The Impact of Development on Source Water and Groundwater Recharge

The water flowing from our taps doesn't originate at a treatment plant; it begins in our watersheds, specifically the upstream land of the greater Occoquan Watershed. When development occurs—such as paving and building within the watershed—we do more than alter the scenery. These actions fundamentally change how water moves through the landscape, how streams withstand drought, and how much pollution ultimately reaches the reservoir.

Development and Its Consequences

Every acre of forest and field that is converted into roofs, roads, and driveways reduces groundwater recharge and increases polluted runoff. This pushes our streams closer to drying up, causing gradual, cumulative, and eventually sudden destruction of our source water.

Currently, there is a proposal to amend the Comprehensive Plan for more than 200 acres west of Thoroughfare Road and south of John Marshall Highway. This proposal would change zoning from A-1 Agricultural to Planned Mixed Residential (PMR), permitting 97 detached homes, 56 attached homes, and the accompanying roads, parking lots, driveways, patios, and other impervious surfaces. Such changes are not neutral for the water supply; they directly impact the land that feeds our streams and, ultimately, the Occoquan Reservoir. It is not just this change it is the destruction of the watershed one comprehensive plan amendment at a time. 

Importance of the Bull Run Watershed

The Bull Run watershed is a vital part of the source-water system for the Occoquan Reservoir, which supplies drinking water to eastern Prince William County and much of eastern Fairfax County. The lower Bull Run area remains one of the most intact and least-disturbed sections of the watershed. Once these headwaters are fragmented and paved, we lose the natural infrastructure that maintains clean water, particularly during droughts. The issues observed in Little Bull Run are not isolated; they serve as a warning regarding the consequences of continued development in western and central Prince William for our drinking water supply.

Groundwater: The Invisible Essential

To understand the ongoing changes, it's important to discuss groundwater, which is invisible yet essential. Groundwater typically provides 30% to more than 50% of the annual flow for streams and rivers. This steady, slow-moving contribution is known as baseflow and acts like a "savings account," sustaining streams over time and providing flow during dry spells.

Drought and Its Effects

Prince William County and most of Virginia are currently experiencing severe drought. While the region usually receives about 44 inches of rainfall annually, last year was about 9 inches below average, and the first half of this water year saw roughly half the usual precipitation- about 13 inches below average. Despite a period of very wet years from 2018 to 2021, followed by near-normal rainfall with dry summers, recent summers have exposed weaknesses in the system.

Stream and Groundwater Dynamics

Streams in the Bull Run watershed are “gaining” streams, receiving direct groundwater supply. For groundwater to feed a stream, the water table must be higher than the stream’s water level, allowing water to seep through streambeds and banks. Excessive groundwater withdrawal or reduced recharge can sever this connection, turning perennial streams into seasonal ones.

Impervious Cover and Stormwater Runoff

As development increases impervious cover—such as roads, pavement, and buildings—the area available for rain and snow to infiltrate the ground decreases. Stormwater velocity and quantity rise, preventing infiltration and increasing flooding, which carries fertilizers, oil, grease, and road salt into rivers and streams. In the past six years, stormwater flooding has occurred adjacent to previously open areas of the county away from the shoreline.

Increased runoff volume and velocity lead to higher peak storm flows, causing flooding, soil erosion, and fast-moving water that carries contaminants. Reduced infiltration means less groundwater, which is essential for maintaining the base flow to streams and rivers feeding the Occoquan Reservoir during dry months.

Groundwater Recharge Trends

Groundwater in the Culpeper Basin is generally replenished annually through precipitation, but increasing impervious surfaces have altered this balance. In the past, water withdrawal matched recharge rates, ensuring adequate supply even during droughts. However, the region’s only nearby US Geological Survey groundwater monitoring well has shown a slow decline in water levels over the past decade and a half, despite varied rainfall conditions.

Watershed Response to Development

In Haymarket, the Bull Run Mountain Conservancy found that perennial streams such as Little Bull Run and Catlett’s Branch were dry during a dry August, with Catharpin Creek reduced to puddles. This was the driest period observed and may signal a new normal during dry times, highlighting how the watershed responds to recent development.

Reduced groundwater levels gradually transform perennial streams into ephemeral ones, disconnecting groundwater from surface water. Once development destroys watershed hydrology, restoration becomes extremely difficult. The Occoquan watershed is essential for regional drinking water, and groundwater is critical for streams and rivers.

Protecting Groundwater and Hydrology

Long-Term Watershed Changes

Development triggers immediate responses in a watershed, but significant ecological and physical changes take 20 to 50 years to fully emerge. Replacing 35–50% of forested areas with impervious surfaces permanently alters water movement. As impervious coverage increases, profound hydrological and ecological shifts occur over time.

Hydrological Alterations

• Runoff reaches streams more rapidly and in larger volumes, causing higher peak floods and more frequent flooding.
• Perennial streams begin to dry out or flow intermittently, eventually becoming ephemeral.
• Impervious surfaces prevent water from soaking into the ground, leading to dropping water tables and dry streams during summer.

Physical and Water Quality Changes

• High-velocity storm runoff causes severe bank erosion, incision, and blowouts, destroying aquatic habitats.
• Stormwater carries oils, heavy metals, road salts, and nutrients directly into waterways, bypassing natural filtration.
• Rainwater flowing over heated pavement raises stream temperatures, threatening aquatic life.
• Pavement, compacted lawns, and buildings prevent groundwater percolation, increasing land temperatures.

The cumulative effects observed now stem from building during the 1990s through 2007.

Recommendations for Protecting Source Water

• Treat the Bull Run watershed as source-water infrastructure—not as leftover land for development.
• Oppose Comprehensive Plan changes and rezonings that increase impervious cover in headwaters feeding Bull Run and the Occoquan system.
• Enforce stronger safeguards: limits on impervious cover, meaningful forest protection, groundwater-recharge preservation, and stormwater designs that mimic natural infiltration.
• Support acquisition and conservation easements for large, connected tracts in the watershed, because once developed, hydrology is permanently changed.

If the watershed continues to be treated as an empty canvas for growth, we risk actively destroying the source water for our regional drinking water supply, incrementally through rezoning, roads, and parking lots. By protecting infiltration, baseflow, and intact stream corridors now, we can safeguard the Occoquan for decades to come.