ML20150C283

Hydrogeologic Evaluation of Radioactive Contamination in Deep Wells at Hanford

ML20150C283
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Issue date:	07/08/1988
From:	Chery D, Nicole Coleman NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
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ML20150B672	List: ML20150B670 ML20150C283
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REF-WM-1 NUDOCS 8807120403
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HYDROGE0 LOGIC EVALUATION OF RADI0 ACTIVE CONTAMINATION IN DEEP WELLS AT HANFORD Neil M. Coleman and Donald L. Chery, Jr.

U.S. Nuclear Regulatory Connission Office of Nuclear Material Safety and Safeguards Washington, D.C. 20555 ABSTRACT Since 1943, the Hanford Reservation in Washington state has been a center for nuclear engineering activities. These activities have produced large volumes of wastewaters containing many radioactive contaminants. The volume of disposed wastewaters greatly exceeded local natural recharge and altered the pre-1944 groundwater system by raising water table elevations. Some of the l

introduced contaminants are highly mobile radionuclides like tritium, l iodine-129, and technetium-99. As environmental tracers, these contaminants provide unique opportunities to study the groundwater flow system (i.e.,

recharge, flow directions, velocities, and contaminant dispersion).

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i Groundwater chemistry has been monitored at Hanford for several decades.

Data from more than 1,000 wells have shown the presence of contaminant plumes in the shallow unconfined aquifer. Recently, data for deeper confined aquifers have shown that contaminants have migrated to depths of 980 ft (300 m) or more 8807120403 PDR 880708 WASTE WM-1 PDC +

. T in fractured basalt aquifers. Nuclear waste disposal sites are located about 1 mi (1.6 km) from wells showing deep contamination. Information from the nearby

. nuclear waste sites was evaluated to identify potential contanination sources, and geologic data were assessed to identify possible paths of interaquifer communication. The data generally show evidence of vertical leakage between confined basalt aquifers in proximity to major geologic structures. Relatively rapid rates of vertical contaminant migration can be inferred from the data, exceeding 21 ft/yr (6.4 n/yr) at sites where deep contamination exists.

INTRODUCTION Since 1943, the Hanford Reservation in south-ceritral Washington (Figure 1) <. F I has been a center for nuclear engineering activities, including research and development for nuclear fuel processing, production of weapon materials, and storage of r.uclear wastes (ERDA, 1975). Over the past decade Hanford has been considered as a possible site for a high-level nuclear waste repository. The Department of Energy (DOE) conducted exploratory hydrogeologic investigations as part of a program 'of site characterization for a repository. Currently, DOE is assessing options for permanent disposal of defense wastes stored at Hanford (DOE,1987).

The Hanford Reservation is in the Pasco Basin, located near the center of the Columbia Plateau. A reach of the Columbia River more than 45 miles long i

occurs within the reservation downriver from Priest Rapids Dam and north of the town of Richland, Washington. The chemical separation areas at Hanford, referred to as the 200 East and 200 West Areas, contain the major radioactive waste storage and disposal facilities on the Hanford Site (Fignre 2). Other e F2 waste disposal sites near the 200 Areas tre shown in F % re 2, including "N l Cribs," sometimes referred to as 200 North, l

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Waste disposal activities at Hanforc have released to the subsurface large volumes of nuclear industrial wastewaters containing varying levels of contaminants (Zimmermanandothers,1986). Most of the disposal activities have occurred near chemical separation facilities in the 200 Areas (Figure 2).

The volume of wastewaters greatly exceeda. local natural recharge (Graham and others, 1981) and has altered the pre-19t4 groundwater system. A groundwater surveillance program has been conducted at Hanford for several decades to monitormovementofcontaminants(NAS,1978). Studying the migration and fate of these contaminants presents unique opportunities to better understand groundwater flow patterns in the fractured rock aquifers at Hanford. As a supplement to hydraulic test data, contaminant migration studies can provide insight about recharge locations ans ' low paths in the groundwater flow system.

InAugust,1987,theDOEreleasedareport(Westinghouse,1987)that contained groundwater surveillance data collected over several decades. This report showed the detection of anthropogenic contaminants in confined aquifers beneath the Hanford Site. The report was of particular interest because it reported good documentation for data from two wells (DC-18 and 08-15) showing iodine-129(I-129)andtritiumcontaminationatdepthsexceeding980ft(300 m). The Westinghouse report includes general observations about possible sources and pathways of contaminants found in deep aquifers at Hanford, but does not include an extensive data analysis. We have pursued a more detailed review by examining the available literature produced by ge scientists at Hanford.

The objectives of this paper are: to evaluate sources of contaminants detected in deep wells, to identify possible transport paths, and to estimate rates of contaminant migration. To meet these objectives,i.e review the geologic setting and groundwater flow system, historical data on wastewater disposal and groundwater monitoring, hydrogeologic effects of wastewater disposal, evidence for interaquifer comunication, and possible contaminant l pathways. General comments are provided about the reliability of past data collection methods. We also examine background levels of I-129 in groundwater l - - - - . . ,. .. _ ___ _

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at the site, and propese a conceptualization of groundwater flow and recharge near the Gable Mountain anticlinal structure.

GE0 LOGIC SETTING The geology in the Pasco Basin and surrounding Columbia Plateau region consists of layered basalts and interbeds of the Colurhia Plateau Basalt Group (Figure 3). Networks of vertical and radiating fractures formed during cooling < F3 of the basaltic lava flows pervade the dense interiors (colonnade and entablaturezones)ofthebasalts.

The Pasco Basin is located along the eastern extent of the Yakima Fold Belt, and the geology within the basin has been significantly influenced by the folding and associated faults. Detailed descriptions of the geology at Hanford are given by Myers and others (1979) and 00E (1982). The 200 Areas are located in a broad synclinal area called Cold Creek Valley that is bordered on the east by the Columbia River and or t',e north by major fold structures that form two ,7 basalt ridges called Gable Butte and Gable Mountain (Figure 4). DOE (1986b) <

identified Gable Butte and Gable Mountain as the surface expressions of a series of westerly trending en echelon anticlines and synclines. These structures were interpreted as second-order folds within the closure of an asymetrical first-order fold. The overall fold structure is asymetrical because the northern flank of the first-order fold dips more steeply than its southern flank. Structural relief on the Gable Mountain structure is about 1,300 ft (396 m) (DOE, 1986b). A number of faults produced during fold deformation exist in proximity to Gable Mountain. Locations of three of these faults are shown in Figure 5. Additional information about local faulting will < F5 be presented in subsequent sections.

In the Pasco Basin the basalts are overlain by alluvial sediments of the Ringold and Hanford Formations. The complex depositional and erosional cycles

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r that produced these suprabasalt deposits are discussed by Fecht and others (1985) and by Tallman and others (1979). Deposition of the Ringold Formation began in the Late Miocene to Middle Pliocene when the course of the ancestral Columbia River was shifted from the west around the eastern end of Umtanum Ridge and into the central Pasco Basin (Fecht and others, 1985). Extensive gravel deposits of the basal Ringold unit overlie the Elephant Mountain Basalt and mark the course of the river in the central Pasco Basin. The only major paleochannel in the Ringold Formation trends southeast from Gable Gap through the northeastern corner of the 200 East Area. An antecedent stream occupied this channel during the uplift of the Umtanum Ridge-Gable Mountain structure, producing an erosional unconformity by deeply incising the basalts (Tallman and-others,1979). Figure 6 presents a cross-sectional view of this paleochannel < f[

betweenwells47-50and50-48(profilelocationgiveninFigure5). Here the channel cut downward through the Elephant Mountain Basalt and the Rattlesnake RidgeInterbedtoanelevationoflessthan280ft(85.4m). Subsequent Pleistocene flooding eroded Ringold sediments from the channel and deposited sediments of the Hanford Formation. The surficial deposits shown in profile B -

B' are mostly those of the Hanford Formation.

