Results of hydrologic research at a low-level radioactive-waste disposal site near Sheffield, Illinois

Abstract

Ten years of hydrologic research have been conducted by the U.S. Geological Survey at a commercial low-level radioactive-waste disposal site near Sheffield, Illinois. Research included studies of microclimate, evapotranspiration, and tritium release by plants; runoff and land modification; water movement through a trench cover; water and tritium movement in the unsaturated zone; gases in the unsaturated zone; water and tritium movement in the saturated zone; and water chemistry. Implications specific to each research topic and those based on overlapping research topics are summarized as to their potential effect on the selection, characterization, design, operation, and decommissioning processes of future low-level radioactive-waste disposal sites. Unconsolidated deposits at the site are diverse in lithologic character and are spatially and stratigraphically complex. Thickness of these Quaternary deposits ranges from 3 to 27 meters and averages 17 meters. The unconsolidated deposits overlay 140 meters of Pennsylvanian shale, mudstone, siltstone, and coal. Approximately 90,500 cubic meters of waste were buried from August 1967 through August 1978, in 21 trenches that were constructed in glacial materials by using a cut-and-fill process. Trenches generally were constructed below grade and ranged from 11 to 180 meters long, 2.4 to 21 meters wide, and 2.4 to about 7.9 meters deep. Research on microclimate and evapotranspiration at the site was conducted from July 1982 through June 1984. Continuous measurements were made of precipitation, incoming and reflected solar (shortwave) radiation, incoming and emitted terrestrial (longwave) radiation, horizontal windspeed and direction, wet- and dry-bulb air temperature, barometric pressure, soil-heat fluxes, and soil temperature. Soil-moisture content, for this research phase, was measured approximately biweekly. Evapotranspiration rates were estimated by using three techniques--energy budget, aerodynamic profile, and water budget. Although monthly totals for each method differed, estimated annual evapotranspiration averages ranged from 630 to 693 millimeters or about 70 percent of precipitation. Tritium concentrations in leaf water from on-site plants were determined for 125 vegetation samples collected during the summers of 1982 through 1986. Concentrations varied significantly among some locations and plant types. Tritium concentrations ranged from the detection limit of 0 .2 to 1,330 nanocuries per liter, with alfalfa (Medicago sativa) having the highest concentrations, followed by brome grass (Bromus inermis), and then red clover (Trifoleum pratense); these variations in concentration are most likely a result of root depth. Runoff and sediment transport were measured from July 1982 through December 1985 in four basins--three comprising almost two-thirds of the 8.1-hectare site and one comprising a 1.4-hectare undisturbed area. Volumes and equivalent weights of collapses were estimated from records of site surficial conditions from October 1978 through December 1985. Runoff showed a direct relation to degree of land modification; lowest mean yields were measured at the undisturbed area, and highest mean yields were measured from the basin composed wholly of trench and intertrench areas. Sediment yield measured onsite averaged 3.4 megagrams per hectare. A total of 315 collapse cavities, corresponding to a cumulative volume of about 500 cubic meters, were documented. Most collapses were recorded after periods of rainfall or snowmelt when soil moisture was near maximum. Almost two-thirds of the collapses, corresponding to 63 percent of the cumulative cavity volume, occurred during February through April. Data for the study of water movement through a trench cover were collected from July 1982 through June 1934. Pressure-head data were collected at four different clusters at depths ranging from 50 to 1,850 millimeters within a selected trench cover. Soil-moisture content for this research phase was measured weekly with a gamma-attenuation moisture probe. The amount of water stored within the trench cover fluctuated in an annual cycle. Moisture contents were greatest in late March or early April, decreased steadily from late spring through the summer, reached a minimum in late August or early September, and then increased gradually from midfall through the winter. Depths of wetting-front movements were a function of initial soil-moisture content and total storm precipitation. Seepage to the trench was estimated by use of four different methods: the Darcy method, the zero-flux phase method, the surface-based water-budget method, and the ground-water based water-budget method. Estimates by the different methods differed considerably. Investigation of the unsaturated zone at the site began in 1981 and is ongoing (1987). Water movement was measured along a generalized vertical section through four trenches. A 120-meter-long, 2-meter-diameter horizontal tunnel provided access below the trenches. The timing of water movement varied temporally and spatially. Vertical flow was inhibited at interfaces between lithologic units of contrasting hydraulic conductivities. Data also indicate that water movement through the sand of the Toulon Member of the Glasford Formation occurs along localized partially saturated to saturated flow paths. Average velocities of water movement through the extent of the unsaturated zone, as estimated by a saturation-tracking method, ranged from 0.04 to 0.34 meter per day. Tritium concentrations in the unsaturated zone varied spatially reflecting the heterogeneity of wastes in the overlying trenches and local hydrogeologic conditions. Tritium concentrations at all lysimeter locations increased with time; however, the increases usually were of small magnitude. Tritium concentrations increased abruptly, from five to nine times previous concentrations, at only 3 of 14 locations. Gas samples were collected from a network of soil-gas piezometers located in the undisturbed unsaturated zone near a waste trench at approximately 70-day intervals during 1984-86. Relative proportions of nitrogen, oxygen plus argon, carbon dioxide, methane, ethane, propane, butane, tritiated water vapor, carbon dioxide, and 222radon were converted to partial pressures, based on a mean atmospheric pressure of 98.6 kilopascals. Methane and 14 carbon dioxide were identified as originating in the waste and having mean partial pressures that generally decreased with horizontal distance from the trench and with vertical distance to the land surface. Partial-pressure gradients for other radioactive gases were not detected in the gas sampling network; definable gradients may occur much nearer to the waste source. Ground-water flow has been studied at the site since 1976. The spatial, stratigraphic, and lithologic complexity of the unconsolidated deposits that compose the shallow aquifer result in the free water surface intersecting nine different lithologic units at the site. Saturated hydraulic conductivities of these units range from about 4x10-12 to 8x10-6 centimeters per second. Three ground-water basins were defined within the shallow aquifer, with flow generally from west to east. Ground-water velocities are highly variable. A tracer test conducted in a pebbly-sand unit resulted in velocities in the range of 640 to 770 meters per year. Estimated velocities for the other units ranged from 2 to 490 meters per year. Tritium was detected in observation wells on-site in 1976 and off-site in 1982. Concentrations ranged from the analytical detection limit of 0.2 to over 300 nanocuries per liter. Seasonal variations in tritium concentrations were observed in most wells, and dilution caused by infiltrating rainfall was observed. Water-chemistry research included the collection and analysis of precipitation, geologic materials, and unsaturated- and saturated-zone water, both on-site and off-site, and in all geologic units, during 1978 to 1984. Precipitation was a calcium-zinc-sulfate type water. Calcium and bicarbonate were the most abundant cation and anion, respectively, in the geologic materials. Mean dissolved organic carbon and tritium concentrations (45 milligrams per liter and 290 nanocuries per liter, respectively) were greater in samples from six lysimeters located directly below waste trenches and one located within a trench, than in samples from all other lysimeters (8.4 milligrams per liter and 17 nanocuries per liter, respectively). Water in the saturated zone was generally a magnesium-bicarbonate type. Tritium was the only radionuclide detected in the saturated zone.