Is Tap, Treated, or Bottled Water Better?

Are We Playing Roulette When We Drink Water? A New Study Of Bay Area Drinking Water Offers Insights.

Photo by Janosch Lino on Unsplash

Drinking water in the San Francisco Bay Area may carry health and aesthetic risks regardless of the source, according to a new peer-reviewed study by researchers at SimpleLab, published this month in PLOS Water.

It is one of the most comprehensive studies looking at bottled water, tap water, and household-treated tap water (HTTW) in the area.  

Worsening public infrastructure, governance failures, and growing environmental pressures have left consumers worried and confused about their drinking water choices.

People often choose to drink bottled water, assuming it’s the best option, but the study authors found that all three drinking water options carry potential health risks, largely driven by trihalomethanes.

Trihalomethanes (THMs) are the result of a reaction between the chlorine used for disinfecting tap water and natural organic matter in the water. These compounds occur when organic matter in the water reacts with chlorine, so essentially the system is not being chlorinated properly, meaning you have too much or not enough.

Many municipalities don’t get to the root of the problem by finding the source of the organic matter. When you know what’s in the water, you can treat it more effectively instead of creating a chemical cocktail mix.

At elevated levels, THMs have been associated with negative health effects such as cancer and adverse reproductive outcomes.

In the study, researchers compared the water quality of 100 bottled water products from 89 brands to that of samples taken at the taps of households across the nine Bay Area counties—603 samples with no further treatment and 111 samples with additional treatment at the household scale either point-of-entry or point-of-use treatment including carbon filters, reverse osmosis filters, and more.

Samples were analyzed for 100 different parameters, including general water quality characteristics, metals, other inorganics and volatile organic compounds (VOCs), in all three water sources, and bacteriological indicators in bottled water samples only.

Potential health risks were assessed by comparing contaminant concentrations to the most conservative established health-protective benchmark concentrations that reflect lifetime exposure risk.

Major Findings from the study include:

Aesthetic Issues: Contaminants that could affect taste, odor, or color were found across all three water options, but less so in bottled water.

Health Risks: Contaminants that present potential human health risks were found across all three drinking water options; at least one health benchmark was exceeded in 53 percent of bottled water samples, 61 percent of household-treated water samples, and 98 percent of tap water samples, with trihalomethanes being the most prevalent contaminants in all three.

Comparison of Drinking Water Options: The potential risk to human health due to all contaminants found in household-treated tap water was similar to that of bottled water (both of which were significantly lower than that of tap water), suggesting that in-home treatment can create water of a similar quality to bottled water.

The findings indicate that while household-treated tap water is comparable to bottled water, proper selection, installation, and maintenance of treatment systems are crucial.

To learn more about the study and offer you more perspective about your drinking water options, we reached out via email to Samantha Bear, senior research scientist at SimpleLab and lead author on the study.

What prompted this study (in this location) and did anything about the results surprise you?

Samantha: The study was prompted by feedback we were getting from customers purchasing kits to test their tap water. We started noticing that a lot of people looking to test their tap water shared that they preferred bottled water, sometimes motivated by off-tastes or other aspects of their tap water, but sometimes without a clear explanation. This prompted us to look into the literature and we found that 1) people often choose bottled water based on the perception that their tap water is unsafe whether or not that is actually true, and 2) there isn't much data out there on bottled water quality!

We realized that if we wanted to find out how bottled water compared to tap water, we would have to do it ourselves.

We decided to keep the scope of the study pretty localized, focusing on the San Francisco Bay Area, so we could really zero in on the drinking water options that are actually available to folks here.

To this end, we took stock of the bottled water products available all over the region and designed the bottled water sample set to be representative of products that are available to people here, while also being reflective of the market share of different product categories purchased in the U.S. (meaning we included representative proportions of purified vs. spring vs. sparkling, etc.).

Keeping it limited to the Bay Area allowed us to be very particular with the bottled water sample set.

Why do you think there's so little data in the academic literature on water quality when it comes to tap water alternatives like bottled water or filtered water?

Samantha: In general, there is not a lot of data at the point of use, so at people’s taps. This data is somewhat difficult to get, as opposed to nationally available sample sets taken at utilities or limited samples at specific locations in academic studies.

We are uniquely suited to study water quality at the point of use because we have this large dataset of (anonymized) tap water results at our disposal, and our data differentiates samples that have undergone further treatment at the household scale, so we can separate data based on whether it receives further treatment.

There are studies out there that look at the mechanistic aspects of household treatment and others that look at a much more limited number of samples, but our dataset of household-treated samples is much larger than those in other studies we've seen.

When it comes to bottled water, as you said, there are very few studies out there that look at the overall water quality of a large number of samples. You'll more often see studies focused on finding specific contaminants, like PFAS, or studying the impacts of things like storage conditions on the water quality. It’s not really clear to me why there isn't more data out there on bottled water quality to be honest. There is a lot more that we can do in this investigation, so it’s not like there aren't interesting avenues remaining.

One takeaway from the study is that in-home treatment can create water of a similar quality to bottled water. Do you have recommendations for how consumers can sort through the many options available for at-home water treatment?

Samantha: It can be tricky with so many different technologies, products, and certifications (or lack thereof). The first and most important thing (which is not going to be a shock if you’ve read our study) is that you need to know what’s in your water so you know what you’re aiming to treat. This will help you narrow down what types of products will work for your situation.

