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Mine Water and the EnvironmentJournal of the International Mine WaterAssociation (IMWA) ISSN 1025-9112 Mine Water EnvironDOI 10.1007/s10230-014-0273-9

MODFLOW-USG: the New Possibilities inMine Hydrogeology Modelling (or What isNot Written in the Manuals)

David Krčmář & Ondra Sracek

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TECHNICAL COMMUNICATION

MODFLOW-USG: the New Possibilities in Mine HydrogeologyModelling (or What is Not Written in the Manuals)

David Krcmar • Ondra Sracek

Received: 6 December 2013 / Accepted: 5 March 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Geological heterogeneity associated with lay-

ers, open mine voids, tectonic faults and fractures can all

make modelling difficult, particularly for mine dewatering.

This heterogeneity is difficult to represent with the tradi-

tional MODFLOW structured grid. A new version of

MODFLOW, called MODFLOW-USG (for UnStructured

Grid), supports a wide variety of structured and unstruc-

tured grid types (Panday et al. in MODFLOW-USG version

1: an unstructured grid version of MODFLOW for simu-

lating groundwater flow and tightly coupled processes

using a control volume finite-difference formulation.

USGS, Reston, 2013) that provide flexibility for modelling

difficult geologic structures. This communication compares

aspects of MODFLOW and MODFLOW-USG, and uses

the Gbely lignite deposit as an example of a situation for

which MODFLOW-USG could be applied.

Keywords Lignite deposit � Model setup � MODFLOW �Unstructured grid

Introduction

MODFLOW is a very popular software program, and is

commonly used in the field of mine hydrogeology. How-

ever, MODFLOW is challenged by complicated geology,

heterogeneities, and artificial structures such as mine voids

that cause flow field changes relative to the surrounding

geological environment.

A typical application is to model water inflow. Singh

and Reed (1988) give descriptions of analytical and

empirical methods for evaluating surface water inflows into

a mine, and Toran and Bradbury (1988) tested the ability of

a ground water flow model to predict the impacts of

underground mining on ground water system. Dewatering

was simulated using a drain package. The drain option

allows variation of dewatering rates and ‘‘turns off’’ auto-

matically when the groundwater head falls below the drain

level, giving this option an advantage over placing dewa-

tering wells in the model. The number and locations of

drains can also be varied between stress periods in a single

run, allowing the sequential addition or reduction of drains

as mining progresses. However, the drain package option

can cause significant problems if improper grid spacing is

applied around the mine workings (Zaidel et al. 2010).

Their simulations showed that seepage rates can be accu-

rately predicted by either applying grid spacing that is

much finer than the diameter of the mine openings or by

using a coarser grid with grid sizes exceeding the charac-

teristic width of openings by a factor of three.

Water discharge from abandoned mines is another situ-

ation that is modelled, due to issues of water resource

degradation (Brown et al. 1998; Flakova et al. 2012; Hiller

et al. 2012; Luo et al. 2012; Moldovan et al. 2008; Ond-

rejkova et al. 2013). Flow in open mine voids is often tur-

bulent, which makes standard modelling tools inappropriate

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10230-014-0273-9) contains supplementarymaterial, which is available to authorized users.

D. Krcmar (&)

Department of Hydrogeology, Faculty of Natural Sciences,

Comenius University in Bratislava, Mlynska Dolina,

84215 Bratislava, Slovak Republic

e-mail: krcmar@fns.uniba.sk

O. Sracek

Department of Geology, Faculty of Science, Palacky University,

17. listopadu 12, 77146 Olomouc, Czech Republic

123

Mine Water Environ

DOI 10.1007/s10230-014-0273-9

Author's personal copy

for predicting groundwater rebound. The scale of the

problem is important, requiring different approaches; water

balance calculations are used at very large scales, while

three dimensional (3D) pipe networks routed through var-

iably saturated porous medium are used at small scales

(Adams and Younger 2001). Case studies of mine dewa-

tering and rebound are presented in Rapantova et al. (2007).

Flow through tectonic faults also needs to be modelled.

A fault may have different functions, e.g. it can act as a

barrier to ground water flow or it can act as a preferential

flow path, usually in a vertical direction. Water flow along

faults has been simulated to model flows to a well

(Anderson 2006), into active mines (Lianchong et al. 2011;

Wu et al. 2003), and into abandoned mines (Frolik et al.