Major changes in the paleodrainage system on the Columbia Plateau occurred during the Pleistocene when catastrophic floods emanated from glacial lakes.

At least four major flooding events have been recorded. During one of these flooding events, the paleochannel of the Columbia that had been established between Gable Mountain and Gable Butte was diverted around the eastern end of Gable Mountain to near its present-day course. The pre-flood river charnel i plugged by a large flood bar formed south of Gable Mountain and Gable Butte (Fechtandothers,1985). The final course of an apparent Pleistocene flood channel occurs south of Gable Mountain and is roughly defined by the 500 ft (152m)topographiccontoursshowninFigure5. This flood channel passed I

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GROUNDWATER FLOW SYSTEM The groundwater flow system at Hanford is complex, involving interactions between dynamic (transient) unconfined aquifers and deeper confined flow systems in the basalts and interbeds. The unconfined aquifer system has been altered significantly by decades of wastewater disposal actiTities, and relevant phenomena are discussed in subsequent sections. This section provides general information about the groundwater flow system.

The unconfined aquifer system enderlying the 200 Areas mainly occurs within sediments of the Hanford and lingold Formations. Although unconfined conditions generally prevail in the surficial aquifer, the lower part of the Ringold may exist under partially confining conditions in some areas. The significant effects of wastewater disposal operations on the unconfined ficw system are discussed in a subsequent section.

Most investigators are in general agreement that, below the unconfined aquifer, confined sedimentary interbeds and basalt flow tcp rubble znnes form the major aquifers 3t Hanford. But there are differing interpretations of the hydrologic role of basalt intraflow and interflow structures (i.e., colonnade and entablature (cooling) fractures, and fault zones). Extensive hydrologic testing by DOE (Strait and Hercer, 1987) has provided little information about the vertical hydraulic properties of these structures and their role in interaquifer communication, To supplement hydraulic testing, studies of contaminant migration can provide additional information about recharge l

I locations and flow paths in complex, fractured rock aauifers.

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The DOE (1986b) presented alternative interp tr ont of groundwater flow in the basalt aquifer system of the Cold Creek 3y- .e . On the basis of available data, thay considered that the best description is one of a flow system having relatively low vertical groundwater flow, bounded by structural discontinuities. Depending on their physical characteristics, these discontinuities may act as barriers to flow or they may vertically connect shallow and deep flow systems. DOE suggested that the Umtanum Ridge-Gable Mountain structure and so-called Cold Creek "barrier" may isolate the Cold Creek valley from hydraulic influences to the north and west by forming boundaries of low, lateral hydraulic condt :tivity. DOE speculated that these boundaries might also be areas of increased vertical groundwater flow between deep and shallow groundwater systems.

Hydrolcgic Interpretations Using Hydrochemistry Numerous investigators have used major ion hydrochemistry data to assess regional flow patterns in the Pasco Basin. DOE (1982) concluded that available hydrochemistry data support a flow system of permeable flow tops and interbeds isolated vertically by dense basalt interiors.

Williams and Associates (1983) demonstrated a quantitative approach using multivariate statistics to evaluate the hydrochemistry :<f aquifer systems at Hantord. They found that groundwater in major formations appeared to be chemically distinguishable, but asserted that this does not necessarily imply hydrologic isolation of the. formations. Mixing could occur between the layers while their respective groundwaters renain chemically distinct. A more complete analysis of aquifer isolation would require an integrated study of vertical hydraulic characteristics, geochemical evolution of interactions between rock and water, and structural analysis of the basalt and interbed units. Steinhorst and Williams (1985) emphasized the need for close consultation between hydrogcologists and statisticians in conducting hydrochemistry evaluations of aqui'er systems. For Hanford, they determined that, since all strata were not sampled in all boreholes, additional work would m - - - -

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require a more complete data base to avoid spatial-stratigraphic confounding.

The quantitative method of Steinhorst and Williams demonstrates how hydrochemical data can be used to gain insight about aquifer hydrodynamics.

Lavenue and Domenico (1986) did a preliminary assessmer.t of regior,al dispersivities for selected basalt flows at Hanford. Their measurements were base 6 on chloride plumes (in the basalt aquifers) that were thought to have formed by upwelling near the so-called Cold Creek "barrier." The "barrier" is a north-south trending zone of pronounced hydraulic head differences located west of the 200 West Area. Geophysical anomalies and an apparent stratigraphic offset have also been detected in this zone.

' Groundwater Monitoring For several decades wells have been used to gather data on the distribution and movement of radioactive discharges in groundwatei beneath the Hanford Site. A series of yearly reports were written, such as Eddy and Wilbur (1981) and Eddy and others (1983). Most of the monitoring was focused on the unconfined aquifer system, but some data were also collected for deeper confined aquifers. During 1980, for example, 317 wells were sampled for radionuclide and chemical contaminants (Eddy and Wilbur, 1981). Overall, thers are about 2.900 monitoring wells, of which 1,100 have been used for groundwater

. sampling (Westinghouse, 1987). Approximately 210 wells have been sampled for iodine-129, including 35 in the confined aquifer system.

Brauer and Rieck (1972) performed radiochemical analyses on well-water samples from Hanford and its vicinity. Their report describes sampling methods and the results of iodine activation analyses as well as gamma-gamma coincidence spectrometry. They noted that since radionuclide , retention by soil is prinarily a cation absorption process, anionic nuclides servt as better groundwater tracers. Consequently, long-lived fission product iodine (i.e.,

I-129) discharged to thu ground in Hanford's 200 Areas was recommended as a l

good tracer in studying the trovement of contaminants in the groundwater. Their

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1 data showed that, for I-129 samples collected from 1962 to 1968, greatest concentrations occurred in wells in the 200 East and 200 West Areas, confirming the entrance of I-129 into the groundwater as a result of waste management practicesinthoseareas(BrauerandRieck,1972). They also showed that the relatively high trace levels of I-129 in meteoric sources at Hanford had the potential to contaminate surface waters and wells. Overall, the data from Brauer and Rieck (1972) have a limited hydrologic application because of possible sample contamination problems and a lack of information about depths from which samples were collected. However, the report was valuable in showing the uset"Iness of aaionic nuclides like I-129 as groundwater tracers. s HYDROLOGIC EFFECTS OF WASTEWATER DISPOSAL The installation of a large network of monitoring wells at Hanford permitted monitoring of water table fluctuations caused by waste disposal.

Zimmerr'an and others (1986) constructed graphs of hydraulic head changes in wells and maps of temporal head chanps, showing how large discharges of wastewaters changed the water ta..M configuration since site operations began in 1943. During the period from 1970 t 980, water table conditions were reported to have reached approximate equilibrium with wastewater sources. Past changes in the water table can be related to the waste-management activities, and the unconfined aquifer system appears to behave in a predictable manner (Zimmerman and others, 1985).

Volumes of Wastewater Disposal Newcomb and others (1972) state that between 1945 and 1959 over 40 billion gallons (0.15 billion cubic meters) of liquid wastes were discharged to groundwaters at Hanford. The Committee on Radioactive Weste Managament (NAS, 1978) reported that, as of January,1975, about 130 billion gallons (0.49 billion cubic meters) of effluent were discharged, largely in and near the 200 l

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' Areas. ' Graham and others (1981) estimated that in 1979 wastewater discharged in the separation (200) areas exceeded natural recharge by a factor of 10.

