FAQ: Performance of the Soil Consolidation Cell

Georgia-Pacific constructed a soil consolidation cell at the mill site between May and September 2009. As described in the questions and answers below, the drainage ditches installed on the east and south sides of the cell will be modified during the 2010 construction season.

To ask additional questions or for more information, email the project team or talk to project staff at the Project Visitor Center. The Project Center is located at 215 South Main Street in Fort Bragg.

Click on a question below to see its answer, or scroll down to view all FAQs.

What is the soil consolidation cell?
How was the consolidation cell designed and constructed?
Is the consolidation cell performing as designed?
Why are there tanks on the mill site?
What will be done with the water in the tanks?
What will be done to improve stormwater drainage around the cell?
When will the drainage improvements be implemented?
Is groundwater around the cell safe until storm water drainage improvements are installed?
Does this issue delay other work ongoing at the mill site?
Where can I find more information about the consolidation cell and other mill site activities?

What is the soil consolidation cell?

The Consolidation Cell is an engineered containment system built in 2009 to receive approximately 13,000 cubic yards of non-hazardous dioxin-containing soil removed from Operable Unit A (OU-A) at the mill site. It is approximately 1.6 acres in size and located more than 1,000 feet away from the coastal bluff. View a cross-section illustration of the cell.

How was the consolidation cell designed and constructed?

The engineering design of the cell was completed and approved by Department of Toxic Substances Control (DTSC) and the Regional Water Quality Control Board (RWQCB) in March 2009. Based on regulatory requirements, the design includes both a bottom and top liner to contain the soils inside the cell, which is described in more detail below.

The cell area was excavated to a depth of approximately 6 feet, and an impermeable 40-mil thick polyvinyl chloride (PVC) liner was placed over the bottom of the excavation. Seams between the large sheets of PVC liner were sealed according to specifications, and a drainage layer and system were then placed on top of the PVC. The soil from OU-A was then placed into the cell and smoothed out to receive the cover system. The cover system, which includes a geosynthetic clay liner (GCL), was placed over the soils, followed by 18 inches of clean soil. A heavy-duty wire mesh is built into the soil cover to prevent access by burrowing animals. Read more details about the consolidation cell design.

Is the consolidation cell performing as designed?

Yes. Based on groundwater monitoring results and a detailed inspection completed in February 2010, the soils that were placed in the cell remain safely secured, and the 40-mil PVC liner that lines the cell is performing as designed.

Groundwater monitoring wells were installed both upgradient and downgradient of the cell so any leaks in the cell could be detected. Groundwater monitoring is conducted quarterly, with all data reported to DTSC. Since installation, there have been no groundwater impacts from the cell. March 2010 data results will be available in a few weeks.

However, the drainage ditches surrounding the cell that were designed to divert stormwater are currently not performing correctly.

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Why are there tanks on the mill site?

The cell is constructed with ditches along its perimeter to divert stormwater runoff. The cell also has its own drainage system. From the time the cell was completed in September 2009 through early January 2010, water in the cell′s drainage system was within normal parameters.

During a period of unusually heavy rainfall in mid-January 2010, the amount of water in the cell’s drainage system increased. It soon became apparent that the stormwater diversion ditches – shown on the right side of the illustration below – were not functioning adequately. Georgia-Pacific mobilized engineers to the mill site shortly after this event to inspect the cell, confirm there was no leak in the liner or cover systems, and investigate the potential cause of the higher than expected volume of water in the cell's drainage system. The inspection and investigation confirmed that there were no leaks, and that excess stormwater from over-saturated stormwater diversion ditches was the source of the excess water in the drainage system. The excess water is currently being pumped out and stored in tanks. Again, there are no leaks, and no impacts to groundwater are expected. This will be confirmed when the March data for groundwater monitoring samples, due in early April, are received.

Cross-Sectional View of the Consolidation Cell Click image to enlarge.

This cross-section illustration of the consolidation cell shows the perimeter drainage ditch and the anchor trench on the right. During the heavy rains of January 2010, these elements of the cell were not able to adequately drain away the excess stormwater. Under the operations, monitoring, and maintenance (OM&M) plan for the cell, improvements to the drainage system will be made during the 2010 construction season.

What will be done with the water in the tanks?

The water being pumped into the tanks is being filtered to remove any particulates that could contain dioxins. A disposal plan for the water is currently being developed in consultation with DTSC and the RWQCB.

What will be done to improve stormwater drainage around the cell?

In consultation with DTSC, the RWQCB, the California Coastal Commission (CCC), and others, the drainage ditches around the entire cell will be modified to improve their capacity to collect and drain away stormwater runoff. Since existing drainage ditches were unable to keep up with the unusually heavy rainfall experienced in January 2010, they will be deepened. As an additional precaution, project engineers also will improve how water drains from the "anchor" trench where the edges of the 40-mil PVC bottom liner and GCL cover liner meet and are anchored and sealed around the perimeter of the cell (shown in the graphic above). Anchor trench drainage improvements will be installed along the north side of the cell, which is where engineers believe such improvements will provide the greatest benefit.

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When will the drainage improvements be implemented?

It is important to complete the drainage improvements before the start of the 2010-2011 rainy season. On behalf of Georgia-Pacific, ARCADIS has proposed the improvements to the DTSC, RWQCB, and CCC so that detailed design of the improvements can be evaluated and associated permits amended and approved.

Is groundwater around the cell safe until stormwater drainage improvements are installed?

Quarterly groundwater monitoring will continue, with results reported to the DTSC as planned and as required by the operations, monitoring, and maintenance (OM&M) plan for the cell. The depth of groundwater near the cell also is monitored, and results indicate that at no time has groundwater been in contact with the consolidation cell. The depth of the cell was specifically designed to remain a minimum of 5 feet above the water table (depth of groundwater), and even during the extreme rainfall this winter the groundwater level did not rise enough to come in contact with the cell.

Does this issue delay other work ongoing at the mill site?

The proposed permanent solutions for improving stormwater drainage around the consolidation cell and for disposal of the excess stormwater already collected are being managed under existing regulations, permits, and the OM&M plan for the cell, and therefore have little impact on other important work at the mill site. For example, preparation of a land use plan for future redevelopment of the mill site continues. And the remedial investigation for Operable Units C and D is nearly complete, with a draft Remedial Investigation (RI) report submitted to DTSC in March 2010, and the associated remedial action plan expected to be ready later this year for DTSC review and public comment.

Where can I find more information about the consolidation cell and other mill site activities?

Additional information, reports, diagrams, maps, FAQs, links, and other resources are available on this project website. As the process for design and approval of the proposed stormwater drainage improvements are completed, updates will be posted to the website. Project reports and other documents are posted on the Technical Reports page, and are available at the Project Center located at 215 South Main Street in Fort Bragg.

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