Occoquan Reservoir Protection Area (ORPA)
What is ORPA?
The Occoquan Reservoir Protection Area is an overlay that is designed to protect the Occoquan Reservoir, which is a drinking water source for over 800,000 people, a regional recreational asset and a water body with a history of impairment and partial restoration. Prince William County encompasses 40% of the approximately 600-square mile Occoquan watershed. In 2020, 43% of the 574,000 people in the watershed resided in PWC. As the most populous jurisdiction in the Occoquan watershed and the one with the largest land area, substantial changes in land use patterns in areas of Prince William County will impact water quality in the watershed and Reservoir.
The 2022 county Comprehensive Plan established the ORPA. The ORPA area contains some of the most environmentally sensitive land in the county and in the watershed area. There are a preponderance of steep slopes, erodible soils, streams and Resource Protection Areas within the overlay. The overlay area contains the highest tree cover per acre in the county. It provides a large percentage of the wildlife corridors in the county. Since the Occoquan River empties into the Potomac River and then the Chesapeake Bay, ORPA is also important for reducing nutrient and sediment loadings into those water bodies. Minimizing land disturbance and maximizing tree cover are key to achieving these goals.
ORPA was delineated along hydrological, geological and watershed boundary lines. Strategies to reduce negative impacts to the reservoir and protect natural resources are incorporated into the overlay. The soil type and geology of the area are particularly important to consider since rocks in the ORPA area are hydrogeologic group D and tend to have moderate water-bearing potential. Ground-water storage tends to be predominantly in the overburden, or the soil area above the rock formations. Wells in this area are most susceptible to drought and tend to be slightly acidic (see notation below).
The soil type and geological formations present in the area also affect septic system function. To properly process the nutrient load and avoid a build-up of nitrogen, which is an environmental and public health issue, there needs to be an acceptable distance between septic fields.
Housing density within the ORPA is recommended as 1 dwelling unit per 5 acres due to all the environmental features mentioned above.
Suggested additional ORPA language:
The ORPA area can be used as something of a testbed for protections that could possibly be used in other environmentally sensitive areas of the county. The following are some strategies that could be employed to enhance the environmental protections:
A minimum 100’ undisturbed buffer should be maintained for intermittent streams similar to what is required for perennial streams. Slopes of 25% or greater should be preserved in an undisturbed state, with minimal disturbance permitted on slopes ranging from 10% to 25%. Wetlands should not be disturbed, and a buffer maintained around them. This means that wetlands can’t be “relocated" to another area. An overall goal throughout the overlay of a Natural Open Space area of ideally 60% but no less than 30% within a parcel for new developments should be established. Conservation easements should be pursued for open space shown in the General Development Plan (GDP) of new applications. In areas where there is currently less than 10% tree coverage, reforestation should be proffered to meet at least the minimum tree cover goal above. To reduce salt input into the reservoir, brine or other low salt methods should be used as pavement deicers. A better outreach program to landowners on proper maintenance of septic systems is needed. A plan to promote low salt product alternatives should be implemented and include but not be limited to detergents, water softeners, and personal care products. Fertilizer best practices should be established and promoted to landowners and lawn/landscaping services to reduce further nutrient load via runoff. A notification for homeowners on RPA requirements should be undertaken by the county.
Notation:
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 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.
In the area of the ORPA is beyond the Culpeper Basin in the Piedmont region. This area of the ORPA is primarily hydrogeologic group D composed of igneous rock formations with limited lenses of hydrogeologic group E that transition at the bounds of the ORPA to group E and then to the Coastal Plain.
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. The igneous rocks have sub horizontal sheeting and near vertical joints overlain by thick overburden. Groundwater wells in the area tend to have yields range from 1.2 to 100 gal/min which has resulted in the development of homes with wells in the area.
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 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 in Prince William County are located in this hydrogeologic group and can be as low as 0.25 gallons per minute up to 70 gallons per minute- but tending towards the low end beyond the limits of what is the proposed ORPA. Homes and businesses in this area have depended on public water supply due to the limitations on well development.