Zimmerman and others (1986) reported that, as of 1985, total accumulated discharges'in the 200 Areas were about 167 billion gallons (0.63 billion cubic meters, or 633 billion liters) (Table 1). These activities created groundwater recharge mounds in the unconfined aquifer system beneath the 200 East and West Areas.

Table 1.-- Wastewater discharges to the separations areas.

Time- Volume (L) Total  % of Total Period 200 East 200 West Volume (L) 200 East 200 West 1943 - 1950 1.006E+10 2.241E+10 3.247E+10 31.0 69.0 1951 - 1955 9.214E+09 6.773E+10 7.694E+10 12.0 88.0 1956 - 1960 6.333E+10 5.779E+10 1.211E+11 52.3 47.7 1961 1965 7.815E+10 4.372E+10 1.219E+11 64.1 35.9 1966 - 1970 7.531E+10 2.532E+10 1.006E+11- 74.8 25.2 1971 1975 6.137E+10 2.308E+10 8.445E+10 72.7 27.3 1976 - 1980 6.836E+10 2.737E+10 9.573E+10 71.4 28.6 TOTAL = 6.332E+11(L)

(Note: Data are derived 1. sm Table 3 of Zimmerman and others,1986)

Newcomb and others (1972) refer to the eastern and western groundwater l rechargt "mounds" and to smaller mounds produced at other lou.tions. The b.

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ll larger mounds roughly correspond to locations of the 200 East and West Areas.

In 1961 the western mound reached a peak height of 60 ft (18 m) above he natural water table and had a basal area of about 15 square mi (39 square km).

This groundwater mound is higher than the eastern mound because of the reportedly lower hydraulic conductivities in the unconfined aquifer beneath the 200 West Area. Hydraulic conductivities in and around the 200 East Area are much higher because of the presence of high-permeability paleochannel sediments. These sediments rapidly conduct excess recharge, generally toward the east and southeast, preventing the formation of a larger recharge mound.

This accounts for the large-scale shape and distribution of contaminant plumes in the unconfined aquifer. Newcomb and others (1972) observed that the rapid (about.10,000 ft/yr, or 3,050 m/yr) movement of tritium an6 ruthenium-106 from the 200 East Area to the Columbia River resulted from the rise of the water table above the top of the Ringold Formation into overlying more permeable glaciofluvial deposits, now known as Hanford Formation deposits.

Figure 7 shows the position of tritium plumes in the unconfined cquifer in < F7 1983 (Westinghouse, 1987), along with locations of wells that were sampled.

Large-scale plumes of other mobile contaminants (i.e., nitrate, I-129, and technetium-99) have also been identified. Studies of the shapes and movements of these plumes have aided in the understanding of groundwater flow patterns in the unconfined aquifer system.

From the shapes of the plumes it is evident that groundwater flow rates in the unconfined aquifer are relatively high, especially in proximity to the 200 East Area. In fact, Brown and others (1962) mapped tritium concentrations in the unconfined aquifer and showei that the tritium plume had reached the banks of the Columbia River by 1962 or earlier. The shape of the plume in Figure 7 f

l shows that groundwater flow is easterly from contaminant sources in the 200

! Areas and is following old Columbia River channels incised into the eroded i upper surface of the low-perreability Ringold Formation. These channels are filled with glacial outwash grovels that have about 100 times the permeability a

of the underlying Ringold Formation (Brown and oth.rs, 1962). Table 2 shows t

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0 1 2 3 4 5 Miles I t I I t I O 2 1 h h Kilometers Figure 7.--Average tritium distribution in the unconfined aquifer in 1983 (Westinghouse,1987).

, 4 l1 ranges of-hydraulic conductivity values for the surficial aquifer based on more than 100 hydrologic tests in the Pasco Basin (Graham and others,1981).

Table 2.-- Estimated hydraulic properties of the unconfined aquifer, ,

Stratigraphic Interval Hydraulic c onductivity ft/ day m/ day l

Hanford Formation 500 - 20,300 150 - 6,100 Undifferentiated Hanford & 100 - 7,000 30 - 2,100 Middle Ringold Unit Middle Ringold Unit 20 - 60C 6 - 180 Lower Ringold Unit 0.1 - 10.0 0.93 - 3.0 (Modified from Table 1 of Graham and others, 1981)

DOE (1987) determined that groundwater velocities in the unconfined system are relatively high and sensitive to recharge. They estimated that travel times over a distance of 5 km (3 mi) ranged from 1 to 25 years, based on varying assumptions about recharge.

Effects of Disposal on Unconfined Flow Directions Becides increasing water table elevations, the large volumes of artificial recharge have influenced directions of groundwater flow. Pre-1944 flow l.

directions in the surficial aquifer within Cold Creek Valley are thought to have been dominantly east-southeast, but under lower hydraulic gradients.

IS Presently, the major"ce nponent of flow is still easterly, but recharge mounds beneath the 200 Areas have increased hydraulic gradients and superimposed radial flow patterns on the pre-1944 water table. A significant northerly flow occurs through.the gap between Gable Butte and Gable itountain, and groundwater containing radioactive tracers thus flows across the fold axis of Umtanum Ridge. Thir.'phenorenon is illustrated by DOE (1987) in a simulation of 1983 water table conditions and flow streamlines (Figure 8) and is supported by the <

presence of tritium in the unconfined aquifer between Gable Butte and Gable Mountain as shown in Figure 7 (Westinghouse,1987). Any recharge to deeper aquifers that occurs along the fold axis in the gap will be tagged with elevated trace levels of contaminants.

Figure 9 shows the configuration of the water table (July, 1980) and flow 4 vectors for the area south of Gable Mountain (Strait and Moore, 1982). Wells DC-18 and D8-15 are shown in relation to the disposal areas of B-Pond and Gable itountain Pond. A groundwater mound appears in the water table beneath the area of B-Pond. Of hydrologic significance is the subsurface expression of the Willa Anticline adjacent to Gable Mountain Pond, producing a northwest trending zone where the basalt surface is above the water table. This zone acts as a partial barrier to flow in the unconfined aquifer and causes the unconfined aquifer north of the barrier and south 'f Gable Mountain to become "channelized" with flew in,a northwesterly direction. Thus, for a period including 1980, the groundwater system in this area included flow away frcm B-Pond, toward DB-15, through the "channelized" unconfined aquifer (south of l' Gable Mountain), past Gable Mountain Pond, and toward Gable Gap and DC-18.

Tritiun. levels in the unconfined aquifer, as shown in Figure 10, verify < F/6 observations about the flowfield, but also show that (circa 1980) the vicinity of well DC-18 is more strongly influenced by contaminant source areas other than B-Pond and Gable Mountain Fond.

