About 90 mature evergreen trees — some up to 100 years old — have died in a patch of mountain forest in the middle of Washington Park, and neighbors are blaming the recycled irrigation water.

The park has been irrigated with recycled water — storm and wastewater cleaned up not quite to drinking standards but OK for lawns and other landscaping — since 2004. But in the last three years the canopy shading what is known as Evergreen Hill has begun to disappear.

Last month alone, 21 dying conifers — mostly blue spruce and Douglas firs — were removed from the hill along Virginia Avenue near Lafayette Street at the north end of the park.