Water quality during the drought - using surface water safely

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

Many regions of the state are in moderate to severe drought, which is impacting water tables and wells. Are you seeing changes to your water table or wells running dry? Are you making changes on your farm because of a dropping water table? 

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There are reports in some areas of wells running low or dry. If your source of ground water is dropping, you might turn to lakes, streams or ponds for irrigation water for your crops. While this type of water can be used for irrigation with caution, surface water should never be used for postharvest uses like washing vegetables, washing hands, or sanitizing and cleaning surfaces.

As a rule, surface waters like ponds are considered the highest risk to produce safety since their quality can be highly variable and they are susceptible to contamination from animals and other sources of fecal contamination. This water is much more likely to contain pathogenic E. coli and Salmonella than well water sources.

Microbiological risks to surface water might include:

• Livestock 
• Birds
• Humans recreating
• Runoff from nearby fields, livestock and roadways
• Dead animals
• Dumping and trash

Testing pond water for bacterial contamination

If using surface water like ponds or streams for irrigation, the following practices can reduce the potential for contamination of your fresh produce

• Take steps to reduce contact of the water with the edible portions of the crop, such as by using drip tape under plastic. This has the added advantage of using much less water than overhead, and also reduces plant disease and mildew that can come with continually wetting plant foliage. Water applied with overhead irrigation is more likely to touch the edible portion. 
• Test the water for bacterial contamination to understand the baseline bacterial load. Know that this level will change rapidly, however, as surface water is open to the environment. It is recommended that surface water be tested 5 times per season. For more info on testing click here.
• Apply to produce that are not likely to be eaten raw. For example, potatoes, winter squash and beets. If there is contamination in the water, the cooking will likely serve as a "kill step" to reduce potential contamination.
• Allow a "die off" period between application of the surface water and harvest of the produce. Bacterial die-off levels in water are highly variable, and the number of days required for adequate die-off varies greatly depending on the initial level of contamination of the water, the crops, the UV and wind conditions, and other field conditions. For this reason, relying on die-off alone to reduce bacterial levels in water is not a good strategy. 
• The FSMA rule allows 4 days from water application to harvest to use water that does not meet initial standards, meaning it has high contamination levels. This assumes a 0.5 log die-off per day.  

Livestock in an irrigation pond

Photo: Wes Kline, Rutgers University