Usually, to seep is to go slowly through a porous material. (example - Blood seeped through the bandage.) Usually, to leak is to go through a hole or a crack. (example - Water leaked from the cracked pipe.) New Oxford American Dictionary seep | sip | verb flow or leak slowly through porous material or small holes leak | lik | verb 1 accidentally lose or admit contents, especially liquid or gas, through a hole or crack

Seepage is a very very very slow leakage / process. Seepage - to leak over time