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SUCCESS STORIES: Virginia Zebra Mussels

Virginia Department of Game & Inland Fisheries (VDGIF)
Contacts: Ray Fernald (804-367-8364), Brian Watson (434-525-7522)

Introduction:
Zebra mussels were identified in Millbrook Quarry, in Prince William County, as the first infestation site of this invasive exotic species in Virginia. An application of potassium chloride over a 3-week period (at 100 mg/l) was used to eradicate zebra mussels from the quarry.

Background:
In August 2002, the VDGIF received a report that zebra mussels were present in Millbrook Quarry, a 12-acre, 93-foot deep abandoned rock quarry extensively used for scuba diving. Following confirmation, a 3 ½ year effort was launched establishing an interagency workgroup to assess the feasibility of eradicating the population. A panel of biologists, chemists, engineers, and human health experts representing seven VA agencies evaluated proposals submitted by companies for the eradication work. Concurrently, other popular dive sites and reservoirs, as well as other potential sites throughout Virginia, were surveyed for zebra mussel infestations.

A comprehensive Environmental Assessment was completed prior to initiation of the treatment, potassium at twice the minimum concentration needed to kill zebra mussels (read complete assessment at: www.dgif.virginia.gov/zebramussels/). The entire quarry was injected with 174,000 gallons of potassium chloride solution over a 3-week period to yield a final diluted concentration of 100 mg/l, which posed no human health risk and did not harm any non-molluscan aquatic wildlife, vegetation, or terrestrial wildlife. Potassium will provide long-term (estimated at up to 33 years) protection of Millbrook Quarry against future infestation by zebra mussels.

Project:
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 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.

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 was provided through a Wildlife Habitat Incentive Program (WHIP) grant from the VA Office of the Natural Resources Conservation Service of the USDA, and through a State Wildlife Grant from the U.S. FWS. The local water authority (Fairfax Water), Prince William County, the City of Manassas, and Dominion Virginia Power contributed the matching funds required for the federal grants. VA Secretary of Natural Resources L. Preston Bryant, Jr. said of the successful eradication, “The existence of zebra mussels in Virginia posed a very real threat to our natural resources and to our economy. The price of eradication was small compared to the potential millions of dollars that would have been needed to control zebra mussels had they escaped into adjacent waters, not to mention the permanent impact on the environment of the Commonwealth.”

Conclusion:
Evaluation of the treatment included several steps:

• Sampling at various depths and locations in the quarry after treatment revealed potassium concentrations ranging from 98 to 115 mg/l and no potassium leakage from the quarry into adjacent waters was detected.
• Over 1,000 mussels were scraped from rocks at numerous sites around the quarry revealing no live mussels.
• VDGIF scuba divers conducted a visual inspection of the quarry searching for live zebra mussels but finding none.
• Aquatic Sciences L.P. (the contractor) conducted extensive video survey and documentation of the dead zebra mussels through use of a robotic camera.
• Eighty bioassays of 100 live zebra mussels each were placed at various locations and depths throughout the quarry and thereby exposed to treated quarry water. After 31 days of exposure, 100% of the test mussels had died; none of the 100 “control” mussels died.

One of the challenges was determining the most appropriate treatment and obtaining approval to use it. Because the selected chemical method, injection of potash (potassium chloride) into the water was not a federally registered pesticide use, VDGIF had to secure approval for that use from VA’s Dept of Agriculture and Consumer Services and the U.S. Environmental Protection Agency. Possible impacts on nearby wells were also of concern. However, the final chloride concentration after treatment was measured at approximately 90 mg/l, well below the EPA/DEQ standard of 250 ppm for potable water.

Report submitted by:
Cathy Martin, Delaware Division of Fish & Wildlife