A lot of Angelenos don’t drink tap water and drink bottled water instead, but bottled water sold in the U.S. is not necessarily cleaner or safer than most tap water, according to a four-year scientific study by NRDC. There has been an explosion in bottled water use in the United States over the past ten years driven in large measure by marketing designed to convince the public of bottled water's purity and safety, and capitalizing on public concern about tap water quality. The NRDC studied more than 1,000 bottles of 103 brands of bottled water. While most of the tested waters were found to be of high quality, some brands were contaminated: about one-third of the waters tested contained levels of contamination including synthetic organic chemicals, bacteria, and arsenic. A key finding of the study was that bottled water regulations are inadequate to assure consumers of either purity or safety, although both the federal government and the states have bottled water safety programs. At the national level, the Food and Drug Administration (FDA) is responsible for bottled water safety, but the FDA's rules completely exempt waters that are packaged and sold within the same state, which account for between 60 and 70 percent of all bottled water sold in the United States (roughly one out of five states don't regulate these waters either). The FDA also exempts carbonated water and seltzer, and fewer than half of the states require carbonated waters to meet their own bottled water standards. Even when bottled waters are covered by the FDA's rules, they are subject to less rigorous testing and purity standards than those which apply to city tap water. For example, bottled water is required to be tested less frequently than city tap water for bacteria and chemical contaminants. In addition, bottled water rules allow for some contamination by E. coli or fecal coliform (which indicate possible contamination with fecal matter), contrary to tap water rules, which prohibit any confirmed contamination with these bacteria. Similarly, there are no requirements for bottled water to be disinfected or tested for parasites such as cryptosporidium or giardia, unlike the rules for big city tap water systems that use surface water sources. Some bottled water then may present a health threat to people with weakened immune systems, such as the frail elderly, some infants, transplant or cancer patients, or people with HIV/AIDS.

Drinking tap water is a better solution, but it isn't perfect. Here is the LA report from NRDC: 

http://www.nrdc.org/water/drinking/uscities/pdf/la.pdf

Tap water, however, can be improved through the use of filters:

http://www.webmd.com/news/20070409/many-tap-water-filters-work-well