Right at the face of the dam where the pipe enters the dirt is gonna be pretty hard to seal off, if it was me I would try to go about ten twelve ft behind that and dig down around the pipe, try to get above but close to one of the seep collars, and compact really good clay along with generous amounts of bentonite and use a plate compactor to pack it really good all the way up past the water level at a minimum.
I have sealed several minor leaks off that way.
At that depth it is more likely to stay moist whereas at the surface it will dry and crack much worse.
Good Luck!
