Distribution of karst regions in Virginia in relation to proposed gas pipeline routes
The two proposed pipelines are the Mountain Valley Pipeline and the Dominion Atlantic Coast Pipeline. The proposed routes as of April 1, 2015, are shown in Figure 1. The map in Figure 1 also shows the areas of different geology that host karst features.
Figure 1.Map of karst regions in Virginia in relation to the proposed natural gas pipeline routes.
The areas of greatest potential impact to karst resources are located in the western portion of Virginia, within the Valley and Ridge physiographic province. The colored areas shown in the map of Figure 1 indicate regions underlain by soluble carbonate bedrock (primarily limestones and dolostones) that contain an abundance of karst features. The gray-to-black clusters of spots indicate the density of mapped closed depressions, or sinkholes, within a 1-square-kilometer region; the darker the cluster, the greater the density of sinkholes.
Dominion Atlantic Coast Pipeline
The proposed route of the Dominion Atlantic Coast pipeline is shown in orange in Figure 1. The two areas outlined in black (labeled A and B) are discussed in further detail below. These areas contain the greatest concentrations of karst features that could be impacted by this pipeline.
In region A, the proposed Dominion Atlantic Coast Pipeline route passes through karst areas in Highland County, as shown in Figure 2. The proposed route passes through a zone of a high concentration of known sinkholes south of Monterey, shown in more detail in Figure 3 and Figure 4.
Figure 2. Route of proposed Dominion Atlantic Coast pipeline route through Highland County, regions with geologic potential for karst, and locations of known sinkholes. Area outlined in black is shown in greater detail in Figure 3 and Figure 4.
Highland County Cave Survey Karst Feature Map PR-4
The proposed pipeline route descends Monterey Mountain on an unnamed ridge west of Hannah Airfield. The karst is composed of Silurian-Devonian carbonates and the structure is the Monterey Syncline. The unnamed stream to the north of the route is a sinking/losing stream, as is the unnamed stream to the south of the route. All 14 streams originating on the east side of Monterey Mountain from Monterey in the north to Vanderpool to the south are sinking/losing streams. None of these sinking points have been dye traced to their spring resurgences. Most of these streams flow into an unnamed valley locally known as Sinking Creek Valley. Sinking Creek Valley is >2 miles long with the limestones continuing approximately another mile and a half to Vanderpool and then beyond (Figure 3).
There are five documented caves in this band of limestone between Monterey and Vanderpool. To the north of the proposed pipeline route is Sawmill Cave. To the south of the route is Sinking Creek Valley Cave, Meeks Cave, 9mm Pit, and Vanderpool Shaft. Sawmill Cave is less than a half-mile north of the proposed pipeline route. It is the sinking point for a small blind valley. It has been tentatively traced to Mackey Spring, approximately 3 miles to the south, when sawdust was pushed into the cave and it resurged at the spring. A formal dye trace should be conducted by competent professionals to confirm this connection.
Figure 3. Highland County Map PR-4, showing the route of the proposed Dominion Atlantic Coast Pipeline and locations of known karst features near where the pipeline crosses west of State Route 220 and south of Monterey.
Sinking Creek Valley Cave is located approximately .25 miles south of the proposed pipeline route. This cave is a sinking point for the unnamed stream above it and has not been dye traced to its spring resurgence. This cave is an atmospheric karst feature that inhales or exhales air depending on the outside temperature and the barometric pressure at the time. This indicates this cave is connected by an air-filled conduit to another open karst feature that may or may not be documented. At the time of this cave’s documentation indications were that this second entrance was higher than Sinking Creek Valley Cave.
Meeks Cave is approximately a mile south of the proposed pipeline corridor. This cave is in a sinkhole and not in a streambed. This cave also is an atmospheric karst feature and the karst feature it is connected to is unknown. At the time of its documentation indications were that the air was flowing to a lower entrance. 9mm Pit is located approximately 1.67 miles south of the proposed pipeline route. Its entrance is on the side of a hill and is not in a sinkhole or a sinking point.
Vanderpool Shaft is located approximately 1.81 miles south of the proposed pipeline corridor. This deep cave reaches the water table and has a water-filled passageway that appears to continue to the south. This cave has not been dye traced to its spring resurgence. This cave has not been inventoried for invertebrate fauna. It does contain Plecotus townsendii, Virginia Big-eared bat, which is on the Endangered Species List (Virginia, 2015) and the cave appears to have suitable Virginia Big-eared bat habitat characteristics.
A known Indiana bat, Myotis sodalis, roost tree is within 3-4 miles of the proposed pipeline corridor (see Attachment 1).
Mackey Spring is located south of Vanderpool Gap. Virginia’s Department of Game and Inland Fisheries (DGIF) has stated that Mackey Spring is extremely vital to a healthy upper Jackson River. It has been described as a 5 “transformational water source to the river” (pers. comm., DGIF, 2015). The DGIF Cold Water Stream data base shows Mackey Spring transforming the Jackson River from a Class VI stream to a Class II stream.
