Abstract: The origin of groundwater recharge and subsequent flow paths are often difficult to establish in fractured, multi-lithological, and highly compartmentalized aquifers such as the Troodos Fractured Aquifer (TFA). As the conjunctive use of stable isotopes and hydrogeochemical data provides additional information, we established a monitoring network for stable isotopes in precipitation in Cyprus. The local meteoric water line, altitude effect and seasonal variation of stable isotopes in precipitation are derived from monitoring data. Stable isotopes and hydrogeochemical data are combined to model water-rock interactions and groundwater evolution along a complete ophiolite sequence. As a result a generic hydrogeologic description for the observed water types is developed. Isotope hydrology was applied in conjunction with hydrogeochemical modelling in Kargiotis Watershed, a major north-south transect of the TFA. PHREEQC was used for hydrogeochemical modelling to establish generic descriptions for observed water types. Mean precipitation-weighted values from 16 monitoring stations were used to calculate the Local Meteoric Water Line (LMWL), which was found to be equal to δ2H = (6.58 ± 0.13)*δ18O + (12.64 ± 0.91). A general decrease of 1.22‰ for δ2H and 0.20‰ for δ18O in precipitation was calculated per 100 m altitude. A generic groundwater evolution path was established: 1. Na/MgClHCO3, 2. MgHCO3, 3. Ca/MgHCO3, 4. Ca/MgNaHCO3, 4a. MgNa/CaHCO3/Cl, 5. NaMg/CaHCO3/Cl, 6. NaHCO3, 7. Na/MgHCO3SO4, 8. NaSO4Cl/HCO3. Hydrogeologic descriptions, consisting of groundwater origin, flow path and possible active water-rock processes, have been realised for the observed water types. The first two water types occur in serpentine and ultramafic-gabbro springs. Type 3 waters represent early stages of recharge and/or short flow paths, in gabbro whereas types 4 and 5 are typical for further percolating waters in gabbro and diabase. Water types 6 and 7 occur both in diabase and in the basal group and represent the regional flow. Water type 8 is the end member of regional, upwelling groundwater in the basal group. The presented descriptions and methods have practical applications in groundwater exploration, characterization, and protection. The methodology can be applied in other complex aquifer systems.

Abstract: A complete hydrogeological understanding of the deep Upper Jurassic carbonate aquifer in the South German Molasse Basin is essential for the future development of this important drinking water resource and geothermally used system. Water chemistry data, δ13CDIC, 14C of the dissolved inorganic carbon (14CDIC) and stable water isotope (δ18O and δD) measurements have been used to evaluate a promising groundwater dating approach with 14C of dissolved organic carbon (14CDOC). The pre-concentration of dissolved organic matter (DOM) was performed by the easy applicable solid phase extraction (SPE) with a styrene-divinylbenzene copolymer sorbent (PPL). Based on the sampling campaign of seven groundwater wells conducted between 2017 and 2019, it was shown that the groundwater is mainly of Ca–HCO3 type with some evidence of ion exchange between Ca2+ and Na+ at two of the investigated wells. The δD values ranged from −89.4‰ to −70.9‰ while δ18O values varied between −12.5‰ and −9.8‰. The obtained stable water isotope signatures indicated that the groundwater is of meteoric origin and was recharged during warm climate (Holocene), intermediate climate and cold climate (Pleistocene) infiltration conditions. The measured 14CDOC activities varied from 5.7 pmC to 51.1 pmC and the calculated piston-flow water ages (ORAs) ranged from 4200 years to 25,248 years using an initial 14C0DOC of 85 pmC. The calculated ORAs showed a very good correlation to the infiltration temperature-sensitive δ18O values which were affirmed with noble gas infiltration temperatures for two wells after Weise et al. (1991) and were also in good accordance with the atmospheric temperature record of the northern hemisphere from Dokken et al. (2015). The results reflect a consistent hydrogeological picture of the carbonate aquifer, which also supports the applicability of the SPE-PPL method for 14CDOC dating in groundwater with a low DOC content (<1 mg/l). In contrast, 14CDIC activities of 1.4 pmC to 21.3 pmC led to geochemically corrected piston-flow ages between 8057 years and >30,000 years and generally to an overestimation of the apparent water ages. This study gives insights into the promising approach of 14CDOC groundwater dating in carbonate aquifers with low DOC contents and allows future sustainable groundwater resource management of the investigated aquifer system.

Abstract: The factors influencing uranium recovery in water-rock systems during acid in-situ leaching (ISL) were studied at the Kujieertai uranium deposit in Xinjiang. Using an ISL unit, a field leach trial (FLT) had been carried out to test the sequential effects of a leaching solution without oxidant (H2SO4 solution 4–8 g/L) and a leaching solution with oxidant (H2SO4 3–7 g/L, and Fe (III) 2–6 g/L). The observation of the leaching process revealed clearly defined stages of uranium release from the solid mineral to solution. Uranium mobilization from solid mineral into solution can be described in four stages. At the beginning of the acid ISL process, there was no oxidant to be added to the leaching solution and the desorption of hexavalent uranyl ions in the open pores, as well as dissolution of hexavalent uranium minerals, led to a short-term peak in the pregnant solution, which happened while pH decreased from about 5.3 to 2.62. Following the depletion of the adsorbed hexavalent uranium and a decline in uranium dissolution intensity, the addition of Fe(III) facilitated the oxidation of tetravalent uranium, which enabled intensive uranium mobilization again. During this process, the dissolution of uranium had a strong positive correlation with the reduction of Fe(III) and Eh in the leach solution. Beside hydrochemical factors, the deportment of uranium was also an important factor affecting uranium recovery. Uranium located in the open pores can be completely exposed to the solution and the mobilization intensity was significantly affected by hydrogeochemical conditions; but the uranium present in microfissures and in the ore matrix could not be fully exposed to the solution, so, their dissolution intensity was primarily controlled by corrosion and permeability of the ore. In general, the hydrogeochemical conditions and the deportment of uranium were the external and internal factors that significantly affected the dissolution and recovery of uranium in the early and middle stages of the FLT. However, in the latest stages, due to uranium depletion, enhancing the chemical potential of the leaching solution, specifically acidity and/or the amount of oxidant, had little improvement on uranium recovery.