Protecting the Occoquan Reservoir: Our Shared Water Source

The quality of our reservoir and the quality of our watershed are interconnected. “It doesn’t take a lot of development to start to see impacts,” explained Dr. Stephen Souza. 

That’s because development changes hydrology, i.e. how water flows during and after storm events. As land is developed, impervious surfaces replace farmland and forests, resulting in an increase in stormwater runoff and a decrease in groundwater recharge.

[Source EPA]

An increase in stormwater means more nonpoint source pollution (NPS), which is the primary pollution source degrading streams. "Over 70% of water quality problems in the US are due to nonpoint source pollution conveyed by stormwater runoff," emphasized Dr. Souza. 

“Watersheds are like a funnel,” he explained, “It all flows downhill, what happens in your watershed eventually affects your reservoir.”  It’s a domino effect that starts with converting naturally vegetated land to impervious surfaces. Even suburban development of one home per acre starts to show impacts. 

According to data from the Virginia DEQ and the Occoquan Monitoring Lab, the Occoquan Reservoir is showing signs of impairment. “The average phosphorus concentration right now is about 0.036mg/L about 36 ppb, anything over 40 ppb - 50 ppb is enough to stimulate and maintain an algal bloom. So you're right there at that threshold,” said Souza, and cautions that once that tipping point is reached, it is difficult to reverse.  

What can we do?

While retrofitting developed areas with stormwater Best Management Practices (BMPs) like rain gardens is helpful, it’s insufficient. “Even the best stormwater systems can only account for 80% of phosphorus and 60% of Nitrogen.” Not developing in the first place is better. Echoing the presentation from last week, protecting undeveloped land, particularly forested land, is the number one strategy to maintaining the quality of our drinking water. 

Fairfax County and Prince William County have implemented different strategies to protect the Occoquan Reservoir watershed. Fairfax protected 5,000 adjacent acres as natural area parkland and in 1982 down zoned 41,000 acres. “That's a big deal and had an impact on maintaining the quality of the reservoir,” Dr. Souza exclaimed. In contrast, Prince William County has protected 70 acres of parkland (Lake Ridge Park) and said the County would protect the upper reaches of the reservoir watershed, which covers most of the Rural Crescent. 

Watch Dr. Souza’s full presentation to learn more about how the quality of our reservoir reflects the condition of the watershed.