Dealing with a manmade pond and a lot (real estate) next to it that's owned be someone else. The pond is about 85 feet across with a dam on one side. The dam is close to the other lot and water is at accumulating just outside the base of the base of the dam. The dam itself does not show any signs of leaking. The dam appears sound, intact and completely dry. My question is . . . How can I know if this water is leaking from my pond or if it's storm drainage water?
Might want to check with a friendly health inspection department. They have techniques for handling complaints about water possibly discharged from septic beds into neighbouring lands. Frequently this involves addition of a dye to water flushed through a toilet bowl and then watching to see if the dye appears at the discharge in question.
One case I was involved in had a discharge pipe in a public park that could have come from a neighbouring house directly uphill from the pipe. The health people came knocking on the door of the house and despite the homeowner's objections added a dye to the house toilet and flushed it. The discharge pipe continued to run clear and so the public was reassured that the water was not contaminated.
Clearly that was a different set of circumstances - the test volume of water was small compared to your pond and there were no fishes or plants to be concerned about. So the nature of the test dye would have to be considered carefully, and the time of testing would need to coincide with a low water level if such can be arranged. Hence the chat with the health people who will have seen many different cases. It sounds like you need a chemo test before and a chemo test after and compare results. A successful test, whatever the result, would lead to better neighbourly relations.
It sounds like an earth dam. The most important part is the size and construction of the spillway ; where water runs out during wet weather. If water over-tops an earth dam .it is finished. Aside from that , we have 3 earth dams in the community : 2 have a body of water at the bottom of the dam so one can not see any seepage. The third ( about 500 ft long) is often damp to wet at the bottom ; this is not unusual for small earth dams. Also "they" recently removed any trees from the dams as tree roots ( after they die) often provide a partial leak channel through the dam.