Millbrook Quarry Zebra Mussel Eradication
What is a zebra mussel?
Zebra mussels (Dreissena polymorpha) are freshwater bivalves (clams) native to the Caspian, Black, and Azov seas of eastern Europe. Since the zebra mussel's initial discovery in the United States in 1988, this bivalve has quickly spread throughout the Great Lakes and Mississippi River Basin states. Reproducing zebra mussel populations currently occupy waters in or adjacent to 25 states and extend westward into Nebraska, Kansas, and Oklahoma.
For more information on zebra mussels, how they are different from native species, and the damage they can cause, please see the zebra mussels section of our Web site.
What is Millbrook Quarry?
Millbrook Quarry, located in western Prince William County adjacent to Virginia Highway 55 and Interstate 66, was established in 1947 to produce road stone for construction of Virginia Highway 55. The 12-acre, 93-foot-deep quarry has been inactive since at least February 1963.
The Dive Shop in Fairfax first began using the quarry for scuba diving in the early 1970's, and has leased the quarry as a training and recreational dive site since 1978. Through The Dive Shop, the site is accessed by more than a dozen dive shops in the northern Virginia and Washington metropolitan region.
Background on zebra mussels at Millbrook Quarry
In late August 2002, the Virginia Department of Game and Inland Fisheries (VDGIF) received a report that zebra mussels were present in Millbrook Quarry in Prince William County. Within days, VDGIF confirmed the species' identification; recognizing Millbrook Quarry as the first infestation site of this invasive exotic species in Virginia.
Since the initial discovery, VDGIF has worked with many federal, state, and local agencies and individuals to pursue eradication of the population. Primary actions included:
- Establishing an interagency Millbrook Quarry Workgroup
- Investigating the hydrologic, geochemical, and biological characteristics of the quarry and infestation
- Inspecting other popular dive sites and reservoirs for zebra mussel infestations
- Evaluating potential avenues for eradicating the infestation
- Surveying other potential infestation sites throughout Virginia
- Surveying Broad Run and Lake Manassas to ensure that zebra mussels had not escaped to those adjacent waters
- Securing funding for the eradication
- Issuing a Request for Proposals to eradicate the infestation
- Selecting a process and contractor to conduct the eradication
- Surveying Broad Run for occurrence of native mussels or other species that might be impacted by potassium seepage from the quarry, and
- Securing environmental review and approvals to eradicate the infestation.
For more information on these topics, please see the Environmental Assessment (PDF).
Why is eradicating zebra mussels important?
Elsewhere in North America and Europe, zebra mussel populations that colonize open or large water bodies are merely managed to reduce economic and ecological impacts, usually at great financial cost and accompanied by long-term loss of natural resources. Numerous water treatment and power facilities must regularly treat their systems to keep them clear of zebra mussels, beaches must periodically remove decaying masses of dead zebra mussels, and bottom-dwelling organisms are often covered by this exotic mussel.
In the United States, congressional researchers estimated that zebra mussels cost the power industry alone $3.1 billion in the 1993-1999 period, with their impact on industries, businesses, and communities exceeding $5 billion. If zebra mussels had not been eradicated from the quarry, and had escaped into adjacent waters, Fairfax Water estimated that they would incur an initial cost of $2-4 million for chemical feed facilities, in addition to $500,000 - $850,000 per year for chemicals and system maintenance. The City of Manassas would likely incur similar expenses to treat zebra mussels at its facility on Lake Manassas, and other private and public facilities throughout the Commonwealth would be at risk.
Many freshwater mussel populations (as well as other aquatic species) have been completely wiped out from areas that zebra mussels now colonize. Fifty-four percent (54%) of the native freshwater mussel species in Virginia are currently listed as endangered, threatened, or of special concern. If zebra mussels become widely established in this state, the effect on native freshwater mussel populations could be devastating.
Given the proximity of Millbrook Quarry to Broad Run and its extensive use as a dive location, it is highly unlikely that the zebra mussel population could simply have been forever isolated. Broad Run has historically flooded the bank separating it from Millbrook Quarry (1972, Hurricane Agnes), and unintentional transport of larvae, or veligers, by divers from the quarry to other state waters would be likely.
Diving in Millbrook Quarry could have been prohibited, restricted, or subject to additional costs or regulation if the infestation of zebra mussels was not eradicated.
