05/25/2010 05:41 pm ET | Updated May 25, 2011

What's in Your Water?

The U.S. Geological Survey (USGS) has been mucking around in wells. Surprise: it's not all muck.

About 105 million Americans -- or around one-third of the nation -- rely on some 140,000 public wells for their drinking water. The USGS's latest installment on the state of our drinking water sheds some light on the quality of that water. Is it good news or bad news? Judge for yourself.

First some quick stats. The study covered:

  • 30 aquifers,
  • 41 states,
  • 932 public wells, and

  • less than one percent of U.S. groundwater-supplied public water systems.
  • Water: Untreated vs. Treated-and-Unhealthy

    Percentage of untreated water samples that had at least one contaminant at levels designated by the USGS to be of potential health concern*:

    This means that the chemicals we use in our agriculture, industry and homes aren't just making their way into surface waters; they are also finding their way into the groundwater that finds its way into our drinking water supply.

    Fortunately, very few of us drink water straight out of the ground. All public water supplies are treated to some extent before consumption. And so most people should be greatly relieved by the following statistic.

    Percentage of treated water samples found to have at least one contaminant at levels designated by the USGS to be of potential health concern: 0 (source)

    Unfortunately, a companion study of private drinking water also found that more than 20 percent of private domestic wells sampled contained at least one contaminant at levels of potential health concern. The important difference being that water from private wells does not undergo additional treatment.

    Water: Untreated vs. Treated-and-Officially-Not-Unhealthy

    I imagine some people may prefer not drinking contaminated water even if it meets health standards. If so, those folks would no doubt find these next statistics of interest.

    Percentage of untreated water samples that had at least two contaminants present but not necessarily at levels of potential health concern: 70 (source)

    Percentage of treated water samples that had at least two contaminants present but not necessarily at levels of potential health concern: 82 (source)

    Note anything strange here? Treated water has more contaminants than untreated water. How come? Two reasons. Most treatment processes are simply designed to disinfect the water. This means that the treatment:

    1. is often ineffective against some types of contaminants, allowing them to pass through to the finished water, and
    2. can add byproducts of the disinfectant process to the finished water.

    Regulated vs. Unregulated Water Supplies

    Number of contaminants identified in USGS study: 337 (source)

    Number of contaminants identified that are regulated by the Environmental Protection Agency via the Safe Drinking Water Act: 58

    Number of contaminants that are not regulated but carry USGS health-based concerns: 135

    Number of contaminants identified that are neither regulated nor have a USGS Health-Based Screening Level* (largely because of a lack of data): 144

    Bottled Water Vs. Tap Water: Not Much of a Contest There

    If all this makes you so concerned that you want to swear off tap water and go on a steady diet of bottled water, that might not be such a good idea.

    Consider that Peter Gleick, fellow member of the National Academy of Sciences and author of Bottled and Sold: The Story Behind Our Obsession With Bottled Water, found that 40 percent of the bottled stuff is from reprocessed municipal water. And for the other half or so that doesn't come from public water supplies? Well, in general, municipal water supplies are much more tightly regulated and monitored than bottled water products (see table below). And, given the paucity of data on bottled water, it's a safe bet that you're better off with tap water even if it means we should still take steps to ensure its safety.


    Tighter regulations surrounding tap water means it's probably overall safer than the bottled stuff.

    For More Information

  • Understanding Contaminant Occurrence in Public-Supply Wells - USGS -
  • EPA's Frequently Asked Questions on Groundwater and Drinking Water -
  • Public Water Systems - information and resources from EPA -
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    *A contaminant is of potential health concern when either the maximum contaminant level of the Safe Drinking Water Act or the USGS's Health-Based Screening Level (HBSL) developed in concert with EPA and other agencies is exceeded.

    The HBSL is a measure of contaminant levels in water that, if exceeded, may be of potential concern for human health. It is a not a regulatory level, but instead is designed to give preliminary information on the potential impacts of compounds in our water.

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