Groundwater chemistry of Prague Castle area
Authors: Weiss Tomáš , Harmáček Jakub , Herichová Iva , Bruthans Jiří 
Published: October 2024

The study examines the groundwater around Prague Castle as an example of an extreme urban environment that has been influenced by human activity since the Neolithic period. The long-term human activity, including several still-functioning drainage adits, resulted in a number of aggradation and degradation processes that have significantly impacted the groundwater regime. Furthermore, building development, particularly large-scale concrete structures, has had a notable effect on groundwater chemistry. This paper presents results of available archival chemical analyses from the area of interest (Fig. 1), along with our own groundwater chemical analyses. The results of recovery test in the anthropogenic alluvium well underneath the courtyard III of Prague Castle (Weiss et al. 2024) indicate that the transmissivity of saturated alluvium is low (8–10 m2/day). However, it is reasonable to assume a higher transmissivity for the entire area. The clayey anthropogenic alluvium near the surface likely has a much lower permeability than the underlying fractured zone of the Letná Formation made of shales (Late Ordovician). Field reconnaissance in the drainage adits indicates that groundwater flows directly from the historic alluvium (presence of bones and other organic remains) into the Ordovician strata. The existence of a clay aquitard, as mentioned by some authors (e.g. Kos 1965), between the historical alluvium and the underlying Ordovician is thus unlikely. The analysed water samples from selected objects can be classified into three distinct groups based on their pH levels: slightly acidic (precipitation water, and the well in the St. George’s Convent – V4), neutral (majority of samples), and alkaline (the environment underneath the courtyard III of Prague Castle). Slightly acidic pH level only applies to samples from precipitation or probable seepage (V4). The concentration of Cl- in the majority of the analysed groundwater samples is higher than the natural background level of around 8–10 mg/l, which indicates an anthropogenic source. The major source of these ions in the area is likely the use of NaCl to salt roads and sidewalks during the winter months, which has been a common practice for decades. The overall analysis of the studied samples identified two main hydrochemical types of water: Type A and Type B. Type A is represented by moderately alkaline water that is rich in sulphate and calcium/magnesium. Type B, on the other hand, is represented by strongly alkaline water rich in hydrogencarbonate and calcium, found mainly under the courtyard III of Prague Castle, which stands on a concrete structure. Type A is the predominant type and is typical of water originating in the pyrite-bearing shales of the Letná Formation. The high sulphate content of these waters may cause “sulphate attack” of concrete, leading to its decomposition and thus affecting the durability of building structures. This study provides valuable insights into groundwater chemistry in urban environments that can be applied to other similar sites.