What is ORPA?
The Occoquan Reservoir Protection Area is an overlay that is designed to protect the Occoquan Reservoir, which is a drinking water source for over 800,000 people, a regional recreational asset and a water body with a history of impairment and partial restoration. Prince William County encompasses 40% of the approximately 600-square mile Occoquan watershed. In 2020, 43% of the 574,000 people in the watershed resided in PWC. As the most populous jurisdiction in the Occoquan watershed and the one with the largest land area, substantial changes in land use patterns in areas of Prince William County will impact water quality in the watershed and Reservoir.
The 2022 county Comprehensive Plan established the ORPA. The ORPA area contains some of the most environmentally sensitive land in the county and in the watershed area. There are a preponderance of steep slopes, erodible soils, streams and Resource Protection Areas within the overlay. The overlay area contains the highest tree cover per acre in the county. It provides a large percentage of the wildlife corridors in the county. Since the Occoquan River empties into the Potomac River and then the Chesapeake Bay, ORPA is also important for reducing nutrient and sediment loadings into those water bodies. Minimizing land disturbance and maximizing tree cover are key to achieving these goals.
ORPA was delineated along hydrological, geological and watershed boundary lines. Strategies to reduce negative impacts to the reservoir and protect natural resources are incorporated into the overlay. The soil type and geology of the area are particularly important to consider since rocks in the ORPA area are hydrogeologic group D and tend to have moderate water-bearing potential. Ground-water storage tends to be predominantly in the overburden, or the soil area above the rock formations. Wells in this area are most susceptible to drought and tend to be slightly acidic (see notation below).
The soil type and geological formations present in the area also affect septic system function. To properly process the nutrient load and avoid a build-up of nitrogen, which is an environmental and public health issue, there needs to be an acceptable distance between septic fields.
Housing density within the ORPA is recommended as 1 dwelling unit per 5 acres due to all the environmental features mentioned above.
Suggested additional ORPA language:
The ORPA area can be used as something of a testbed for protections that could possibly be used in other environmentally sensitive areas of the county. The following are some strategies that could be employed to enhance the environmental protections:
A minimum 100’ undisturbed buffer should be maintained for intermittent streams similar to what is required for perennial streams. Slopes of 25% or greater should be preserved in an undisturbed state, with minimal disturbance permitted on slopes ranging from 10% to 25%. Wetlands should not be disturbed, and a buffer maintained around them. This means that wetlands can’t be “relocated" to another area. An overall goal throughout the overlay of a Natural Open Space area of ideally 60% but no less than 30% within a parcel for new developments should be established. Conservation easements should be pursued for open space shown in the General Development Plan (GDP) of new applications. In areas where there is currently less than 10% tree coverage, reforestation should be proffered to meet at least the minimum tree cover goal above. To reduce salt input into the reservoir, brine or other low salt methods should be used as pavement deicers. A better outreach program to landowners on proper maintenance of septic systems is needed. A plan to promote low salt product alternatives should be implemented and include but not be limited to detergents, water softeners, and personal care products. Fertilizer best practices should be established and promoted to landowners and lawn/landscaping services to reduce further nutrient load via runoff. A notification for homeowners on RPA requirements should be undertaken by the county.
Notation:
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 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.
In the area of the ORPA is beyond the Culpeper Basin in the Piedmont region. This area of the ORPA is primarily hydrogeologic group D composed of igneous rock formations with limited lenses of hydrogeologic group E that transition at the bounds of the ORPA to group E and then to the Coastal Plain.
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. The igneous rocks have sub horizontal sheeting and near vertical joints overlain by thick overburden. Groundwater wells in the area tend to have yields range from 1.2 to 100 gal/min which has resulted in the development of homes with wells in the area.
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 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 in Prince William County are located in this hydrogeologic group and can be as low as 0.25 gallons per minute up to 70 gallons per minute- but tending towards the low end beyond the limits of what is the proposed ORPA. Homes and businesses in this area have depended on public water supply due to the limitations on well development.
Your browser does not support viewing this document. Click here to download the document.