Mishnayos Mikvaos Perek 5 Mishnah 1

מעין שהעבירו על גבי השוקת – which is a utensil, and not the trough/grooved stone that is in the rock.

פסול – to immerse in the water that is within the trough and in the water that leaves and passes from the trough outwards, since all of the waters of the spring spill into the trough. But if the waters of the spring pass on the rim of the trough a bit, even though that some of them pass into the trough, it is kosher/fit to immerse in the waters that are on the rim of the trough or outside of it, for the spring purifies with a miniscule amount.

והפסיקו – for the waters of a pool are not attached to the spring.

הרי הוא כמקוה – but it doesn’t purify with a miniscule amount until there will be there forty Seah.

חזר והמשיכו – that he attached the waters of the spring to the pool.

פסול לזבים – that already stopped up their living [waters]. But regarding those with gonorrhea and lepers and waters of purification [mixed with the ashes of the red heifer for those made impure through contact with a corpse], it is written concerning them [that they require] potable, running spring water (see also Tractate Mikvaot, Chapter 1, Mishnah 8).