Effects of Wastewater Dispos Q on Confined Aquifers

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N The significant changes that occurredsin water table elevations in response to liquid waste disposal may also have affected the underlying basalt ,

and interbed aquifer system. Data collected by the Basalt Waste Isolation s Project (BWIP) from piezometer clusters at DC-19, -20, and -22 show decreasing heads with depth from the surface down to the Priest Rapids member of the HanapumBasalt(SwansonandLeventhal,1984). Well locations are shown in ,

Figure 11. A plot of the vertical head profile for well DB-19 is presented in < f//

Figure 12. These decreasing heads indicate a downward hydraulic gradient with < gy a corresponding potential for recharge from the surface. Downward hydraulic gradients have also been observed at other well locations, including RRL-2 and DC-16A (00E, 1982), and Strait and Moore (1982) noted that well 08-15 occurs in an area having hydraulic heads that favor downward flow of groundwater. Figure 13 shows the hydrograph from wel! 699-60-60 (shown as well 60-60 in Figure 11), < h located in Gable Gap about 0.6 mi (1 km) west of well 00-18 and over 2.5 mi (4.0 km) from the 200 Areas. Even at this distance, the water table was perturbed significantly by wastewater disposal operations to tne south. Water table elevations in this well ranged from 394 ft (120 m) in 1947 to a peak in the late 1960s at over 406 ft (124 m), a rise of over 12 ft (3.7 m). This increase in water table elevations in Gable Gap favored downward flow of groundwater and contaminants from the uc. confined aquifer to the basalts.

l Although the potential for recharge exists near the above wells, actual recharge rates cannot be irmsured due to a lack of information about vertical hydraulic properties. It is probable that a major component of the observed vertical hydraulic gradients was caused by decades of wastewater disposal near l the 200 Areas. If this interpretation is correct, then current conditions are a transient phenomenon and heads measured in the Saddle Mountains Formation may not be representative of pre-1944 steady-state conditions. Hindcasts of pre-1944 water table conditions at Hanford indicats that heads rear the location of DC-19 were less than 410 ft (125 m). If wastewater disposal operations in Cold Creek Valley ceased, we estinate that hydraulic heads for the Basal Ringold, Rattlesnake Ridge Interbed, and the Mabton Interbed will l

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420 440 460 480 380 400 Hydraulic Head Elevation MSL (Ft)

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\5 return to inferred steady-state values approximated by the dashed line in Figure 12.

The observed downward gradients in the Saddle Mountains and upper Wariapuin Basalt; are probably influenced by hydrologic conditions deeper in the Wanapum Formation. The Hanapum has some of the most transmissive aquifers at Hanford (i.e.,PriestRapidsBasalt). In the confined aquifer system underlying the 200 Areas, the Wanapum Basalts generally have the lowest potentiometric surfaces, and thus this formation may act as a horizontal flow divide that "drains" deeper and shallower aquifers. The Wanapum may thereby receive groundwater flow from both above and below and conduct groundwater horizontally-to regional discharge zones along the Columbia River. A number of wells adjacent to the river (i.e., DC-15, 08-1, 08-2, DDH-3) have shallow potentiometric surfaces in various horizons, indicative of an area of regional discharge.

Previous studies near Hanford have shown that hydrologic conditions in basalt aquifers can be altered significantly by artificial recharge. La Sala and othe s (1973) observed that in the area of the Columbia Basin Irrigation Project natural recharge has been ove"shadowed by recharge from irrigation.

This artificial recharge resulted in head increases and changes in water quality in basalt aquifers. La Sala and others suga naa that similar recharge of basalts may be occurring at the Hanford Reservation.

Washington Public Power Supply System (WPPSS, 1981) presented additional information about hydrologic changes in the Columbia Plateau caused by the Columbia Basin Irrigation Project. They reported that water table eleYations in the central Columbia Plateau have risen hundreds of feet in some areas.

They identified one well where an increase of 440 ft (134 m) occurred.

EVIDENCE FOR INTERAQUIFER COMMUNICATION

i . },

In this section, both hydraulic and radiochemical evidence for interaquifer communication is discussed. Information about contaminant sources and background data for I-129 and tritium are presented to introduce the discussion of deep contamination measured in wells 08-15 and 0C-18.

Hydraulic Evidence Graham and others (1984) investigated aquifer intercommunicetion in the area south of Gable Mountain, including the vicinity of West Lake, Gable Mountain Pond, B-Pond, and the 200 East Area. Wells DC-18 and 08-15, both of which contain anthropogenic contamination at depth, are located nearby. Graham and others (1984) concluded that erosional "windows" through a confining bed provide direct interconnections between the unconfined and Rattlesnake Ridge aquifers. Downward gradients from the unconfined aquifer to the Rattlesnake Ridge Interbed were identified in the immediate vicinities of Gable Mountain Pond and B-Pond. Well 08-15 occurs within the area of downward gradient surrounding B-Pond. Flow 'firections in the Rattlesnake Ridge Interbed were westerly, based on a potentiometric map for the area south of Gable Mountain and east of 200 East. Graham and others used barometric well efficien-fes to assess the relative degree of confinement of the Rattlesnake Ridge aquifer.

They reported that a group of wells located south of Gable Mountain near 200 East have relatively low barometric efficiencies (13 to 28%) that suggest minimal confinement. These wells are located near known or suspected areas of erosion that may allow vertical communication between aquifers.

Water level responses to the south caused by the drilling of well DC-23W (at DC-23 in Figure 11) provide evidence of vertical communication between aquifers on the flank of the Untanum Ridge structure. This well is located about one mi (1.6 km) south of the anticlinal crest of the Umtanum Ridge-Gable Butte structure, and was drilled by 00E to provide hydrogeologic data for the southern flank of the anticline. During the drilling of DC-23W, water-level and pressure changes were recorded within 18 piezometers in the 00-19, DC-20, andDC-22(Figure 11)wellclusters(Spane,1986). Pressure changes measured

0 l'l es in nine piezometers within the Wanapum Basalts were definitely correlated with drilling activities at DC-23W. These responses were caused by large losces of drilling fluid during the drilling of highly transmissive zones in the Wanapum.

Responses in the middle and lower Wanapum were observed before penetration of these zones at DC-23W, suggesting a degree of vertical comunication.

No related piezometer responses occurred at other nearby observation wells in the Wanapum (i.e. , DB-11 and 08-12). Well 08-11 is located about 2.2 mi (3.5 km) northwest of the DC-22 well cluster. With respect to DC-23W, these wells are located beyond known hydrogeologic teatures (i.e., Cold Creek Barrier and Umtanum Ridge) and appear to have been hydrologically isolated from the drilling activities. Spane (1986) stated that the cause of the piezometer responses was not conclusively known but that enhanced vertical comunication caused by geologic factors (i.e., increased flow top thicknesses for the Roza Flow) in the Wanapum may exist at DC-19, -20, and -22. A possibie lack of piezometer integrity (i.e., lack of aquifer isolation) in the wells was suggested as a possible cause of the apparent vertical comunication. An alternative to Spane's interpretations is that enhanced vertical comunication exists on the flank of the anticline at DC-23W, and that hydraulic effects of injected drilling fluids were propagated from that well horizontally within the I various Wanapum Basalts.

Radiochemical Evidence of Interaquifer Communication The presence of anthropogenic contamination in the system of confined aquifers provides significant evidence of interaquifer comunication at Hanford. BrauerandMcFadden(1975)tabulatedmeasurementsofI-129,

{

cobalt-60, and ruthenium-106 in Hanford groundwater during the period from 1962 to 1974. Measurements were also reported for off-project wells, lakes, and local rain and river samples. They reported on five wells drilled in 1973 and 1974 for the purpose of collectirg samples to test whether radioactive effluents had migrated down through the basalts. The possible penetration of contaminants downward into the basalt layers to depths of over 800 ft (244 m) l l

L

. ID 7

was indicated by the results, and Brauer and McFadden recommended that additional wells and sampling would allow a more detailed mapping of contaninant movements. Based on their sampling, they concluded that fission product iodine had proven to be a good tracer for studying groundwater novement.

liestinghouse (1987) summarized data about I-129 in Hanford groundwater that was collected from 1959 through 1986. The report refers to previous efforts in the 1960s and 1970s to determine whether groundwater contaminants had moved offsite. These efforts included sampling from confined aquifers because it was believed that the uppermost confined aquifers might act as conduits for offsite transport of radionuclides. Results from offsite data wers considered inconclusive because of potential sampling and contamination i problems (Westinghouse, 1987).

