Effects of the vadose zone on groundwater flow and solute transport characteristics in mountainous karst aquifers – the case of the Javorniki–Snežnik massif (SW Slovenia)

Abstract

Tracer tests are rarely applied in mountainous karst aquifers due to difficult or limited basic data acquisition, problems of access and expected strong tailing effects in tracer breakthrough. They are, however, a straightforward method for learning about the characteristics of underground water flow. The main aim of this study was to better understand the processes of flow and storage in the karst aquifer and its unsaturated (vadose) zone, which is several hundreds of metres thick. A tracer test was conducted with injection from the surface into a shaft in the central part of the aquifer. The results were compared to a previous tracer test with similar injection characteristics (the same tracer, injection into shaft vs karren) but with a significantly different vadose zone thickness. For this purpose, direct information obtained through tracer breakthrough curves analysis was modified in accordance with the distance factor, normalised as a function of tracer mass injected, and expressed as a residence time distribution. A conceptual model of flow and transport duality through the karst system was elaborated: part of the tracer flows rapidly through the primary drainage channels, while the other part is predominantly characterised by prolonged retention in less permeable zones. The results obtained support the idea that the thickness of the vadose zone has a very strong influence on residence times and dispersion. The subsequent outflow of the tracer and its detection greatly depend on the meteorological conditions. Our study suggests that the greater the thickness of the vadose zone, the better its protective function. In addition, the study was also relevant from the regional perspective, in that it enabled us to identify more closely the location of the Adriatic–Black Sea watershed. Key words: karst hydrogeology, tracer test, unsaturated zone,conceptual model, groundwater flow and storage, source protection. Vplivi nezasičene cone na značilnosti toka podzemne vode in prenosa topnih snovi v gorskih kraških vodonosnikih - primer Javorniško - Snežniškega masiva (JZ Slovenija) Sledilni poskusi sodijo med metode, ki neposredno prikazujejo značilnosti toka podzemne vode. Zaradi oteženih ali omejenih dostopov do osnovnih podatkov, zahtevnosti terenskega dela in pričakovanega dolgotrajnega podaljšanja pojavljanja sledila se sledilni poskusi v gorskih kraških vodonosnikih redko izvajajo. Glavni namen te študije je bil boljše razumevanje procesov toka in skladiščenja v kraškem vodonosniku in njegovi nezasičeni (vadozni) coni, ki je lahko debela več sto metrov. Opravljen je bil sledilni poskus v osrednjem delu vodonosnika z injiciranjem v brezno. Rezultate smo primerjali s preteklim sledilnim poskusom s podobnimi lastnostmi injiciranja (enako sledilo, injiciranje v brezno oz. škraplje) toda s precejšnjo razliko v debelini vadozne cone. V ta namen so bile neposredne informacije, pridobljene z analizo sledilne krivulje, spremenjene v skladu s faktorjem razdalje, normalizirane kot funkcija količine uporabljenega sledila in izražene kot porazdelitev zadrževalnega časa. Izdelan je bil konceptualni model dvojnosti toka in prenosa topnih snovi skozi kraški sistem: del sledila se pretaka hitro skozi primarne drenažne kanale, medtem ko je za del tega značilno podaljšano zadrževanje v manj prepustnih območjih. Dobljeni rezultati podpirajo trditev, da debelina vadozne cone močno vpliva na zadrževalne čase in disperzijo. Poznejši odtok sledila in njegovo zaznavanje sta v veliki meri odvisna od meteoroloških razmer. Naša študija kaže, da je z večjo debelino vadozne cone njena zaščitna funkcija večja. Poleg tega je bila študija pomembna tudi z regionalnega vidika, saj nam je omogočila, da natančneje opredelimo jadransko-črnomorsko razvodnico. Ključne besede: kraška hidrogeologija, sledilni poskus, nezasičena cona, konceptualni model, tok in skladiščenje podzemne vode, zaščita vodnih virov.

Keywords

• karst hydrogeology
• tracer test
• unsaturated zone
• conceptual model
• groundwater flow and storage
• source protection