Most of our drinking water is gained from groundwater. Will climate change affect the quality of groundwater? We analysed historical data of several aquifers with a focus on temperatures and oxygen concentration levels. We worked on the assumption that rising temperatures accelerate biological, geological and chemical processes which can lead to lower oxygen concentration levels and alter the redox conditions in the water. As a result, certain substances may precipitate and cause problems in pump stations. To secure drinking water supplies, it is therefore important to understand how climate change influences temperatures and oxygen concentration levels in groundwater.
In a first step, the historical data was reconciled: missing figures were interpolated, outliers were removed and even-time intervals were established. To analyse long-term trends and abrupt changes in the time series, we relied on the standard method of time series analysis. Cross correlation functions and regression models were used to analyse how climate, river and groundwater variables are interrelated. Statistical modelling methods formed the basis of our predictions of how groundwater temperatures might develop. We created linear regression models, a box model as well as a transfer function model which model groundwater temperature in relation to air temperatures. On the basis of existing predictions for air temperatures in Switzerland, we calculated groundwater temperatures till the end of the 21st century.
The predictions show that the temperatures in the analysed aquifers will rise during the 21st century. The extent of the rise depends on the climate scenario and the type of aquifer. The temperature in aquifers fed by rivers will rise by 2 to 4 degrees Celsius. The groundwater temperature in aquifers fed by precipitation will only increase by up to one degree Celsius on average. The time series relating to oxygen concentration in aquifers fed by rivers offer various insights. On the one hand, they reveal that oxygen concentration levels have sunk in the course of several years, which is probably linked to a strong rise in river and groundwater temperatures. On the other hand, they show sudden increases in oxygen concentration levels which seem to be unrelated to water temperatures. Instead, these increases coincide with spates. We assume that the spates influence the permeability of riverbeds. As a result, infiltrating water passes more quickly through the microbiologically active zone, which in turn increases the oxygen concentration in the groundwater.
Our study shows that groundwater temperatures in aquifers fed by rivers will rise significantly in the near future. It is possible that these aquifers, supplying 25% of our drinking water, will suffer from lower oxygen concentration levels which will negatively impact the quality of groundwater and lead to infrastructure problems at pump stations. We recommend intensifying the monitoring in areas where drinking water is gained from such aquifers.
Response of Swiss groundwaters to climatic forcing and climate change: analysis of the historical instrumental record