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Cavernous weathering forms have long been studied and discussed as enigmas in geomorphology. Recently, their evolution has been shown to be controlled by moisture patterns, which are still poorly understood. For the first time, capillary water and vapor fluxes were characterized in detail at tafone in a temperate climate of central Europe using a wide range of methods adapted from soil hydrology. Time domain reflectometry showed that moisture flows from the rock interior to the evaporation front in the shallow subsurface of both — the backwalls and the outer surface. When overland flow occurs on the outer surfaces (after heavy rains), 10 mm/day can infiltrate and flow toward the backwalls. The main sources of water for tafone are the influx of water from the rock interior and the infiltration of overland flow after heavy rains, while condensation of air humidity is a minor source. Influx from the rock interior is coupled to the evaporation rate, which varies between 100 and 300 kg/m2/year in summer and less than 15 kg/m2/year in winter. More water evaporates from the backwall of the tafone than from the outer surface, and more salt is deposited in the backwalls, resulting in predominant salt weathering in the backwalls. The tafoni studied thus evolve, and the cavities deepen. Tafoni in arid and semi‐arid environments generally show a much higher contrast between evaporation rates from backwalls and outer surfaces than tafoni and honeycombs in temperate and coastal environments. Tafoni in temperate settings are therefore more susceptible to degradation when evaporation decreases or inflow to the tafone increases. This study also shows that microtensiometers can be used to determine moisture content with high spatial resolution, while time domain reflectometry allows accurate characterization of moisture patterns with depth. [ABSTRACT FROM AUTHOR]

Erosion in loess is a widespread phenomenon, as loess covers about 10% of the Earth's land surface. While erosion of loess soil has been intensively studied, the mechanisms controlling erosion in gullies and pipes in loess are poorly understood. Cohesion plays an important role in erosion in loess. Interactions between erosional processes and stabilizing mechanisms in loess are poorly understood. This study focuses on the interaction between air slaking and stabilization by confinement to improve leaching techniques for identifying cohesion sources in undisturbed loess from four sites in Czechia. The decrease in tensile strength of samples after leaching in distilled water, dithionate, HCl, and hydrogen peroxide was used to selectively remove cohesion sources such as Fe-Al oxides and hydroxides, carbonates, and organic matter. Experiments showed that confinement and overburden stress are important but neglected stabilizing mechanisms in loess. Leaching of unconfined loess samples gave misleading results because of ubiquitous air slaking. To reliably identify cohesion sources, samples confined in compacted sand were used. Leaching of confined samples showed that Fe-Al oxides and hydroxides are major sources of cohesion (60-90%), while carbonates and organic matter are of minor importance (0-30%). To avoid misleading results, examination of loess structure after leaching is critical to identify samples with damaged structure due to enhanced air slaking caused by bubbles generated during leaching. Air slaking is a powerful and rapid damage mechanism, but it occurs only in dry or semi-dry loess near the soil surface. In contrast, chemical weathering is able to remove cohesion sources in deeper parts of the loess profile. [ABSTRACT FROM AUTHOR]

The origin of highly permeable flow paths in carbonate-siliciclastic rocks, such as large-aperture fractures in aquifers in the Eastern Bohemian Cretaceous Basin (EBCB), is poorly understood. The karst potential was assessed from the rock carbonate content and the degree of disintegration after leaching in HCl. Surprisingly, dissolution of calcite in EBCB usually did not lead to rock disintegration until calcite > 78%. Instead, porosity increased significantly. High-porosity rock is held together by microns-thick secondary silica cement with a foam-like structure and considerable tensile strength. Three types of conduits occur in the EBCB: (i) bedding-parallel conduits associated with calcite-rich layers, (ii) subvertical fracture swarm conduits that develop on damaged zones of fracture swarms, and (iii) conduits formed by dissolution of calcite veins by groundwater flow. These are ghost-rock karst features where calcite is leached from the rock in the first phase and the residue is washed out by conduits under steep hydraulic gradients in the second phase. Very similar features have been described in Minnesota and Wisconsin, USA. Research has shown that fractures with sharp-edged walls that give the impression of an extensional tectonics origin may actually be ghost-rock karst features in which dissolution and piping have played an important role in their enlargement. [ABSTRACT FROM AUTHOR]

