The plastic water bottle contains BPA that leaches into the water you are drinking! BPA stands for Bisphenol A. BPA is an industrial chemical that has been used to make certain plastics and resins since the 1960s. Flynn, S. W. (2008, April 17). Health & Wellness. Retrieved May 21, 2013, from She Knows Health & Wellness: http://www.sheknows.com/health-and-wellness/articles/803505/avoid-plastics-to-avoid-bpa-health-hazards
While the plastics industry insists BPA is harmless, reports suggest it has an estrogenic-mimicking effect that interferes with hormone levels and cell signalling systems. Long-term exposure may put you at risk for a gamut of health hazards including breast cancer and uterine fibroids in women, and prostate cancer and decreased sperm counts in men. Infants and children could face behavioural problems such as hyperactivity or an early onset of puberty.  Zeratsky, K. (n.d.). What is BPA? Should I be worried about it? - Mayoclinic.com. Retrieved May 21, 2013, from Mayo Clinic: http://www.mayoclinic.com/health/bpa/AN01955
The National Toxicology Program at the Department of Health and Human Services says it has "some concern" about the possible health effects of BPA on the brain, behaviour, and prostate gland of fetuses', infants and children.
Water from springs & aquifers may not be filtered at source to remove bacteria & natural sediments like arsenic.
By their own admission, it often is merely tap water or tap water that has been de-mineralized or other wise treated to change the taste.
Considerably more water than it can hold is wasted in the production of each bottle. "To manufacture a bottle it requires a staggering 6.74 times as much water as in the bottle" Pablo Paster  Pablo. (2007, February 5). What's the True Environmental Cost of Fiji Water? Retrieved May 21, 2013, from Triple Pundit: http://www.triplepundit.com/2007/02/whats-the-true-environmental-cost-of-fiji-water/
Over 38 million plastic bottles go to the dump each year in America. (This is not referring to pop/soda bottles) Plastic bottles take approximately 700 years to begin composting. The plastic toxins leach into the soil much, much sooner & affect the water table.  Gunzelmann, D. (2008, August 2008). Plastic Bottle Facts Make You Think Before You Drink | Greenup GRADER. Retrieved May 21, 2013, from Greenup GRADER: http://greenupgrader.com/3258/plastic-bottle-facts-make-you-think-before-you-drink/
Water bottles are expensive. Whether you are paying 25 cents or $2.50 per bottle your yearly costs would be significantly reduced by purchasing a quality water purification system and/or a BPA free portable water bottle with a built in purification filter!