Calibration and Reliability in Groundwater Modelling (Proceedings of the ModelCARE 96 Conference held at Golden, Colorado, September 1996). IAHS Publ. no. 237, 1996. 343

Modelling of strongly coupled groundwater brine flow and transport at the Konrad radioactive waste site in Germany

ALFONSO RIVERA, RUSSELL T. JOHNS1, MARC SCHINDLER, SIMON LOW & GEORG RESELE Colenco Power Consulting Ltd., Division of Nuclear Technology and Safety, Mellingerstrasse 207, CH-540S Baden, Switzerland

Abstract The former Konrad iron ore mine at Salzgitter, Germany, is proposed to be used as a radioactive waste repository. The mining drifts of the Konrad site are covered by thick clay layers and are bounded laterally and below by salt domes that provide an excellent low permeabi­lity geological barrier. We present simulation results on a 40 X 2.5 km vertical 2D model of the Konrad site that include the effects of salt dissolution on groundwater flow. The simulations were done in steady state with the finite element code NAMMU. The main objective of this work was to evaluate quantitatively the effects of the downward increase in salinity of the groundwater on the vertical flow through the disturbed zones (e.g. back filled shafts and boreholes) in the clay barrier. The presence of salt caused a significant decrease in the vertical flow velo­cities of the major aquifers at a location far from the waste site, which in turn decreased horizontal flow near the waste site. The net effect of the decreased aquifer flow and increased gravity forces was to slightly increase vertical flow in the intact clay barrier above the waste site. The presence of salt, however, did significantly reduce upward velocities by over one order of magnitude for disturbed zones that may penetrate the geological barrier. This means that even hydraulically strongly disturbed zones would have no negative effect on the long-term safety of the proposed site.

INTRODUCTION

Situations may arise in groundwater formations where the introduction of solute causes changes in the groundwater density that are sufficiently large to affect the flow dynamics of the system. Recently, attention has been focused on long-term salt dissolution and brine transport in connection with salt formations that are under consideration as radioactive waste repositories (Herbert et al., 1988; Vogel et al., 1990; Oldenburg & Pruess, 1995). Three examples of these are the Konrad and the Gorleben sites in Germany and the WIPP site in the USA.

The former Konrad iron ore mine at Salzgitter, northern Germany, is proposed to be used as a potential repository (for the disposal of radioactive wastes with negligible 1 Now at: the University of Texas at Austin, Department of Petroleum and Geosystems Engineering, Austin, Texas 78712-1061, USA.

344 Alfonso Rivera et al.

heat generation). Hence, the mine has undergone very extensive R&D investigations to assess its suitability as a repository. The Konrad mine lies some 12 km to the southwest of the city of Braunschweig, at a depth of 800-1200 m in the Malm formation. The former mining drifts are covered by thick clay layers and are surrounded by salt domes at a distance of less than 5 km. Salt is also believed to exist at a maximum depth of about 2.5 km below the Konrad site and to be in contact with a very permeable limestone formation, the Muschelkalk. Measurements in the Konrad mine indicate an increase in salinity with depth from 160 g l"1 at a depth of about 500 m to 220 g l"1 at a depth of about 1300 m (Klinge et al., 1992).

Because of the proximity of the salt domes, the density of the groundwater depends on solute concentration, and can be 20% greater than that of freshwater. Therefore, flow and transport are strongly coupled. This coupling causes significant nonlinearities in the flow and transport equations and poses significant challenges for numerical simulation.

Numerical models have been used as tools for studying local and regional ground­water flow conditions at the Konrad site. However, most of them were based exclusively on freshwater calculations, thus neglecting the presence of high salinities in the ground­water system. Gilby et al. (1988) made three-dimensional simulations of the ground­water flow system in a 450 km2 region around the Konrad site. The results of their study were the basis for selecting the two-dimensional model used in this work. For the present study we used the groundwater flow and transport numerical program NAMMU to model the Konrad site (Hartley & Jackson, 1992).

