YAKIMA, Wash. (AP) — The Environmental Protection Agency is beginning to sample groundwater in the lower Yakima Valley this week to find the source of contamination.

Residents there have been complaining about nitrate and bacterial pollution in private wells. Many believe the problem is chiefly caused by the application of dairy manure and commercial fertilizer to crops.

Tom Eaton, director of EPA's operations in Washington, says the agency is assembling all available information to identify possible sources of contamination and has started a sampling program.

EPA has been working with several state agencies over the past year to compile a report on the problem first reported by the Yakima Herald-Republic newspaper.

Copyright 2010 The Associated Press.