Seepage from proposed Black Rock Dam could affect U.S. nuclear site

A Bureau of Reclamation report finds that construction of the proposed Black Rock Dam and reservoir in central Washington could result in seepage and groundwater migration toward the federal government’s contaminated Hanford Nuclear Reservation.

�Since the potential Black Rock Dam could retain water up to about 600 feet deep, there is concern that hydrostatic pressure could result in a significant increase in aquifer head along the western boundary of the Hanford Reservation,� the technical water seepage study said. �The increased head condition could, in turn, cause migration of contaminants from the Hanford Reservation to the Columbia River.�

Black Rock is one of two alternatives BuRec and Washington’s Department of Ecology are evaluating as opportunities to increase water storage capacity and improve conditions for fish in the Yakima River Basin. (HNN 1/15/07) The other alternative would involve a proposed Wymer Dam and reservoir. Both projects could include hydropower plants.

The report, released Sept. 18, estimates a range of volume and direction of seepage that could be expected if Black Rock Dam and reservoir were constructed. Modeling in the report describes the potential for reservoir seepage into underlying aquifers and the effects on aquifer head, groundwater flux, and groundwater discharge into creeks, drains, and springs. It considers previous hydrogeologic studies and incorporates results of geologic drilling and aquifer testing.

Estimated seepage rates are highest for the first 13 months after the reservoir begins filling, and then decline rapidly over the next four years. Annual reservoir seepage rates in the first year or so after the reservoir begins filling range from 72,900 to 121,000 acre-feet. Five years after the reservoir is first filled, total reservoir seepage is expected to have declined to between 32,100 and 54,300 acre-feet annually.

Drilling at Black Rock revealed fractured and faulted basalt in an area that abuts the south side of the proposed dam, the report said. While the fractures are not expected to affect the stability or safety of the dam, the report states extensive fracturing could cause water to flow in the subsurface, east toward Hanford Reservation and south toward the Yakima River.

Hanford: �World’s largest environmental cleanup project�

The 586-square-mile Hanford site is located along the Columbia River in southeastern Washington. The U.S. Department of Energy describes Hanford, a plutonium production complex with nine nuclear reactors and associated processing facilities, as the �world’s largest environmental cleanup project.�

The report makes no conclusions or recommendations as to whether seepage at Black Rock should eliminate the site from further consideration for water storage. However, it does say additional multi-well aquifer testing is essential to reduce uncertainty in model predictions of reservoir seepage.

It said additional testing also would help determine what mitigation measures would be most effective in controlling seepage rates. It said mitigation could include cut-off walls, grout curtains, drainage systems, and pumping wells.

BuRec will use the seepage analysis to help identify potential effects to the Hanford Reservation and possible mitigation measures in a draft planning report/environmental impact statement, scheduled for release in January 2008.

Congress authorized BuRec in 2003 to initiate the Yakima River Basin Water Storage Study to examine the feasibility and acceptability of storage augmentation in the Yakima River Basin. That authorization included the study of a potential Black Rock dam and reservoir, in Washington’s Black Rock Valley.

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