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 longterm human activity, including several stillfunctioning drainage adits, resulted in a number of aggradation and degradation processes that have significantly impacted the groundwater regime. Furthermore, building development, particularly largescale 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 Clin 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 pyritebearing 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. [ABSTRACT FROM AUTHOR]

This study is focused on the base flow decrease due to surface water and groundwater evapotranspiration (ET) in wetlands of the Liběchovka catchment. Evapotranspiration in wetlands can significantly affect stream discharge, and its influence will probably still increase in the future due to the global rise of temperatures caused by climate change. The study site is located in the upper part of Liběchovka catchment (Fig. 1). This wetland hosts a small stream supplied by rather steady groundwater inflow, representing the favourable site for detection of the effect of ET. V notch weir and two piezometers were used to monitor stream discharge and wetland water table in 30 minute periods (Fig. 2). Diurnal periodical oscillations of water flow and water table caused by ET were observed (Fig. 3). They occurred in the summer part of the year, especially on sunny days without rain. The amplitude of water table level oscillation was increasing with increasing temperature (Fig. 4). Calculation showed that evapotranspiration reduced the water flow, on average, by 15 % on the warm sunny days. The maximum daily reduction was up to 32 %. Direct measurements were compared with the potential ET calculated using the Oudin's method (Oudin 2005). The ET from the measurement was 45 % of the potential ET calculated by Oudin's method. In the next step, the Oudin's method was used to calculate the potential ET of all wetlands in Liběchovka catchment upstream from the Želízy village, where gauging station of ČHMÚ is situated. The original discharge not reduced by the wetland ET was defined as the sum of the wetland potential ET and the mean annual measured discharge. Due to the ET from wetlands (years 2015-2020), the discharge was reduced on average by 13 %. If only the summer months of July and August were considered, it was reduced, on average, by 26 % and by 39 % in the driest day recorded in this period. To estimate the role of groundwater abstraction on the Liběchovka stream during the period of 2015-2020, the sum of measured stream discharge, groundwater abstraction and wetland ET was calculated. From this sum, 71.5 % represented the directly measured water flow, 18.2 % was the groundwater abstraction, and 10.3 % was the wetland ET (Fig. 8). [ABSTRACT FROM AUTHOR]

Groundwater inflows showing notable natural radioactivity have been found at the northern end of the Holešovice cable tunnel close to the Čimice Grove. This groundwater originates from the Barrandian Neoproterozoic slates of the Blovice accretionary complex, Belt I (Šárka-Zbiroh Belt) at depths approximately 100 meters below the surface, occurring along 50 meter profile of the cable tunnel. Water springs are dispersed in the spring area, with the total discharge of 2 L/s. Groundwater is cold (T ~ 11.1 - 12.3 °C), neutral (pH ~ 6 - 7.48), suboxic to oxic (Eh (ORPH) ~ 177 up to 418 mV), of a hydrochemical type Ca-Mg-HCO3, total mineralization being around 702 mg/L. Radon activity (222Rn) varies between 415 and 1195 Bq/L and radium activity (226Ra) ranges between 0.18 and 5.3 Bq/L. Activities of Rn and Ra are relatively high. Currently, this is the most radioactive water within Prague agglomeration. This water is relatively aggressive as well, causing damage to the concrete lining of the cable tunnel and also allowing formation of calcite precipitates with an increased radium (226Ra) activity. [ABSTRACT FROM AUTHOR]