The band of karst exposed in Sinking Creek Valley is the western limb of the Monterey Syncline trough which is covered in the center of the valley by shale and sandstone, and which reemerges on the western slope of Jack Mountain as another band of karst in the eastern limb. While these bands of karst recharge spring resurgences along the Jackson River, they also recharge the deeper aquifer containing the Town of Monterey’s municipal wells. Well #1 and Well #3 are approximately 1.78 and 1.56 miles respectively from where the proposed pipeline corridor crosses this band of karst. The pumps are approximately 427 feet and 865 feet below the lowest point where the proposed pipeline corridor crosses this band of karst.
Highland County Cave Survey Karst Feature Map PR-5
The proposed pipeline route crosses Route 220, the Jackson River, and Jackson River Valley and ascends Jack Mountain on an unnamed ridge. The carbonates are Devonian-Silurian and the structure is the Monterey Syncline. The seven streams to the north of the proposed pipeline corridor are sinking/losing streams and the five streams to the south of the proposed pipeline corridor are sinking/losing streams. None of their sinking points have been documented and none have been traced to their spring resurgences (Figure 4).
Figure 4. Highland County Map PR-5, showing the route of the proposed Dominion Atlantic Coast Pipeline and locations of known karst features near where the pipeline crosses east of State Route 220 and south of Monterey.
Needle’s Eye Cave is .4 of a mile south of the route.
Eight known Indiana bat, Myotis sodalis, roost trees are within 3-4 miles of the proposed pipeline corridor (see Attachment 1).
The band of karst exposed on the western flank of Jack Mountain is the eastern limb of the Monterey Syncline trough which is covered in the center of the valley by shale and sandstone, and which reemerges on the eastern slope of Monterey Mountain as another band of karst in the western limb. While each of these bands of karst recharge spring resurgences along the Jackson River, they also recharge the deeper aquifer containing the Town of Monterey’s municipal wells. Well #1 and Well #3 are approximately 1.70 and 1.95 miles respectively from where the proposed pipeline corridor crosses this band of karst. The pumps are approximately 568 feet and 1006 feet below the lowest point where the proposed pipeline corridor crosses this band of karst.
Summary for Highland County
The proposed pipeline corridor has sinking/losing streams to its north and south. Indications are that an undetermined karst feature greater than 3 miles in length, stretching from Sawmill Cave (and possibly farther north and west) to Mackey Spring, lies under Sinking Creek Valley and the hills south of it. Nothing is yet known of the invertebrate fauna of this karst feature or its associated satellite karst features. Habitat alteration due to sedimentation is potentially a threat that can be caused by construction of the proposed pipeline in this area of relatively steep slopes. Any major release of sediments or slope failure can potentially change conduit habitat, block recharge sites, or alter flow volume and velocity. Siltation can drastically modify gravel riffle and pool habitats and contaminated sediments can have detrimental effects on cave life (USDA, 2001). Due to the Jackson River being a Class II trout stream down gradient of the proposed pipeline, the discharge of hydrostatic test water must be prohibited within the Jackson River Valley and must be confined to holding ponds in a non-carbonate area. All sinking points down gradient of the proposed pipeline corridor must be identified and protected from potential engineering failures that could cause catastrophic releases of sediment, or possible slope failures. Their companion springs must be located and sampled for invertebrate fauna. The proposed pipeline corridor may be the recharge area for more than one spring. All possible springs must be identified and their recharge area protected. Several caves are known to exist in the vicinity of the pipeline route. Two of these caves have active air currents that inhale surface air from one entrance and exhale it from another. Methane from a leak on the east side of Monterey Mountain or in Sinking Creek Valley could be sucked into either of the two known caves that suck air or their unknown connection points and produce explosive atmospheres which would be detrimental to cavers. Potential methane entrapment within caves would present a significant hazard.
Approved surveys must be conducted for the presence of the Indiana and Northern Long-eared bats. All slopes of greater than 25% must be limited to open trenches of 500 feet or less. The Cave Board recommends that Dominion Transmission Inc. maintain a 100-foot buffer around all karst features when blasting, drilling, digging, or trenching. Any engineering failures in Sinking Creek Valley may cause irreparable damage to the underground karst drainage conduits and the health of the Upper Jackson River and the Town of Monterey’s municipal wells. The sinking points in Sinking Creek Valley must be dye traced to their spring resurgences and the caves and resulting springs must be inventoried for invertebrate fauna. The spring recharge areas must be protected. All sinking points down gradient of the proposed pipeline corridor must be identified and protected from potential engineering failures. Extra time, money, and oversight must be put into any plan to cross this valley with a construction project of this magnitude. Even with the best “Best Management Practices,” this segment of this route may be too risky and it is recommended the proposed pipeline corridor be re-routed around Sinking Creek Valley and Mackey Spring.
The concentration of known sinkholes in this region suggests intensive karst development is present in this portion of Highland County. The Cave Board recommends application of heightened practices for environmental protection when constructing the pipeline through this area (see section “General recommendations on pipelines through karst regions” below), and local re-routing of the route to avoid passing directly over karst features. Particular attention needs to be paid to ensuring slope stability onsteep slopes above karst valleys and within karst areas.
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