Following earlier offsite sampling work, the Confined Aquifer Sampling Program (CASP) was begun. Thirteen new wells (DB series) were installed to measure radionuclide concentrations in selected confined aquifers and to further assess potential for offsite migration (Westinghouse, 1987). The report emphasizes that drilling, development, and sampling procedures for the DB wells were not well documented. Wells 0B-15, DB-7, and the recently drilled DC-18 were cited as exceptions, in that good records were kept about development history and sampling methods. Because of this reportedly good

! documentation, we have focused our review on these wells.

l l

l General Sources of Contaminants l

1 To assess the migration of contaminants from source areas to wells 08-15

) and 00-18, it is necessary to identify nearby waste disposal activities and facilities. During the past four decades, many radioactive and non-radioactive i

contaminants have entered the groundwater from infiltration cribs, trenches and ponds, injection wells, accidental leaks from defense waste storage tanks, and other accidental releases. Major releases have occurred withir or near the 200

'. \Q East and West Areas. Some examples of contaminants detected in Hanford's groundwater include nitrate, technetium-99, I-129, and tritium.

A report by the General Accounting Office (GAO,1986), titled "Nuclear Waste - Unresolved Issues Concerning Hanford's Waste Management Practices,"

gives a good summary of the number and kinds of nuclear waste sites on the Hanford Reservation. Hanford has 39 active and at least 337 inactive low-level waste disposal sites and 35 transuranic waste sites. These figures do not include several hundred accidental release sites or 149 high-level waste tanks.

Of these tanks, it is suspected that 60 have leaked. GA0 (1986) reported that the tanks leaked about 492,000 gallons (1,862 cubic meters) of high-level waste and other contaminants to the soil. The largest single leak was 115,000 gallons (435 cubic meters) over a 2-month period in 1973. Hanford officials could not estimate how many accidental release sites exist over the entire Reservation. Overall, the GA0 report shows that many contaminant sources exist at Hanford, not all of which can be located. However, locations are reasonably well established for waste sites having significant inventories of radionuc~lides, as docunented in ERDA (1975) and DOE (19FJ).

Zimmerman r; others (1986) reported the following information about major waste.ater disposal areas in or near 200 East:

B Plant - One of the original fuel separations facilities, operated between 1945 and 1952. In 1960 it was converted to a waste fractionizationplantandisinoperation[1986].

l

• A Plant (PUREX) - An irradiated fuel-processing plant, operated between 1955 and 1972. The PUREX plant resumed operations in i 1983 and is continuing to operate [1986).

l t

• 200-East Evaporator - An evaporator used to reduce the volume of liquid waste stored in tank farms. This evaporator began operating in 1977 and is in operation today [1986].

f

• B-Pond - This pond began receiving wastewater in 1945. Two expansion l

l

bb O

ponds were constructed in 1984 adding 50% more capacity.

• Gable Mountain Pond - This pond began receiving wastewaters in 1957.

Deconnissioning of this pond began in 1984, but it continues to receive reduced wastewater discharges [1986].

• Cribs or Trenches - Certain cribs and trenches received significantly larger volumes of wasteweter than others; these include A8, B12, B55, B62, and PUREX sribs (see Figure 2).

Background Iodine-129 and Tritium Levels To assess the significance of measured levels of subsurface contamination, Ever it is helpful to review background information about I-129 and tritium.

on a geologic time scale, 1-129 is a long-lived nuclide with a half-life of 1.57 x 10 years. It has both natural and anthropogenic sources, one source being artificially induced fission processes. Natural sources include interactions of high-energy particles with xenon in the upper atmosphere, ano Higher

' the spontaneous fission of U-238 (Eisenbud, 1987) in geologic environs.

' natural concentrations of I-129 would be expected in geologic environments

, enriched in uranium (i.e., granitic terranes, and rocks bearing uranium ores).

Atmospheric levels of I-129 were significantly increased by atmospheric l

This nuclide in anionic form is highly mobile in l

thermonuclear tests.

! aquifers; however, it has been reported that 1-129 can accumulate in top soil andplantcommunities(BrauerandStrebin,1982).

Background levels of I-129 will vary regionally because natural sources of j

this nuclide exist in the subsurface. We examined available data fron Westinghouse (1987) to estimate a useful background range for groundwater at l

i Hanford. The data included Columbia River water samples collected from 1976 to

! 1986 at Priest Rapids Dam, located about 14 mi (22 km) northwest of the 200 pCf/1 I-129 West Area. Based on 61 measurements, a geometric mean of 1.1 x 10-5 was obtained. This value was compared with data frem rain and snow samples from Washington state for the period from 1966 to 1971, which yielded a geometric mean of 5.6 x 10~4 pCi/l I-l'.9. Thus, the geometric mean for the meteoric data

1 21 is more than an order of magnitude greater than the geometric mean for I-129 data from Priest Rapids Dam.

Our interpretation of the above data is that in the Columbia Plateau the Columbia River receives considerable groundwater baseflow comprised of pre-1952 groundwater (with relatively low I-129) that dilutes the I-129 bombpulse fluxes carried downriver from watershed areas. The river samples appear to be strongly influenced by the regional groundwater influx, albeit somewhat elevated by watershed contributions, and thus may provide an upper estimate for groundwater background values of I-129. We propose an upper background range that includes the 95% confidence interval for the geometric mean of the samples from Priest Rapids Dam, i.e., from 8.9 x 10-6 to 1.4 x 10-5 pCi/1. Due to the mixing-body nature of a large river like the Columbia, the range obtailled in this way is really a rough upper estimate for much of the Columbia Plateau region. Future studies based entirely on deep groundwater sampling in areas remote from waste disposal sites should refine this estimate downward, perhaps by several orders of magnitude.

Like I-129, tritium is also a valuable groundwater tracer. This nuclide is a radioactive isotope of hydrogen with a half-life of 12.35 years. It is formed naturally in small amounts from interactions of cosmic rays with gases in the upper atmosphere (Eisenbud, 1987). It exists mainly as water vapor and precipitates as rain or snow, Natural concentrations of tritium in lakes, rivers, and potable waters prior to ruclear testing were estimated to be about i 5 to 25 pCi/l (UNSCEAR, 1982, vide Eisenbud, 1987). Tritium concentrations in the environment were greatly increased by atmospheric thermonuclear tests which began in 1952. As recorded at Ottawa, Canada, concentrations of tritium in precipitation peaked in 1963 at about 9,600 pCi/1, or 3,000 tritium units (AECL, 1968, vide NCRP, 1975). Actual tritium values vary with both time and location.

Tritium is an excellent tracer because it directly forms part of the water molecule. Starting in 1952, elevated amounts of tritium in meteoric water were

'. 2R introduced to groundwater recharge areas over much of the world, and this phenomenon can be useful in hydrogeologic studies. Elevated tritium concentrations in an aquifer show that post-1952 recharge han entered the system. Given the short half-life of tritium, pre-1952 groundwaters with significant aquifer residence time and minimal vertical mixing should have tritium levels at or below detection limits. Brauer and others (1978) reported that activities as low as 10-1 pCi can be measured using the gas proportional counting method, and detection 'imits can be extended using enriched samples.

In appendix F of Westinghouse (1987), it was stated that, with 'espect to Hanford groundwater, tritium values at about 0.1 tritium unit (0.3 pCi/1) can be considered elevated.

Contaminants in Well 08-15 As shown in Figure 2, well 08-15 is located about 1.1 mi (1.8 km) east of the 200 East Area. The drilling history of DB-15 is given by Diediker and Ledgerwood(1980). Two major areas of wastewater disposal occur near DB-15.

