I wonder if it has something to do with the fact that the Womb is the largest and deepest of the pools. Perhaps the water tends to get a bit more stagnent than the other smaller pools.
Isn't the only source of water coming from the Anniversary Pool above? I can't remember if there is another source coming from below or from the rocks above the womb.
Or another question might be... "Who peed in the pool?!?!?"