aerator drainage problem

I have an aerator system where the drainage line is about 500 feet and eventually feeds into a stream. The first 100 feet are on a gradual downhill plane from the house, then it straightens out for a hundred or so feet, then more downhill to the stream. Since spring thaw, I have seen pools of water at the bottom of where the first level part is. The ground is very moist back up the hill towards the house for about 20 feet, then it is dry. I suspect a clog or broken pipe, but don't have a clue as to where to start digging. Not sure to start at where I first see wetness, back up the hill where it's dry or down the hill.
Also, at this time there is no backup at the last point of the aerator.
Any ideas would be appreciated.

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