Application and comparison of different methods for aquifer test analysis using TTim

Abstract

Hydraulic properties of aquifer systems are usually estimated by conducting field experiments, which are called aquifer tests. Different approaches have been applied to simulate the drawdown data, such as graphical type curves, analytical solutions, and grid-based models. Since the type of groundwater system varies greatly from location to location, only a few general solutions have been developed. Computation of hydraulic properties are limited by the size and time step of grid-based model. Determining the boundary conditions is also difficult in actual groundwater system. Thus, in order to better simulate various aquifer systems, semi-analytic approaches have been promoted and used in aquifer test analysis. In this report, three different softwares aiming at aquifer tests simulation are presented. Both MLU and TTim are based on semi-analytical solutions and need users to build their own aquifer models, while AQTESOLV provides a base model for adding a variety of data and a stack of choices containing both analytical and semi-analytic solutions. Some benchmark analyses have been performed to assess the performance of TTim with limited types of aquifer systems. This research focuses on the application of TTim to different aquifer systems and the investigation of TTim's performance compared to the other two softwares. In this study, fifteen aquifer tests have been simulated with TTim. Test 0 uses hypothetical data to verify TTim's capabilities of retrieving specified parameters and reporting accurate confidence interval. Test 1 through Test 10 are pumping tests which are taken from reported field experiments and are grouped as confined systems, leaky systems, and unconfined systems. Test 11 through Test 14 present four slug tests with different top boundaries and well construction. The values and confidence intervals of the calibrated parameters are compared to the results of AQTESOLV and MLU. Improvement of conceptual models is carried out by model structure adjustments and parameters set adjustments. Different models' performance are assessed by root-mean-squared-error and AKAIKE Information Criterion (AIC). Most of the pumping tests and slug tests can be conceptualized using either ModelMaq and Model3D within TTim. Model3D is recommended when conceptualizing unconfined systems. The top boundary needs to be specified as `confined', and an additional thin aquifer needs to be added to simulate the specific yield, which is calibrated separately. The performance of TTim is similar, in general, to AQTESOLV and MLU. Well construction parameters cannot be calibrated with AQTESOLV, and only one aquifer system is available. TTim is more flexible and accurate than AQTESOLV when the groundwater system has information of multi aquifers and well construction. Modifications of parameters to be calibrated and model structure have been carried out to improve TTim's performance. It is concluded that aquifers with multi subdivided layers perform better when the well is partially penetrating or the observation wells have different depths. Calibration of well construction parameters may also contributes to a better simulation, but they are usually sensitive to the initial values. It is important to note that adding parameters may give better results, but whether this is significant needs to be tested by the AIC criterion.