Effects of anthropogenic impacts on urban groundwater fauna

Groundwater is an important global resource, providing water for industry, irrigation, geothermal uses and potable water. Moreover, groundwater harbours the world's largest terrestrial freshwater biome. Climate change and anthropogenic activities cause diverse changes in groundwater systems. Particularly, these processes lead to an increase in groundwater temperature under densely populated urban areas. While physico-chemical effects have been widely studied, the consequences for groundwater ecosystems are scarcely understood. Thus, understanding how these sensitive ecosystems respond to stressors, such as temperature increase and oxygen depletion, is crucial for sustainable groundwater management, especially in cities.

Our work aims to provide a spatial and temporal overview of groundwater fauna (stygofauna) in two cities in Germany to identify alterations in groundwater fauna due to natural or anthropogenic impacts. To this end, groundwater fauna and several (hydro-)geological, site-specific, climatic and physico-chemical (water) parameters are analysed in 39 monitoring wells in Karlsruhe and 406 wells in Berlin, respectively.

In Karlsruhe, statistical analyses indicate a connection between abiotic groundwater characteristics, such as temperature and dissolved oxygen, and land use. The groundwater temperature shows a warming trend towards densely built-up areas within the study area, yet no substantial change in temperature can be observed over time. In contrast, the oxygen content shows spatially and temporally unstable conditions, with a significant decrease over time, presumably due to degradation processes and a low oxygen input. Also, differences in the spatial distribution of groundwater fauna species due to abiotic groundwater characteristics are identified. Over time, the groundwater fauna community in Karlsruhe has remained largely stable. However, the number of individuals has decreased significantly, which coincides with decreasing contents of dissolved oxygen.

In Berlin, six investigation sites were selected for a detailed assessment of relevant influences on stygofauna. No correlation with groundwater temperature was found based on the data from the individual sites, although a warming trend towards Berlin's city centre is visible. Land use, dissolved oxygen content and exchange with surface waters are the main factors impacting the faunal colonisation of monitoring wells. Generally, urban sites with low oxygen levels and certain levels of pollutants show unfavourable living conditions. In contrast, sites outside the city centre, in nature reserves and close to surface waters contain a more diverse faunal community with more individuals. Based on these findings, a conceptual model was developed to showcase processes and interactions in the groundwater of Berlin.

The results of our study reveal heterogeneous and time-varying conditions in urban groundwater as a habitat. The influence of temperature and, thus, potential geothermal energy systems on groundwater fauna could not be statistically proven for both cities. However, both study areas share the dependence of the groundwater fauna on the content of dissolved oxygen. Contrary to Karlsruhe, the land use in Berlin influences the fauna composition, as the fauna in Berlin depends on the surface water impact. Information on urban groundwater ecosystems should be integrated into urban and energy planning for sustainable subsurface use. In addition, studies in other cities with large-scale, repeated measurement campaigns are necessary to verify our results.