REDUCTION OF RADIONUCLIDE TRANSPORT IN STORM WATER IN DP CANYON, LOS ALAMOS, NEW MEXICO

Treated radioactive liquid waste was discharged into upper DP Canyon in Los Alamos, NM from 1952-1985, with prior discharges of untreated radioactive liquid waste at the same location. The majority of the radiological material remaining in the treated discharge water (⁹⁰Sr, ²⁴¹Am, ^238&239/240Pu and ¹³⁷Cs) preferentially bound to sediment in the canyon and was then available for remobilization and transport by canyon flows from storm-water runoff. Small amounts of soluble contaminants, such as ⁹⁰Sr, remained in solution and have impacted shallow groundwater. DP Canyon is a tributary of Los Alamos Canyon, which flows into the Rio Grande upstream of a newly constructed water intake intended to supplement the City of Santa Fe, New Mexico’s drinking water supply. Remedial actions (soil/sediment/rock removal) were completed below the liquid waste outfall point in 1996-7 and again in 2002-3. A grade-control structure was installed slightly down-canyon of the outfall location in early 2010 to limit sediment transport from the upper reach of the canyon. A limited set of historical storm-water quality and flow data from DP Canyon just above the confluence with Los Alamos Canyon is available from 1967-8, 1996, 1998-2000, 2006. Recent samples were taken in 2010. These data span the various remedial actions and demonstrate an overall decrease in the levels of radionuclides transported in the canyon by storm-water runoff. The gross beta activity in the suspended sediment portion of the storm-water samples shows an overall decrease through time from 537 pCi/g in 1968 to 19.0 pCi/g in 2006 (mean activity of time-series samples during single events). The ⁹⁰Sr activity in solution also demonstrates an overall decrease through time from 852 pCi/L in 1968 to 8.1 pCi/L in 2006. The overall decrease in radiological contaminants being transported out of DP Canyon is likely the results of source term removal, dilution and radioactive decay.