Exercise developed and modified by Karen Martinez, Sandra Teruya and Saul Montoya

This example is a simulation of a mining dump which is a continuous source of concentrated leachate.

Hydrogeological complexities to consider: heterogeneity of the aquifer and the presence of fractured rock.

Groundwater system parameters:

Conceptual groundwater flow model

Groundwater flow is directed from the waste dump downstream to the valley. Travel time on alluvial material is shorter than travel time on deeper rock as shown in the graphic below.

It is known that the amount of precipitation increases with elevation, thus in this exercise a regression equation dependent on elevation will be considered. Evapotranspiration will be worked likewise but inversally proportional. FAO Climate Interpolation software (Loc Clim) has been used to define precipitation, recharge (as percentaje of precipitation) and evapotrasnpiration rates.

It is considered that the porous media has a porosity of 0.25 % for a volcanic rock type, discarding dual high porosity due to fracturing. It is difficult to make assessments on the hydraulic conductivity of internal fractures.

Flow model with MODFLOW

1. Download and open the file Angascancha.gpt

2. You can see the following model geometry.

3. To see the calculated heads click over the Data Visualisation bottom, in the option Color Grid, choose Head_P1_S1

and you will see:

Particule tracking with MODPATH

4. Turn on the MODPATH package; to do this you have to go to Model/MODFLOW Packages and Programes.

then you go to Version 6 options,

5. Double-click on the “Botadero” (Waste Dump) object, then select in MODFLOW Features:

6. We run the particles model (MODPATH), for this we will go toFile/Export/MODPATH Input Files.

7. Then you go to the Visual data bottom on the Color Grid option, we choosenone and click on Apply.

8. To see the pathlines, go to MODPATH Pathlines and upload the fileAngascancha.path file

therefore, you have,

Contaminant transport modeling with MT3DMS

Turn on the MT3DMS package

9. Go to Model/MODFLOW Packages and Programes and turn on the following:

10. On BTN: Basic Transport Package define the mobile species:

11. On the ADV:Advection package you choose the option Third-order TDV and keep the other default options.

Time

12. The evaluation time flow will be analyzed at two stress periods. The first period is static state -1 to 0, with this the initial water table is calculated. The second interval is transient, used for calculating changes by installing the waste dump from time 0 to a thousand years (31536000000 seconds).

13. Go to Model/MODFLOW Time

on the MT3DMS option

Time for objects that define the boundary conditions

14. Now define times for objects that define the boundary conditions. Let’s start with “Botadero” (Waste dump), double click the object, then on MODFLOW Features we enter the following

Also, turn on the SMS option for Botadero.

15. Lets go to the button Show or hide objects, select CHD: Carga_Constante_Perimetral and define the time:

16. Repeat the previous step for the objects within DRN, EVT and RCH(Angascancha_Basin_Extension_1 only), for which the time (Starting time: -1 andEnding time: 31536000000)

Other parameters of transport

Apply and Close the window.

Run the flow pattern of pollutants

18. Save the project and run the model flow or File/Export/MT3DMS Input Files or:

9. This process takes about a few hours, depending on the processor of the computer you work with.

20. To import the results go Import and display model results, then upload the fileAngascancha_MT_Arsenico.ucn and Ok to the dialog box. This way the outcome will be: