Distribution heads

Figure 5. Hydraulic-head distribution in the Middle Devonian Keg River Formation of northern Alberta. Geology after McCamis and Griffith (303).

Prior to tracking particles, the groundwater flow equation must be solved in order to determine the hydraulic head distribution within the aquifer. The two-dimensional, steady-state groundwater flow equation is... 

The groundwater velocity field is determined by Darcy s law after the head distribution from solution of (1) has been determined ... 

The hydrogeology of the Aspo site is controlled by flow in fractures (Figure 15). The hydraulic conductivity of the rock mass is 10 -10 ° m s while the hydraulic conductivity of the fractures is 10 " -10 m s (Rhen et al., 1997). The irregular water table is locally controlled by sparsely distributed, poorly interconnected fractures. At depth, the hydraulic pressure head distribution becomes more regular due to control by fracture zones and larger... 

The hydraulic-head distributions are determined mainly from extrapolated drill-stem pressure data (Earlougher, 1977). In all cases, test results have been rejected if the initial and final maximum reservoir pressures differed by more than 5%. Pressure determinations are expressed as equivalent freshwater hydraulic head in meters above sea level at 15.6°C. Maps of the potentiometric surfaces for eight major geological units have been prepared from this data set (Fig. 4). Because potentials may have been influenced by gas and oil production (or groundwater in the case of the Milk River aquifer), the location of major fields is indicated on the figure. 

Fig. 2. A. Regional stratigraphy and hydraulic-head distribution. B. Regional distribution of total dissolved solids (TDS).

Contaminants may migrate within saturated residual or transported soils, within soil macropores, or within small interconnected pores where they find their way into groundwater. In either case contaminant dispersal will be limited to distinct groundwater subbasins defined by fixed or transient groundwater divides, by the hydraulic-head distribution within the flow system, and by geological barriers which restrict flow such as the presence of poorly permeable soil or bedrock units. 

The output of a flow model consists of the head distribution in time and space. Darcy s Law is used to convert the head distribution to a velocity distribution suitable for input to a contaminant transport model. In a two-dimensional application, Darcy s Law is used to compute two sets of velocity components ... 

Flow Modeling. The flow component of the random walk model was used to produce the head distribution shown in Figure 3a. The hydraulic conductivity of the aquifer was set equal to 200 ft/day (61 m/day). The saturated thickness of the aquifer is equal to the elevation of the water table above the Impermeable bedrock the water table elevation is adjusted automatically during the iteration process used to solve the flow equation. 

The steady state head distribution shown in Figure 3a was used to calculate the velocity distribution, using a hydraulic conductivity of 200 ft/day (61 m/day) and an effective porosity of 0.30. Velocities ranged from 0.8 to 1.2 ft/day (0.24 - 0.37 m/day) but were around 1.1 ft/day (0.34 m/day) beneath most of the field. Groundwater flows in a westerly or northwesterly direction, as can be inferred from the groundwater potentials shown in Figure 3. 

A steady state flow field was assumed when the heads do change in response to fluctuations in recharge rate. Hence, the head distribution used in the model is an approximate one. 

The velocity field is not exactly reproduced. The model assumes constant hydraulic conductivity and effective porosity and uses an approximate head distribution to compute the velocity field. 

A finite-element mesh was constructed as shown in Figure 5. The hydrological properties of the different units within the finite-element model was first estimated from local geohydrological field data and then calibrated to match observed head distribution and inflow into the open drift. An initial stress was assigned according to = 10 MPa, Oh = 15 MPa, and On = 30 MPa, where Oh is oriented 45° from the axis of the FEBEX tunnel (Figure 5b). These values are within the range of stress measurements in the GTS area (Pahl el al.. 1989). 

A thermo-mechanically coupled, transient ice sheet model (Boulton and Payne, 1994), coupled with the Earth model of Lambeck et al., (1998) has been driven by the climate function over a prescribed topography of North America with a 10 km resolution. The model computes the temperature at the base of the ice sheet and the rate of basal melting in time and space. This is used to compute the spacing between channels that are required to exist at the ice/bed interface to discharge meltwater that cannot be discharged by groundwater flow (Boulton et al., 2(X)1), and the head distribution at the ice/bed interface. 

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