🇸🇮 Sprehodite se do Straže pod Pirničami in si oglejte edini še ohranjen termalni izvir v Ljubljanski kotlini. Domačini mu pravimo kar Topli izvir. Po novi kolesarski stezi Medvode-Vikrče se lahko pripeljete tudi s kolesom. Za vpis najdbe boste potrebovali termometer!
Naloge za vpis najdbe zaklada
Terenska oprema: kuhinjski ali stenski termometer!
1. kaj vpliva na temperaturo vode termalnega izvira?
2. kaj je najverjetnejši glavni vir vode, ki napaja topli izvir v Pirničah?
3. omočite si roko v curku, ki izteka iz cevi. Se vam zdi voda topla ali hladna? Zakaj menite, da je tako?
4. s termometrom izmerite temperaturo curka vode, ki izteka iz cevi. Posnemite sliko termometra (po možnosti z rezultatom meritve) in izvirom v ozadju in jo objavite v dnevniškem zapisu. Zapisi brez slike bodo izbrisani.
Pomembno!
Preden vpišete najdbo, mi pošljite odgovore v Središču za sporočila (Message Center). Potem lahko najdbo na spletu vpišete brez dodatnega dovoljenja. Če bo z odgovori kaj narobe, se vam bom oglasil. Vpisi brez spremljajočega sporočila, ki vsebuje pravilne odgovore, in/ali objavljene fotografije bodo izbrisani brez opozorila! Vpisi, ki vsebujejo odgovore, bodo prav tako izbrisani!
🇬🇧 Take a short hike to Straža near Pirniče, and visit the only preserved thermal spring in Ljubljana Basin. Locals call it simply Warm Spring. You can also come by bike using the new cycling trail Medvode-Vikrče. You will need a thermometer to log the find of this EarthCache!
Logging tasks
Field equipment: kitchen or wall thermometer!
1. which factors determine the temperature of thermal springs?
2. what is the most likely principal source of water that emerges as Warm spring in Pirniče?
3. wet your hand in the water that comes from the pipe. Does the water feel cold or warm? Explain your observation.
4. measure the temperature of the water coming from the pipe with a thermometer. Take a photo of the thermometer (showing the water temperature if possible) and the spring in the background, and attach it to your Found It log. Logs without the photo will be deleted.
Important!
Before you log the find, please, send me a message with the answers to the logging tasks through the Message Center. After you send the message, you can log the find without waiting for my reply. If there is something wrong with the answers, I will contact you. I will delete the logs without accompanying message or without attached photo without warning. I will delete the logs containing the answers as well.
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Topli izvir Straža je razmeroma močan termalni izvir sredi polj pod Spodnjimi Pirničami. Temperatura vode se giblje med 17 in 23 °C, pretok vode pa med 6 in 8 l/min. Temperaturna razlika med toplo vodo in mrzlim zrakom je najbolj opazna pozimi, ko se iz vode dvigajo meglice. Že od nekdaj so ga, posebej pozimi, uporabljale perice iz bližnjih vasi. Straža je kot krajinska posebnost zaščitena kot naravni spomenik. V 20. letih prejšnjega stoletja so tu, daleč od drugih znanih rastišč v Primorju, našli redko sredozemsko praprot venerini lasci. Nikoli ne bo znano, ali je prišla sama ali so jo sem zanesli akvaristi, ki so v tistih časih v topli vodi gojili tropske ribice, ker je bilo rastišče z gradbenimi deli okoli izvira uničeno.
Voda iz izvira se danes steka v večji bazen, ki ga je Turistično društvo Pirniče pred leti uredilo in očistilo, da je poleti primeren tudi za kopanje. Izvir z okoliškimi potmi med polji in travniki ob Savi (in novo pridobitvijo, kolesarsko stezo med Vikrčami in Medvodami) je priljubljen cilj sprehajalcev, tekačev in kolesarjev, ob njem so urejene klopce, kjer si boste po opravljenih nalogah lahko privoščili zasluženo malico. V neposredni bližini ni primernih parkirišč. Lahko pridete s kolesom ali peš iz Pirnič, Vikrč, Tacna ali Medvod. Znajdite se.
