Sentence:  Whenever the muddy water would accumulate in the <WSD>swag</WSD> the water from the well in question would become muddy... After the water in the <WSD>swag</WSD> had all disappeared through the sink-hole the well water would again become clear. 
Meaning of the word marked with <WSD> tags:  A low point or depression in land; "especially", a place where water collects.