The Iowa Department of Natural Resources is warning Dixon residents against using the city's water for infants younger than 6 months old because of elevated nitrate levels.

The DNR found nitrate levels of 11 milligrams per liter during routine testing Thursday, higher than the recommended limit of 10 milligrams per liter for infants younger than 6 months. The city is working with the DNR to find and fix the cause of the problem, which could come from natural, industrial or agricultural sources, including septic systems and run-off.

Depending on the source of the problem, elevated levels could persist for several weeks or longer. This is the second time in four months testing has shown high nitrate levels. In September, the DNR recorded a level of 13 milligrams per liter.

Nitrate isn't a concern for children older than 6 months of age or adults because they can process nitrates.

Tap water should not be used to mix formula or juice or be given to children under 6 months of age, DNR said. The water should not be boiled, frozen, filtered or allowed to stand because those actions don't reduce nitrate levels. In fact, boiling could increase the nitrate levels as the water evaporates and nitrates are left behind.