Thursday, May 19, 2016

Groundwater of Prince William County

from USGS
About 25 years ago the U.S. Geological Survey studied the groundwater systems within Prince William County, Virginia. The following is based on that report.

Private wells draw their water from groundwater. Geology, climate, weather, land use and many other factors determine the quality of the groundwater. Within Prince William County Virginia there are four distinct geologic provinces: (1) the Blue Ridge, (2) the Culpeper Basin, (3) the Piedmont, and (4) the Coastal Plain. The U.S. Geological Survey divides the four geologic provinces of the county into seven hydrogeologic groups based on the presence and movement of the ground water calling them groups: A, B, B1, C, D, E and F.

The quantity and quality of ground water in Prince William County varies across the county depending on the geologic and hydrogeologic group you are in. The rocks in the Blue Ridge, Piedmont, and Coastal Plain contain minerals that are resistant to weathering, and the ground water tends to be acidic having low concentrations of dissolved constituents. Generally, ground water is soft (slightly acidic) to moderately hard in the Blue Ridge, Piedmont, and Coastal Plain and soft to very hard in the Culpeper Basin. Hydrogeologic group A is within the Blue Ridge formation; hydrogeologic groups B, B1, and C are in the Culpeper Basin; hydrogeologic groups D and E are in the Piedmont; and hydrogeologic group F includes unconsolidated material of the Coastal Plain and overburden in the other provinces.

Hydrogeologic group A underlies the northwestern part of Prince William County on Bull Run Mountain, which is part of the Blue Ridge geologic province, and consists of Early Cambrian metasedimentary rocks. Because of the thin to absent cover of overburden, ground-water storage predominantly is in the fractures in the bedrock. Areas underlain by Quaternary mountain-wash deposits along the base of Bull Run Mountain may have ground water stored in these deposits.

Hydrogeologic group B underlies 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 and this is where I bought a house. 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 B1 is a subset of group B with similar rock types, structure, and water-bearing potential; except that B1 is beneath group B at 500 feet below grade and evaporitic minerals tends to increase at depths. The predominant mineral is gypsum (CaSO4) though it does not appear to impact taste.

Hydrogeologic group C, which is interspersed throughout the area of groups B and Bl, 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 likely to have a dry well and these wells tend to become mineralized and loose flow over time. The best wells are in the basalt.

Hydrogeologic group D is located within the Piedmont formation and consists of three igneous plutons in the eastern part of Prince William County: the Goldvein, Lake Jackson, and Occoquan Plutons. Rocks within hydrogeologic group D tend to have moderate water-bearing potential and ground-water storage tends to be predominantly in the overburden. Wells in this area are most susceptible to drought and tend to be slightly acidic.

Hydrogeologic group E is also in the Piedmont formation in the eastern part of the county, and consists of metasedimentary, metavolcanic, and other metamorphic rocks. Rocks within hydrogeologic group E tend to have poor to moderate water-bearing potential, and thin- to thick cover of overburden. Similar to the rocks of hydrogeologic group D, ground-water storage tends to be predominantly in the overburden. Some of the poorest yielding wells are located in this hydrogeologic group.

Hydrogeologic group F is an approximately 5 mi in width, at the very eastern edge of Prince William County. This area is east of the Fall Zone in the Coastal Plain. The geology consists predominately of sand, silt, clay, lignite, gravel, soil, and weathered bedrock. Because of the sand and gravel this area tends to have very good to excellent water-bearing potential and wells in the Potomac Formation of the Coastal Plain tend to have high yields. There is a possible interconnection between the aquifers and the Potomac River.

Generally speaking, the groundwater in the county is recharged in elevated areas between stream valleys and channels and discharges to streams and estuaries. The paths and duration of groundwater flow are different between consolidated rocks and unconsolidated material. Groundwater in the consolidated rocks flows through the system of fractures following a circuitous path before discharging to a stream or estuary. In unconsolidated material, ground water generally follows a direct path from the recharge area to the discharge area. Well yields tend to be highest in the rocks of hydrogeologic groups B and B1, which can be attributed to the closely spaced fractures, joints, and bedding-plane of this fractured rock system. This area is most likely to produce a reliable and water rich well. Well yield in hydrogeologic group C range widely with the best yield coming from the basalt..

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