Scientific Investigations Report 2006–5236

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5236

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Introduction

The Idaho National Laboratory (INL) encompasses about 890 mi2 of the eastern Snake River Plain in southeastern Idaho (fig. 1). Names formerly used for this site, from earliest to most recent, were National Reactor Testing Station (NRTS, 1949–74), Idaho National Engineering Laboratory (INEL, 1974–97), and Idaho National Engineering and Environmental Laboratory (INEEL, 1997–2005). Facilities at the INL are operated by the U.S. Department of Energy (DOE) and are used in the development of peacetime atomic-energy applications, nuclear-safety research, defense programs, advanced energy concepts, and environmental research. Since 1952, radiochemical and chemical wastes generated at these facilities have been contained in wastewater discharged to infiltration ponds, lined evaporation ponds, disposal wells, or a combination thereof. Liquid and solid radiochemical and chemical wastes also have been buried at the INL. Disposal of wastewater to infiltration ponds and infiltration of surface water at waste-burial sites resulted in formation of perched ground water in basalts and in sedimentary interbeds that overlie the Snake River Plain aquifer. Perched ground water is an integral part of the pathway for waste-constituent migration to the aquifer.

The U.S. Department of Energy (DOE) requires information about the dilute radiochemical- and chemical‑waste constituent mobility in perched ground water at the INL to monitor the possible movement of these constituents to the Snake River Plain aquifer. Waste‑constituent mobility in part is determined by (1) hydraulic properties of saturated and unsaturated basalts and sedimentary interbeds, (2) location, quantity, and method of waste disposal, (3) waste-constituent chemistry, and (4) geochemical processes taking place in perched ground water. This study was conducted by the U.S. Geological Survey (USGS) in cooperation with the DOE’s Idaho Operations Office.

Purpose and Scope

In 1949, the U.S. Atomic Energy Commission, which later became the DOE, requested that the USGS describe the water resources of the area now known as INL. The study’s purpose was to characterize the water resources before development of nuclear-reactor testing facilities. Since that time, the USGS has maintained water-level and water-quality monitoring networks at the INL to determine hydrologic trends and to delineate the movement of radiochemical and chemical wastes in ground water and in the Snake River Plain aquifer.

This report presents an analysis of water-level and water-quality data collected from selected wells completed in perched ground water at selected INL facilities during 1999–2001 as part of the continuing hydrogeologic investigations by the USGS at the INL. The report describes the distribution and concentration of selected radiochemical and chemical constituents in perched ground water and the history of waste disposal at the Reactor Technology Complex (RTC) (known as the Test Reactor Area [TRA] until 2005), Idaho Nuclear Technology and Engineering Center (INTEC), and Radioactive Waste Management Complex (RWMC). Perched ground water also has been detected beneath infiltration ponds and ditches at other facilities at the INL, but is not discussed in this report because of the relatively small quantity of wastewater and associated radiochemical and chemical constituents discharged. An analysis of water-quality and water-level data collected from wells completed in the Snake River Plain aquifer during 1999–2001 is described in Davis (2006).

Previous Investigations

The extent of perched ground water at the RTC and the distribution of selected wastewater constituents in perched ground water are discussed in a series of reports describing the hydrology of the INL. This series includes reports by Barraclough, Teasdale, and Jensen (1967), Barraclough, Teasdale, and others (1967), and Barraclough and Jensen (1976). An analysis of perched ground water at the RTC is presented in a comprehensive discussion of conditions related to the disposal of wastewater to the subsurface at the INL (Robertson and others, 1974). Later reports present data on perched ground water at the RTC, INTEC, and RWMC: hydrologic conditions during 1974–78 were described by Barraclough and others (1981); during 1979–81 by Lewis and Jensen (1985); and during 1982–85 by Pittman and others (1988). Cecil and others (1991) discussed mechanisms responsible for formation of perched ground water and described the distribution of chemical and radiochemical constituents in perched ground water at the RTC, INTEC, and RWMC during 1986–88. Distribution of selected radiochemical and chemical constituents in perched ground water during 1989–91 was described by Tucker and Orr (1998); during 1992–95 by Bartholomay (1998); and during 1996–98 by Bartholomay and Tucker (2000).

