A new study led by Washington State University researchers in Guatemala found that drinking water sources widely believed to be clean and safe often contain harmful bacteria.
The research focused on Guatemala's Western Highlands, where scientists compared what people think about their drinking water with what laboratory testing actually revealed. Many residents viewed bottled water sold in large refillable jugs as the safest choice. However, testing showed that among the 11 water sources examined, bottled water was the most likely to contain coliform bacteria, which signal fecal contamination. The study was published in the Journal of Water and Health.
"We found there is a clear disconnect between what people believe about water safety and what's actually happening in their homes, and that can have major public health implications," said Dr. Brooke Ramay, lead author and assistant research professor in the WSU College of Veterinary Medicine's Paul G. Allen School for Global Health.
Why Unsafe Drinking Water Remains a Global Threat
Unsafe drinking water continues to be a serious global health issue. More than 4 billion people around the world lack access to safely managed water, and fecal contamination contributes to millions of cases of diarrheal illness each year, especially among children. Contaminated water is also increasingly linked to the spread of antimicrobial-resistant bacteria.
"Understanding how people view water safety is key to improving public health," Ramay said. "When people believe their water is safe, they don't take extra precautions, but when they see a risk, they change their behavior."
The research team surveyed 60 households, split evenly between urban and rural areas, and collected water samples from several sources, including bottled, piped, well, spring and filtered water. The samples were analyzed for coliform bacteria, Escherichia coli, and antibiotic-resistant organisms such as extended-spectrum beta-lactamase-producing (ESBL) and carbapenem-resistant Enterobacterales (CRE). These bacteria can survive many commonly used antibiotics and cause infections that are difficult to treat. Although ESBL- and CRE-producing bacteria often live harmlessly in the human gut, they can cause severe illness if they enter the urinary tract or bloodstream.
Bottled Water Showed Unexpected Contamination
Despite being widely trusted, bottled water was six times more likely to test positive for coliform bacteria than other water sources. Only 17% of bottled water samples met World Health Organization standards for safe drinking water.
Bottled water serves as the primary drinking water source for many households in Guatemala. Large jugs are commonly filled at local purification facilities or neighborhood refill stations, where the water is treated, sealed and delivered to homes and businesses.
"The problem isn't usually with how the water is bottled -- it's what happens afterward," Ramay said. "These jugs can be stored improperly, and dispensers aren't cleaned regularly, and we think this can create ideal conditions for bacteria to grow."
Widespread Bacteria Found Across Water Sources
Across all samples tested, coliform bacteria were detected in 90% of water sources, E. coli in 55%, and ESBL in 30%. CRE bacteria were less common but were found in some household piped water samples. The presence of these organisms in drinking water is particularly concerning because they can spread antibiotic resistance even when they do not cause immediate illness.
Which Water Sources Were Safest
Water drawn directly from protected municipal wells had the lowest contamination levels, even though residents ranked these sources lower in perceived safety. These sealed, chlorinated community wells showed no coliform contamination and no detectable E. coli, ESBL or CRE bacteria.
However, once municipal water was piped into homes, contamination increased sharply. More than 65% of household samples contained coliform bacteria, while 28% tested positive for E. coli (28%), 11% for ESBL and 11% for CRE (11%).
How Beliefs About Water Safety Shape Behavior
The study also highlighted how cultural beliefs influence everyday behavior. People who trust their water source are less likely to boil or treat it, and those who rely on bottled water often skip cleaning dispensers.
"Our results suggest that beliefs about water safety may actually contribute to contamination because people don't take the same hygienic steps with sources they trust that they might with other less trusted sources," Ramay said.
