Midrasch zu Wajikra 15:16

(Fol. 14a) Our Rabbis taught that: "The sea which Solomon made was as large as one hundred and fifty Mikvaoth." Now, let us see what the size of a Mikva is? Forty Seali, as we are taught. Then shall he bathe his whole body in water (Lev. 15, 16), i.e., In water of a Mikva; His whole body (Ib.), i.e., so much water shall be used that it shall cover the whole body. How much does that require? A square cubic by three cubics in height. And the Rabbis have estimated the quantity of the water in a Mikva to be forty Seah.

1) (Vayikra 15:16) ("And a man, if there issue from him semen, then he shall bathe all of his flesh with water, and he shall be unclean until the evening.") "And a man": excluding a minor. I might think to exclude, then, a minor of nine years and one day, (who may have viable semen); it is, therefore, written "and a man" (to include the foregoing).

2) "if there issue from him semen": (He does not become unclean) until his semen comes outside his flesh. From here they ruled: One who was eating terumah and felt the onset of a zav discharge holds his organ (to prevent it from issuing forth) and swallows the terumah.