With groundwater testing nearly complete and the extent of the tritium plume determined, the U.S. Department of Energy, Brookhaven National Laboratory and the Environmental Restoration Division are prepared to begin operation of a groundwater remediation system.
BNL has completed the installation of an interim system designed to intercept the tritium plume, but, at the request of Suffolk County, will delay operation of the system until additional characterization and modeling are completed. This is expected to take approximately 2-3 weeks. The system uses a pump-and-recharge system to prevent tritium above the U.S. Environmental Protection Agency's drinking water standard of 20,000 picocuries per liter (pCi/L) from leaving the Lab site.
Pump-and-recharge
Under the plan, groundwater will be pumped from about 150 feet below the ground and piped 3,000 feet northward to an existing recharge basin within the BNL site. Three groundwater extraction wells have been installed 3,500 feet south of the High Flux Beam Reactor, where tritium concentrations are approximately 6,500 pCi/L. Each well will pump tritiated groundwater from the aquifer at a rate of about 40 gallons per minute.
Before being recharged, the water will pass through carbon filters to remove chemical contamination that is also present in groundwater in the area due to other past BNL activities. When the water enters the recharge basin, the tritium concentration is expected to be approximately 3,500 pCi/L, well below the drinking water standard. Samples will be analyzed on a regular basis to determine the tritium concentrations being recharged.
Dilution and decay
Once the water has re-entered the ground, it will flow southward, taking approximately 19 years to reach the BNL site boundary. By that time, natural decay and dilution will have diminished tritium levels to nearly undetectable levels. Monitoring wells located at the Lab boundary will assure that no tritium above the drinking water standard is leaving the BNL site.
The pump-and-recharge remediation is being conducted as a fast-track removal action to prevent tritium above the drinking water standard from moving further south. It also will give BNL and DOE time to study alternative remediation technologies and prepare a plan to address the high levels of tritium found immediately south of the HFBR.
The long-term treatment of the plume will be included in the Operable Unit III study, currently under way. The interim action has been reviewed by regulators at county, state and federal levels, and has been discussed with the community at four public information sessions and more than a dozen local civic meetings.
Understanding the plume
The tritium investigation began in January, when a monitoring well installed just south of the HFBR detected tritium at levels above the drinking water standard. The contamination's suspected source is the pool in the HFBR's lower level that is used to store the reactor's spent fuel rod assemblies.
After a three-month investigation that included the installation of more than 100 monitoring wells and analysis of more than 1,400 water samples, BNL now has a comprehensive understanding of the tritium plume's extent and depth. The plume is completely within the BNL site, as confirmed by recent results from the site boundary that show no detectable tritium.
The portion of the plume where tritium concentrations exceed the drinking water standard extends about 2,200 feet south of the HFBR, at depths ranging from 40 to 150 feet below land surface (see illustration). Tritium concentrations range from 660,400 pCi/L immediately south of the HFBR down to about 6,440 pCi/L 3,585 feet south of the HFBR.
All the findings are consistent with general knowledge about groundwater in the area, which has a general southward flow rate of about a foot per day, with some downward movement and natural dilution as it moves. The decrease in concentration further from the HFBR is also partly due to tritium's natural radioactive decay, which cuts its concentration in half each 12.4 years.