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.

Popular Abstract

The degree project is focused on an important water resource for southern Sweden, that is lake Bolmen. In particular, an evaluation of the amount of water being transferred from the aquifer in the lake’s basin towards lake Bolmen is carried out with an appropriate software named MODFLOW. The latter is a software for numerical estimation of groundwater flow and it can work only after a simplification of the study area is introduced by dividing it in a grid made of squared cells with a defined dimension.
The work has been performed to reproduce the natural environment found in the area of the lake, to investigate its connection with other types of water occurrence, particularly rivers, channels and groundwater. Results of this research suggest that the path that water follows when it enters the area, for example in the form of rainfall, is mainly directed from the underground environments below the soil surface towards lake Bolmen. However, the path of water is not the only interesting information obtained with the project work, since also the amount of water moving is of great importance. From the calculations, it has been found that most likely tens, maybe hundreds of thousands cubic meters of groundwater are naturally transferred to lake Bolmen in a way that is not directly visible with human eyes.
Smart management of water resources is essential to maximize the benefits provided by their use, benefits that can include among others the production of electrical energy, the supply of drinking water and the maintenance of vital ecosystems. A considerable amount of study involves lake Bolmen, since it has been affected during the last decades by a problem that is also encountered in almost all Nordic regions, that is brownification. This means that the colour of the lake’s water is turning browner and browner, causing issues related to the treatment of the water to make it potable, to the fishery and to the lake’s aesthetic (it is indeed an attractive touristic area!). So, the study of the water movement is important to address the quantity of biological and chemical compounds that may affect brownification.
The model created inside the project work can be intended as a further tool to reach a sufficient understanding of the processes taking place in the area. A flaw of the model is that it tends to either overestimate or underestimate the observed values of groundwater levels in some parts of the Bolmen’s basin. Usually, the goodness of a model is evaluated according to its capacity of reproducing real-life events or dynamics. This means that only when a satisfactory correspondence between what the model foresees and what actually happens is obtained, it will be possible to use it as technical support on the decisional phase. To achieve this, possible improvements include a data collection in the study area throughout field tests, among which there is the measure of the groundwater level in correspondence to drinking water wells, and the use of alternative software to build up the model.

• author: Favaro, Massimiliano
• supervisor: Gerhard Barmen; Clemens Klante; Matteo Camporese
• organization: Civil Engineering (M.Sc.Eng.); Engineering Geology
• alternative title: Beräkning av inflöde av grundvatten till sjön Bolmen
• course: VTGM05 20211
• year: 2021
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: Groundwater, modelling, MODFLOW, GMS, hydrogeology
• publication/series: ISRN
• other publication id: ISRN LUTVDG(TVTG-5171)/1-62/(2021)
• language: English
• additional info: Examiner:Jan-Erik Rosberg
• id: 9055925
• date added to LUP: 2021-06-17 13:45:42
• date last changed: 2021-06-17 13:45:42