The effect of geochemical processes on groundwater in the Velenje coal basin, Slovenia: insights from mineralogy, trace elements and isotopes signatures

Abstract

This study investigated the mineralogical and isotopic composition of groundwater and precipitation to identify and constrain geochemical processes within stacked Pliocene and Triassic aquifers in the Velenje coal basin. Scanning electron microscopy combined with energy-dispersive X-ray spectroscopic analysis revealed that suspended matter in the Pliocene aquifer consists of feldspars and quartz, while dolomite, calcite and feldspars are present in the aquifer dewatering Triassic strata. The concentrations of trace elements in Triassic and Pliocene aquifers range from highest to lowest Zn > Fe > Ni > Al > Ba > Mn > B>Li > Mo > As with the majority of trace element concentrations below international drinking water health guidelines. Multivariate principal component analysis indicated that concentrations of Mn, Ba, Eu, Cs, Y, Li and T, pH, conductivity and dissolved oxygen in samples were the best chemical parameter for distinguishing the two aquifers. A significant positive correlation (p < 0.05) was found between Ni, Mn, Co, Zn, As and Mo. Groundwater in the Pliocene aquifer likely has an external source of carbon based on the δ 13 C CO2 values (− 12.3 to − 3.6‰). The groundwater also has detectable levels of dissolved methane with isotopic values (− 77.7 to − 51.4‰ δ 13 C CH4 ; − 247 to − 162‰ δ 2 H CH4) consistent with microbial methanogenesis. The groundwater in the Triassic aquifer has tritium values (up to 4.1 TU 3 H) characteristic of modern recharge (< 50 years), while the lack of detectable 3 H (0 TU) in the Pliocene aquifer is consistent with longer residence times.

Exploring the 2013–2018 degassing mechanism from the Pesje and Preloge excavation fields in the Velenje Coal basin, Slovenia: insights from molecular composition and stable isotopes

“…So far, no studies of gas-derived shales from the Velenje Basin were performed, but it would be possible that both gas reservoirs: shales deposited in the Pre -Pliocene basement and the main lignite strata (Figure S1), could be connected along the active Velenje fault and this hypothesis should be tested in the future. Groundwater also has detectable levels of dissolved methane with isotope (δ 13 C CH4 from −77.7 to −51.4 ‰; δ 2 H CH4 from −274 to −162 ‰) consistent with microbial methanogenesis [36,37]. These values are comparable to δ 13 C CH4 values found at excavation fields investigated in this study.…”

An Integrated Approach to Characterising Sulphur Karst Springs: A Case Study of the Žvepovnik Spring in NE Slovenia

“…No significant difference was observed between the seasons (Figure 6), with mean δ 2 H and δ 18 O values of −60.3‰ and −8.85‰, respectively, which fit along the global meteoric water line (GMWL; δ 2 H = 8 × δ 18 O + 10; [60]). No monitoring of isotopes in precipitation is performed regularly in this area, but some data do exist for the nearby Velenje basin for the period 2012-2015 [61]. The isotopic values of the Žvepovnik spring fall into the range of Velenje precipitation and were very similar to the average weighted values of Velenje precipitation (−64.9‰ and −9.14‰ for δ 2 H and δ 18 O, respectively).…”

“…Boreholes made in Pliocene sediments from the Velenje area [63] typically have more negative values that plot slightly higher left above the GMWL and may be due to recharge under different (cooler) climatic conditions (e.g., paleorecharge), recharge of predominantly winter precipitation, or from higher elevations [61]. δ 2 H and δ 18 O groundwater values from Triassic boreholes from Velenje area were slightly more negative than our data indicated [63], which likely reflects the mixing of precipitation and shallow groundwater.…”

Multielemental composition of some Slovenian coals determined with k0 -INAA method and comparison with ICP-MS method

“…Arsenic speciation studies and the different forms of calcite present in the coal suggest that bacterial activity was a significant factor during sedimentation of the basin (Kanduč & Šlejkovec, 2005;Kanduč et al, 2018;Kanduč et al, 2019a). The analysis of other geological matrixes such as coalbed gas (Kanduč & Pezdič, 2005;Kanduč et al, 2012, Sedlar et al, 2014 and groundwater (Kanduč et al, 2014;Kanduč et al, 2019b) reveal more evidence of bacterial activity during sedimentation of the basin.…”