No product is going to work perfectly for every single contaminant possible so having in mind what you care most about is going to be very helpful.

I know this sounds very convenient coming from a company that tests water, but there’s really no way around it without making a blanket recommendation to install a treatment train that is going to be overkill for most people.

[Editor’s note: We’ve given this same advice for years and many great labs exist for testing water. You can see this list from the EPA.]

When it comes to the product themselves, as a company, we don’t endorse specific products, but rather point people to look for certifications by organizations like NSF, WQA, and IAPMO. These organizations certify water treatment products to standards (NSF/ANSI standards) for the reduction of specific contaminants, so you have to make sure that the product you’re looking at is certified for the reduction of the contaminant you’re targeting.

Just the fact that a product is certified does not necessarily mean it’s certified to reduce the contaminant that you care about! Products can be certified for all kinds of things, including that the product itself is lead free (not even that it reduces lead in water), so just be aware that a certification alone does not support every claim the manufacturer makes on the box.

I, personally, always check the websites of the certifying bodies to make sure the product I'm looking at has the certification it claims.

It is important to note that just because a product isn’t certified doesn’t mean it doesn’t work. However, there’s really no good way for us to confirm the claims about what a product does and doesn’t do without certifications, so we tend to lean on them for lack of other options.

Trihalomethanes (THMs) are the contaminants with the most frequent health benchmark exceedances across all samples tested. While we know this is a problem in tap water, why do you think it showed up in bottled water? Can you talk more about the health risks associated with THMs?

Samantha: For the most part, we’re seeing THMs in bottled water because that bottled water started out as tap water. This is not the case for all bottled water, especially products that are labeled as “spring,” “artesian” or “mineral,” which are designations that mean the water was drawn directly from the ground. However, most products labeled as “purified” are just tap water that has undergone further treatment which, as we have found, does not completely remove THMs. 

However, we did detect THMs in three bottled water samples in which we did not expect to find them—one labeled “mineral,” one labeled “artesian,” and the last labeled “spring.” Chlorinating (which is what creates THMs) is not common practice for bottled waters with designations like “spring,” etc., and you'd think the water wasn't chlorinated before they drew it out of the ground, so these are a bit of a mystery to us. We were surprised to find THMs in these products.

As for the health effects of THMs, we want to be super clear about the fact that it is highly unlikely that any of these waters would cause acute problems for anyone.

The levels of THMs (and other contaminants) in all of these samples were quite low for the most part, and mostly within regulations governing water quality.

However, what we are concerned with is the impacts of these low concentrations over long time scales, in this case over a lifetime. Even at very low concentrations, if THMs are consumed in water over the course of a lifetime there is the potential that they could cause health issues.

The most prominent, and perhaps most worrisome, potential impact is an increased risk of cancer, specifically bladder cancer. Again, this is not a pronouncement that if you drink this water for your entire life, you will get cancer. This is just saying that if you drink this water for your entire life, you have an increased risk of getting cancer. Other documented potential impacts of THM exposure include developmental and reproductive effects, and liver impacts.

This conclusion from the study is something we agree with and talk about often:

Persistent aesthetic issues and potential toxicity in HTTW (home-treated tap water) could be addressed by designing household-scale treatment to specifically address identified issues, perhaps in partnership with water systems. This approach is not common and represents an area for future research and policy design to achieve safe water goals. 

Any ideas on where to start? We'd love to see more research and policy muscle going in this direction too. 

Samantha: This is actually already happening at some utilities! Not with a focus on THMs, though, the examples we’ve looked at are utilities working with households to reduce lead exposure, but it's a great model to look at when thinking about utilities working directly with consumers. For example, Denver Water, the utility serving Denver, Colorado, delivered free pitcher filters to households to address potential lead contamination. So we know that this type of collaboration can happen, it’s a matter of which contaminants people decide to focus on, among other factors.

Anything else from the study you'd like to expand on for our readers? 

Samantha: We would like to emphasize to your readers, and any readers of the study, that just because contaminants with potential health risks were found in all three drinking water options, this does not mean that these waters are inherently unhealthy.

In fact, for the most part, the water here is pretty darn good. The concentrations of contaminants we focused on are very low, typically within regulations, and may not ever cause problems for the majority of people, as susceptibility to these risks is very unique to each individual and depends on many factors. We’re comparing potential health risks, not guaranteed outcomes.

We don’t want to take away from the impacts of our findings here, we just want to make sure that people understand the subtleties and don’t walk away with the (mis)understanding that all water is bad.

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One last thought about water quality. My friend and water quality treatment expert, Bob Bowcock, is the first to emphasize that water treatment professionals are trained to say, “This water meets or exceeds all federal and state safe drinking water requirements.”

Yes, the water might be “compliant” when they tested it one day, but is it safe drinking water? Even if drinking water meets federal standards, it might not be safe if it is contaminated with an unregulated substance or tainted with contamination through some other scenario (for example, contamination resulting from residential piping).

I’m grateful for the work of these scientists to give us a more clear picture of what is happening at our taps, and to continue to use this data to help inform the general population about any risks associated with our drinking water.

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Keep the conversation going in the comments below. Comments are open to all subscribers today. Are you surprised by this new study? What did you learn?