Pumping Test Analysis of Multi-Layered Aquifers
Overview
AquiferTest Pro v.2016 features a significant advancement in pumping test analysis capabilities. Using the new new Multi-Layered Aquifer analysis solution, based on the method published by Hemker and Maas (1987), you can efficiently estimate hydraulic parameters for complex multi-layred aquifers and aquitards. This advanced analytical solution allows you to run parameter estimates and predict future drawdown in seconds, and offers a viable alternative to using 3D-numerical models which can be time-consuming and complex to design. In order to demonstrate these new capabilities in AquiferTest, we benchmarked this new solution to a 3D numerical model solution generated using Visual MODFLOW. The scenario is illustrated below.
Benchmark Comparison
A two-aquifer system was setup with simple T and S values for the aquifer and aquitard. The model specifications are the following:
- 2000m * 2000m
- 102 columns * 112 rows
- 6 layers (elevation from 0 – 65 m, varying layer thicknesses)
- Topmost layer contains constant head (no drawdown)
- 3 aquitards, 2 aquifers
- Constant Head boundaries located 1000 m from the pumping well
- Pumping well screened in layer 5
- Pumping rate: -10 L/s, for 30 days
- 5m * 5m grid cell size at pumping well
There are 4 observations wells added to the model:
- 2 obs. wells in pumped aquifer (20 m and 100 m from pumping well)
- 2 obs. wells in unpumped aquifer (20 m and 100 m from pumping well)
The model properties are illustrated in the cross section and table below.
The properties for the aquifers and aquitard are summarized in the table below.
After the numerical model was run, the time-drawdown data was exported from the 4 observation well locations. This data was then imported into an AquiferTest Pro project. The well locations were defined and the constant discharge rate for the pumping well was also defined.
The Multi-layer conceptual model was configured with the same layer order/types as the Visual MODFLOW model.
The parameter values where then configured using the equivalent values from the Visual MODFLOW model; at this point, some conversions needed to be applied due to the differences in parameter conventions (Kx (in Visual MODFLOW) vs. T (in AquiferTest), etc.
When viewing the drawdown derivative data, the diagnostic plots provide an effective tool for identifying the flow regimes/phases throughout the test, for the wells in the pumped and unpumped aquifers.
In the analysis graph below, you can see a reasonable comparison between the theoretical drawdown curve estimated by the Multi-Layer solution (in AquiferTest) and the observed time-drawdown data series generated by Visual MODFLOW.
AquiferTest Pro will produce summary statistics of the quality of the fit between the type curve and the data series; in this example, you can see there is a very low error between the Visual MODFLOW data series and the type curve from the Multi-Layer solution.
For more details on this method, please see the AquiferTest Pro page on our website.
References
Hemker C.J. and C. Maas, 1987. Unsteady Flow to Wells in Layered and Fissured Aquifer Systems, Journal of Hydrology, vol. 90, pp. 231-249
Hemker, C.J. (1999) Transient well flow in layered aquifer systems: the uniform well-face drawdown solution. Journal of Hydrology, 225: 19-44