Like people have said it is hard to tell from a distance and it is likely to be the plastic top tank. They get brittle and crack (usually length ways). Here is some good news though. If you can be certain that it is the top tank they can be replaced without buying a whole new radiator. Or it may be the seal between the tank and the radiator, even cheaper. I have had some success with repairing the plastic cracks by emptying the coolant cleaning the %&#^ out of the area, sanding and then using a 2 part epoxy resin to glue a patch over the top.... I have considered fiber glass also but not tried it......
If all else fails, use a can of black pepper to fill the hole. Pour it into the rad, as she's warming up, close her up and let it idle. Works great on copper and brass rads, and I'm pretty sure its non-reactive with aluminum, since aluminum pans and pots withstood pepper for decades (AKA WearEver).
It takes a lot to make me sneeze. I eat onions and garlic raw and when we cook I'm in charge of grinding the pepper.
Fine saw dust will also work. I have used egg whites in demo cars(no I never demo'd a wagon, only a rotted out 68 Impala 4 door, 85 Delta, 77 Imperial, 78 Granada and a 85 Honda Accord) Not sure about tryin egg whites in a daily driver, might plug stuff up, but then again, you have the pepper added, just wait for some to float under the filler neck and stab it with a fork and have some breakfast.
The thing is... Murphy's Law- If you try stopping up the leak it, most likely, will work great. Until... You are in the middle of nowhere at night, in pouring rain without cell phone signal. At this point the #@%$!&^^ radiator will catastrophically fail... (yes, it happened to me )
