Long-term groundwater transport of radionuclides from seepage basins at the Savannah River Site

The Savannah River Site (SRS) in South Carolina produced tritium and plutonium between 1953 and the beginning of the 1990s. The site released process wastewater containing plutonium, tritium, uranium, and fission products into seepage basins with the intent that short half-life radionuclides would decay in the years required for the groundwater to transport the waste materials to surface waters and that activity levels in the surface waters would not exceed levels considered appropriate in the 1950s. Between 1955 and 1988, the process operations at the F-area lead to the discharge of more than 12×106 cubic meters of low-level liquid radioactive waste solutions into unlined seepage basins. The waste contained longer half life nuclides that did not significantly decay during groundwater transit such as 3H (t½ = 12.28 years), 90Sr (t½ = 28.6 years), 99Tc (t½ = 2.13×105 years) and 129I (t½ = 1.57×107 years). Remediation started with the capping of the basin in 1990 followed by active plume pumping between 1997 and 2003. In 2004 a groundwater barrier was installed and in situ pH neutralization started in 2005. Tritium monitoring detected migration to Four Mile Creek by the end of the 1950s. Other radionuclides such as 90Sr, 99Tc and 129I have also been detected in groundwater seeping into the creek, and tritium levels and conductivity were well correlated at the seepage line. The seepage basin contaminated groundwater plume surfaced at seepage faces near a creek with a pH of 3. This acidity combined with high ionic strengths associated with nitrate mobilized contaminants such as 90Sr. The high levels of tritium, low pH and high conductivity at the seepage line show the likely importance of density driven flow for the salts of the plume and the limited dilution by groundwater flow. The Savannah River Site requires remediation to minimize radionuclide migration off-site, and there has been an extensive monitoring program of process waste discharges, groundwater levels of contaminants and migration to surface waters. This rich data set, which includes concentrations of longer half-life radionuclides, provides an opportunity to quantify reactive contaminant migration under field-scale conditions applicable to other sites. The monitoring record since 1955 is being mined to quantify the various mechanisms responsible for contaminant migration during waste disposal operations and various phases of remediation.