Study finds timing, amount of manure applied affects nitrates in groundwater

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Applying dairy manure to fields as fertilizer can be a major source of nitrate pollution in groundwater, depending on the timing and amount of manure applied, according to a new study out of Whatcom County.

Like the Lower Yakima Valley, Whatcom County has a lot of dairy farms and a significant percentage of wells near them have been found to have nitrate levels above federal drinking water standards. Understanding how farming practices can influence groundwater is an important part of solving the nitrate pollution problem for both communities.

To get an in-depth view of the relationship between using manure as fertilizer and groundwater contamination, hydrologists at the state Department of Ecology teamed up with Washington State University’s Livestock Nutrient Management experts to monitor one 22-acre field where a farmer was growing grass to feed his cows.

The amount of nitrate that can move through the soil and into the groundwater depends on how much the plants can take up and use, Barb Carey, a hydrogeologist for Ecology explained. Heavy rain or too much irrigation water flush the nitrate out of the soil and into the groundwater.

She and her colleagues monitored groundwater below the field for four years and found that the nitrate contamination levels varied greatly, even though the farmer was growing the same crop. Two years ago, the water below the farm had nitrates exceeding federal standards, but during the other two years the levels were fine.

The conclusion for farmers, according to WSU’s Joe Harrison, is that keeping careful records of manure applications, testing the nitrogen content in the manure, and using good estimates of crop yield can help them keep nitrogen in balance and protect the groundwater.

Virginia Prest, manager of the Dairy Nutrient Management Program for the state Department of Agriculture said he study does a “good job of illustrating the relationship between excess nitrate-nitrogen and the excess water that it moves with,” and offers some good recommendations.

The good news, she added, was that in Yakima and other parts of Eastern Washington, farmers can manage irrigation water to reduce nitrate pollution.

Tom Eaton, director of Washington operations for the Environmental Protection Agency, said the findings were helpful but that local studies focusing on the farming practices here would be more valuable to solving the Lower Valley’s nitrate problem.

“Not surprisingly, if you’re over-applying manure, you get nitrates moving into groundwater,” Eaton said. But, local conditions matter, he said, so “it’s hard to take findings (from Whatcom County) and directly translate them to Yakima.”

Yakima’s Groundwater Management Area committee is in the process of designing local studies that will help them find the best ways to reduce nitrates in the Lower Valley’s groundwater.

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