I agree with Theo, you might have a "channel" along the pipe created by the pipe moving in the fill clay. A collar 10-15' into the dam would probably stop the water from working its way along the pipe.

If the gap of the channel is very small, then the swelling bentonite MAY work. If the gap is too large, then the bentonite may just wash out.

Is the pipe just a lateral with a slight slope? How far underground is your pipe? Those parameters will probably change the advice if you have to go with an "invasive" repair.