Termalni izviri
Zanimivo, da enotne opredelitve, kaj je termalni izvir, v literaturi ni, najpogosteje pa kot termalni izvir štejejo vsak izvir s temperaturo vode vsaj 20-21 °C. Nekateri viri glede na temperaturo vode ločijo vroče in tople izvire, pri čemer pa izraz termalni izvir ni sinonim za vroči izvir. Voda iz termalnega izvira se ogreje geotermalno, kar pomeni s toploto, ki nastaja v Zemljinem plašču. V področjih z veliko vulkansko aktivnostjo, kjer je raztaljena kamnina prisotna razmeroma blizu površja, se podtalnica ogreva zaradi bližine nakopičene magme. Temperatura vode v takih izvirih je lahko zelo visoka. V področjih, kjer vulkanske aktivnosti ni, pa pride do ogrevanja vode zaradi geotermalnega gradienta, tj. naraščanja temperature kamnin z globino. Voda, ki pronica dovolj globoko, se ogreje ob stiku z vročimi kamninami. Do tega pojava pride večinoma ob prelomnicah, kjer razpoke omogočajo, da površinska voda ali podtalnica prodreta dovolj globoko.
Termalni izviri v Sloveniji
V Sloveniji aktivnih vulkanov (še) nimamo. Izvire tople vode pri nas lahko razvrstimo v dve skupini s podobnimi značilnostmi. V prvi so termalni izviri, ki pritekajo na površje iz dolomita in apnenca že tako dolgo, da jih štejemo za trajne. Majhna vsebnost mineralnih snovi in razmeroma stalna sestava kažeta, da gre za podzemeljsko kroženje vode (večinoma padavinskega oziroma meteornega izvora), ki s površja pronica v globino, se geotermalno ogreje in se na površje vrne kot termalni izvir. Temperatura vode izvira je odvisna od geotermalnega gradienta (ta ni po vsej Sloveniji enak, glej sliko) in stopnje stika s površinskimi vodami. V drugo skupino so zajete termomineralne vode, ki so jih odkrili z vrtinami med iskanjem nafte v Pomurju. Imajo visoko vsebnost mineralnih snovi in so akumulirane v mlajših poroznih peščenih plasteh. Njihovo obnavljanje in trajnost sta (morda) vprašljiva.
Geotermalni gradient: zemljevid temeperature v globini 3000 m v Sloveniji
Topli izvir Straža spada v skupino termalnih izvirov v Ljubljanski in Radovljiški kotlini. Straža je edini ohranjen topli izvir v Ljubljanski kotlini (izvir v Preski pri Medvodah naj bi bil zazidan), sicer pa v to skupino spadajo še izviri na Bledu, v Zgornji Besnici, Hotavljah in pri Vrhniki. Njihova skupna značilnost je, da pritekajo na površje ob stranskih plitvih prelomih, navadno več kilometrov stran od glavnih prelomov, in imajo temperaturo od 19 do 23 °C. Takšna temperatura je posledica nizkega geotermalnega gradienta. Kroženje vode je zaradi tega lahko zelo globoko, temperatura pa vendarle ostane nizka. Z vrtanjem na Bledu so šele v globini 550 m izmerili temperaturo 19 °C. Zelo verjetno pa je, da k razmeroma nizki temperaturi vode v izvirih prispeva tudi mešanje s podtalnico.
Topli izvir na Straži geologi bolj ali manj redno spremljajo od 50. let prejšnjega stoletja. Leta 1972 so izvrtali tri orientacijske vrtine, ki so pokazale, da topla voda priteka iz globine ob stiku triadnega dolomita z neprepustnim terciarnim laporjem. Najvišja ob tej raziskavi izmerjena temperatura je bila 20,5 °C, kar je bilo za 2,6 °C manj od temperature izvira ob izjemni suši leta 1971. To kaže, da se termalna voda pred pojavom na površju najverjetneje meša s hladno v razmeroma plitvi coni. Geologi so bili takrat mnenja, da bi bilo umestno z zajetjem ločiti termalno in hladno vodo, vendar je ostalo le pri tem. In naj pri tem tudi ostane.
Viri:
– Fundamentals of Physical Geology, Sreepat Jain, 2014.
– Izvor in kemijska sestava mineralnih in termomineralnih vod v Sloveniji, Andrej Lapanje, Geologija 2006; 49: 347-70.
– Termalni in mineralni vrelci v Sloveniji, Anton Nosan, Geologija: razprave in poročila, Ljubljana 1973.
– Evolucija Zemlje in geološke značilnosti Slovenije. Ur. Miha Jeršek, Prirodoslovni muzej Slovenije, 2009.
