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Abstract:

Groundwater samples from a post-mining environment of a gold deposit in Romania (Zlatna mining site, Apuseni Mountains) were analysed for carbon isotopes in order to estimate the age, recharge conditions and flow transit velocities. Three distinct types of groundwater have been considered: springs, domestic wells and mine water. The previous stable isotope results (²H/¹H and ¹⁸O/¹⁶O) showed that the recharge source of the groundwater is local meteoric water. All obtained per cent modern carbon (pMC) values are less than 100% which indicates old groundwater or mixture of old groundwaters and modern water coming from precipitation. The distribution of pMC data clearly shows an increasing trend from 40.86% in mine water to values as high as 93% in phreatic water sampled in domestic well. Springs show an intermediate position with pMC values varying from 54.03% to 79.41%. Mean calibrated date vary from 6065 BC to 2553 BC in mine water, from 3754 BC to 186 AC in springs, and from 563 AC to 1354 AC in phreatic water. Non-renewable paleo-waters could not be recognized in the study area. Correlation between pMC values and δ¹⁸O values of groundwater allowed to recognize inputs from the late-glacial (Younger Dryas) recharge in the case of mine water and several springs, as well as contributions from modern precipitation in the case of other springs and phreatic water sampled in domestic wells. Using radiocarbon dating for pairs of sampling points allowed the calculation of flow transit velocities of groundwater. For the groundwater sampled in springs we obtained a mean flow transit velocity of 2.4 m/year.