2004). Faults are often simulated in MODFLOW with the

horizontal flow barrier (HFB) package (Hsieh and Freckl-

eton 1993), where conductivity and width of the HFB is

specified. However, this approach does not allow simula-

tion of vertical flow within faults.

MODFLOW USG: the New Features

The US Geological Survey (USGS) developed MODFLOW

based on a structured grid (rectangular finite difference grid),

in which each cell is connected to six neighbouring cells.

Connections are easily determined by row, column, and layer

numbering. However, irregular boundaries cannot be fitted

precisely with a rectangular grid. If high accuracy is

required, a large refinement is necessary, which requires a

large numbers of cells for the model setup. There are also

problems when grid resolution refinement is only needed in

some parts of the modelling domain. Row and column width

can vary, but the dimensions of a particular row or column

have to be the same through the entire modelling domain.

Efforts to deal with this limitation led to the development of a

nested grid approach. Nested grids usually use a parent grid

with coarse resolution and child grids with fine resolution,

which are then coupled in an iterative manner.

A new version of MODFLOW–MODFLOW USG

(UnStructured Grid, Panday et al. 2013) was developed

based on an unstructured grid approach (control volume

finite differences, or integrated finite differences). This

approach allows the user to create flexible grids of virtually

any shape, which can be refined in areas of interest.

Groundwater flow is then simulated at all grid connections

using fully implicit solutions with a single matrix.

MODFLOW-USG also supports packages that transfer

water between different grid nesting levels, such as the

Stream Flow Routing Package or the connected linear

Fig. 1 The Gbely lignite mine area

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network (CLN) Package. The new CLN package is roughly

equivalent to the Multi Node Well Package or MODFLOW-

Surfact’s Fracture Well Package, but it is much more general

(Rumbaugh 2011). The CLN package also replicates some of

the capabilities of the Conduit Flow Process module for

MODFLOW-2005 (Shoemaker et al. 2008). The CLN cells

can represent wells, pipes, fractures, underground excava-

tions, tunnels, rivers, or other linear features, and CLN cells

can be vertical, horizontal, or tilted. The formulation is gen-

eral enough that different types of features can transmit water

into each other. For instance, pipes can transmit water to/from

open channel features, which have small cross sectional areas

relative to groundwater cells, without requiring groundwater

cell refinement (Panday et al. 2013).

Case Study: Gbely Lignite Deposit

Significant lignite deposits are found in the northern part of

the Vienna basin, intruding from Austria into southern

Moravia (Czech Republic) and the southeast tip of Slovakia.

The Dubnany coal seam is part of the youngest (Pontian)

sediments of the Vienna basin. There are two main mine

sites, one near Hodonın in the Czech Republic and another

near Kuty and Gbely in the Slovak Republic. The Gbely

deposit is a part of the Kuty trough, tectonically bounded by

the Farsky fault in the north (amplitude of tectonic

Late Panonian, Záhorské strata: gray clays and sandy clays

Early Pontian, Sekulské strata:

sandy aquifers and clay aquitards

Pontian Dub an coal seam with clay layers

Late Pontian, Jánske strata:

sandy aquifers and clay aquitards

Dacian, Gbelké strata: green, brown and

patchy clays

Quarternary sediments

South NorthINFILTRATION ZONEFig. 2 Schematic

representation of the Dubnany

seam around the infiltration

zone (for location, see Fig. 1)

fau

lt

coal seam

aquifer

aquitard

layer 1

layer 2

layer 3layer 4

layer 5

layer 6

layer 7

layer 8layer 9

Fig. 4 Model grid, northern part

movement along tectonics80 (m)

movement along tectonics60 (m)connected

tectonic blocks

-10 m asl

180 m asl

1

23

blocknumber

Fig. 3 3D view of the Eastern

lifted block (only position of

coal seam is displayed)

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movement approximately 300 m), the Svatojansky fault in

the northeast, and the Cary fault in the west. The Gbelsky

fault splits the deposit into two parts: a central block and the

eastern lifted block. There are also smaller inner faults with

tectonic movements up to 50 m (Fig. 1).

The Gbely deposit consists of marine clay sediments of

the Late Panonian Zahorsky strata. Early Pontian Sekulske

strata are above the marine clay and comprise two sandy

aquifers (1–15 m thick) separated by a clay aquitard

(1–28 m thick). A clay aquitard is also present directly

below and above the coal seam. The thickness of the coal

seam is relatively uniform and varies from 3.5 to 5.6 m.

