Integrative assessment of urban groundwater quality in the city of Munich, Germany: Spatio-temporal patterns of hydrochemical and selected microbial indicators

Urban areas are expanding worldwide. Consequent changes to the environment, such as surface sealing, underground structures, and alterations to vegetation, do not only affect the surface but also impact the subsurface. Groundwater, a crucial natural resource for humankind, an essential water resource to plants, and a huge aquatic ecosystem, is potentially threatened by these changes. In particular, the hydraulic conditions and the water quality in shallow urban aquifers are being altered significantly. This study focuses on the shallow aquifer of the city of Munich, Germany, examining it in terms of urban land use classes (dense and discontinuous sealed surfaces, parks, forests, and agricultural sites) and typical physical and chemical measures used in standardised groundwater monitoring. In addition, the groundwater quality assessment is complemented by selected microbiological indicators. The objective of the study was to evaluate if physical, chemical and microbiological variables are suited for a routine groundwater monitoring in an ecological manner and if they show the same distribution patterns in an urban environment. Groundwater below the five land use categories distinguished showed differences in variables such as chloride concentration, temperature, and total bacterial cell counts. However, the considerable natural variation in the depth of the groundwater table across the city partly masked the effect of urban land use on groundwater hydrochemistry. Bacterial activity in shallow urban groundwater, measured as cellular ATP concentrations, on average, was in the range of clean surface waters rather than near-natural groundwater. Concentrations of dissolved organic carbon (DOC) and nutrients, on the other hand, were overall low. In summary, Munich's shallow gravel aquifer mirrors an energy-limited, oligotrophic ecosystem. Strong correlations were observed between bacterial cell counts and DOC concentrations, with groundwater temperature being a significant influencing factor alongside the concentration of major ions. Some of the physical–chemical variables and microbiological measures exhibited variations between two sampling seasons, as well as between well water and pumped groundwater. In conclusion, the consideration of land use classes provided useful information on the impact of urbanisation on groundwater hydrochemistry and microbiology. Both sets of criteria sensitively indicated deviations of the urban to rural groundwater characteristics. Finally, seasonal effects and differences in the type of water sampled need to be considered for the setup of a routine integrative monitoring scheme.