminated groundwater. I suspect the technology has improved since that time!

Here is a recent copy/paste for you.

Why use fluorescent dyes?

Groundwater tracing using fluorescent dyes is the best method to determine the connectivity of recharge and discharge points and allows for determination of direction and velocity as well. Dye tracing is a vital tool in understanding groundwater flow and the resulting information is used in a variety of ways to protect groundwater and surface water quality.

Fluorescent dyes make excellent tracers for a variety of reasons, including:

They are water soluble.
They are highly detectable (some below 10 parts per trillion).
They have extremely low toxicity ratings.
They are inexpensive.
They are fairly stable in natural waters.
They have been used successfully for tracing for over one hundred years.

I don't know if you will see dye at your sump, but I don't see any harm in trying. (After you determine the forest pool is in fact a groundwater pond.)

We had injection wells at the outside perimeter of the pollution plumes. I think in one case we had dye show up in a groundwater producing well more than 100 days after it was injected!