GRANGER, Wash. (AP) -- A long-awaited federal study says Lower Yakima Valley dairies and several farms are "likely" sources of nitrates contaminating private drinking water wells.

However, federal officials said the Environmental Protection Agency study released Thursday was limited and doesn't prove a larger trend of nitrate contamination across the entire 576-square-mile area southeast of Yakima known for fruit, livestock and a diverse array of other crops.

The report says action is needed, but that it could take years to reduce nitrate levels in residential drinking water wells to safe levels due to the extent of contamination in the Lower Valley.

The report singles out five dairies, including several whose lagoons it estimates have leaked millions of gallons of manure into the underlying soil each year.

Authors of the report, however, do not blame the region's entire nitrate problem on the dairies.

"The dairies that we sampled are the only ones we could draw this conclusion about because we didn't look at all the dairies," said Mike Cox, an EPA scientist who managed the report and who was on hand at two gatherings held in Granger on Thursday to explain it.

EPA officials are now negotiating a binding agreement with the dairy owners on ways to stop contamination and ensure clean water for downstream wells, and to drill monitoring wells, said Tom Eaton, the EPA's Washington operation manager.

The agency does not plan to levy fines.

In the unincorporated areas of the Lower Valley, about 24,000 residents -- a third of the region's population -- rely on private wells for drinking water.

The EPA began probing contamination after a 2008 Yakima Herald-Republic series detailing how up to one in five of those wells, many of them serving low-income residents, were contaminated by nitrates. Excessive nitrates can harm infants and people with compromised immune systems and can also indicate the presence of other contaminants, such as bacteria and pesticides.

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Information from The Yakima Herald-Republic: http://www.yakima-herald.com

Copyright 2012 The AP.