Indicative mean transit time estimation from δ¹⁸O values as groundwater vulnerability indicator in karst-fissure aquifers

Ideally, groundwater vulnerability assessment should take into account potential contaminant breakthrough curve, characterized by transit time, relative change of contamination concentration (input/output) and duration of overstepped concentration. Still, a majority of groundwater vulnerability assessment methods that tend to be more physically based count only with the value of mean transit time (MTT), the elapsed time when groundwater can exit the aquifer after its recharge. In this paper, indicative values of MTT derived from single δ¹⁸O sampling values were used to estimate intrinsic vulnerability of individual springs occurring on the Muránska planina Plateau (Slovakia). For 295 springs sampled within 10 days, δ¹⁸O was found in the range between −10.90 and −7.32 ‰, without signs of altitudinal effect. According to International Atomic Energy Agency (IAEA) Global Network of Isotopes in Precipitation (GNIP) stations, significant contrast in precipitation δ¹⁸O values was found several weeks before sampling. Indicative MTT value, e _maxMTT (estimated longest possible mean transit time), was calculated from sine wave course of monitored precipitation isotopic composition, δ¹⁸O value in the spring groundwater sample and altitudes of sampled point and precipitation station. Estimated e _maxMTT values suggest that heavier oxygen isotopes in samples probably reflect quick circulation and a more vulnerable environment as influenced by enriched summer precipitation. Depleted δ¹⁸O suggests lower vulnerability represented by higher e _maxMTT values.