Use of H and O stable isotopes in recognition of preferable groundwater flow paths in bedrock fracture zones

Introduction

The stable isotopes of oxygen (¹⁸O/¹⁶O) and hydrogen (²H/¹H) were studied in the Talvivaara mining area in Northeastern Finland to find hydraulic connections between surface water and groundwater and to investigate the flow of the groundwater in bedrock fracture zones. The study was based on the different isotopic characteristics of waters from different sources and on isotopic similarities, which arise from mass discrimination effects between heavier and lighter elements and kinetic fractionation between two phases of the same substance or two substances. It is important to know the locations and hydraulic connections of water conducting fracture zones, particularly in mining areas, because they can transport adverse substances outside the mining area (Kittilä 2015). In order to locate the water conducting main fracture zones and to gain more detailed understanding of the geochemical composition of water flowing in these fracture zones, geophysical methods and geochemical analyzes were used and interpreted (Eskelinen et al. 2013, Forss et al. 2013, Pasanen et al. 2014).

The study area in the mining district is located approximately between the gypsum ponds to the north and the Kortelampi dam to the south, and the primary heaps in the east (Figure 1) (Kittilä 2015, Pasanen et al. 2014). The urge for studying the hydraulic connections between surface water and groundwater and the hydraulic conductivity of the bedrock fracture zones was prominent after a leakage in the gypsum ponds in November 2012. The leakage released about 1.2 Mm³ of acidic, metal-containing waste water, from which a majority was dammed in the study area. The isotope composition of 39 groundwater and surface water samples was analyzed with cavity ring-down spectroscopy (CRDS) method, and the results helped to determine the possibility of transportation of adverse substances outside the mining area through bedrock fracture zones (Kittilä 2015).

Methods

The methods used in the study can be divided into preliminary geophysical methods and investigative geochemical methods. With the geophysical methods (gravimetric, magnetic and refraction seismic surveys, and electrical resistivity tomography (ERT) and ground penetrating radar (GPR) surveying) the main fracture zones in the bedrock were located and their course, continuation and water content roughly evaluated, and the thickness of the overburden was estimated. The results of the geophysical methods were interpreted in the reports of the Geological Survey of Finland (GTK) (Eskelinen et al. 2013, Forss et al. 2013, Pasanen et al. 2014) and were then used as a preliminary material in the master’s thesis of Kittilä (2015).

Based on the geophysical interpretations of the locations of the main fracture zones 8 new bedrock wells and 5 new shallow groundwater observation wells in sediment were installed. The geochemical composition of groundwater was then sampled from these new wells and other suitable wells already existing in the area. Also surface water was sampled. In total there were 29 sampling sites (Figure 1), from which 15 were bedrock wells, 7 shallow wells, 6 surface water sites, and one spring (dug well) (Eskelinen et al. 2013, Kittilä 2015). The geochemical composition of the waters was studied with following parameters: dissolved and total concentration of elements, ferrous iron (Fe²⁺), total and dissolved organic carbon (TOC and DOC, respectively), anions (SO4ˉ, Clˉ, Fˉ, Brˉ and NO3ˉ), solids content, alkalinity and phosphate-phosphorus (PO₄-P) (Pasanen et al. 2014). The geochemical studies also included isotope analyzes of the stable isotopes of oxygen (¹⁸O/¹⁶O) and hydrogen (²H/¹H) that were interpreted in detail by Kittilä (2015). In total there were 39 isotope samples, because the bedrock wells were sampled from various depths in order to distinguish groundwater with possibly different composition originating from different fractures.

Figure 1. Sample sites in the study site at the Talvivaara mine area. Fracture zones that were interpreted from lineaments are marked with grey lines. The distance from the study site to the two sample sites near lake Kolmisoppi (smaller map in he top right corner) is approximately 7.5 km to the northeast, and the distance between those two sample sites is about 2.7 km. From Kittilä (2015).

Results and conclusions

In the study made in the Talvivaara mining area stable isotopes of oxygen and hydrogen did not indicate surface water contamination or mixing with bedrock groundwater. The sampled surface waters that could have possibly been a source for adverse substances due to the leakage in the gypsum ponds showed clear signs of evaporation, whereas the bedrock groundwaters did not show any major signs of surface waters (evaporation) as they were plotted along the local meteoric water line (LMWL) (Figure 2). This was not expected because earlier geochemical data had indicated contamination with process chemicals in some of the bedrock wells, and it was interpreted that groundwater contamination with smaller amounts of water is possible, just not with so high amounts that it could had changed the isotope compositions of the bedrock groundwater significantly. In addition to this, similarities between groundwater from different depths of the same well and also between different wells along same fracture zones were found, indicating hydraulic connections between some of the bedrock wells and fracture zones in the study area. Based on the isotopical similarities and geochemical composition of elements that were linked with process chemicals it was concluded that a NE-SW oriented fracture zone passing through the area has the most important role in collecting and transporting groundwater outside the mining area. This fracture zone possibly collects the groundwater from intersecting fracture zones within the study area, so further studies of hydraulic connections of this NE-SW fracture zone, for example surface water-groundwater interactions, outside the study area were recommended. (Kittilä 2015)

Figure 2. Isotopic composition of the sampled waters. Global meteoric water line (GMWL) is from Craig (1961) and the local meteoric water line (LMWL) is from Kortelainen (2007). Evaporation line is drawn for the surface water samples. From Kittilä (2015).

References

Craig, H. 1961. Isotopic variations in meteoric water. Science 133, 1702-1703.

Eskelinen, A., Forsman, P., Hendriksson, N., Pasanen, A. & Kittilä, A. 2013. Vesinäytteenotto Talvivaaran kaivosalueella (In Finnish). Geological Survey of Finland, Report of investigation, 8 p.

Forss, H., Lerssi, J., Huotari-Halkosaari, T., Pasanen, A., Eskelinen, A. & Kittilä, A. 2013. Talvivaara, Geofysikaaliset tutkimukset 2013 (In Finnish). Geological Survey of Finland, Report of investigation, 49 p.

Kittilä, A. 2015. Groundwater flow paths in the bedrock fracture zones revealed by using the stable isotopes of oxygen and hydrogen in the Talvivaara mine gypsum pond area, Northeastern Finland. Master’s thesis, University of Helsinki, Department of Geosciences and Geography, Division of Geology, 60 p.

Kortelainen, N. 2007. Isotopic fingerprints in surficial waters: Stable isotope methods applied in hydrogeological studies. Geological Survey of Finland, Espoo, 39 p.

Pasanen, A., Eskelinen, A., Räisänen, M.-L., Lerssi, J. & Kittilä, A. 2014. Talvivaaran kipsisakka-altaan vuodon pohjavesivaikutusten selvitys ja leviämisen ja haitan arviointi. Geological Survey of Finland, tutkimusraportti, 50 p.