The main objectives of this work were to evaluate quantitatively the effects of the downward increase in salinity of the groundwater on the vertical flow through disturbed zones (e.g. back filled shafts and boreholes) in the clay barrier downstream of the proposed site. Secondary objectives were to develop a qualitative and quantitative under­standing of the predominant physical processes and transport mechanisms of variable density flow and to understand the numerical problems encountered during the simulation of the complex model better.

HYDROGEOLOGICAL MODEL

A plan view of the region around the Konrad site is shown in Fig. 1. Section A-B depicted in the figure was chosen as the 2D cross-section; the limits of the 3D model are shown with straight lines on that figure. The length of the cross-section is about 45 km; its orientation is south-north-northeast such that its length is approximately parallel to the principal groundwater flow direction obtained from the freshwater 3D model from Gilby et al. (1988). (Recharge occurs in the south and discharge to the north).

The Konrad region is part of a rim synclinal system east of the salt domes Broistedt-Wendeburg-Rolfsbutell and Gifhorn (Fig. 1) and is characterized by a distinct sequence of aquifers and aquitards. Below the uppermost Quaternary aquifer additional aquifers exist which are separated from the uppermost aquifer by the thick very low permeability Lower Cretaceous clay layer. These aquifers may be intersected by tectonic faults or high permeability fault zones, although no tectonic faults were explicitly included in this model.

The base of the 2D section is the top of the nearly impermeable Zechstein salt formation. There is some uncertainty as to the depth at which the salt exists. We chose

Modelling of strongly coupled groundwater brine flow and transport 345

Gifhorn

Broistedt-Wenderburg-

Rolfsbuttel

Lebenstedt

Wolfsburg

J3raunschwieg

cJ2Z3> Salt Domes

Salzgitter Hôhenzug Flaohstôckheim 10 km

Fig. 1 Plan view of geographical and geological setting.

the top of the Muschelkalk as the base of the 2D section. To improve numerical conver­gence, the top of the model was taken to be the base of the Quaternary aquifer. Heads in the Quaternary layer were assumed to be vertically constant, so that the surface elevation was used to prescribe heads at the top of the model.

Figure 2 shows the model geometry on an exaggerated vertical scale with the nine main hydrogeological units (see Table 1 for unit abbreviations). Disturbed zones were

- 45 km, C = 1.0, no flow

Fig. 2 Cross-section with main hydrogeological units and boundary conditions.

346 Alfonso Rivera et al.

Table 1 Input parameters.

Hydrogeological unit

Alb (aquitard)

Hilssandstein, hits (aqu

UntereKreide, km (aqu

Kimmeridge, jo (aquife

Oxford, ox (aquifer)

Dogger, D (aquitard)

Combrash, C (aquifer)

Lias, L (aquitard)

Rhât, ko (aquifer)

Fluid/salt

ifer)

itard)

r)

Hydraulic kf(m s"1)

i o - 1 2

IO"6

IO""

IO"9

IO"8

IO"12

IO"8

IO"12

IO'7

conductivity

freshwater density max. solution density diffusion

Porosity

0.15

0.25

0.15

0.10

0.10

0.10

0.10

0.10

0.15

Longitudinal dispersivity aL (m)

50

125

50

125

125

50

125

50

125

1000 1200 1.2 x 10'9

Transverse dispersivity ar (m)

10

50

10

50

50

10

50

10

50

(kg m-3) (kgm;3) (n? s"')

included in the model in several sensitivity analyses. The hydraulic and transport parameters are given in Table 1. The boundary conditions used in the 2D cross-sectional numerical model are presented in Fig. 2.

NUMERICAL MODEL

Details of the hydrogeology can be found in Johns et al. (1994). The permeabilities and porosities were taken from the 3D freshwater model of Gilby et al. (1988) and modified slightly to match the freshwater results of Gilby et al. (1988). The aquifers and aquitards were assumed to be isotropic with respect to dispersion. The values of dispersivities shown in Table 1 were chosen to obtain longitudinal Peclet numbers of about 10. Ideally, transverse dispersivities 1/10 the longitudinal values were assumed, although several values were increased because of numerical convergence problems.