They are B-Pond, 0.8 mi (1.3 km) to the south, and Gable Mountain Pond, 1.4 mi (2.2 km) to the northwest. Gable Mountain Pond was constructed in 1957 to receive process cooling water and waste condensates from sources in the 200 East Area (Strait and Moore, 1982). In 1964, an accidental release from the PUREX Plant in 200 East added about 100,000 Ci of fission prodccts to Gable Mountain Pond and nearby 8-Pond. This release significantly added to groundwater contaminant levels in the vicinity of these ponds. In addition, due to the restart of the PUREX Plant in 1983, a subsequent increase in groundwater mounding beneath B-Pond was predicted by Zimmerman and others (1986).

In 1979, groundwater was sampled from various horizons in well 08-15, and subsequent analyses included measurement of I-129 and tritium concentrations.

The distribution of I-129 concentrations and their relation to stratigraphic units are shown in Figure 14. Elevated trace levels of I-129 were fcund in the < F/4 Rattlesnake Ridge Interbed and in deeper geologic units of the Saddle Mountains and Wanapum Formations to a depth of over 1,400 ft (427 m). The increase below

1 Estimated 1-129 Background con =,e (upper range) '

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Figure 14.--Depth distribution of Iodine-129 in relation to 08-15 stratigraphy (after Westinghouse, 1987).

kJ the Mabton Interbed is not consistent with an interpretation involving downward migration from the surface due to casing or other defects, where a continuous decrease with depth would be expected. Westinghouse (1987) reported that tritium concentrations in well DB-15 appear to be positively correlated with the I-129 concentrations, and the ratio of I-129 to tritium is similar to that observed in the unconfined aquifer in the vicinity of 200 East. Westinghouse suggested that a natural pathway may explain how the deep, confined aquifers can be in communication with near-surface groundwater.

Tritium in Well DC-18 Well DC-18 was drilled at the western margin of Gable Mountain, near the crest of the Umtanum Ridge anticline (Figure 4). It was designed to study the effects of geologic structures in that area on vertical aquifer commur.! cation.

Well DC-18 was also constructed to provide high-quality hydrochemistry data by using only water instead of drillirg muds. C; emical tracers in the drilling make-up water were used to assess the efficiency of clean-up and development of the well. Results of well development showed a thousand-fold attenuation of tracers, indicating good clean-up. Later sampling showed tritium concentrations in deep units on the order of 1 to 10 tritium units (Westinghouse,1987).

As discussed earlier, tritium values shown in Figure 10 suggest that, circa 1980, the unconfined aquifer near 0C-18 was more strongly influenced by contaminant sources other than Gable Mountain Pond. These sources, which cannot be differentiated using available data, would include sources in the 200 North, 200 East, and 200 West Areas. The 200 North Area (N Cribs) disposal facilities are closest to DC-18, about 1 mi (1.6 km) to the southwest of this well. Large volumes of wastewaters were discharged in the 200 North Area from l

1943 to 1952 (Zimmerman and others, 1986). These disposal activities were l discontinued in 1952 (Graham and others, 1984).

1 L

Figure 15 shows the distributitm of tritium concentrations in relation to < F lE stratigraphic units in DC-18. Elevated. trace values of tritium occur at depths of over 980 ft (300 m). Westingnouse (1987) stated that the only plausible explanation for the deep contanination is an interconnection with surface sources of tritium. Tritium concentrations in the unconfined aquifer near well DC-18 were reported at about 2,400 tritium units. As shown in Figure 15, fau'lt zones in the Wanapum Formation were penetrated by well DC-18, indicating that structural discontinuities exist in the vicinity. Westinghouse (1987) recommended additional characterization work to assess the hydrologic role of fault zones in the Umtanum Ridge anticline.

Contaminants in Other Wells There is also evidence of contamination in other deep wells at Hanford.

However, Westinghouse (1987) points out that information regarding the construction and sampling of these wells is poorly documented, and resulting data are thus questionable. Well 08-7 was identified as a well having reliable data showing elevated I-129 levels, but it was pointed out that anthropogenic I-129 should be accompanf t:d by tritium. The reported absence of tritium in l

DB-7 raises' questions about an anthropogenic origin for the I-129 in this well.

l Its location is also suspect, because DB-7 is located over 11 mi (18 km) S-SE from the 200 Areas. Future environmental monitoring at Hanford should assess whether disposal sites have identifiable ratios of I-129 and tritium (or other nuclides), perhaps making it possible to link subsurface contamination with identifia e source areas.

CONTAMINANT MIGRATION PATHWAYS l

l Both natural and artificial pathways may exist for transporting contaminants down from the unconfined aquifer to the deeper confined systems.

Natural pathways are discussed in a later section. Artificial pathways include I

I PREUMINARY DC-18 STRATlGRAPHY TRmVM WITH FAULT / SHEAR ZONES . PROFILE v

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l Figure 15.--Preliminary stratigraphy and groundwater tritium concentrati:+, in well0C-18(afterWestinghouse,1987).

I

As injection wells, interaquifer communication through open boreholes, and migration through paths caused by drilling or fau.ty well construction.

Artificial Pathways Westinghouse (1987) stated that no evidence was found to support the possibility of deep injection. At least nine ve11s were used to inject wastes and wastewaters at Hanford (ERDA, 1975), but available information doesn't show whether any of these wells were used to inject wastes deep into the basalts.

Smith (1980) describes construction detafis for an inactive injection well (216-B-5) in the 200 East Area and gives results of a program of environmental surveillance. Wastes were discharged from this well to unconsolidated deposits of the Ringold Formation located above the basalts.

Another potential pathway for interaquifer communication is via open boreholes. Graham and others (1984) discussed the migration of wastes by density flow through open boreholes in the vicinity of 200 East. This resulted [

in flow from the unconfined aquifer to the Rattlesnake Ridge Interbed. We examined available information to see if any suspect wells that deeply penetrate the basalts occur near wells DB-15 or DC-18. Acc'ording to DE l (1986a), two relatively deep wells (DH-9A and 08-8) occur less than one mile (1.6 km) from 0B-15. Well DH-9A was drilled in 1979 for groundwater monitoring purposes to a total depth of 222 ft (68 m). This well was reportedly grnuted back to casing to prevent contamination from rearby B-Pond. Well DB-8 (shown j

! in Figure 11) was one of the "0B" series wells of the CASP study, designed for deep groundwater sampling. It was drilled in 1977 to a total depth of 1,092 ft l

I (333 m). It was reportedly cased to 937 ft (286 m) and cemented to 902 ft (275 m)(00E,1986a).

Based on available information, it appears unlikely that either of the above wells contributed to the deep contamination at 08-15. Well DH-9A was drilled in tne same year that 08-15 was sampled, and seems too shallow to have played a role in deep contamination. Well 0B-8 was drilled two years before f

}

A DB-15wassampled,butthedeepcasing[cementedto902ft(275m)]shouldhave prevented most contaminants from the unconfined aquifer and nearby B-Pond from reaching dceper zones. We recognize, however, that some contamination may have reached deeper zones duri.ig the initial drilling of well 08-8. This contamination may have affected water samples collected at DB-8 during the CASP study if initial well development had not been 3dequate.

