Resource Protection Areas Maps and Regulations
Fairfax County: A thorough discussion can be found on the website of the Northern Virginia Soil and Water Conservation District.
Find your borders in relationship to the nearest park and Resource Protection Areas (RPA) on this map. Note: when entering your address, don't include the word "street" or "drive" or whatever. For example, enter 21 Main, not 21 Main Street. Be patient - the RPA map takes a very long time to load initially.
Loudoun: Go to Weblogis - Online Mapping System, click on Environment then also on Hydric Areas.
Arlington: Go to the Watershed and Resource Protection Areas map. Learn more here and here. Permits are not needed to remove invasive vegetation from RPAs in Arlington except for trees over 3 inches in diameter (assuming you are not causing soil disburbance.)