If the water is clear enough, it would be easiest to turn on the water pump, and "feed" a frozen lump of milk to see where it's drawn in.
Otherwise, suggest you borrow a saltwater fisherman's gaff, and drag the area with the shortest "run" to the water pump. (That is, if a garden rake didn't work due to rocks crowding the bottom).
Alternatively, the foot valve would be bronze, and an underwater metal detector should find it.
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