HARRISBURG — State regulators say tests of water downriver from some treatment plants that handle natural gas-drilling wastewater are raising no red flags for radioactivity.

Samples were taken in November and December from seven rivers. The state Department of Environmental Protection said today that all samples showed levels at or below the normal naturally occurring background levels of radioactivity.

The department says it installed the sampling stations last fall to monitor the impact of Marcellus Shale gas drilling on water quality. Some wastewater from gas drilling is handled by treatment plants that discharge into rivers that also supply drinking water for many Pennsylvanians.

Tests were from the Monongahela; South Fork Ten Mile Creek; Conemaugh; Allegheny; Beaver; Tioga; and the West Branch of the Susquehanna.