Grid discretization

A series of trial simulation runs were made with three different grid sizes and for various values of diffusion to determine the mesh size needed. Initially, we used a two-dimensional grid, 45 X 2 1cm, with element sizes on the order of 100-200 m in the vertical direction, and a maximum size of 1000 m in the horizontal direction. Hence, the initial grid contained about 900 elements. The results from this coarse grid (pressure and concentration) were then used to calculate two-dimensional grid Peclet numbers for each grid block as defined in Johns et al. (1994).

Modelling of strongly coupled groundwater brine flow and transport 347

The grid was then refined based on the two-dimensional grid Peclet number so that the Peclet numbers were less than about four. A new simulation was run with the new grid until the solution converged to the minimum parameter values possible for that grid. This process was repeated three times to a base case grid of nearly 6000 elements and 18 000 nodes. An additional refinement of the base case grid, four times larger, was made to check for grid convergence.

Numerical solution using NAMMU

The finite element code NAMMU (Hartley & Jackson, 1992) was used to model the Konrad site. NAMMU was chosen because it has been extensively verified against analytical solutions and other numerical codes for problems of coupled flow and solute transport (i.e. Herbert et al., 1988; Jackson & Farmer, 1989). NAMMU uses a standard Galerkin formulation of the finite element method.

The steady state nonlinear equations used are discussed in Herbert et al. (1988) and Johns et al. (1994) and are not repeated here for brevity. The discretized nonlinear equations are solved using Newton-Raphson iterations. As a result, the convergence of the Newton-Raphson scheme depends highly on the initial guess of the residual pressure and salt concentration and may not converge even for low Peclet numbers. Parameter stepping techniques are used to provide updated initial estimates of the unknown parameters and improve convergence. For this study, we updated the initial estimates by continuously decreasing the pore diffusivity and dispersivities (and increasing the numbers of elements) until the solution converged to a low level of pore diffusion and dispersion. The difficulty in solving the nonlinear equations arises from the dependence of the density on the relative concentration and the dependence of dispersion on velocity.

SIMULATION RESULTS WITHOUT DISTURBED ZONES

A series of 14 simulations were performed including one without the presence of salt (i.e. freshwater). We present here only the results from the base case run and a run in which a disturbed zone above the Oberjura was included as a vertical back filled borehole. For brevity, we only compare freshwater and saltwater results at one profile (see Fig. 2).

Residual pressure distribution

A comparison of the freshwater heads (Fig. 3) and the residual pressures (Fig. 4) shows that the residual pressures are substantially increased everywhere when salt is present (the residual pressures can be converted to equivalent hydraulic heads by dividing by p0g). For example, in the area of the proposed waste site, the residual pressures in the Oxfordian increase by approximately 0.7 MPa, and the residual pressures increase from about 0.3 MPa in the Hilssandstein formation to about 2 MPa in the Rhàt formation at the bottom. Only at the main recharge zone to the south where the pressure is not affected by the variations in density is the pressure similar to the freshwater case.

348 Alfonso Rivera et al.

Fig. 3 Hydraulic head distribution for the freshwater run (isolines are in m).

Fig. 4 Residual pressure distribution for the base case (isolines are in MPa).

Figure 5(a) compares the residual pressure between the freshwater and the salt water models along the selected profile.

Salt concentration distribution

The concentration distribution for the base case is shown in Figure 6 and is nearly linear with depth. Only in the Oxford and Cornbrash do iso-concentration lines deviate from the overall trend. Figure 5(b) shows measured values of concentration (from Klinge & Jaritz, 1990) that were projected into the selected profile. Their data shows a linear trend below the hils. From the hils to the surface layers (i.e. above the model surface), however, the concentration drops rapidly to zero. Except for one value, the measured concentrations are generally lower than those predicted from the model. The smaller measured concentrations below the jo are probably due to the imposed boundary condition of 1.0 in the RMt formation (Keuper). The possibility of a larger concentration above they'o may be due to lateral salt input from the east-west salt domes not included

Modelling of strongly coupled groundwater brine flow and transport 349

- -600

-900

Q_ œ D

-1200

Freshwater

Saltwater

Darcy Velocity, x [m/s]

-1500 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Residual Pressure [MPa]

-300

-600

-900

1200

(C)

, " .