How were zebra mussels eradicated from Millbrook Quarry?
To kill the zebra mussels through exposure to potassium, the entire quarry was injected with 174,000 gallons of potassium chloride solution over a 3-week period from January 31 to February 17, 2006. The solution was delivered each morning to the site, and then pumped from land-based storage tanks through a floating supply line to a 22-foot work boat outfitted with a specially designed diffuser assembly on its bow. Potassium concentrations throughout the quarry and in adjacent surface waters were measured each weekend during the treatment. The target concentration was 100 milligrams of potassium per liter of water (mg/l, or parts-per-million - ppm); far below the level that would invoke environmental or human health concerns, but more than twice the minimum concentration needed to kill all the zebra mussels. Sampling at various depths and locations in the quarry after treatment revealed potassium concentrations ranging from 98 to 115 ppm, and no potassium leakage from the quarry into adjacent waters has been detected.
No land disturbance was required, as the staging area and setup occurred within the disturbed uplands surrounding the quarry. No disturbance of substrate or bottom sediments within the quarry occurred. No land disturbing activities in or adjacent to Broad Run took place, though Broad Run is being monitored for groundwater infiltration of potassium from Millbrook Quarry.
How do you know the treatment was successful?
Concentrations of potassium were monitored at various depths along transects established throughout the quarry, both during and after "charging" of the quarry, to ensure that lethal concentrations were achieved and maintained. Then, several weeks after treatment was completed, four separate methods of confirming eradication of the infestation were implemented. First, over a thousand mussels were scraped from rocks at numerous sites around the quarry during informal assessments, revealing no live mussels. Second, VDGIF scuba divers who had documented the extent of the infestation during pre-eradication studies conducted a visual inspection of the quarry, searching for live zebra mussels but finding none. Third, Aquatic Sciences L.P. conducted extensive video survey and documentation of the dead zebra mussels through use of a robotic camera. Finally, eighty bioassays of 100 live zebra mussels each were placed at various locations and depths throughout the quarry and thus exposed to the treated quarry water. After 31 days of exposure to the treated quarry water, 100% of the test mussels had died. None of the 100 "control" zebra mussels held in untreated water drawn from Broad Run died during their bioassay period. In dramatic contrast, other aquatic wildlife including turtles, fishes, aquatic insects, and snails continue to thrive in the quarry.
Are there any risks to people or other wildlife?
No. At concentrations used in the quarry (100 parts-per-million) potassium poses no human health risks, nor will it harm any non-molluscan aquatic wildlife, vegetation, or terrestrial wildlife inhabiting the project site. In fact, you would need to drink about 19 gallons of Millbrook Quarry water to consume your daily recommended dose of potassium.
Will it affect nearby wells?
No. It is anticipated that there will be negligible, if any, impact on drinking water. There is no federal or Virginia water quality standard for potassium, but potassium chloride is widely used in home water softeners, and many health benefits are attributed to diets rich in potassium. The final chloride concentration in Millbrook Quarry after treatment has been measured at approximately 90 ppm, well below the EPA/DEQ standard of 250 ppm for potable water.
How long will this treatment protect the quarry from zebra mussel infestation?
Potassium will provide long-term (estimated at up to 33 years) protection of Millbrook Quarry against future infestation by zebra mussels. For more information, please see the Environmental Assessment (PDF).
Is scuba diving still allowed in the quarry?
Unrestricted use of Millbrook Quarry for recreational and instructional diving resumed on May 6, 2006.
How much did the eradication cost, and who funded it?
The contract awarded for the eradication and bioassays totaled approximately $365,000, with another $54,000 awarded in contracts for post-project monitoring. Primary funding for the eradication was provided through a Wildlife Habitat Incentive Program (WHIP) grant from the Virginia Office of the Natural Resources Conservation Service of the U.S. Department of Agriculture, and through a State Wildlife Grant from the U.S. Fish and Wildlife Service. The local water authority (FairfaxWater), Prince William County, the City of Manassas, and Dominion Virginia Power contributed the matching funds required to facilitate receipt of the federal grants.
Detailed information about the eradication project is available in the Environmental Assessment (PDF). General information about zebra mussels can be found in the zebra mussels section of this Web site.