Where and When Irrigation Water is Tested Could Affect Food Safety

Researchers find bacteria is tied to water depth, distance from shore, time of day, and other complicated factors, providing insight into designing water testing rules

PhD Student Jackie Smith collects water samples from an irrigation pond at Wye Research and Education Center.

Image Credit: Matt Stocker

January 25, 2024 Kimbra Cutlip

When it comes to food safety, it’s important for farmers to know what’s in the water they irrigate their fields with. But standards for when and where they should test the water quality in their irrigation ponds are currently limited, mostly because no one knows enough about how water quality varies within a single pond to adequately guide such testing.

To bring some clarity to this murky issue, a recent study by a University of Maryland microbial ecologist and colleagues from USDA, reveals that water quality and the diversity of microbes in agricultural irrigation ponds are interconnected and vary in different parts of the pond at different times of day.

The work highlights the need for standardization in water quality testing and for better understanding of the biological systems in these small water bodies. The study was published in the March 2024 issue of the journal Science of the Total Environment (online version available January 2024).

“Our study shows that where and when we take samples impacts not just water quality but how we understand the underlying biology,” said Ryan Blaustein, assistant professor of food and nutrition science and senior author of the study. “We are beginning to look at how the whole system within a small pond works.”

Previous research showed that sampling location and time impacts general water quality, but this study looked at how specific water quality measures such as pH, dissolved oxygen and temperature, are associated with the diversity and abundance of the many microbes present. It is a first step toward understanding how changes in microbial communities may be either influencing or responding to changes in water quality.

To conduct the study, Blaustein and his colleagues collected a variety of water samples from an agricultural irrigation pond at the Wye Research and Education Center (a part of the Maryland Agricultural Experiment Station). The samples were taken from multiple times during the day from locations along the shore and farther from shore at depths of roughly zero, three and six feet.

The researchers found that although the dominant microbes were the same throughout the pond, the microbial community as a whole varied by location and changed with every sample. As the day progressed, microbial diversity increased, as did water temperature and dissolved oxygen. Abundances of some bacteria increased while other bacteria decreased. The results also showed that nutrients such as ammonia, nitrates, phosphates, chlorophyll and other measures of water quality were closely tied to abundances of different bacterial species in the water.

Although E. coli levels were low in the study pond, the team found an association between e-coli levels and certain other bacterial species and water quality factors. E. coli monitoring is important for food safety because it poses potential health risks to humans.

The study highlights how a better understanding of the complex interactions between water chemistry and bacterial communities could lead to easier ways of monitoring for E. coli and other specific microbes. The methods Blaustein and his colleagues employed could eventually extend beyond irrigation ponds to other small water bodies used for recreation, and even to other sources of water used in growing food such as wells and city water, which is also variable in quality.

“By understanding the underlying biology and how microbial communities and water quality interact we can start to design water quality testing methods and rules that make sense for a given system,” Blaustein said.