SEATTLE — The Washington Department of Health plans to test several hundred water systems in the state for trace contamination of more than a dozen chemicals found in some firefighting foams. The chemicals are called per- and polyfluoroalkyl substances, or PFAS. They already have been found in five Washington drinking-water systems at levels over the Environmental Protection Agency’s guidelines, as well as dozens of private drinking-water wells near firefighting training areas where the foams were used. Department officials will use the test results to help assess the scope of the problem as they work with the Washington State Board of Health to develop possible state standards for some of the chemicals. Washington drinking-water contamination is part of a much larger PFAS pollution problem at sites across the country.