Yoma, Chapter Five, Mishnah Six
This mishnah continues to discuss the sprinkling of the blood on the altar and then teaches what is to be done with the leftover blood.
1) Then he sprinkled the top of the altar seven times.
2) And he would pour out the remainder of the blood at the western base of the outer altar.
3) And [the remainder of the blood sprinkled] on the outer altar he poured out at the southern base.
4) Both mingled in the aqueduct and flowed into Nahal Kidron, and they were sold to gardeners as manure and by using them one transgresses the law of trespass (meilah).
Section one: Leviticus 16:19 states, And the rest of the blood he shall sprinkle on it with his finger seven times. This means, according to the mishnah, that he sprinkles the blood on the top of the altar.
Section two: The remainder of the blood is poured out on the western base of the altar. This is the same place that the blood of the high priests bull (not a Yom Kippur sacrifice) is poured out according to Leviticus 4:7.
Section three: There are other sacrifices that are offered on Yom Kippur on the outer altar (see Numbers 28:7-11). The blood of these sacrifices is spilled on the southern base of that altar.
Section four: The extra blood from all of the altars flows into the aqueduct that runs through the Temple and then the blood flows out to Nahal Kidron (Wadi Kidron). The blood, assumedly mixed with some water by this time, would be sold to gardeners to use to fertilize their fields and the proceeds would go to the Temple. The mishnah notes that one who benefits from this blood without paying for it is guilty of trespassing, which means illegal use of Temple property. The blood retains its holiness even after having flowed out of the Temple.