User's guide of Proceessing Modflow 5.0

282 Processing Modflow
6.5.4 Cut-off Wall
Folder: \pm5\examples\geotechniques\geo4\
Overview of the Problem
As shown in Fig. 6.53, a highly contaminated area is located in the first stratigraphic unit of an
unconfined aquifer. To the west and east of the aquifer exist fixed-head bounaries with the
hydraulic head h = -0.4 m and +0.5 m. The aquifer consists of five stratigraphic units. Each unit
is horizontally isotropic with uniform thickness. The elevations and horizontal hydraulic
conductivities are illustrated in Fig. 6.53. The vertical hydraulic conductivities are assumed to
be a tenth of the horizontal hydraulic conductivities. The effective porosity of the aquifer is
-8
0.15. The recharge rate is 1 × 10 m/s.
Because of the unpayable cost, the contaminants cannot be removed. Your task is to
develop a strategy to isolate the contamination. There are four steps to be done.
1. Construct a groundwater flow model and perform a steady-state flow simulation by using
the data given above and the model grid given in Fig. 6.53.
2. Geotechnical measures, such as cut-off wall, impervious cover, drain etc., can be
considered as an alternative. Calculate flowlines for the case that a cut-off wall has been
built to a depth of -8m and the recharge rate within the cut-off wall is reduced to zero by an
impervious cover. The location of the cut-off wall is given in Fig. 6.53. When calculating
the flowlines, particles should be started from the contaminated area.
3. Repeat step 3 for the case that the cut-off wall reaches the depth -10m.
4. Use a pumping well located in the cell [6, 12] to capture the contaminants. Calculate the
required pumping rate and penetration depth.
Modeling Approach and Simulation Results
The aquifer is simulated using a grid of 5 layers, 23 columns and 23 rows. All layers have the
same layer type 3: confined/unconfined (Transmissvity varies). The cut-off wall is modelled
by using the Horizontal-flow Barriers package. An impervious cover can be easily simulated by
reducing the recharge rate. Fig. 6.54 and Fig. 6.55 show the flowlines by performing forward
and backward particle tracking with PMPATH. The particles are initially placed in the center of
each cell, which is located in the first model layer and within the cut-off wall. It is obviously
that the contaiminants will be “washed out”, even if the cut-off wall is going deeper.
The contaminated area can be captured by using a pumping well (located in the cell [6,12])
6.5.4 Cut-off Wall