FLOOD01419 - Laserfiche WebLink

Home Browse Search

Water Quality Control at Rocky Flats: A Wet Detention Model for Stormwater RadionucHde Removal Ian Paton, Rocky Mountain Remediation Services, L.L.C. (RMRS) George S. Squibb, RMRS and Water Measurement Technologies, Inc. Gregory A. Wetherbee, RMRS and Water Measurement Technologies, Inc. Abstract The removal of radionuclides from stormwater using a wet detention pond was modeled for South Walnut Creek and Pond B-5 at the Rocky Flats Environmental Technology Site (Site), a Department of Energy facility formerly used for the production of nuclear weapons components. Site wet detention ponds are used to remove plutonium activity, predominantly bound to particulates, from stormwater. A model was developed to relate particle size and particle settling velocity within the pond using a variation of Stokes' Law. The model was used to estimate the minimum particle size that would be removed by settling, given estimated overflow rates based on the flow of stormwater through the pond. Using the plutonium activity in the storm water influent to the pond, the maximum allowable inflow rate that can occur without discharging plutonium in excess of stream standards was estimated. This settling model can be applied to other contaminant removal applications in wet detention facilities. Verification of the modeled results will facilitate more cost effective stormwater management at the Site. Introduction and Obiectives The Rocky Flats Environmental Technology Site (Site) is a United States Department of Energy (DOE) facility, formerly used in the production of nuclear weapons components and now undergOing decommissioning and remediation. All surface water flowing over the Site's core industrial area is managed in a system of wet detention ponds before being released offsite. One of these pond systems was modeled to determine its effectiveness in removing plutonium, the contaminant of greatest concern, from Site stormwater runoff. The purpose of the modeling effort described herein is to assess the effect of an individual detention pond in removing plutonium through settling. taking into consideration known hydrologic characteristics of the drainage, including plutonium activity in the stormwater runoff. The ultimate goal of the project was to assess the maximum allowable flow rate through the pond while remaining in compliance with a given plutonium activity discharge standard. Backeround The Site is under close scrutiny by local communities because it is the Nation's largest repository of special nuclear material (SNM), namely plutonium-239,240. Past accidents, management practices, and waste disposal at the Site created contaminated areas which present potential risks to environmental quality. Stormwater runoff from the Site Industrial Area (IA) transports radionuclides from contaminated areas to detention ponds; which are monitored for compliance with State stream standards. The stream standard for plutonium-239,240 is 0.05 picocuries I Liter which is very close to analytical detection limits (Minimum Detectable Activity = 0.01 picocuries I Liter). Pu-239,240 activity in stormwater runoff in North Walnut Creek and South Walnut Creek has been measured between 0.07 pCiIL and 5.0 pCiIL. Pu-239,240 activities in the detention ponds have been below the 0.05 pCiIL limit for the past five years, except for one large storm on May 17, 1996 which passed through the detention ponds without adequate time for settling of the particulate-bound Pu-239.240.