Late Pontian Janske strata are above the coal seam and

comprise three sandy aquifers (6–20 m thick) separated by

clay aquitards (0–20 m). In some areas, aquitards are not

evolved and aquifers are connected. The Dacian Gbelske

strata are above the Janske strata, forming an impervious

layer and separating the coal seam from Quaternary

fau

lt

layer 1

layer 2

layer 3

layer 4

layer 5

layer 6

layer 7

layer 8

layer 9

layer 1

layer 2

layer 3

layer 4

layer 5

layer 6

layer 7

layer 8

layer 9

flow of wateralong layers

Fig. 5 Model grid, central part,

structured grid design

fau

lt

2

3 4

6

7 8

10

11 12

14

15 16

18

19 20

22

2423

26

27 28

30

31 32

1

5

913

17

21

25

29

33 34

35 36

flow of wateralong connected cells

Fig. 6 Model grid, central part,

unstructured grid design

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sediments. However, the coal seam reaches the surface in

the south (Fig. 2), allowing precipitation to infiltrate

through the Quaternary sediments (aeolian sand and sandy

or clayey loams) and provide recharge for aquifers (Fig. 1).

The eastern lifted block is divided by inner faults into

three sub-blocks. In the north, along the Farsky fault, sub-

blocks are connected. In the south, the sub-blocks rise

along inner tectonic faults, with variable amounts of uplift

(Fig. 3).

The Gbely mine workings are currently active in the

central block and there are plans to mine the eastern block.

Active mined areas are progressing towards the north

(Fig. 1). Mine workings have to be dewatered because of

inflow from overlaying and underlying sandy aquifers.

Recent exploration has been conducted to determine an

optimal dewatering plan for the eastern block. The

MODFLOW-USG with its new capabilities seems to be a

great tool for fulfilling the project goals. This article tries to

emphasise its benefits in the model setup.

In a typical MODFLOW (i.e. using a finite difference

grid) approach, connections between neighbouring cells are

determined by row, column, and layer numbering. This

fact, that flow is directed only, for example, by the layer

number is not particularly emphasised in the manual, and

occurs even if the cells are not physically connected in the

model setup. An inexperienced user could easily fall in

trouble in complicated geological settings. This paper tries

to show how to deal with or overcome such situations.

In the northern Gbely area, there is no problem with the

classical approach because the sub-blocks are physically

connected (Fig. 4). The model grid can consist of layers

representing the aquifer, aquitards, and the coal seam. The

only unknown would be the hydraulic properties of the

fault zone, which could act as a hydraulic barrier; this

could be represented in the model by an HFB element.

Further south, blocks are shifted upwards along tectonic

faults, and the magnitude of vertical movement is different

for each block. In such a case, we face problems with the

classical finite grid approach (Fig. 5) as the model incor-

rectly assumes that water is transmitted along layers with

the same numbers. In the classical approach, ‘‘dummy’’

layers could be added on both sides, and the layers

renumbered so as to guarantee the expected water flows.

This method is cumbersome and could result in errors in

the model design. Using MODFLOW-USG and its

unstructured grid, we can directly specify connections

between cells in all directions (Fig. 6). Cells may have

more neighbouring cells along one principal direction, e.g.

cell no. 18 on its right hand side has two neighbouring

cells, no. 19 and no. 27, and water flow is split between

flow of wateralong layers

fau

lt

Fig. 7 Model grid, southern part

pinching of layers reaching surface

pinching of inner layers

Fig. 8 Representation of layer

pinching with unstructured grid

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them. The model also considers contact areas between

connected cells and calculates correct water flow accord-

ingly, which is not possible with the classical approach.

This fact is not usually mentioned in the manual. Fault

zones can be represented in the same way as in the classical

approach with an HFB element.

Even further south, the blocks (model layers) are totally

disconnected. In the classical approach, the model is still

transmitting water along layers with the same number,

which is inaccurate (Fig. 7).

Pinching out of layers poses another problem. In Fig. 2,

two cases of pinching out appear, one where layers reach

the surface and the other inside the Janske strata where

aquifers are connected. Again, the classical approach has

problems representing such phenomena, but an unstruc-

tured grid can include pinching out in the grid design

(Fig. 8).

