Štěnec – Jenišovice


site 34

Where is the site located?

GPS: 49° 56′ 7.46″ N, 16° 2′ 30.87″ E

The area lies in the northwestern part of the Svitavská pahorkatina Hilly Land, in a gently rolling landscape, mostly used for agriculture purposes. It contains scattered vegetation and picturesque villages. This area slopes to the East Bohemian Table in the north and rises towards the Iron Mountains in the south. A very special place for the local inhabitants lies in a near bosquet – the Měrkovec spring, supplying the wide vicinity with groundwater.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the eastern part of the Bohemian Cretaceous Basin, which is represented here by Upper Cretaceous sediments of the Chrudim Cretaceous. The original horizontally lying sediments of largely marine origin were modelled by subsequent geological processes. The present setting is very favourable for the creation of groundwater.

What happened at this site in the past?

– 95 million years

In the course of the Mesozoic, the supercontinent of Pangea was falling apart into separate lithospheric plates, which drifted atop the elastic asthenosphere.

In the latest Mesozoic, this area was lying on the Northern Hemisphere, becoming a part of the Eurasian Plate. The climate was very warm, controlled by the greenhouse effect which induced a rise of global sea level. Flooding occurred over large portions of dry land.

In the Late Cretaceous (a part of the last period of the Mesozoic), this site was a shallow sea passage, progressively flooded by warm sea waters. It was located in a shallow near-shore zone, locally dominated by sand, abundantly colonized by bivalves, gastropods, crustaceans and fish. With prolonged flooding, progressive deepening of the depositional area occurred.

What does the site display today?

This site is built by sediments of Upper Cretaceous age, which constitute the eastern margin of the Chrudim Cretaceous. They comprise formations of marine sediments, originally unconsolidated, which were subjected to diagenetic changes (hardening) through geological time. The lowermost and the oldest sediments pertain to the Peruc-Korycany Formation of Cenomanian age, which are about 20 m thick. It is formed by grey sandstone with darker claystone intercalations.

It is overlain by finer sediments, deposited in Lower Turonian. Then, another rise in sea level occurred, accompanied by deepening of the depositional area. This sedimentary complex pertains to the Bílá hora Formation and consists of light grey siltstone and fine-grained sandstone ca. 60 m thick. The last formation was deposited in Mid Turonian the Jizera Formation. It is largely composed of yet finer sediments having the character of silty marlstone and fine siltstone.

The original almost horizontally lying formations of marine sediments were later affected by tectonic forces, which induced their weak flexure towards the basin centre and disintegration into separate blocks.

The Upper Cretaceous formations pose a significant groundwater storage. They display the so-called multi-aquifer hydraulic system: individual formations are more-or-less naturally separated by horizons almost impermeable for groundwater – by aquitards. These are mostly represented by intervals of sediments with a high proportion of clay component. The Peruc-Korycany Formation, the Bílá hora Formation and the Jizera Formation are defined as aquifers A, B and C, respectively.

Groundwater flow is also considerably affected by the presence of tectonic structures (faults), which accelerate the flow in a fractured environment. This area provides a typical example of the effect of a fault on groundwater circulation. The fault functions as a natural drainage for groundwater of the highest aquifer C. Drained water supplies a local stream through hidden inflows. This is visible especially in winter time: intervals free of ice, with permanently green vegetation cluster around these places even if temperatures drop deep below zero. The character of the youngest Quaternary sediments was equally affected by the groundwater regime: the sediments include the so-called meadow chalk of black colour, and a layer of peat. These soils require copious amounts of water for their origin.

What was affected by man?

The existence of spring issues attracted our ancestors already: they used the springs for the foundation of ponds. Landscape character changed after splitting of the land into plots under the Austrian-Hungarian Empire. The ponds were dried, leaving behind only massive dams, such as those to the north of the Měrkovec spring. The bottom of the pond was turned into agricultural land.
Groundwater withdrawal from the Měrkovec spring dates to the 1970s: then, a shallow well was excavated here to supply the near agricultural centre with water. Hydrogeological survey intended to find a better source of groundwater for a water-supply system was conducted in this area some ten years later.

What was discovered?

In the first stage, hydrogeological survey focused on the quantification of groundwater discharge to the stream near the Měrkovec spring. The stream discharge was found to increase by about 25 l/s over a distance of ca. 400 m. Then, borehole JS-1 was drilled to a depth of 50 m. Tests at the borehole proved that sufficient amounts of groundwater can be withdrawn from aquifer C, i.e. from the Jizera Formation. As time passed, however, water pumping induced a reduction in both the stream discharge and the capacity of the shallow well. Moreover, substances adversely affecting the quality of water (ammoniac, nitrates and nitrites) started to advance from the near manure repository to the borehole. It was clear that groundwater from the topmost aquifer – aquifer C – will not be suited for water-supply purposes. Another hydrogeological survey followed: borehole JS-2 drilled to the depth of 147 m reached deeper-lying aquifers B (Bílá hora Formation) and A (Peruc-Korycany Formation). Aquifer A was found to be of low yield and undesirable groundwater chemistry, especially due to high amounts of iron compounds naturally derived from the ambient sediments.

The “winning prizeˮ in this contest was finally awarded to the mid-positioned aquifer – aquifer B (Bílá hora Formation) – which showed favourable quantitative parameters as well as qualitative parameters. Groundwater in this borehole forms an artesian system: it is under pressure, being spontaneously discharged onto the earth surface. Such boreholes are also called “flowing wells”. Relatively high volumes of groundwater are spontaneously discharged: about 10 l/s. In addition, the use of this aquifer is advantageous in terms of its easy protection. As it is sealed from above by an aquitard (lying between aquifers B and C), there is no need to regulate land use by wide groundwater protection zones.

“The Iron Mountains – a geologically significant region” project of 2014

An information panel was manufactured within the project of “The Iron Mountains – a geologically significant region”. It was erected near the road between Štěnec and Jenišovice at the point where a concrete panel road diverges towards the withdrawal area, which is located some 150 m to the north. Access to the withdrawal area is permitted only when accompanied by the operating company staff.