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Warm spring Straža is a thermal spring in the fields below Spodnje Pirniče, with water temperature 17-23 °C and water flow of 6-8 l/min. Temperature difference between water and cold air is best observed in winter, when the mist rises above the pool. Washerwomen from nearby villages had been using it as a lavoir for centuries, especially in winter. Today, Straža is protected as a natural monument. In 1920's, a rare Mediterranean fern (Adiantum capillus-veneris or venus hair fern) has been discovered here, far away from other known sites in Slovenian Littoral region. It could have been introduced by aquarists breeding tropical fish in the warm spring water. We will never know because the site was destroyed by the construction of the basin around the spring.
Today, water from the spring flows into a larger pool, a popular summer bathing spot for local children (of all ages). Trails and footpaths (and the newest addition, bike trail from Vikrče to Medvode) in the fields and meadows towards the Sava river are popular with hikers, joggers and bikers. Tourist Association Pirniče has placed the benches and trash bins in the area of the spring and the pool so you can celebrate another geocaching success with a small picnic. There is no convenient parking nearby. You can reach the spring on foot or bike from Pirniče, Vikrče, Tacen or Medvode. You will find a way.
Thermal springs
Interestingly, there is no universally accepted definition of a thermal spring. Often, sources define it as a spring with water temperature of at least 20-21 °C. Others use water temperature to categorize springs as hot and warm, with thermal spring as a general term and not a synonym for hot spring. Water from thermal spring is heated geothermally, with the heat produced in the Earth's mantle. In the areas of vulcanic activity, where molten rock is located closer to the surface, the water heats in the proximity of collections of magma. Water temperature in this type of springs can be extremely high. In the areas without volcanic activity, the water heats because of the increase of the temperature with depth (geothermal gradient). Water that percolates deeply enough in the Earth's crust is heated in the contact with hot rock. This process generally takes place near the faults, where fissures and shattered rock areas allow water to circulate to greater depths.
Thermal springs in Slovenia
There are no active volcanoes in Slovenia (yet). Thermal springs in our country can be broadly divided in two main groups. The first group are thermal springs, emerging from dolomite and limestone for such a long time that they are considered permanent water sources. Low mineral content and relatively stable composition indicate an underground circulation. Water (mainly precipitation water) percolates from the surface into depth, heats geothermally and emerges on the surface again as a thermal spring. Water temperature of the spring depends on geothermal gradient in the area (not uniform; see the map below) and the extent of contact with colder waters near the surface. The second group are thermomineral waters, discovered during exploratory drilling for oil in Pomurje region. They have high mineral content and are accumulated deep in younger porous sandy beds. Their natural recharge and permanence are subject of speculation and debate.
Geothermal gradient: map of temeperature at the depth of 3000 m in Slovenia
Warm spring Straža belongs to a group of thermal springs of Ljubljana and Radovljica basin. Straža is the only preserved thermal spring in Ljubljana basin (spring in Preska near Medvode was reportedly walled in when the settlement construction extended to the area), while other springs of this group are in Bled, Zgornja Besnica, Hotavlje and near Vrhnika. Their common feature is that they emerge near shallow secondary faults, often kilometers away from the main fault, with water temperature from 19 to 23 °C. Low temeperature is related to low geothermal gradient. Water could permeate very deep, but the temperature remains low. During research drilling in Bled, the temperature of 19 °C was first recorded at the depth of 550 m. Mixing with cold ground water is probably a contributing factor as well.
Geologists have been more or less systematically recording the state of the spring at Straža since 1950's. In 1972, three exploratory bores revealed that the thermal water emerges from the depth along the junction of Triassic dolomite and impervious Tertiary marl. The highest temperature recorded during this research was 20,5 °C, which was 2,6 °C lower than temperature recorded during extreme drought in the summer of 1971. This observation indicates that thermal water is mixing with cold ground water in the shallow zone before emerging on the surface. Geologists expressed the opinion that it could be worthwile to continue the exploration and separate the thermal water from ground water by additional bores. But there was no further action. Let's hope it stays that way.
References:
– Fundamentals of Physical Geology, Sreepat Jain, 2014.
– Izvor in kemijska sestava mineralnih in termomineralnih vod v Sloveniji, Andrej Lapanje, Geologija 2006; 49: 347-70.
– Termalni in mineralni vrelci v Sloveniji, Anton Nosan, Geologija: razprave in poročila, Ljubljana 1973.
– Evolucija Zemlje in geološke značilnosti Slovenije. Ur. Miha Jeršek, Prirodoslovni muzej Slovenije, 2009.