Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch
Sulphur is an abundant element in nature and it participates in many chemical reactions because it can possess an oxidation state from +6 to -2. The main sulphur compounds are sulphate and sulphide minerals, dissolved sulphate (SO42-), dissolved sulphide (HS–), and hydrogen sulphide gas (H2S). There is also organic sulphur which is a component in organic compounds, for example in humic substances, kerogen and hydrocarbons (Clark & Fritz 1997). Sulphur has four stable isotopes: 32S, 33S, 34S and 36S. However, the 34S/32S ratio is most often used in hydrogeological studies, because 32S and 34S are the most abundant isotopes (SAHRA 2015). The sulphur isotope compositions are reported in δ34S values, which is a ratio of 34S/32S in per mil (‰) relative to the standard VCDT (Vienna Canyon Diablo Troilite).
Sulphur has many sources, each of which has a specific type of isotopic fingerprint. When sulphur compounds from various sources participate in the geochemical evolution of groundwater, the isotopic compositions and variations in both groundwater and possible sources can be used to interpret and trace the flow path and source of the groundwater or salinity (e.g. Sacks & Tihansky 1996, Clark & Fritz 1997, Halas et al. 1998, Otero & Soler 2002, SAHRA 2015).
Description of the method
The isotopic composition of sulphur in groundwater is measured from sulphate (SO42-) or sulphide (H2S or HS–) (Clark & Fritz 1997). Sulphate, however, is a very common pollutant in groundwaters. Standard chemical methods say little about the origin of sulphates, but the sources can be elucidated with δ34S and δ18O measurements. Combining the isotopic compositions with SO42- concentration and other geochemical data can give an indication of sources and processes in groundwater systems (Halas et al. 1998). For example, 34S is highly fractionated between different sulphur compounds due to the biological cycling, and similarly, the 18O content of sulphate is an important tool to trace the sulphur cycle (Clark and Fritz 1997).
The oceans are huge sink for sulphur, and the δ34S (sulphate) of modern seawater is approximately 21‰ (and δ18O is about 9.5‰). The range of the δ34S values for the oceans has, however, changed over the geological eras (Clark & Fritz 1997, SAHRA 2015). Gypsum [CaSO4•2H2O] and its unhydrated polymorph anhydrite [CaSO4] are principal constituents of marine evaporites and can be significant contributors to groundwater sulphate by dissolution (Clark & Fritz 1997). The isotope fractionation of sulphur and oxygen is small in dissolution and precipitation of sulphate minerals (Clark & Fritz 1997, Krouse & Mayer 2000), although slight enrichment of the evaporitic gypsum or anhydrite might occur compared to the seawater from which it precipitated (Strauss 1997).
Precipitation of evaporates from seawater is one of the two main processes in the geochemical sulphur cycle; the other process is biologically mediated reduction of seawater sulphate and the subsequent formation of sedimentary pyrite [FeS2]. Pyrite is formed via bacterial sulphate reduction, and it is usually strongly depleted in 34S because the sulphate reducers preferentially utilize the lighter 32S isotope. This leads to more or less negative δ34S values of sulphides (Strauss 1997), whereas the remaining solution enriches in the heavier 34S isotope (Halas et al. 1998). Most clastic sediments (mudstones, shales, siltstones) contain pyrite (Strauss 1997), and as a consequence, the oxidation of pyrite and other sulphide minerals can produce significant amounts of sulphate, metals and acidity to surface waters and groundwaters, as happens, for example, in acid mine drainage (Clark & Fritz 1997, Krouse & Mayer 2000). The oxidation of reduced sulphur, either in the form of sulphide mineral or organic sulphur, can increase the amount of the lighter 32S isotope in the solution (Sacks & Tihansky 1996). In comparison to the dissolution of marine evaporites (marine sulphate), the sulphate formed by oxidation of sulphide can be considered terrestrial (Clark & Fritz 1997).
