Hydrological and meteorological data for an unsaturated zone study near the Radioactive Waste Management Complex, Idaho National Engineering Laboratory, Idaho; 1985-86

Abstract

The hydrologic properties of the unsaturated zone and amount of net recharge to the system must be determined to field-calibrate a mathematical model to predict the long-term migration of radionuclides in the unsaturated zone. This study is being conducted to provide that necessary data for a specific area. Radioactive waste has been buried at the RWMC (Radioactive Waste Management Complex) at the INEL (Idaho National Engineering Laboratory) since 1952. In 1985, the U.S. Geological Survey and EG&G Idaho, Inc., in cooperation with the U.S. Department of Energy, began a study of the geohydrology of the RWMC to provide a basis for estimating the extent of and the potential for migration of radionuclides in the unsaturated zone beneath the waste burial trenches and pits. Two test trenches were installed in the surficial sediment adjacent to the RWMC burial ground to collect hydrologic data from undisturbed and disturbed soil. Hydrologic data collected during 1985 and 1986 included measurements, taken every 12 hours, of soil temperature and soil-water potential from 30 sensors placed at selected depths to about 6 meters using thermocouple psychrometers; and soil-moisture content measurements collected weekly in 9 neutron-probe access holes with a neutron moisture depth gage. Meteorological data averaged every 6 hours included wind speed, wind direction, relative humidity, air temperature; solar radiation and precipitation were totaled over the 6-hour period. (fig. 1). The RWMC is managed by the U.S. Department of Energy and operated by EG&G Idaho, Inc., a Department of Energy contractor at the INEL. Radioactive waste has been buried at the RWMC sijnce 1952. From 1952 to 1970, low-level radioactive and transuranic waste were buried in pits and trenches excavated into a veneer of surficial sediment. The sediment is underlain by a thick sequence of basaltic lava intercalated with sedimentary deposits. Since 1970, low-level radioactive waste has been buried and the transuranic waste has been stored on above-ground asphalt pads in retrievable containers. From 1952 to 1986, abou|t 180,000 m3 of low-level and transuranic radioactive waste containing abo|ut 9.5 million curies of radioactivity were buried at the RWMC. An estimated 335,000 liters of organic waste were also buried before 1970 (D.E. Kudera, EG&G Idaho, Inc., written commun., 1987). Radionuclides have been detected in core arid drill cuttings from several boreholes drilled into the surficial sedjiment and underlying basalt at the RWMC. Americium-241, plutonium-238, and plutonium-239,-240 (undivided) were detected in core samples from the 34 m deep sedimentary deposit. Because of the potential for the migrajtion of radionuclides from the RWMC to the Snake River Plain aquifer about 177 m below land surface, a comprehensive study to determine extent of and potential for migration was undertaken by the U.

Hydraulic and Geochemical Framework of the Idaho National Engineering and Environmental Laboratory Vadose Zone

“…Often the tracers used in such studies preexist in the environment from natural or artificial causes (e.g., Cecil et al, 1992; Busenberg et al, 2001; 1993). Measurements of water content and matric pressure with time (e.g., Davis and Pittman, 1990; Pittman, 1989; Pittman, 1995; Perkins, 2000; and McElroy and Hubbell, 2003) provide data for calculation of fluxes, and through changes with time may indicate fluxes directly. The dynamics of variably saturated conditions (e.g., perching, mounding of water on the aquifer, fluctuation of measured water levels) can suggest vadose zone flow rates and directions (e.g., Orr, 1999).…”

INEEL Subregional Conceptual Model Report Volume 3: Summary of Existing Knowledge of Natural and Anthropogenic Influences on the Release of Contaminants to the Subsurface Environment from Waste Source Terms at the INEEL

“…The test trench was designed to investigate infiltration in natural, undisturbed sediments; disturbed (excavated and backfilled) sediments; and simulated waste zones. Using tracers and extensive instrumentation (i.e., thermocouple psychrometers, lysimeters, tensiometers, and neutron moisture probes), the USGS quantified soil moisture and variability with depth, time, and temperature and estimated soil hydraulic conductivity, soil-moisture flux, and evapotranspiration rates (Pittman 1995).…”

FLASH: A finite element computer code for variably saturated flow

“…The surficiai sediments are primarily composed of eolian and alluvia_ sand, loess, and lacustrine deposits (Barraciough, et al, 1976). Moisture movement through the sediments occurs at different rates depending on depth (Pittman, 1989;Davis and Pittman, 1990;o McEiroy and Hubbell, 1990). For example, in the upper O.1 to 1 m of sediments, moisture movement is very dynamic and responds quickly to precipitation events as well as to periods , of evaporation.…”