Groundwater dynamics between Planinsko Polje and springs of the Ljubljanica River, Slovenia

Abstract

Caves reaching the (epi)phreatic zone may provide the only access to groundwater in the internal parts of karst aquifers. Modern instruments enable unattended high frequency measurements of groundwater parameters in such caves. A network of distributed observation points may give new information on temporal and spatial pattern of groundwater flow. Such network, recording water level, temperature and specific electrical conductivity was established in all major ponors and active water caves of the karst aquifer between Planinsko Polje and springs of the Ljubljanica River, Slovenia. Up to three and a half years long records were analysed in view of available and relevant geological, speleological, meteorological and hydrological information, and basic hydraulic principles of the epiphreatic flow. The result of the interpretation were models of possible conduit networks of the three main flow subsystems draining Planinsko Polje. For each subsystem a hydraulic model comprising the main features of the inferred conduit network and inputs was made. The models were adjusted until their response to the typical flood situations gave a good qualitative fit to the response of the observation network. The results provide new insights into the mechanism of flood propagation through the system and its relation to the known conduit geometry. The hydraulic role of known cave passages is assessed and new flow pathways are proposed. Back-flooding within the aquifer causes an increase of head in the polje, followed by the activation of higher located ponor zones and flow pathways related to them. Hydrographs recorded in two caves located northwest from Planinsko Polje revealed pos sible high transmissivity zone of the Idrija Fault Zone and the mechanism of estavelles located at the polje’s north-western border. The approach taken in this work offers new insights into the relation between the conduit network geometry and flood mechanism. It can be used elsewhere in conduit-dominated systems with multiple accesses to groundwater flow and well constrained recharge.

Key words: karst aquifer, epiphreatic zone, groundwater monitoring, hydraulic models, Planinsko Polje, Ljubljanica River.

Dinamika podzemne vode med Planinskim poljem in izviri Ljubljanice, Slovenija

Jame v epifratični coni običajno predstavljajo edini dostop do podzemne vode znotraj kraškega vodonosnika. Sodobni merilniki omogočajo kakovostne zvezne meritve različnih parametrov tudi v takšnih jamah. Merilna mreža z več lokacijami znotraj izbranega vodonosnika lahko ponudi nove podatke o časovni in prostorski dinamiki toka podzemne vode. Takšna merilna mreža, z meritvami višine, temperature in elektroprevodnosti vode, je bila vzpostavljena v vseh pomembnejših ponorih in vodnih jamah na območju med Planinskim poljem in izviri Ljubljanice. Do tri in pol leta dolgi nizi podatkov so bili analizirani z vidika razpoložljivih geoloških, speleoloških, meteoroloških in hidroloških podatkov ter osnovnih hidravličnih načel o epifreatičnem toku vode. Rezultat interpretacij so poenostavljeni modeli z domnevno razporeditvijo podzemnih kanalov v treh glavnih podsistemih, ki odvajajo Planisko polje. Za vsak podsistem je bil napravljen hidravličen model z dotokom in mrežo kanalov, ki najbolje ponazarja resnično stanje. Tekom raziskave so se hidravlični modeli nadgrajevali do takšne mere, da je simulacija vodnega toka zadovoljivo ponazarjala dogajanje, izmerjeno v resničnem okolju. Rezultati so prinesli nove ugotovitve o širjenju in dinamiki poplavljanja skozi sistem ter njihovo povezavo s poznano geometrijo proučevanega vodonosnika. Ocenjena je bila hidravlična vloga poznanih jamskih rovov, na podlagi njih pa interpretirane nove smeri pretakanja vode. Ugotovljeno je bilo tudi zajezitveno poplavljanje znotraj vodonosnika, ki vpliva tudi na vodostaj na polju in nadalje na aktivacijo višje ležečih požiralnikov ter njim sledečih kanalov. V dveh jamah severozahodno od Planinskega polja so višinski hidrogrami nakazali na obstoj bolj prepustnega območja Idrijske prelomne cone ter delovanje estavel na severozahodnem obrobju polja. Postopek, ki je bil uporabljen v pričujočem delu, omogoča nova dognanja o razporeditvi podzemnih kanalov in s tem povezano dinamiko poplavljanja v kraškem vodonosniku. Predstavljeno metodo je mogoče uporabiti tudi v drugih kraških sistemih z dobro razvitimi kanali, z več dostopi do toka podzemne toka vode in dobro poznanem dotoku v sistem.

Ključne besede: kraški vodonosnik, epifreatična cona, spremljanje podzemne vode, hidravlični modeli, Planinsko polje, Ljubljanica.