Another important source of sulphur in groundwater is atmospheric sulphur, although it comprises only minor contribution compared with the addition of sulphate in the subsurface. Hence, the atmospheric sulphur affects particularly the δ34S composition of shallow groundwaters (Clark & Fritz 1997). Atmospheric sulphate’s isotopic composition is mainly controlled by: i) emissions from fossil fuel combustion, ii) sulphate from sea spray in coastal areas, and iii) volcanic exhalation. In general, the δ34S composition of atmospheric sulphur is between -3‰ and +9‰, and the δ18O composition between +7‰ and +17‰ (Clark & Fritz 1997, Krouse & Mayer 2000). Anthropogenic sulphur in the atmosphere can comprise more than two-thirds of the total atmospheric sulphur, and particularly in northern industrial areas its influence can be greater (e.g. Clark & Fritz 1997, SAHRA 2015). In addition to these three sources of sulphur in the groundwater (marine, terrestrial and atmospheric), some local, anthropogenic sources could exist, for example, fertilizers or mining effluents (e.g. Halas et al. 1998, Otero & Soler 2002). Nevertheless, the distinct δ34S and δ18O compositions of sulphates from different sources can be recognized and the groundwater evolution and its flow path studied (Clark & Fritz 1997).
Sulphur and especially sulphate isotopic compositions can be useful in determining different sources of sulphur, except where redox reactions have altered original sulphate isotopic ratios. The oxygen isotopic composition of sulphates is often affected by soil zone processes, and redox cycling of sulphur in soils can also change the isotopic composition of the original compound. The isotopic compositions of both oxygen and sulphur in sulphates must therefore be interpreted carefully. In general, however, the method is robust and well established (Robinson & Bottrell 1997). The main applications of measuring the isotopic composition of dissolved sulphate in the study of the chemistry and evolution of hydrologic systems and groundwater pollution are i) determining the source of sulphur, and ii) understanding chemical reactions occurring in groundwater systems (Vo Thi Tuong Hanh 2013). Additionally, unlike conventional chemistry, the δ34S signature of waters is not sensitive to high dilution of original brines. Sulphur isotopic signature can also be used to trace an anthropogenic source even when it is highly diluted and cannot be detected by basic chemistry (Otero & Soler 2002).
A case study of the use of sulphur isotopes in assessing impacts of mining on natural waters is presented in the following Closedure web page:
Clark, I.D. & Fritz, P. 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers, Boca Raton, 328 p.
Krouse, H.R. & Mayer, B. 2000. Sulphur and oxygen isotopes in sulphate. In: Cook, P.G. & Herczeg, A.L. (Eds.) Environmental Tracers in Subsurface Hydrology. Kluwer Academic Publishers, Norwell, 529 p.
Halas, S., Trembaczowski, A. & Soltyk, W. 1998. The influence of brown coal exploration in Poland on the groundwater pollution as determined by isotopic analyses of sulphate. In: IAEA. Application of isotope techniques to investigate groundwater pollution. IAEA-TECDOC-1046, 81-93.
Otero, N. & Soler, A. 2002. Sulphur isotopes as tracers of the influence of potash mining in groundwater salinisation in the Llobregat Basin (NE Spain). Water Research 36, 3989-4000.
Robinson, B.W. & Bottrell, S.H. 1997. Discrimination of sulphur sources in pristine and polluted New Zealand river catchments using stable isotopes. Applied Geochemistry 12, 305-319.
Sacks, L.A. & Tihansky, A.B. 1996. Geochemical and isotopic composition of ground water, with emphasis in sources of sulfate, in the Upper Floridan aquifer and intermediate aquifer system in Southwest Florida. U.S. Geological Survey, Water-Resources Investigations Report 96-4146, 54 p.
SAHRA. 2015. Sulfur. Site visited 24.4.2015. http://web.sahra.arizona.edu/programs/isotopes/sulfur.html#9
Strauss, H. 1997. The isotopic composition of sedimentary sulphur through time. Palaeogeography, Palaeoclimatology, Palaeoecology 132, 97-118.
Vo Thi Tuong Hanh. 2013. Using isotope techniques 34S/32S (δ34S) to determine the origin of dissolved sulphate in Thuan My mineral water. In: IAEA. Isotopes in Hydrology, Marine Ecosystems and Climate Change Studies, Volume 2. Proceedings of the International Symposium held in Monaco, 27 March-1 April 2001, Proceedings series, 636 p.