Strata slope must also be accounted for. If the slope is too

high and discretisation is too coarse, then neighbouring cells

are not connected (Fig. 9). Paradoxically, in the classical

approach, the model transmits water correctly (according to

layer numbers) but could face numerical instability because

there is too much difference in water levels between cells. In

an unstructured grid, there can be problems with cell con-

nections. The data for unstructured grids are usually prepared

by software (e.g. Groundwater Vistas), which calculates

connections automatically (see Fig. 9), and can incorrectly

assume that neighbouring cells are connected. There is an

option to enter connections manually in the setup file, but this

is an error-prone procedure. It is possible to estimate

‘‘physical’’ connectivity of neighbouring cells before the

grid setup by comparing the angles of strata slope (angle a in

Fig. 9) with the diagonal of the cell (angle b). When b B a,

the cells are disconnected, when b[a, cells are connected.

You can calculate a maximum allowed cell width to assure

connectivity between neighbouring cells by at least one half

of a cell thickness (Fig. 10):

width ¼ thickness

2 tanað1Þ

At Gbely, the maximal slope of the strata is in the

recharge zone and is equal to 7.3�, which means that for an

average aquifer thickness of 10 m, the maximum cell width

should not exceed 39 m.

Simply preparing the data for such complicated geo-

logical conditions can be difficult. At Gbely, more than 70

boreholes have been drilled, most with filters in the Janske

or the Sekulske strata. Some were installed in tectonics

features to investigate their hydraulic properties (Fig. 11—

colour circles represent functional wells, empty circles

represent abandoned wells). To produce a contour map of

the coal seam bed, the grid included elevations of the coal

seam bed using an ArcGIS interpolation tool called ‘‘Top

to Raster’’. However, this does not consider breaks in the

continuity of geological features, such as faults. It was

necessary to interpolate grids separately for each block and

then add them together with a ‘‘Raster Calculator’’ (see

Supplemental Fig. 1, which appears with the on-line ver-

sion of this paper, and can be downloaded for free by all

journal subscribers).

The thickness of all layers, including the coal seam, clay

aquitards, and sandy aquifers, as determined from the bore

logs, is consistent throughout the area (tectonics moved the

sub-blocks long after sedimentation) and could be inter-

polated over the whole area at the same time. After inter-

polation, thickness was added to the coal seam bed (if the

layer was above the seam) or subtracted from it (when the

layer was below the seam) with the help of the Raster

Calculator. Where the clay aquitard was not present, a

minimum thickness of 0.1 m was entered to be able to run

the interpolation; this allowed the modelling software to

automatically pinch out layers below the entered value.

flow of wateralong layersstructured grid

α β

layer1

layer2

flow of wateralong layersunstructured grid

Fig. 9 The slope of the strata versus discretisation, b = a, cells are

disconnected

α β

layer1

layer2

flow of wateralong layers

Fig. 10 The strata versus discretisation, b[a, cells are connected

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Conclusion

Modelling water flow in mine workings represents a

challenging problem. We can deal with many complicated

features, such as discontinuities (geological faults, mine

openings, etc.), steep and uneven slopes, or pinching out of

layers. This paper describes possible problems that could

arise by creating a model setup with the classical approach,

using the Gbely lignite deposit as an illustrative example.

The problem is how to represent the features and water

flow in the model correctly. If the cells are physically

disconnected in the model, modelling software may not

know that and could transfer water inappropriately. The

other problem is discretisation. Users should be aware of

errors that could develop due to coarse discretisation of the

model domain. We recommend that you estimate the

maximal cell width based on the slope of the strata and

thickness of the layer. Doing this, you can evaluate correct

discretisation of the model. In the software manuals, spe-

cial cases such as these are not usually fully covered and it

was our goal to describe them using real case examples.

This paper also tried to introduce the new MODFLOW-

USG, which is a great tool for complicated conditions, with

many improvements over the classical structured grid

approach. MODFLOW-USG is very useful for modelling

water flow in mined areas because it deals more appropri-

ately with many problems that the classical approach strug-

gled with. But there are also some cases, as we can see in the

case of rough discretisation, when automatic creation of

connections between cells could fail. So deep understanding

of the whole process is vital; we can all learn the most by

solving real life problems and by sharing our knowledge.

Acknowledgments The research was performed with the help of

funds from the Ministry of Education, Science, Research and Sport of

the Slovak Republic under contract VEGA 1/0921/11, ‘‘Optimization

of mine discharge with help of numerical modeling’’.

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