Information on recharge and flow area of thermal waters is commonly limited as these waters are normally accessible for study only in discharge area. The same is true for thermal waters in the Teplice nad Bečvou Spa, flowing from hypogenic karst in Paleozoic limestones, deeply buried under greywackes, and partly also under Carpathian Flysch Nappes. Increasing interest in hydrogeology of this area in last years has been due to the presence of the Hranice Abyss, currently the deepest underwater cave in the world, and secondly due to the planned dam construction across the Bečva River, only several kilometers upstream from the Spa (Geršl 2016, Geršl - Konečný 2018, Sracek et al. 2019, Vysoká et al. 2019, Klanica et al. 2020). The objectives of this contribution are as follows: i) to estimate the natural yield of thermal water; ii) to delineate river segments which are hydraulically connected to karst aquifer in surroundings of the Spa wells based on their hydraulic response to the Bečva River oscillation; iii) to localize the recharge area of thermal water. The total yield of thermal water was estimated from the change of electric conductivity (EC) of the Bečva River, where a portion of thermal water directly flows. EC was measured along riverbanks during exceptionally low flow conditions in November 2018. Major inflows of thermal water to the Bečva River were localized by thermal camera (Figs. 1, 2; Table 1). Reevaluation of all available data from archive wells enabled to delineate the thermal water occurrence in the discharge area and define boundaries with plain cold groundwaters (Fig. 1). Based on the measured EC and the mixing equation, the natural yield of thermal waters is 14 l/s, which is close to the results of the collective pumping test in the Spa. Based on the natural yield and temperature of thermal water and the comparison to terrestrial heat flux, the extent of the flow area of thermal waters is at least 11 km2, but more likely several tens of km2. Ideas of some previous authors that source of thermal water is derived from the Bečva River several km upstream of the Spa thus can be excluded (c.f. Švajner 1982). It is also unlikely that dominant part of thermal water is recharged via carbonate outcrops of the Maleník Hill due to their small extent. Flysch rocks act as regional aquitard hosting gas reservoir not far from the study area. Most likely, the recharge area of thermal water is located in the Odra Hills in wider surroundings of Potštát town to the North, where active flow was observed in greywackes in depth of 1300 m below the surface. Greywackes are underlain by limestones. This is in agreement with O and H stable isotopes of thermal waters, which are depleted in heavy isotopes indicating higher altitude of recharge area (Sracek et al. 2019, Vysoká et al 2019). Water level fluctuation of the Spa wells with thermal water can be relatively well predicted from fluctuation of the Bečva River level (Figs. 3-5). Based on the elevation of the river water level along its stream course (Table 2), the studied wells R1, R3, HV301 are hydraulically connected to a river segment spanning between 1.2 km above and 0.5 km below the river gauging station, thus in close spa surroundings. Inflows of thermal water to the river traceable by escaping CO2 bubbles were observed in the same river segment in the past. Hydraulic effect of river level oscillation on wells is traceable as far as 400 m from the riverbed also in areas which have been recently occupied by cold groundwater (Fig. 1). This shows that groundwater table is very flat, and limestones are strongly karstified in wider river surroundings. [ABSTRACT FROM AUTHOR]

[Display omitted] Caves in coastal cliffs in the Apostle Islands were analyzed for factors controlling their origin and evolution. Even though the Bayfield Group quartz sandstones were found to be of relatively low tensile strength, the traditional explanation attributing speleogenesis to localized zones of lithological and tectonic weakness does not adequately explain all geomorphic features of the caves. The caves were found to be formed by coalescence of smaller arcade-shaped cavities serially developed along subhorizontal discontinuities. In concordance with the previously formulated "arcade principle", positions and shapes of the cavities are defined by areas of reduced stress within the gravity-induced stress field. Evacuation of material from the rock massif can be attributed to wave action including the effect of water-pressurized air, and frost action. Compound cavities above the discontinuities tend to form "higher-order" arcade-like cavities/caves while those beneath more resistant strata rather develop into elongated overhangs similar to ordinary coastal notches. Propagation of the largest cavities, formed along steeply dipping joints and faults, is attributed to mass wasting and roof collapses. Similar examples of discontinuity-related cavities in coastal settings, suspectedly also controlled by gravity-induced stress distribution, can be found worldwide, being mostly concentrated to areas covered by ice sheets in the Pleistocene. The interplay between post-glacial isostatic uplift and Holocene lake-level rise is responsible for the development of mature forms of coastal caves in the Apostle Islands where present cliffs are estimated to have been exposed to the effects of wave action for the last 3–5 ky. [ABSTRACT FROM AUTHOR]