Groundwater Modelling 2020-2021

Setting
A three-layered aquifer system is potentially threatened by groundwater pollution. A map view of the
area is presented in Figure 1 showing its dimensions (6000 m by 4000 m). A N-S cross section is shown
in Figure 2. The topographical elevation decreases towards the north where a river flows from west to
east. In the southeast of the domain, a granite intrusion of more or less 0.6 km2 is present throughout
the entire depth of the system. Furthermore, two pumping wells are to be installed in the area around
the same time an unspecified contaminant starts leaching into the groundwater (in reality, this will
probably never be the case). Your task is to investigate where the contamination from the pollution
site eventually ends up. You will do this by building a manually calibrated and validated groundwater
flow and solute transport model.
Table 1 shows initial estimates of the hydrogeological properties of the layers. The top layer is a fine
sandy layer which is separated from the deeper coarser sand unit by a regional clay layer. The thickness
of the top layer varies from 10 m in the north to 25 m in the south. The clay and deep sand unit have
constant thicknesses of 7 and 23 m each. Incorporate your own surface topography but respect the
gradient shown in Figure 2 and the maximum (15 m in the south) and minimum (0 m in the north)
elevations. One kilometre to the south of the model, hydraulic head in each unit is assumed to be
constant at values of 12 m (layer 1), 10 m (layer 2) and 8 m (layer 3). The river in the north is westeast oriented and is assumed to have a constant river stage at 2.5 m below the top surface. The bottom
elevation is estimated at 5 m below the top surface with a thickness of 1 m. The river is assumed to be
straight and having a total length of 6 km. The width is constant at 5 m. Vertical riverbed hydraulic
conductivity is assumed to be 0.1 m/d. Two pumping wells are installed in the area: one in the deep
aquifer in the southwest (X = 1187.5 m, Y = 687.5 m) pumping at 500 m3
/d year-round and one in the
shallow aquifer in the northeast (X = 4812.5 m, Y = 3062.5 m), pumping at 300 m3
/d intermittently for
6 months.
A single unknown contaminant is leaching into the groundwater from a subsurface storage (X = 4562.5
m, Y = 2812.5) in the upper aquifer. The contaminant concentration of the solute at the point of entry
has a constant value of 0.1 mg/m3 (neglect concentrations smaller than 0.1 μg/m3
). Longitudinal
dispersivity at the scale of the area is estimated at 12.2 m for all layers. Horizontal and vertical
transverse dispersivity ratios are estimated at 0.1 and 0.01, respectively. Density changes and
geochemical reactions are not considered.
Hydraulic heads and solute concentrations are measured every 6 months in 5 multi-level piezometers
in the area over 10 years. Their locations are given in Table 2. The measurements are handed to you
in separate CSV files and “*.complete_obs” files, the latter being a PMWIN format which allows all
observation information for a measured variable to be read from a single file. Length units are meters,
time units are days and mass units are milligrams.
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Figure 1: Map view of the area. The red square indicates the pollution site. The blue line is the river.
Figure 2: North-south cross-section of the area.
Table 1: Initial estimates of hydrogeological properties of the layers (Kh = horizontal hydraulic
conductivity, Kv = vertical hydraulic conductivity, Ss = specific storage, Sy is specific yield).
Layer Lithology Thickness (m) Kh (m/d) Kv (m/d) Ss (m-1
) Sy (-)
1 fine sand 10 to 25 8 8 1.00E-05 0.25
2 silty clay 7 0.05 0.0005 1.00E-03 0.05
3 coarse sand 23 25 25 1.00E-06 0.3
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Table 2: Locations of the observation wells. Both hydraulic heads and solute concentrations are measured.
name layer X (easting) (m) Y (northing) (m)
pz_a_1 1 4812.5 3062.5
pz_a_3 3 4812.5 3062.5
pz_b_1 1 1312.5 687.5
pz_b_3 3 1312.5 687.5
pz_c_1 1 4687.5 3187.5
pz_c_3 3 4687.5 3187.5
pz_d_1 1 4562.5 3062.5
pz_d_3 3 4562.5 3062.5
pz_e_1 1 1187.5 812.5
pz_e_3 3 1187.5 812.5
Answer following questions related to the modelling task and show relevant figures and values where
necessary.
1. Discuss the conceptualization of your model. If you had unlimited cells and layers, how would
you refine the model resolution?
2. Discuss the long-term natural groundwater flow without anthropogenic influences.
3. Simulate and discuss the 10-year groundwater flow and solute transport after the installation
of the pumping wells and the leaching of the solute, before and after manual calibration (use
the TVD scheme for the MT3DMS software for advection simulation). Discuss the fate of the
solute particles (point of exit, hydrodynamic characteristics, flow paths,…), before and after
manual calibration. Is this realistic given the geological setting?
4. For the calibration, discuss what parameters were most sensitive. Show and discuss the final
parameter estimates and relevant goodness-of-fit statistics. Only calibrate groundwater flow
parameters, not solute transport parameters. What is the maximum value of the vertical
hydraulic conductivity so that there is no influence of either pump observable across the clay
unit (i.e. pumping in the deep aquifer is not measurable in the shallow aquifer and vice versa)?
5. State how you validated your model and discuss the validation.
6. Discuss the mass balance of the model.
7. How long does it take for the water from the regional recharge area to reach the river? What
are the most sensitive hydraulic parameters for this process and why? Does all the water from
the top of the recharge zone pass through layer 3? Discuss the flow paths.
8. What do you think the strengths and weaknesses of this model are for simulation of
groundwater flow and solute transport for the situation given here?
Good luck.

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