Strathmore Development in Nokesville

The Planning Department has recommended approval of Strathmore, a request to rezone ±55.5 acres from A-1, Agricultural, to PMR, Planned Mixed Residential. The planned mixed development will be for up to 168 residential dwelling units, consisting of single-family detached and townhomes, and with associated development waivers and modifications. The project area is located on the north side of Vint Hill Road, on the west side of Pioneer Drive, and northwest of the intersection of Pioneer Drive and Vint Hill Road. The immediately adjoining properties to the north, east and west of the subject property are zoned A-1, Agricultural, and also designated RN-2, Residential Neighborhood.

I was asked by a neighbor of the development if their well might be impacted. This one development is unlikely to have specific impact; however, a significant portion of the area between Broad Run and kettle Run along Vint Hill Road is planned for residential development and will impact this watershed.

In general adding roads and buildings that prevent the infiltration of precipitation into the ground will reduce the replenishing (recharge) of the aquifer.  It is planned for the developments to be connected to the public water supply, but groundwater is an essential portion of the hydrology. This potentially impacts the stability and quality of the water supply to the Occoquan. The combination of reduce recharge and water filtration by eliminating forested areas and replacing them with compacted soils, pavement, buildings and over time the aquifer will be reduced.

Groundwater is used for water supply and serves to support steam flow between rain storms. Groundwater comes from rainwater and snow melting and  percolating into the ground. Typically, the deeper the well (thousands versus hundreds of feet) the further away is the water origination and the older the water. The groundwater age is a function of local geology, the amount of precipitation and the rate that water is pumped out of the aquifer. Geology also determines the ease with which water and contaminants can travel through an aquifer and the amount of water the land can hold. The thickness of the overburden and the soils of the overburden impact the rate of recharge.  The land surface through which groundwater is recharged must remain open and uncontaminated to maintain the quality and quantity of groundwater.

Groundwater availability varies by location within Prince William County (Nelms and Richardson, 1990). Precipitation and soil type determines how much the shallower groundwater that is tapped for private wells and hydraulically connected to the streams and rivers is recharged annually. The volume of water that can be stored is controlled by the reservoir characteristics of the subsurface rocks. The amount of groundwater removed from an aquifer needs to be sustainable and should ideally match the recharge rate which is reduced as open and forested land is replaced by development and the impervious surfaces it brings. Reducing the recharge rate by diverting surface flow and adding pavement and roads will result in changes in the local or regional hydraulic balance- a reduction in discharge to surface water at some other location, an increase in recharge from surface water, or a loss of storage in the aquifer by falling water table or some combination of these effects.

That area in Nokesville between Broad Run and Kettle Run is a mixture of diabase and shale and are interspersed lenses of Hydrogeologic group B and group C.  Hydrogeologic group B underlies a large portion of western 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.

frrom DEQ

The rocks dip to the west and northwest at 0 to 70° with the slope progressively steepening toward the western border fault. Closely spaced fractures and bedding-plane partings with a high frequency of intersections overlain by a thin cover of overburden are characteristic features (Nelms and Richardson, 1990). Rocks within hydrogeologic group B tend to have moderate to excellent water-bearing potential because of the close spacing and high frequency of intersections of fractures and partings. Ground-water storage tends to be predominantly in the fractures and partings of the rock because of the extremely thin cover of overburden. The highest reported yields in the county are from wells finished in the rocks of hydrogeologic group B.

A significant portion of the area around the Strathmore project area is Hydrogeologic group C . The rocks of group C are Early Jurassic in age and include: the Mount Zion Church, Hickory Grove, and Sander Basalts; an unnamed diabase; and thermally metamorphosed rocks. 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. Better yields have been obtained from wells finished in areas where the diabase is intersected by cross-strike lineaments (Nelms and Richardson, 1990, p. 25) and in areas underlain by basalt which also exist in Nokesville.

Based on hydraulic elevations it appears that the natural flow of groundwater would be east southeast as is common in Prince William County. However, there is considerable variation and surprises in the flow as documented by monitoring at several cleanup sites in the county. This one development is unlikely to impact the groundwater generally in the area, but it is part of a massive change in land use in the county in the last 20 years and the next. We need to monitor the condition of our water resources before we impair the water supply to any Prince William resident either through impaired groundwater or reduced flow of Broad Run and Kettle Run. Important tributaries to the Occoquan River, the Reservoir and ultimately the water supply for nearly a million residents of our region.

As the Department of Public Works pointed out in 2021 there are only two monitoring well in Prince William County that track groundwater levels. One well in the Culpeper Basin near the Loudoun County Line and one well in Forest Park in the Potomac Aquifer within the Coastal Plain. The data is publicly available for both wells, but they are not part of any state or local program of monitoring.

Those wells are not adequate to know if the groundwater supply is sustainable. Analysis by Loudoun County of the only groundwater well in the Piedmont region is that since 2004 the water level at that well has been falling a foot a decade.  The stream monitoring program run by PWSWCD has noted the significant change in the visible indicators of stream health in Broad Run and Kettle run over the same period. In 2010 the lower Broad Run watershed was only 8% covered in impervious surfaces. We are planning significant changes to the land use in the Lower Broad Run watershed. We need to understand the impact this will have on the water supply not only for the remaining well users but also for the entire region.

from BroadRunReport-Sept2012 Complete - scanned.pdf