Robertson (1977) used a three-segment numerical model to simulate flow and transport of chemical and radionuclide constituents through perched ground water at the RTC. The model included effects of convection, hydrodynamic dispersion, radioactive decay, and adsorption. Hull (1989) developed a conceptual model that described migration pathways for wastewater and constituents from the radioactive-waste infiltration ponds (commonly referred to as the warm-waste ponds) at the RTC. Orr (1999) described the development of a transient numerical simulation used to evaluate a conceptual model of flow through perched ground water beneath wastewater infiltration ponds at the RTC. Anderson and Lewis (1989) and Anderson (1991) correlated drill cores and geophysical logs to describe a complex sequence of basalt flows and sedimentary interbeds in the unsaturated zone underlying the RWMC, RTC, and INTEC. This stratigraphic sequence provides the geologic framework where perched ground water formed. Ackerman (1991) analyzed data from 43 aquifer tests conducted in 22 wells to estimate transmissivity of basalts and sedimentary interbeds containing perched ground water beneath the RTC and INTEC.

Ground-Water Monitoring Networks

Ground-water monitoring networks at the INL are maintained by the USGS to characterize the occurrence, movement, and quality of perched ground water beneath INL facilities. Periodic water–level and –quality data are obtained from these networks. Data from these monitoring networks are on file at the USGS INL Project Office and are available on the USGS National Water Information System (NWIS) Web site at http://waterdata.usgs.gov/id/nwis/nwis.

Water-Level Monitoring Network

The USGS perched water-level monitoring network was designed to estimate the extent of perched ground water and the volume of perched water in storage. Water levels in 42 wells (fig. 2) were monitored during 1999–2001. At the RTC, the network included 22 wells to monitor deep perched ground-water levels and 11 wells to monitor shallow perched ground-water levels. Shallow perched ground water is considered water perched in surficial sediment deposits, and deep perched ground water is water perched at greater depths. Perching mechanisms are attributed to contrasting hydraulic properties between sedimentary interbeds and basalts or between low-permeability basalt-flow interiors and overlying fractured basalt. At the INTEC, the network included seven wells to monitor perched ground-water levels around the INTEC infiltration ponds and one well to monitor the water-level changes in deep perched ground water beneath the INTEC. Perched ground water at the RWMC was monitored in one well. Well locations and frequency of water-level measurements as of December 2001 are shown in figure 2.

Water-Quality Monitoring Network

The radiochemical and chemical character of perched ground water beneath INL facilities was determined from analyses of water samples collected as part of the USGS water-quality monitoring network to identify contaminant concentrations and define the pattern of waste migration in perched ground water and in the Snake River Plain aquifer.

Type, frequency, and depth of ground-water sampling generally depend on the information needed in a specific area. Water samples routinely are collected from selected wells and analyzed for concentrations of tritium, strontium‑90, cesium‑137, cobalt-60, plutonium-238, plutonium-239 and plutonium-240 (undivided), americium-241, dissolved chromium, sodium, chloride, sulfate, nitrate, volatile organic compounds (VOCs), and measurements of specific conductance, pH, and water temperature. Water samples were analyzed for concentrations of radiochemical constituents at the Radiological and Environmental Sciences Laboratory (RESL) and for chemical constituents at the National Water Quality Laboratory (NWQL) in Lakewood, Colo. Well locations in the USGS water-quality monitoring network for perched ground water beneath INL facilities during 1999–2001 and the frequency of sample collection are shown in figure 3 and table 1. A sample schedule that lists constituents analyzed at each site is available in a report by Mann (1996, attachment 1).

Water-Quality Sampling Methods and Quality Assurance

Methods used to sample and analyze for selected constituents generally followed guidelines established by the USGS (Goerlitz and Brown, 1972; Stevens and others, 1975; Wood, 1981; Claassen, 1982; W.L. Bradford, USGS, written commun., 1985; Wershaw and others, 1987; Fishman and Friedman, 1989; and Wilde and others, 1998).

Water samples were collected according to a quality-assurance plan for water-quality activities conducted by personnel at the INL Project Office (Mann, 1996). Water samples collected for dissolved constituent analysis are filtered through a 0.45-micron membrane filter. About 10 percent of samples collected generally are for quality assurance. Quality-assurance samples collected by the USGS INL Project Office include equipment blanks, splits, and replicates. Nine quality-assurance replicates were collected for wells sampled for this study; results are included in the tables in this report. Comparative studies to determine agreement between analytical results for individual water-sample pairs by laboratories involved in the INL Project Office quality-assurance program were summarized by Wegner (1989), and Williams (1996, 1997). Additional quality-assurance studies by personnel at the INL Project Office included:

  1. An evaluation of field sampling and preservation methods for strontium-90 (Cecil and others, 1989);
  2. A study comparing pump types used for sampling VOCs (Knobel and Mann, 1993);
  3. An analysis of tritium and strontium-90 concentrations in water from wells after purging different borehole volumes (Bartholomay, 1993);
  4. An analysis of effects of various preservation types on nutrient concentrations (Bartholomay and Williams, 1996); and
  5. An analysis of two analytical methods to determine gross alpha- and beta-particle activity (Bartholomay and others, 1999).

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