Groundwater recharge evaluation in a glacial buried valley using time series analyses of water levels.

Groundwater hydraulics and aquifer processes are largely investigated using techniques that involve some kind of borehole test pumping. Such techniques are well established in the engineering and scientific community but are resource demanding. Time series analysis offer a collection of alternative methods that can be used to evaluate groundwater processes like recharge by using natural groundwater levels as input data. In this work, time series analyses were used to increase the hydrogeological knowledge of an area characterised by a glacial buried valley in southern Sweden. The hydrogeological environment is characterized by an upper aquifer (often clay till) and a deep aquifer consisting of different types of sediment (fine sand and gravel). The two aquifers are often separated by dense clays or massive low-permeability tills. The work aimed to implement time series analysis for estimating the amount of groundwater infiltrating at several points of the buried valley, which is one of the most important groundwater reservoirs in southern Sweden. The goals were to (a) contribute new knowledge about groundwater recharge and (b) evaluate the use of time series analysis to answer hydrogeological questions in this particular geological setting. The data processing steps included barometric compensation, Fast Fourier Transform (FFT) for identification of dominant unwanted frequencies, median filter to remove any disturbances in the signal, and corrections for possible signal shifts. Data filtering or correction was in some cases also applied to precipitation data and other meteorological parameters used to evaluate the reliability of the results. Recharge calculations were carried out using the water table variation method. The results suggest that for the period 2020 – 2021 the recharge was in the order of 350 mm for the upper aquifer and that around 10% of the precipitated water may be available for further infiltration to the deeper high-permeable sediments. Attempts to compare the obtained results with calculations made with the water budget method brought uncertainties as calculating the parameters for the water budget method implied using data with variable quality, scale and accuracy. However, using time series analysis has the potential to qualitatively monitor the recharge process in the area. The reliability of such recharge estimations can be supported by evapotranspiration measurements.