According to []E (1986a), three relatively deen wells occur less than one mi (1.6 'm) fron 3C-18. Well 08-9, another of the CASP wells, was drilled in 1977 to a total depth of 589 ft (180 m). It is reportedly cased to 461 ft (141 2,; andsc<.enedfrom490ft(149m)to589ft(180m)(DOE,1986a). Well DH-8A was drilled in 1976 to a tntal depth of 249 ft (76 m). It is, reported to be uncased, but plugged at a depth of 234 ft (71 m). Well DH-8B was drilled in 1976 and later deepened to a total depth of 327 ft (100 m) in 1980. None of the three wells appears to be deep enough to have significantly contributed to the contap.ination in well DC-18 at depths of over 980 ft (300 m). We would, however, recommend careful evaluation of the casing integrity at 08-9 as part of any future study of contaminant migration at DC-18. Integrity testing had been proposed for 08-9 and a number of other t. ells at Hanforo (Brown,1985).

f(aturalPathways Westinghou. (198/) noted that the occurrence and distribution of I-129 in the confined aquifer appear to be localized in the vicinity of geologic fracture zones. Areas of suspected structurally controlled aquifer intercommunication occur south of Gable Butte and Gable Mountain and extdad far south as the 200 Areas. Well DB-15 occurs in one , these areas. Elavated ,

levels of I-129 occur in this well at depths exceedin9 1,300 ft (396 m). The f I 1.5 depth profile for this well shows that elevated trace levels of I-129 occur in both the Saddle Mountains and Wanapum Formations. Westinghouse (1987) states that well 06-15 is located near a geologic structure where fracture networks might permit vertical movement of I-129 and tritium. This type of flow concept was also proposed to explain the relatiuly high trace levels of

.- - ~

M tritium at depths excceding 980 ft (300 m) in well DC-18. As mentioned earlier, fault zones in the Wanapum Basalts were penetrated by well DC-18.

Figure 16 shows top-of-basalt elevations for the area south of Gable < Fl s .

• Mountain (Strait and Moore, 1982). In Gable 3ap the subcrop surface was deeply incised by paieochannels of the ancestral Columbia River. At DC-18, entire basalt units were removed by erosion, including the Elephant Mountain and Pomona Basalts, and the intervening Rattlesnake Ridge Interbed. A portion of the Esquatzel Basalt was also removed. These relationships are shown along profile A - A' in Figure 17, where the principal zones of erosion are located < Fl7 at or belcw the 350 ft (107 m) contours in Gable Gap ar.d adjacent to Gable Mountain (Figure 16). The location of profile A - A' is shown in Fioure 5, and a cross-sectional view of the paleochannel was previously shown in Figure 6.

The400ft(122m)closedcontour(Figure 16)nearGableMountainPondreflects tne presenca of the Willa Antic!ine (shown in Figure 4). The net result is that upper units of the Saddle Mountains Basalts were eroded and provide direct avenuas for groundwater infiltration and interaquifer communication.

Geologic Structures Near Gable Mountain l A number of faults are known to be associated with the Gable Butte-Gable Mountain structure. Golder Asrociates, Inc. (GAI, 1981) investigated the relationship between faulting ana folding near Gable Mountain and Gable Butte l

by studying local faults. The faults were produced during fold deformation, l

and the dimensions of the faults are probably constrained by the dimensions of folds they are associated with. The subsurface configurations of the faults were determined by trer.ch and borehole investigations. The so-called south fau!' and associated shear zones of the north-dipping reverse fault were found I

to intersect most of the basalt and interbed units of the Saddle Mountains Formation (Figure 18). ihe subsurface expression of the Central Fault, which < f / h W similarly deformer' ant intersected geologic units of the Saddle Mountains l

Formation, is depicted in Figure 19. These fault zones servt as models of the < flh 1

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comunication between basalt and interbed aquifers.

Myers and others (1979) reported the detection of two subsurface f ault zones in well 08-10, located south of Gable Mountain about 1.4 mi (2.2 km) northeast of well 08-15 (Figure 20). The fault zones were located at depths of < hk{ -

400 ft (122 m) and 575 ft (175 m) and were interpreted to be reverse faults with about 160 ft (49 m) of combined displacement. Fault strikes could not be determined because the core orientations were unknown. Myers and others speculated that the top-of-basalt high where DB-10 was drilled might be caused by faulting in addition to sharp folding. Other subsurface expressions of possible faulting (i. e., tectonic breccias) have been reported at Hanford (Landon, 1985), but reported depths of these zones exceeded 1,000 ft (305 m).

CONCEPTUAllIATION OF THE GROUN0 WATER FL0ll SYSTEM NEAR GABLE MOUNTAIN Given the presence of significant structural deformation in the vicinity of Gable Mountain, vertical contaminant .nigration can be postulated or this area. Contaminants originating from waste disposal sites near the 200 Areas migrate in the unconfined aquifer in all directions following radial recharge patterns caused by B-Pond and Goble Mountain Pond. Northerly groundwater flor f

) is toward the structurally complex fold axis and southern flank of the Umtanum l Ridge-Gable Butte-Gable Mountain structure. In the area south of Gable Mountain in the vicinity of 200 East, water table elevations raised by decades of wastewater disposal exceed the potentiometric surfaces in basalts of the Saddle Mountains Formation, and potential for recharge to the basalts exists.

As discussed in the previous section, known geologic discot.tinuities in this l

area illestrate potential paths of interaquifer comunication. Geologic ,

features of this kind can account for the deep contaminatien observed in wells l

D8-15 and 0C-18.

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The artificially raised water table has caused an increased flow of groundwater toward Gable Gap. As groundwater moves toward the Gap and across the anticlinal crest of Umtanum Ridge, recharge is preferentially introduced to zones of higher hydraulic conductivity. These zones would include undetected faults that are exposed on the basalt horizon buried beneath sediments of the Hanford and Ringold Formations. Some contaminants could also be introduced via the extensive system of cooling fractures that pervade the basalts.

Groundwater contaminants would thus be introduced to a system of geologic structures that in placas are known to vertically connect basalt aquifers of the Saddle Mountains Formation. Interflow structures may also connect the Saddle Mountains with the deeper Wanapum Basalts. Contaminants following horizontal paths away from deep recharge zones would ba detectable in deep wells like DC-18 and 08-15.

The above conceptualization is supported by the presence of anthropogenic contamination in deep basalts, anomalous piezometer responses to drilling on the flank of Umtanum Ridge, and the presence of major fault zones in the Umtanum Ridge-Gable Butte-Gable Mountain structure.

Estimation of Contaminant Migration Rates Rates of contaminant migration from source areas to d2ep zones in wells DB-15 and 0C-18 can be estimated using the above conceptualization of flow.

Our method is a simple one based on the length of time that disposal operations have occurred. Estimates of mininial migration rates can be made by assuming l

short travel times of contaninants in the unconfined acuifer and by also l as;uming that ell migration occurs in a vertical path in the saturated zone l

from the unconfined aquifer to deeper zones.

I l The largest waste disposal site near well DB-15 is B-Pond, 0.8 mi (1.3 km) to the south. B-Pond has been in operation since late 1945. Based on the f

water table maps of Strait and Moore (1982) and Graham and others (1984), a l

significant groundwater recharge mound exists beneath B-Pond, and the imposed

_.  ?

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radial flow pattern includes flow vectors toward 08-15. Thirty-four years had passed fron the start of B-Pond operations in 1945 until 1979, when 08-15 was sampled for I.129 content. Given the presence of I-129 in 0B-15 at depths of over 1,300 ft (396 m) below the surface and over 1,200 ft (366 m) below the water table, a downward migration rate of at least 31 ft/ year (9.4 m/yr) can be obtained for a vertical path in the saturated zone. .This value would be larger if the contamination events affecting 08-15 occurred more recently than 1945.

liigration rates would also be greater if contamination reached deep zones

( before 1979 or if paths of migration were highly tortuous.