^ T \ :

_LS

= TT i i i i i i

Alb

kru

Jul

OX

~D~

L

5 5

-300

-900

.Q

£ - 1 2 0 0

-1500 10'

0.4 0.6

Concent ra t ion

L -

• • " " ; : : : : : : : : : ! i

_IZX ,

(d) Alb

kru

ox

D

L

"to

10"1 10-1 10"1' 10"''

Darcy Velocity, z [m/s]

Fig. 5 (a) Pressure; (b) concentration; (c) horizontal Darcy velocity; and (d) vertical Darcy velocity plots along selected profile.

in the model (see Fig. 1), or could be an indication for a larger Peclet number (i.e. lower diffusion and dispersion).

Fluxes

The groundwater flow patterns are strongly affected by the presence of salt as compared to those of the freshwater simulation. Figures 7(a) and 7(b) show the velocity vectors for the fresh and salt transport flow in an enlarged view near the proposed site. Figures 5(c) and 5(d) show the principal components (i.e. northern horizontal and upward vertical) for the Darcy velocity for the selected profile (downward vertical and southern horizontal velocities are set to zero in the figures).

In general, fluid flow is significantly vertically oriented and slightly larger in the upward state, reduced flow in the aquifers at the northern end of the model implies that horizontal flow in the aquifers near the southern end must also be reduced to some degree. The reduction of flow in the aquifers caused an increased driving force and slightly greater upward flow in the area of the waste site.

The net effect on upward vertical flow is for the flow to slightly increase in the salt water model as compared to the freshwater model. That is, the difference between the

Alfonso Rivera et al.

Fig. 6 Relative salt concentration isolines for the base case.

(b)

T '

\

-UTT^—. iZZ——~ï

'

/ y

7^~ <^-—__

s—

Waste S te

" .

/ -

~~f~~~ = 5 ~^~-__^.

/

/

—̂

't3

m

L — ^ i

} E

. _̂

i ~ _ J i

1 ^

Hil

l

•̂

--* /

\\\

Fig. 7 Enlarged view of flow vectors near the Konrad site for (a) freshwater, and (b) base case.

Modelling of strongly coupled groundwater brine flow and transport 351

increased upward flow in aquitards due to reduction of velocities in the aquifers minus the reduction of upward velocities in the aquitards due to the gravity effect of salt is greater than zero.

INCLUSION OF A VERTICAL HIGH PERMEABILITY ZONE NEAR THE WASTE SITE

A disturbed zone of approximately 330 m in thickness, 1 km to the north of the Hilssandstein and above the Oberjura, was included to simulate the effect of weak zones in the geological barrier (faults, boreholes and/or shafts). The disturbed zone was assumed to have a conductivity of 10~8 m s 4 . Due to the large permeability contrast between the disturbed zone (10"8 m s4) and the Alblkru (10~12 m s"1), a buffer zone of intermediate permeability (10~10 m s"1) was included on each side of the disturbed zone.

The results of this simulation are shown in Fig. 8 for a vertical profile inside the disturbed zone, and are compared with updated freshwater results. Figure 8 shows that the upward velocities with salt are substantially reduced by over one order of magnitude as compared to the freshwater velocity. The reduction is expected because the Peclet number is large.

These calculations indicate that, as a result of the variable density of the groundwater, the presence of disturbed zones downstream from the proposed site does not have a negative impact on the site.

o

IL -1°O CD

a -200 t

W -300 CD

E -400 s I "500 CD

^ -600

g -700

-800

1 0 ' " 10 '1 3 10"12 10~11 10''° 10-9

Darcy Velocity, z [m/s]

Fig. 8 Vertical Darcy's velocities along the disturbed zone.

