State environmental scientists are in the midst of a lengthy probe into the poor condition of aquatic insects in two major Fairfax County watersheds, a problem that could have broader implications to the health of local streams.

The Difficult Run and Accotink Creek watersheds have both been listed as “impaired” because of the compromised health of their aquatic life since the mid-1990s, said Bryant Thomas, water monitoring planning and assessment supervisor for the Virginia Department of Environmental Quality’s Northern Virginia region.