Groundwater is the world's most important source of raw drinking water. However, the potential impact of climate change on this vital resource is unclear because of a lack of relevant long‐term data. Here we statistically analyze over 20 years of groundwater temperature data from five Swiss aquifers fed predominantly by river‐bank infiltration. The results reveal an abrupt increase in annual mean groundwater temperature centered on 1987–1988 that can also be observed in air and river temperatures. We associate this temperature increase with the Northern Hemisphere late 1980s climate regime shift (CRS), which itself is related to an abrupt change in the behavior of the Arctic Oscillation. Because temperature affects redox conditions in groundwater, groundwater biogeochemistry in aquifers fed by river‐bank infiltration is likely to depend on large‐scale climatic forcing and will be affected by climate change.
The evolution of the atmospheric noble gas composition during the past few decades has hardly been studied because, in contrast to many other atmospheric gases, systematic time-series measurements have not been available. Based on theoretical considerations, the atmospheric noble gas isotope composition is assumed to be stable on time scales of up to about 10 6 years, with the potential exception of anthropogenic changes predicted for the He concentration and the 3 He/ 4 He ratio.However, experimental assessments of the predicted changes in the atmospheric He isotope composition are controversial. To empirically test these assumptions and predictions, we analysed the noble gas isotope composition in samples of the * Corresponding author Cape Grim Air Archive, a well-defined archive of marine boundary layer air in the southern hemisphere. The resulting time series of the 20 Ne, 40 Ar, 86 Kr and 136 Xe concentrations and 20 Ne/ 22 Ne and 40 Ar/ 36 Ar ratios during 1978-2011 demonstrate the stability of the atmospheric Ne, Ar, Kr and Xe composition during this time interval. The He isotope data provide strong evidence for a decrease in the 3 He/ 4 He during the same time interval at a mean rate of 0.23-0.30 per year. This result is consistent with most model predictions of the rate of decrease in the atmospheric 3 He/ 4 He ratio associated with mining and burning of fossil fuels.
Long-term changes of 14 water constituents measured in continuously and water discharge proportionally collected samples of four Swiss rivers over a period of 39 years are analyzed using several statistical techniques. Possible drivers and causes for the identified trends and shifts are explained by consideration of catchment characteristics and anthropogenic activities. Water temperatures increased by 0.8-1.3 °C, whereas water discharges remained largely unchanged. Concentrations of alkalinity, total hardness, Ca, and Mg regulated by dominant carbonate lithologies in catchments increased by up to 10%. We attribute this change to an increase in the partial pressure of CO in the subsurface, provoked by increasing temperatures. Re-oligotrophication processes in lakes also influence the behavior of alkalinity and silicic acid. In contrast to concentrations, most loads did not change significantly, due to their large variances. Therefore, no changes in overall weathering rates of carbonate rocks can be detected. The outgassing of CO in rivers from the place of carbonate dissolution to measurement stations amounts up to 6% (mean) of CO sequestered (mean 1.1 mol m a) by the weathering of rock minerals. Changes in alkalinity/Ca/Mg ratios indicate an increase in calcite precipitation over time. Total nitrogen concentrations and loads peaked at the end of the 1980s and then decreased up to 50%, while NO concentrations showed almost no changes. This dynamic matches the changes in the agricultural N balance. Concentrations and loads of Na and Cl increased up to 60% due to an increase in the various uses of rock salt.
Although temperature is an important determinant of many biogeochemical processes in groundwater, very few studies have attempted to forecast the response of groundwater temperature to future climate warming. Using a composite linear regression model based on the lagged relationship between historical groundwater and regional air temperature data, empirical forecasts were made of groundwater temperature in several aquifers in Switzerland up to the end of the current century. The model was fed with regional air temperature projections calculated for greenhouse-gas emissions scenarios A2, A1B, and RCP3PD. Model evaluation revealed that the approach taken is adequate only when the data used to calibrate the models are sufficiently long and contain sufficient variability. These conditions were satisfied for three aquifers, all fed by riverbank infiltration. The forecasts suggest that with respect to the reference period 1980 to 2009, groundwater temperature in these aquifers will most likely increase by 1.1 to 3.8 K by the end of the current century, depending on the greenhouse-gas emissions scenario employed.
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