A multiparameter analysis of environmental gradients related to hydrological conditions in a binary karst system (underground course of the Pivka River, Slovenia)

Abstract

Chemical and bacterial gradients under different hydrologi­cal conditions were studied in a well-developed underground karst system. Water samples were collected from the main un­derground drainage conduit of the Pivka River from October 2013 until June 2016. The system responds quickly to external pulses (precipitation events), and is also impacted by human interventions, as is demonstrated mainly by fluctuations of sul­phates, chlorides, and occasionally elevated concentrations of organic and faecal pollutants. Chemical and bacterial param­eters showed a monotonous trend of decreasing concentrations from the ponor towards the interior of the karst massif during stable hydrological conditions, and a significant change dur­ing high water conditions. High flow events tend to equilibrate chemical and bacterial parameters in the underground river. Concentrations of chlorides, TOC (total organic carbon) and nitrates were the most indicative parameters describing the for­mation of the gradient. Stable isotopes of hydrogen and oxygen in water indicated that the main karst conduit collects isotopi­cally different waters from the aquifer. The river water collected after nine kilometres of underground flow was always isotopically lighter than the waters collected from the upstream sites. Multiparameter analysis proved to be a useful tool for providing a more comprehensive understanding of the dynamics of the underground water, which influence both the underground environment and the ecology of the biome.

Key words: karst, hydrology, water chemistry, nutrients, stable isotopes, PCA, bacteria.

Multiparametrska analiza okoljskih gradientov, povezanih s hidrološkimi razmerami v binarnem kraškem sistemu (podzemni tok reke Pivke, Slovenija)

V dobro razvitem podzemnem kraškem sistemu smo pri različnih hidroloških pogojih preučevali kemijske in bakterijske gradiente. Vzorce vode smo odvzeli iz glavnega podzemnega toka reke Pivke med oktobrom 2013 in junijem 2016. Sistem se hitro odziva na zunanje impulze (padavinski dogodki) in je tudi podvržen človekovim posegom, kar dokazujejo predvsem nihanja v koncentraciji sulfatov in kloridov ter občasno povišane koncentracije organskih in fekalnih onesnaževal. Spremljanje kemijskih in bakterijskih parametrov v stabilnih hidroloških razmerah je pokazalo monotoni trend zniževanja koncentracij od ponora proti notranjosti kraškega masiva. Razmere se izrazito spremenijo v času visokih vod, ko pride v podzemnem vodotoku do izenačenja tako kemijskih kot bakterijskih parametrov. Kloridi, TOC (skupni organski ogljik) in nitrati so bili najbolj indikativni parametri za opis nastanka gradienta. Stabilni izotopi vodika in kisika v vodi so pokazali, da vodotok glavnega kraškega kanala zbira izotopsko različne vode iz vodonosnika. Voda podzemne reke po devetih kilometrih toka v podzemlju je bila vedno izotopsko lažja kot vode iz gorvodno vzorčevanih mest. Multiparametrska analiza se je izkazala kot uporabno orodje za celovitejše razumevanje dinamike podzemnih voda, ki vpliva tako na podzemno okolje kot ekologijo bioma.

Ključne besede: kras, hidrologija, kemija vode, hranila, stabilni izotopi, PCA, bakterije.