First Maryland-wide well water study finds E. coli in 10% of private wells

In the first-ever statewide study of well water on Maryland’s farms, water quality specialists at the University of Maryland School of Public Health (SPH) and UMD Extension found both harmful bacteria and a wealth of information about the factors that affect well contamination. 

The study, published in PLOS Water, tested 79 wells around the state and found 10% of them were contaminated with E. coli and 43% were contaminated with total coliforms, bacteria that indicate the presence of fecal matter in the water and raise health concerns. 

“Unlike municipal water, private wells in the United States are not required to be tested. So this study fills a critical gap for Marylanders – it offers practical information about the key factors increasing contamination risks in wells across the state,” said Dr. Rachel Rosenberg Goldstein, associate professor in the SPH Department of Global, Environmental, and Occupational Health. 

The study analyzed contamination levels against several factors: geographic location, precipitation, well characteristics such as depth, well cover material and maintenance intervals.

The percentage of wells contaminated with E. coli was higher than in previous studies, said Rosenberg Goldstein, possibly because of the larger sample size or the fact that all samples came from farms, which have multiple possible sources of contamination. 

Significant increases in the odds of E. coli presence in wells were found in the far western part of the state such as Allegheny and Washington counties, compared to wells in the Atlantic Coastal Plain Province, such as Worcester and Wicomico counties. The disparity could be due to the different geologic makeup of each area, said Rosenberg Goldstein. Precipitation had a somewhat surprising effect – where rainfall was higher in the two weeks before testing, E. coli was less likely to be present. Precipitation usually increases contamination risks, but this new finding could point to the more complex interaction of dry and wet periods preceding rain,  where bacteria can be washed away in consistent wet periods. 

Wells with covers made of concrete were significantly more at risk of E. coli presence than those made of plastic or metal. And deeper wells were less likely to have E. coli than shallow wells, defined by the researches asless than 9 meters deep. 

Rosenberg Goldstein worked with SPH doctoral student Cameron Smith and colleagues at UMD Extension, including the recently retired Dr. Andy Lazur, State Extension Specialist in Water Quality, andbiostatistician Dr. Raul Cruz-Cano, a UMD alumnus who is now an associate professor at Indiana School of Public Health-Bloomington. 

Though the sample size is limited, participant demographics were similar to Maryland farmers overall, when compared to the 2022 Census of Agriculture from the U.S. Department of Agriculture, says Rosenberg Goldstein, suggesting that the results could apply more widely. 

“The results highlight the need for private well testing in Maryland and beyond. If we understand the factors that affect contamination, we can better prevent health risks among private well owners,” said Smith, a graduate assistant in SPH’s Water Quality, Outreach and Wellness (WOW) laboratory.  

Rosenberg Goldstein’s team also tested Potomac river water after a historic sewage spill early 2026.