CONCLUSIONS

We made a preliminary examination of the effects of variable density on the regional groundwater flow patterns in the surrounding areas of the Konrad mine. The simulations were done in steady state with the finite element code NAMMU. The main conclusions from the work are: (a) the upward velocities in disturbed zones (e.g. faults, shafts, boreholes) downstream of the proposed site were substantially decreased from the freshwater case; and (b) the effect of the salt on the groundwater movement in the intact

352 Alfonso Rivera et al.

clay barrier were contrary to our expectations. That is, groundwater flow slightly increased vertically upward in the aquitards as compared to the freshwater simulation. The primary explanation for the increase is that the presence of salt substantially decreased the flow velocity in the aquifers at the discharge zone, which increased flow upwards in the aquitards. The consequences of the former are that, due to the down­wardly increasing salinity, even in the presence of strongly disturbed zones in the geological barrier there is no negative effect on the long-term safety of the proposed site.

Acknowledgement We wish to thank the Federal Office for Radiation Protection (BfS), Germany, for supporting this study. In particular, we would like to thank G. Arens and J. Wollrath.

REFERENCES

Gilby, D., Hagenbucher.T., Hiirlimann, W., Job.D., Klemenz, W., Resele,G. &Schindler, M. (1988)Modellrechnungen mit clem Programm FEM301 fur ein alternatives geologisch.es/hydrogeologischesModell (Model calculations with the FEM301 program for an alternative geological/hydrogeologicalmodel)./4«/rrags-A'/\ 9K 352126.39, Motor Columbus AG, Baden Switzerland.

Hartley, L. J. & Jackson, C. P. (1992) NAMMU User Guide Release 6.1. Harwell Report AEA D&R 0182, Theoretical Studies Dept, Radwaste Disposal Division, AEA Decommissioning and Radwaste. DOE(92).

Herbert, A. W., Jackson, C. P. & Lever, D. A. (1988) Coupled groundwater flow and solute transport with fluid density strongly dependent upon concentration. Wat. Resour. Res. 24, 1781-1795.

Jackson, C. P. & Farmer, C. L. (1989) Modelling saline intrusion for repository performance assessment. Safety studies for NIREX Radioactive Waste Disposal.

Johns, R. T., Klemenz, W., Low, S., Resele, G., Rivera, A. & Schindler, M. (1994) Barrierewirkung der geschichteten salzhaltigenTiefengrundwâsser (Barrier effect due to increasing salt content in deep groundwater). Colenco Technical Report 3739/14, Baden, Switzerland.

Klinge, H. & Jaritz, W. (1990) Stellungnahme zur Auswirkung des Salinars in Mittleren Muschelkalk auf die Tiefenwasserhydraulik im Gebiet der Schachtanlage Konrad (Comments on the effects of the rock salt layer on the hydraulics of the middle Muschelkaikofthe deep groundwater at the Konrad mine). BGR-Bericht, Archiv Nr. 106640, TagebuchNr. 110176/90; Tk25, 3828.

Klinge, H., Vogel, P. & Schelkes,K. (1992) Chemical composition and origin of saline formation waters from the Konrad mine, Germany. In: Water-Rock Interaction (ed. by Kharaka& Maest). Balkema Rotterdam.

Oldenburg, C. M. & Pruess, K. (1995) Dispersive transport dynamics in a strongly coupled groundwater-brineflow system. Wat. Resour. Res. 31, 289-302.

Vogel, P., Schelkes, K., Klinge, H. & Geissler, N. (1990) Analysis of density-dependentdeep groundwater movement in northern Germany influenced by high salinity. In: ModelCARE 90: Calibration and Reliability in Groundwater Modelling (ed. by K. Kovar) (Proc. The Hague Conf., September 1990), 373-382. IAHS Publ. No. 195.