The fractured basalt underlying the Eastern Snake River Plain in southeast and south-central Idaho acts much like a giant underground sponge. Ground water fills in the cracks and crevices in the rocks, creating the Eastern Snake River Plain Aquifer. Just as a sponge can hold germs and cleaning chemicals after being used, the aquifer also holds contaminants from the INL. Several "plumes" of ground water contamination reside under the INL and are direct results of INL activities. The plumes contain both hazardous and radioactive components.

The locations of the plumes and the concentrations of contaminants in them are tracked through routine monitoring and special studies. Monitoring shows the overall concentrations of contaminants in all the plumes are decreasing and none of the plumes are above drinking water standards outside the INL boundaries. Some of the plumes are being treated to remove contaminants, while others are simply being monitored. Cleanup activities at the INL are helping to decrease contaminant concentrations, as are the natural processes of radioactive decay and chemical breakdown.

Ground water monitoring wells help scientists track the progress of a plume, and determine what is happening with the contaminants in the water. Monitoring wells also help us figure out what types of material—rock, sand, clay—are under the ground at a specific location. Although we would like to have more information, we don't drill wells indiscriminately. They're expensive to drill—at least $100 per foot on the INL—and are typically 500 to 700 feet deep. If not drilled properly, wells can also provide a pathway for contamination to travel from the surface of the earth or between zones of the aquifer.

The INL Oversight Program, the U.S. Geological Survey, the U.S. Department of

Energy, and other agencies and organizations continuously monitor ground water, surface water, and drinking water on and near the INL and in the Magic Valley area. This monitoring allows scientists to track the plumes of contamination from the INL.

Most of the liquid radioactive effluent injected into the aquifer at the INL was tritium. Tritium disposal at the INL began in 1953 and continued steadily until the 1980s and 1990s, when it dropped off sharply.

Because of the effect of half-life (tritium has a 12.3 year half-life, meaning half of the tritium present decays every 12.3 years), the concentration of tritium has been decreasing ever since.

Testing of ground water for tritium began in 1956, and became routine in 1961. Also in 1961, the U.S. Geological Survey first mapped the contaminant plumes in the groundwater beneath TRA and INTEC. A 1970 map shows that tritium plumes beneath TRA and INTEC were continuing to spread. In 1977, the U.S. Geological Survey found that the TRA and INTEC plumes had merged into one. This plume, the largest under the INL, is now effectively shrinking instead of expanding. While the pollutants are still being carried further and further from their source at TRA and INTEC, they are decaying and becoming diluted to a point that they are no longer detectable, which makes the plume of contamination appear to shrink.

No contaminants from the INL have been found in or near the Magic Valley, and it is highly unlikely they ever will. Cleanup programs on the INL, better disposal methods, and less contaminated wastewater to begin with, coupled with natural dilution, chemical breakdown, and radioactive decay of the contaminants, should cause the detectable plumes to continue to shrink under the INL.

Although the injection wells have been closed down, and in many places lined ponds have replaced percolation ponds, the contaminant legacy will remain in the Eastern Snake River Plain Aquifer for many years. However, the healing process in the aquifer has begun, and technologies now being studied may help bring the aquifer back into good condition.