The Savannah River Site is reducing the flow of hazardous and radioactive metal contaminants to South Carolina’s rivers and streams by injecting a mix of clean water and baking soda into the site’s groundwater. The base mix neutralizes groundwater that has become acidic as a result of SRS’s chemical separations work, helping restrict the flow of contaminants.
According to Savannah River Nuclear Solutions (SRNS), since it became the Department of Energy’s management and operations contractor for SRS in 2008, 126.4 million gallons of base solution have been injected into the groundwater at the site’s F Area, and 46.3 million gallons at H Area.
The source: Low-level radioactive waste solutions were generated from SRS’s now shuttered F Area Canyon and the still active H Area Canyon chemical separations facilities. For decades, the waste was disposed of in pond-like pools of water known as seepage basins, which was the accepted method at the time. With advances in waste processing, this disposal method became outdated. The last of the basins were backfilled, capped, and closed in 1991, but 33 years of use had resulted in hazardous and radioactive contamination of the groundwater beneath the basins.
As part of post-closure care and corrective actions that were initiated after the basins were closed, two water treatment units were built to clean the contaminated groundwater by extracting, treating, and re-injecting the clean water. This method, however, was costly to operate, produced large volumes of radioactive sludge that was expensive to dispose of, and became less effective as the groundwater cleanup progressed.
A different approach: In 2004, SRS transitioned to a phased groundwater cleanup approach employing innovative remedial technologies. Underground barrier walls were installed to redirect and channel groundwater flow toward base injection zones to make the groundwater less acidic and reduce the migration of contaminants.
The new, more passive system provides for improved remedial effectiveness, is more cost-effective, and doesn’t create radioactive waste that has to be managed and disposed of, SRNS said.
“The groundwater under the basins is acidic from nitric acid present in the waste solutions,” said Jeffrey Thibault, a hydrogeologist at SRNS. “The acidic property of the water allows some contaminants to remain dissolved and slowly migrate with the groundwater toward a nearby stream. Our system uses a wall to direct the water into open spaces, called gates, where a base solution is injected into the water to increase the pH and slow down the movement of the contaminants.”
The process: The environmentally harmless base solution is made by mixing clean water, from Savannah River’s drinking water system, with a base concentrate and baking soda. A system of pumps, sensors, and piping, mounted on a small metal skid platform, blends the concentrate with water in precise proportions prior to delivering the solution through underground pipes to a network of 24 injection wells at F Area.
The wells are turned on when treatment is needed to neutralize the acidic water. Once enough base solution has been injected, the wells are turned off and remain off for a period of 12 to 18 months, until sampling data show that treatment is required again. A similar system is in use in H Area.
“We’ve seen great results from this method,” Thibault said. “It’s effectively a passive system, meaning we only run it every few months, when testing shows that the acidity is increasing in the soil again. It’s economically friendly, and, most importantly, it’s environmentally friendly. It restores the pH of the groundwater to more natural conditions and lets nature essentially take care of itself.”