Green Infrastructure is not Preserving our Essential Watersheds

There seems to be a huge misunderstand in some department of Prince William County that “green infrastructure” can substitute for maintaining natural open space and not developing more than 5%-10% of the Occoquan Watershed to preserve our essential water resources and the source water for the Occoquan Reservoir. This is not true. Green infrastructure emphasizes design elements to manage stormwater and enhance urban environments. Natural open space is essential to preserve existing ecosystems-our essential Occoquan Watershed.

Traditional "gray" stormwater infrastructure is designed to move urban stormwater away from the built environment and includes curbs, gutters, drains, piping, and collection systems. Generally, traditional gray infrastructure collects and conveys stormwater from impervious surfaces, such as roadways, parking lots and rooftops, into a series of piping that ultimately discharges untreated stormwater into a local water body.  This would indeed be disastrous to our local drinking water supply. “Green" stormwater infrastructure is designed to mimic nature and slow stormwater movement. Trying to capture rainwater where it falls in stormwater ponds or slowing the velocity of stormwater with some natural elements. Green infrastructure reduces and somewhat treats stormwater to reduce localized flooding, reduce the costs of stormwater infrastructure, and improve aesthetics.  

Though, “green” infrastructure solutions can be applied on different scales. On the local level, green infrastructure practices include rain gardens, permeable pavements, green roofs, infiltration planters, trees, and tree boxes, and rainwater harvesting systems.  Generally, it does not reduce the impact of urban sprawl and impervious surfaces on the groundwater that is an essential element of our hydrology and  water supply. Changing the use of the land, covering it with buildings, driveways, roads, walkway will reduce groundwater recharge in the surrounding area increasing stormwater runoff velocity and quantity.

Only at the largest scale, the preservation and restoration of natural landscapes (such as forests, floodplains, and wetlands) is it possible to restore the function of the land. Prince William Forest Park which took over a hundred years to restore from monoculture agriculture to an Eastern Piedmont forest may be an example. There is a huge difference between letting a forest reclaim agricultural fields and trying to restore urbanized land. There is no example of restoration of land from urbanization back to forest.

Changing land use from open land either forest or agriculture can reduce the water supply over time. Many studies have found that an increase in impervious surface reduces base flow to our rivers and streams. This is because impervious surfaces prevent infiltration, thereby reducing groundwater recharge and base flow. As groundwater levels fall, perennial streams that feed the rivers become ephemeral. The groundwater becomes disconnected from the surface water network.

No study has found that green infrastructure prevents this from happening. The quantity of impervious surfaces, those surfaces that prohibit the infiltration of water from the land surface into the underlying soil, turns out to be the most critical indicator for analyzing impacts of urbanization on the water environment. Once the hydrology is destroyed by development, it cannot be restored. 

In the Water Infrastructure Improvement Act, of 2019,  green infrastructure is defined as "the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters." Though the principals are related to maintaining a functioning watershed, these steps are inadequate to preserved a functioning watershed as the impervious surfaces expand beyond 10%. Stream quality degrades and can only be slowed, but not stopped by green infrastructure.

Impervious surface increases the frequency and intensity of downstream runoff and decreases water quality. Increasing urbanization has resulted in increased amounts of impervious surfaces - roads, parking lots, roof tops, and so on - and a decrease in the amount of forested lands, wetlands, and other forms of open space that absorb and clean storm water in the natural system. This change in the impervious-pervious surface balance has caused significant changes to both the quality and quantity of the storm water runoff, leading to degraded stream and watershed systems.

Furthermore several studies have documented that the quantity of impervious surfaces is directly related to the water quality of a watershed- the drainage basin and it’s receiving streams, lakes, and ponds. Increase in impervious cover and runoff directly impact the transport of non-point source pollutants including pathogens, nutrients, toxic contaminants, and sediment. Impervious surfaces also collect and accumulate pollutants that are deposited on roadways and other impervious surfaces  from the atmosphere, leaked from vehicles or derived from other point sources. During storms, accumulated pollutants are quickly washed off these surfaces and rapidly delivered to aquatic systems. As the area under impervious cover increases, more water reaches the Chesapeake Bay and ocean as surface water run-off. Stormwater runoff picks up pollutants as it flows across land surfaces. Pollutants include sediment, pesticides, asphalt, fertilizers, bacteria and other disease-causing organisms from failing septic systems; petroleum products such as oil and grease.

Finally, the areal extent of impervious surfaces may significantly influence urban climate by altering the sensible and latent heat fluxes within the urban areas as was found in the study of temperature variation study in Westmoreland County . Areas with a large amount of impervious surfaces are also susceptible to  higher ambient air temperatures because the man made roads, parking lots, concrete surfaces and buildings absorb and trap more heat than natural environments. Plants use the energy of the sun while man made surfaces absorb and radiate the energy of the sun. These clustering of heat absorbing manmade surfaces and structures create Urban Heat Islands that can impact a community’s environment and quality of life increasing energy consumption for cooling, increase emissions of air pollutants and greenhouse gases, and impaired water quality.

Using 320 air temperature measurements at 20 sample sites on July 10, 2022 and the Random Forest model in ArcGIS Pro they were able to extrapolate temperatures across the region, ultimately identifying non-heat islands, heat islands, and urban heat islands. The data found that 3.57% of the landmass of the region (approximately 32,700 acres) was an EPA classified urban heat island.  The heat island results were clustered in Fredericksburg and surrounding areas. The variation was found to be a  17-degree Fahrenheit difference from forestland temperatures and heat island.