SITE CHARACTERIZATION

Drilling characterization wells at the Shoal test area.

Gathering data and developing a comprehensive understanding of a groundwater system is complex, time-intensive, and expensive. The wells drilled at the Shoal test site penetrated at least 1 300 feet through solid granite. The cost of drilling wells through granite is especially high, and drilling is further compounded with safety and monitoring requirements associated with a potentially radioactive environment. Thus, designing investigative approaches to make optimal use of existing data is a crucial element for environmental remediation investigations.

The 1996-1998 field campaign was designed to accomplish four objectives: (1) describe the groundwater gradient in the test area under undisturbed conditions; (2) measure permeability and porosity of the granite rock; (3) determine recharge conditions; and (4) assess contaminant migration from the testing zone. These objectives were set with the specific goal of further evaluating groundwater flow and transport depicted in the 1960s conceptual model. For example, the presence of a hydrologic divide was well demonstrated in the early studies, but its location was unclear.

Videographic image obtained during downhole well-logging operations.

Groundwater flow through the granite is confined to fractures and faults. These features were characterized in the boreholes using video logs, acoustic logs, and surface mapping. Highly specialized instruments were used to measure water chemistry, groundwater flow, and rock density, and a 3-inch video camera was used in the wells to capture information about rock fractures. Determining the direction, size, and location of fractures contributed to the overall picture of the groundwater system. Through well-logging and aquifer tests conducted in the four characterization wells (known as HC - 1, -2, -3, and -4) drilled in 1996, DRI researchers established that the groundwater divide was located west of the nuclear test. This suggested that groundwater flow from Shoal moves essentially eastward-perpendicular to the hydrologic divide. Based on the data, permeability and porosity were confined to fractures in the granite.

Even though bulk and discrete measurements of hydraulic conductivity were collected, it was not possible to quantify porosity. Recharge was estimated using temperature profiles. Absence of radionuclides in the inferred downgradient well, HC-3, indicated a lack of significant radionuclide transport, although low levels of test-derived nuclides were found in well HC-4, close to the test cavity.

Field data from the 1996-1998 field campaign definitively placed the hydrologic divide, demonstrated in the early studies, west of the Shoal underground nuclear test site.

Understanding the hydrogeological characteristics of the site was important in predicting transport properties-how radionuclides and contaminants released from the test would move through the groundwater. Therefore, samples of the granite rock from the drilling muck pile were collected for laboratory experimentation. Transport properties were characterized to estimate how the rock served to retard the movement of contaminants in the groundwater system by binding with contaminants.

Although fieldwork produced significant data in the 1996-1998 campaign, it was not sufficient to meet DOE objectives. Uncertainties regarding porosity and recharge primarily necessitated the 1999-2000 campaign. A two-well, forced-gradient tracer test and deep hydraulic characterization well nest (cluster or grouping of hydraulic characterization wells), which involved drilling four additional characterization wells (HC-5 6, 7, and 8), were selected as the methods whereby additional data would be collected. Both campaigns, including supporting laboratory investigations, were ultimately sufficient to fulfill DOE objectives of establishing contaminant boundaries and developing a groundwater flow and transport model.

Aerial view of the Shoal test area showing locations where original wells, 1996 wells, and 1998 wells were drilled.

DOE's general purpose in conducting the two-well, forced-gradient tracer test was to determine subsurface hydraulic properties including effective porosity; however, the test also addressed other objectives including effective porosity and hydraulic properties of the Shoal granite aquifer, dispersion coefficient at the 30-meter scale and field-scale sorption for weakly sorbing solutes, and hydraulic properties of the fractures.

The two-well tracer test was conducted between wells HC-6 and HC-7, located approximately 30 meters apart to reflect the scale of the numeric model. These two wells were drilled approximately 350 meters southwest of the Shoal cavity. They were approximately 375 meters deep and had sections of well screen measuring 35 meters long beginning 40-70 meters below the static water table at a depth of 299 meters. A submersible pump was set near the bottom of the well screen in well HC-7, and a temporary pump was used in well HC-6 for recirculating the tracers during and immediately after injection to prevent density stratification.

Large injection of bromide, PFBA, and lithium.

The initial tracer was injected after a quasi-steady-state flow field was established. This injection consisted of a small volume of sodium iodide to evaluate rapid transit times and to verify the data collection procedures for the main test. The larger main injection containing multiple tracers, including bromide, pentafluorobenzoate (PFBA), and lithium, occurred on November 10, 1999 . Additional injections of cesium and microspheres occurred later, but neither injection was detected in the pumping well.

Breakthrough (detection of the tracers in the pumping well) of bromide began about one month into the test. The weak dipole was maintained until the last sample was collected from well HC-7 on September 24, 2000 . The tracer test ran for 319 days, or approximately 7 630 hours. Water levels slowly declined through the duration of the test but have been slowly recovering.

The two-well tracer test, involving wells HC-6 and HC-7, was designed to determine subsurface hydraulic properties. Each black line represents a separate fracture. Click on graphic to open animation and click a second time to initiate.

Multiple tracers allowed evaluation of flow porosity, dual-porosity transport involving matrix diffusion, and sorption. Results and interpretations of the tracer test indicated that a dual-porosity conceptual transport model should be used to describe transport of dissolved radionuclides. The lithium breakthrough curve from the two-well tracer test indicated that sorption occurs both in the matrix and on fracture surfaces, although sorption in the matrix is much greater. The tracer test also revealed the importance of matrix diffusion during transport processes through the granite of the Shoal site. This was observed by comparing the breakthrough of bromide with the breakthrough of PFBA-a large molecule that has a lower free diffusion rate as compared with bromide.