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Other publication (non peer-review)

Book The impact of climate change on groundwater temperature and oxygen concentration in Swiss aquifers
Publisher ETH, Zürich
DOI 10.3929/ethz-a-010085885

Open Access

Abstract

Groundwater is an important drinking water resource in Switzerland and throughout the world. Despite the importance of groundwater, research on the impact of climate change on groundwater has attracted interest only recently. However, most of the research studies on this topic have focused on the potential impact of a changing climate on groundwater quantity rather than quality. The basic research task of this thesis was to determine past and future impacts of climatic forcing and climate change on some aspects of groundwater quality in Switzerland. To accomplish this task, time-series of historical records measured in the groundwater of several Swiss aquifers were analyzed. The best available data with regard to length and temporal resolution consisted of time-series of groundwater temperature and groundwater dissolved oxygen (DO) concentration in aquifers recharged by riverbank infiltration (RBF). Both groundwater temperature and DO concentration are important determinants of groundwater quality, in particular at RBF sites. The time-series were used to analyze the impact of the 1980s climate regime shift on groundwater temperature, to build models which enabled forecasts of groundwater temperature up to the end of the current century to be modeled, and to estimate the impact of a changing climate on groundwater DO concentration. To investigate the impact of climate variability on groundwater temperature in five aquifers recharged by RBF, the impact of the late 1980s climate regime shift on groundwater temperature was revealed using three statistical methods. The late 1980s climate regime shift is associated with a shift in the Arctic Oscillation to a strong positive phase. This shift led to an abrupt, strong increase in air and river-water temperatures in Switzerland in spring and summer from 1987 to 1988. In groundwater, the temperature increase was found in all seasons because the climate signal was damped and delayed. Although the size of the abrupt temperature shift as well as the behavior of groundwater temperature after the late 1980s climate regime shift were not homogeneous from aquifer to aquifer, the study confirmed that groundwater temperature at RBF sites responds strongly to large-scale climatic phenomena. Forecasts of groundwater temperature, which were calculated using two linear regression models for seven Swiss aquifers, indicated that groundwater in aquifers recharged by RBF is likely to undergo substantial warming by the end of the current century. Depending on the greenhouse-gas emissions scenario employed, groundwater at these sites is predicted to warm by 1 to 3.5 K with respect to the reference period 1980-2009. For aquifers which are not recharged by RBF but by the percolation of precipitation only, the models predicted a maximum increase of 1 K but the performance of the models was comparatively poor. With regard to these aquifers, however, the time-series used as training data for the calibration of the regression models started later and were therefore shorter than those available from the RBF sites. Because the model performance depends strongly on the training data, it is not clear whether the poor performance of the linear regression models and the small predicted increase for aquifers recharged by the percolation of precipitation resulted from a weak response to climatic forcing or from the inadequate length of the training data set. The strong warming of groundwater and river water observed in the past probably led to an increase in microbial respiration and reduced oxygen solubility, resulting in multi-annual periods of decreasing groundwater DO concentration at the five RBF sites analyzed. By contrast, the DO concentration also underwent some large, abrupt increases, which were caused presumably by strong changes in local hydrological conditions related to river discharge, groundwater pumping rates or riverbed clogging. Taking into account these findings and the groundwater temperature projections, it can be concluded that groundwater DO concentrations at RBF sites will undergo a further decrease in the future, but that the occasional occurrence of strong changes in hydrological conditions will result in increases in DO concentration that will prevent the groundwater from turning permanently anoxic. The main conclusions of this thesis are that climate change will affect groundwater temperature and DO concentration at RBF sites, and that this will have negative consequences for groundwater quality. These consequences will not be so grave as to render groundwater from these sites unusable as a source of raw drinking water; nevertheless, under certain conditions groundwater quality inaquifers recharged by RBFis likely to be reduced periodically, making it necessary to take counter-measures. To further examine the impacts of climate change on groundwater quality at Swiss RBF sites, further co-ordinated research and long-term monitoring will be necessary.
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