Assessment of Groundwater Flow to Lake Bolmen

Abstract: This work reports the research and findings concerning the processes of groundwater flow and distribution to lake Bolmen in the Småland region, Sweden. The watershed of Bolmen is of high interest since it provides water for drinking and energy purposes, representing a resource of notable importance for the society of southern Sweden. Among the main objectives there is the estimation of the water balance in a sub-area of the main Bolmen’s catchment, the estimation of plausible values of hydrological parameters as for instance hydraulic conductivity and recharge rate characterizing the area and the development of a three-dimensional model simulating the real hydrological processes taking place in the area. A hydrogeological conceptual model has been built to reproduce the actual hydrological dynamics existing in the study area of Bolmen’s sub-catchment. The water budget of the lake is particularly in focus and the influence of the top unconfined aquifer composed mainly by Quaternary deposit. The information used to build up the conceptual model has been downloaded from the database of the Swedish Geological Survey. The conceptual model includes a description of the spatially heterogeneous elevation of the ground surface as well as of the gneiss bedrock, assumed as impermeable. Along with the dynamics of groundwater flow governed by the Darcy’s law, also the effect of surface waters with tributaries and outflowing rivers or channels has been considered. A finite difference grid has been created in the software MODFLOW to solve the groundwater flow equation. Before running the numerical model, the information of the conceptual model has been imported into the grid by means of GMS, a software providing a user interface to the solver. Steady state simulations have been run with different plausible values of hydrological parameters as the recharge rate of the unconfined aquifer or the hydraulic conductivity to compute a value of groundwater level for every cell of the grid. The results of each simulation have been compared to observed in-situ data in order to adjust the parameters driving the flow dynamics and to obtain a model well representing the actual conditions. The resulting model effectively reproduces the distribution of the groundwater table in certain portions of the study area, while it either overestimates or underestimates the hydraulic heads in the majority of the grid cells. This is due not only to a lack of data regarding the precise values of hydrological parameters characterizing the Quaternary deposits, but also to a lack of data with respect to the thickness of the deposits. A further reason is the non-homogeneous collection in time of the groundwater levels used for calibration. According to the computed water balance, the major flow direction is from the unconfined aquifer towards lake Bolmen. Possible developments on this study include the use of other MODFLOW solvers, the reduction of data uncertainty through field tests at least collecting measurements of groundwater levels in water wells and the use of other modelling approaches as for instance finite element approximations to improve the quality of the results.