Last week the Prince William Service Authority held a virtual meeting to update the community on the progress of the modification to the Bull Run Mountain/ Evergreen water system that are necessary to bring the water system up to Virginia Department of Health standards and increase the potentially available water supply so that the system will be able to respond to and recover from emergency situations such as water main breaks.
The project will include:
- Adding a new well on Evergreen County Club land to increase the water supply;
- Modifying existing and new wells to add chemical feed for disinfection; and
- Connecting the new well to the existing distribution system
The Bull Run Mountain/ Evergreen Water System currently consists of a series of groundwater wells and a distribution systems serving Bull Run Mountain and Evergreen Estates some of the wells and much of the distribution system are more than 40 years old, others parts are much younger. The Service Authority took over ownership and operation of the system in 1990. According to the Service Authority, the system is experiencing diminishing yields from some of the wells. The Service Authority did not explain the causes of the diminished yield which can be due to age of the wells, biological or mineral encrustations, or diminishing yield from the groundwater in the fractured sedimentary rock system.
According to the Service Authority two major factors are affecting the yield of the existing wells: All wells have a “safe yield” pumping rate, which is the rate that water can be extracted from the aquifer over a long period of time without producing unacceptable effects. As the geology has changed over time and the area has seen more development, the yields of the wells have decreased; possibly due to declining aquifer transmissivity. Age is also a factor. Some of the wells are more than 40 years old and their yields have been impacted by encrustation and in part, by the development of other private wells around them.
The quantity and quality of groundwater in Prince William County varies across the county depending on the geologic and hydrogeologic group you are in. The wells of the Bull Run Mountain/ Evergreen supply system are east of Bull Run Mountain in the western part of Prince William County. The Service Authority hired Emery & Garrett Groundwater, Inc. to study further development of the groundwater resources to expand the water supply for the system.
Hydrogeologic group B underlies most of the western part of Prince William County and consists of sedimentary rocks of the Culpeper Basin. The predominant rock types are conglomerates, sandstones, siltstones, shales, and argillaceous limestones. Rocks within hydrogeologic group B tend to have moderate to excellent water-bearing potential because it is a fractured rock system with very little overburden. The highest reported yields in the county are from wells located in this hydrogeologic group B. The downside is that the hydrogeologic group is susceptible to contamination- the fractures that carry water can easily spread a contaminant and without adequate overburden spills could flow to depth through a fracture.
Hydrogeologic group C, which is interspersed throughout this area of group B in the western part of the County, consists of igneous rocks (basalt and diabase) of the Culpeper Basin. The rocks of group C are Early Jurassic in age. The predominant rock types are basalt, sandstone, siltstone, diabase, hornfels, and granofels. Rocks within hydrogeologic group C tend to have generally poor water-bearing potential because of the wide spacing between fractures, mineralization of fractures, and random fracture orientations. In other words, unless you hit a good fracture, you are unlikely to have the water yield the system needed.
Thus, in this area of Prince William County it is necessary to drill test wells to identify the locations with the best yield. The Service Authority was looking to locate wells that will provide a total of 40 to 100 gallons per minute of capacity for the system serving Bull Run Mountain and Evergreen Estates and to improve the system redundancy by augmenting water supply. The Service Authority converted two of the test wells drilled in 2019 into production wells. One well located on Evergreen Country Club and drilled to 330 feet below grade produced 60 gallons per minute and will be the new supply well. The second well located north west of the Country Club only yielded 10 gallons per minute and will be closed and held as a reserve well.
The Service Authority ran a 72 hour pump test to determine if pumping this well would have an impact on nearby private wells. The aquifers underlying Bull Run Mountain and Evergreen are regional and interconnected. Every well that draws water from these aquifers has some effect, even if very small, on other wells drawing water from the same aquifers. This impact can take years to express. The 72 hour pump test was run to see if a cone of depression would for drawing down the water level in nearby wells in the short term. The Service Authority found “no impact” during 72 hour pumping tests to the nearby wells they were granted permission to monitor. True long term impact will not be seen for years and the Service Authority should maintain monitoring well or wells in the area to document any changes in the groundwater over decades to come to have a greater understanding of the sustainability of the groundwater use.
This project does not expand the service area for the Bull Run Mountain/ Evergreen water system. It is intended only to ensure the continued level of service to existing customers and to provide sufficient water for others within the existing service area who decide to become Service Authority customers.