15 LONG-TERM COVER DESIGN FOR LOW-LEVEL
RADIOACTIVE AND HAZARDOUS WASTE SITES AS
APPLIED TO THE ROCKY FLATS ENVIRONMENTAL
TECHNOLOGY SITE SOLAR EVAPORATION PONDS
Sandra E. Stenseng, Senior Lead Engineer
Parsons Engineering Science, Inc.
Denver, Colorado 80290
Philip A. Nixon, Project Manager
Parsons Engineering Science, Inc.
Denver, Colorado 80290
INTRODUCTION
The U.S. Department of Energy (DOE) operated five lined solar evaporation ponds (SEPs) at
the Rocky Flats Environmental Technology Site (RFETS) in Jefferson County, Colorado from
1953 until 1986. Figure 1 presents a location map of the RFETS. The SEPs were used primarily
to store and evaporate low-level radioactive and hazardous process wastes. Operation of the
SEPs has resulted in contamination of the surrounding soils, and may also provide a source of
groundwater contamination. The DOE proposes to close the SEPs by consolidating the contaminated material beneath an engineered cover. The primary objective of the closure of such hazardous and radioactive sites is to limit the exposure of the general public to the contaminants for
time periods ranging from 100 to 10,000 years.1,2,3 The goal of the SEPs engineered cover is to
isolate hazardous and low-level radioactive soils for a minimum of 1,000 years.
Since there is currently no existing regulatory design guidance for a 1,000-year engineered
cover, the proposed design of the SEPs engineered cover is based on research and testing that has
been conducted for many years at various DOE facilities in the United States. Approximately 10
years of testing and research have been conducted at the Hanford Site in Washington State,
where a 5-acre prototype cap (using the 1,000-year design theories) was recently constructed to
cover a decommissioned wastewater disposal facility.4 Monitoring of this prototype cover will
yield valuable information for the future design of such closures. Scientists at the Los Alamos
National Laboratory (LANL) in New Mexico have also been researching and testing cover design theories, focusing on the effect of capillary barriers on percolation, methods to control erosion, and strategies to prevent biological intrusion. Testing has also been conducted at the
Nevada Test Site, the Idaho National Engineering Laboratory (INEL), the Savannah River site,
and other DOE sites around the nation.
The above-mentioned programs, along with other published research papers and reports, have
been investigated and used to enhance the design of the proposed engineered cover for the
RFETS SEPs. This paper discusses the main design theories of the proposed engineered cover
for the closure of the SEPs, and how the research and test results of these other programs have
been used to arrive at the final cover configuration, the material selections, the component layering, layer thicknesses, and the balance and interaction between components to establish an overall effective cover system.
ENGINEERED COVER MATERIALS AND CONFIGURATION
Selecting materials that will maintain their integrity over the design life of the engineered
cover is very important in demonstrating that the engineered cover will be protective of human
health and the environment. Because of the 1,000-year design life of this engineered cover, the
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50th Purdue Industrial Waste Conference Proceedings. 1995, Ann Arbor Press. Inc.. Chelsea, Michigan 48118.
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