Parah, Chapter 8, Mishnah 9. This section of the chapter is occupied with a single practical question: which water may be drawn for the mei chatas, the water into which the ashes of the parah adumah are placed? Not every water source qualifies, and our Mishnah disqualifies two categories by name.
The Mishnah begins: "Hamayim hamukin pesulin", smitten waters are unfit. And it immediately defines the term: "Eilu hein hamukin? Hamluchim vehaposhrim", these are the smitten waters: salty water and tepid water. A spring whose water comes up briny, or a spring whose water comes up lukewarm rather than fresh and cool, is considered water that has been struck, damaged in its very nature, and such water may not be used for the chatas.
The Mishnah then turns to a second disqualification: "Hamayim hamichazvim pesulin", failing waters are unfit. What are failing waters? "Hamichazvim echad bashavua", waters that give out once in a seven year period. A spring is supposed to be a source of living water, mayim chayim, water that flows continuously. If this spring is known to stop flowing once within any seven years, it has shown itself to be undependable, and it is pasul for the mei chatas. A spring whose interruption comes at longer intervals than that is not classified as failing water, and it remains fit.
One more clause treats a spring that halts for reasons imposed on it from the outside. The Mishnah speaks of water that ceases "befulmasiyos", during the passage of troops, and "uvishnei vatzaron", in seasons when rain fails; the ruling on such a spring is "kesheirin", it is fit, while Rebbi Yehuda holds "posel", that it is unfit. An army marching through a region will draw heavily from whatever springs it finds and may even ruin them in the process, and when the heavens withhold rain the water table sinks and springs thin out. Neither of these stoppages reveals any defect native to the spring; the spring is simply being squeezed by conditions around it. For that reason the Mishnah keeps it fit for use. Rebbi Yehuda takes the opposite view and rules that such a spring is disqualified too.
So the Mishnah has given us two independent tests for the water of the chatas: the water must be sound in its quality, neither salty nor tepid, and the spring must be reliable in its flow, not one that gives out within a seven year cycle.