The article mentioned about the water being pure for use based on the laws of that particular religion of that time. That is the water is pure and safe for bathing. It seems that the people during that time goes to great length to make the bath water 'pure' for ritual purposes.
2000 years ago the planet is definitely less populated and the environment is cleaner without much pollutions created by modern man made pollutants. I guessed rainwater was quite clean as well.