The Census doesn’t count basements, so nobody can honestly say how many Chantilly houses have one. Fairfax County’s soil guide does explain what a basement here is up against: the county’s west end, along the Loudoun line, sits on the red sandstone, siltstone and shale of the Triassic Basin, where the soil over the rock is often shallow, 2 to 10 feet to bedrock, and large flat areas drain slowly and poorly.
The county even maps a soil named Chantilly: dark red ground mixed, graded and compacted as the subdivisions went in, and slow to let water through. On one natural soil of the basin’s level uplands, the guide puts a seasonal water table perched on the bedrock 18 to 30 inches down, and says foundation drains and waterproofing are often needed to keep basements dry. Those drains usually empty into the pit.
In 20151 the typical owner’s house went up in 1987, and about 1 in 4 homes is a townhouse. The county’s soil and water district says Virginia groundwater tends to run highest in February and March, after snowmelt and rain. That’s when a worn pump meets its hardest weeks, so a dry day in January is the time to find out it’s tired.
Both sides of Chantilly send the water the same way: out, never into the sewer. Fairfax County calls an unpermitted sump pump tied into the sanitary sewer illegal, since that water can overload the system and push raw sewage into streets, streams and basements. In South Riding, where the typical owner’s house dates from 2006 and nearly half the homes are townhouses, Loudoun Water says sump pumps shouldn’t connect to its sewer lines and wants the water out on the yard, far enough from the foundation that it can’t soak back into the drains. With a narrow townhouse yard and a neighbor close by, that takes planning.