Complex sentence: Each had it's own set of pumps to keep the workings dry and these lowered the water level in the whole area, but over the last fifty years (1986) the collieries around Chatterley Whitfield closed one by one.
Split into simpler sentences: Each had it's own set of pumps to keep the workings dry and these lowered the water level in the WHOLE area. But over the last fifty years (1986), the collieries around Chatterley Whitfield closed one by one.