A project supported by the Subsurface Contaminants Focus Area has achieved a major milestone—deployment in June 2000 at Brookhaven National Laboratory in Upton, New York. By installing a viscous liquid barrier (OST/TMS ID 50) at Brookhaven LINAC Isotope Producer (BLIP) facility, MSE Technology Applications of Butte, Montana helped SCFA achieve the first hot deployment of a technology whose development and demonstration the focus area has supported since FY96 (see August 1996 and Winter 1998 issues of Initiatives). MSE is now testing the subsurface barrier’s performance to ensure that the gelled material—colloidal silica (CS)—is indeed containing the contaminated soil zone and preventing tritium, sodium-22, beryllium-7, iron-55, and manganese-54 from migrating to the groundwater. The barrier’s expected life span is 25 years, which far exceeds the half-life of the primary contaminants of concern.

The soils underlying the BLIP building were contaminated as a result of isotope production for medical and research purposes. The contamination extends about 6.5 feet outward from a tank under the building and includes about 95 cubic yards of activated soil. The barrier, which was installed as part of a interim action under Superfund, is designed to encapsulate the activated soil and prevent rainwater from seeping through.

Although DOE investigated three general types of barrier fluids, all of which are chemically and biologically inert, colloidal silica was chosen for the BNL deployment. Developed by Lawrence Berkeley National Laboratory in California, CS is a silicon-based substance that gels to form a hydraulic barrier after it’s injected into the subsurface. The VLB technology works by displacing the water and air that reside in the pore spaces of soil with a chemical grout that permeates the soil matrix and seals the pore voids. The barrier fluid containment performance is controlled by the gel time, which depends on factors such as pH, temperature, chemistry of the grout mixture, and geochemistry of the interstitial pore waters and the soil matrix.

"We injected a series of bulbs—each created with a predetermined quantity of material. We controlled how much we pumped down there to control the size of the bulb. Then the process is repeated by pushing down to the next horizon and doing the same thing again." North-Abbott also explained how previous lab and field work contributed to MSE’s knowledge of how CS behaves underground. "We demonstrated it first in big sand tanks in the laboratory. We packed the tanks with sand from BNL, put them into a load cell (large press), and applied pressures that would simulate those experienced at 30 feet below ground surface. Once the injected grout material was cured, we tested samples and then excavated the sand in the tanks to see the actual size and shape of the bulbs."

The CS material was emplaced using permeation grouting, a low-energy and minimally invasive method of injection that doesn’t disturb the soil matrix or damage structural features like tanks, pipes, or cables. The low pressures of permeation grouting—plus the fact that CS gels around infrastructure like the BLIP tank, piping, and footings—mean that installation of a VLB can be less problematic at facilities where existing structures must be protected.

To avoid sampling the actual barrier and destroying it in the process, MSE is using a test barrier that the company installed adjacent to the BLIP building and at a slightly shallower depth just prior to the construction of the actual barrier. The test barrier, consisting of three columns of three bulbs each, was formed with the same materials and construction process that MSE used for the actual barrier.

MSE has adopted a wait-and-see attitude. An early set of permeameter tests was performed about 60 days following installation to determine if lower subsurface temperatures were affecting the grout’s gel time. With these early tests, MSE validated that the cooler subsurface temperature was slowing gel time and that it would be necessary to run a set of tests after the grout had fully cured. "Because of the colder temperatures in the subsurface, we don’t feel it has cured long enough. We didn’t get the results that CS is capable of achieving, so we’ll wait and test again. This also happened in the lab. Once the grout enters the subsurface, it just takes longer to cure." The higher hydraulic conductivity data was also anticipated because the permeameter test locations were on the outer edges of the test barrier. "The outer edges of the barrier and the intersections of the grout bulbs are the worst case areas to test. They are likely to have higher hydraulic conductivity values because that part of the barrier is on the fringe of the bulb, where the grout doesn’t completely fill the pore space. In contrast, the central part of a bulb, near the injection point, which we will test during the next effort, should have a lower hydraulic conductivity."

MSE and the Subsurface Contaminants Focus Area have high hopes for this new technology and its ultimate application at other DOE sites where radiologically contaminated areas require some type of interim or long-term containment. Based on estimates from a cost model MSE previously developed for DOE, costs for this low-maintenance technology are approximately six times less than that for the baseline of excavation and disposal. Another advantage of the technology is that a VLB is emplaced using a low-energy delivery system, which prevents damage to fragile infrastructure, lowers the risk of bringing contaminants to the surface, and minimizes worker exposure.