A similar assessment can be made for tritium in well 0C-18. This well is located in Gable Gap 1 mi (1.6 km) northeast of the 200 tiorth Area, 1.6 mi (2.6 l km) northwest of Gable Mountain Pond, and 2.6 mi (4.2 km) north of the 200 East Area. Although principal unconfined flow directions from the 200 Areas are easterly, some flow is known to occur northward throuth Gable Gap to the Columbia River. Northerly flow discharges through the Gap may have been larger in the past when greater wastewater volumes were disposed in the 200 Areas. As previously shown in Figure 13, the water table at nearby well 60-60 peaked in

1968 at over 406 ft (124 m) MSL. In any event, it is established that flow vectors (circa 1980 to 1983) have existed to carry contaminants from disposal aret.9 to the vicinity of well 0C-18. As in the case of 08-15, we shall consider migration rates in the unconfined aquifer to be rapid (00E, 1987) and shall assume a maximum period of vertical migration because the actual arrival i time of tritium in the deep subsurface is unknown. Beginning around 1945 when major discharges were oce.urring in the 200 areas, about 40 years passed until well 0C-18 was sampled in the mid.1980's. Assuming instantaneous arrival at the well, and given the presence of tritium in 00-18 at depths below the water table of over 860 f t (262 m), a downward migration rate of at least 21 f t/yr (6.4 m/yr) can be obtained for a vertical path in the saturated zone.

We emphasize that these estimates of contaminant migration rates are not predictions based on hydrodynamic flow models. Rather, they are minimal estimates of migration rates based on known instances of subsurface

< (

L - - -

_ __ )

?

Q anthropogenic contamination. Although minimum migration rates can be estimated in this way, vertical groundwater fluxes cannot be determined because there is a lack of information about vertical hydraulic properties in the basalts. We believe that this information can help guide future hydrogeologic work at Hanford by emphasizing the need to link studies of aquifer hydrodynamics with studies of hydrochemistry.

SUMMARY

AND REC 0ftitENDATIONS Dispost.1 of nuclear industrial wastewaters in volumes that greatly exceed natural recharge has changed hydraulic gradients and flow directions in the surficial aquifer. The increase in hydraulic gradients may also have altered conditions in the confined aquifers of the Saddle Mountains Formation.

Groundwater tagged with contaminants has migrated in the unconfined aquifer radially away from artificial recharge sites, including flow vectors north from the 200 Areas to the Columbia River, across the structurally deformed fold axis of the Umtanum Ridge-Gable Butte-Gable Mountain structure. Available hydrochemical data from wells rear Gable Mountain show evidence that contaminants have deeply penetrated the basalt formations near this structure.

Specific contaminant pathways have not been confirmed, but probably include faults and shear zones associated with regional folding. A series of such fault zones is assoct 'ed with the Gable Mountain structure, intersecting most of the geologic units o +he Saddle Mountains Basalts. These fault zones illustrate the kinds of features in anticlinal areas that can provide vertical comunication between the unco.1 fined aquifer and deep basalt units. On a regional scale, interaquifer communication may occur via the ubiquitous networks of cooling fractures that pervade the interiors of basalt flows.

Many deep wells at Hanford show evidence of anthropagenic contamination, but drilling and sampling procedures in earlier years were not carefully documented and results are therefore questionable. Case histories for wells l

Q f

0C-18 and DB-15 are better documented and support a conceptualization of recharge of deep basalts via major geologic structures associated with the Umtanum Ridge anticline. Anomalous hydrologic responses associated with the ,

drilling of DC-23W also provide evidence of vertical hydrologic comunication near the anticline. Nuclear waste sites occur as close as 0.8 mi (1.3 km) from DB-11 and 1 mi (1.6 km) from DC-18. Contaminants detected in these wells at deptls exceeding 980 ft (3"O m) are postulated to have migrated laterally in the unconfined aquifer, downward through intraflow structures in the basalts, then laterally to zones sampled at 08-15 and DC-18. As the recipient of numerous waste disposal effluents, the unconfined aquifer south of Gable Mountain acts as a broad, diffuse source of contaminants, making it difficu.

to link groundwater contamination in a well with specific waste disposal sites.

Relatively rapid rates of downward migration from the unconfined aquifer can be inferred ftom the data, exceeding 21 ft/yr (6.4 m/yr) at well DC-18 and 31 f t/yr (9.4 m/yr) at well 08-15.

I To better understand scenarios of aquifer interconnection at Hanford, and f.

to help rule out artificial pathways, the following kinds of site work would be needed:

• continua hydrochemical sampling to obtain direct evidence of interaquifer comunication and to better understand background levels of ,

contaminants conduct studies of long-term effects of deep, previously bncased boreholes to determine if significant interaquifer flow occurs

• continue assessments of piezometer integrity in deep wells

• obtain estimates of vertical hydraulic conductivities in interflow and intraflow structures witnin the basalt formations

• conduct large-scale aquifer tests to assess the hydrogeologic role of Uk.jor features like the Umtanum Ridge Anticline and the Cold Creek Barrier J

33 ACKNOWLEDGMENTS The authors thank Dr. Tilak Verma, Hydrogeologist at NRC, for providing a technical review of this paper. His st.:19stions and encourtgement were valuable in developing this evaluation cf groundwater flow and hydrochemistry.

DISCLAIMER The views expressed in this paper are the opinions of the authors, and do not necessarily represent the official policies of the U.S. Nuclear Regulatory Comission.

SELECTED REFERENCES Brauer, F. P. , Goles, R. W. , Kaye, J. H. , and Rieck, H. G. ,1978, Sampling and measurement of long-lived radionuclides in environmental samples:

American Chemical Society Proceedings, 4th Joint Conferen*e on Sensing of Environmental Pollutants, p. 330-335.

Brauer, F. P. and McFadden, K. M., 1975, [ Draft]I-129,Co-60,andRu-106 measurements on water samples from the Hanford Project environs (1962-1974): Pacific Northwest Laboratories, Richland, Washington (available from Nuclear Regulatory Commission file 101.0, Washington, DC).

Brauer, F. n. and Rieck, H. G. , Jr. ,1972, Radiochemical analyses of Hanford well-water samples: BNWL-CC-1800 83, Pacific Northwest Laboratories, Richland, Washington.

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l l

CH0-BW-SA-318P, Rockwell Hanford Operations, Richland, Washington, 55 p.

?

1. I 36 a

GAI (Colder Associate.s, Inc.), 1981, Gable Mountain - structural investigations and analyses: Appendix 2[0] in "Preliminary Safety i Analysis Report,-Skagit/Hanford Nuclear Project, Puget Power, Vol. 4."

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Czimer, W. J.,

l 1 Davidson, N. J., Edwards, R. C., Fecht, K. R., Holmes, G. E.,

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J. T., Lor,], P. E., Mitchell, T. H., Price, E. H.,

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t

.el' _

Washington.

Myers, C. W., Price, S. M., Tallman, A. M., Lillie, J. T., Fecht, K. R.,

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Price, E. H. , Holmes, D. J. , Mitchell, .T. H. , Kunk, J. R. . Ault, T. D. ,

and Cochran, M. P.,1981, Subsurface geology of the Cold C eek syncline:

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! Rockwell Hanford Operations, Energy Systems Group, Richland, Washington, 119 p.

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l DC-19, DC-20, and DC-22 during construction of DC-23W: SD-BWI-Ti-313, Rockwell Hanford Operations, Richland, Wasnington.

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Strait, S. R. and Moore, B. A., 1982, Geohydrology of the Rattlesnake Ridge interbed in the Gable flountain Pond area: RH0-ST-38, Rockwell Hanford Operations, Richland, W6shington, 73 p.

w. ~ %

b t')

~

37 Swanson, L, C. and Leventhal, B. A. 1984, Groundwater monitoring data and

('

borehole descriptions for the Hanford site monitoring network wells:

SD-BWl-DP-042, Rockwell Hanford Operations, Richland, Washington.

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WHC-EP-0037, Intercontractor Working Group, Westinghouse Hanford Company, Richland, Washington.

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3 J