Sulphur is an essential nutrient for vegetation. It is also a major element of seawater and marine sediments. Its four main oxidation states ranging from +VI to –II make it both an electron acceptor and donor redox reactions. Major forms of sulphur in the subsurface include sulphate and sulphide minerals, dissolved sulphate (SO₄^2–), dissolved sulphide (HS^–) and hydrogen sulphide gas (H₂S). Organic sulphur is a component of organic compounds such as humic substances, kerogen and hydrocarbons. Its oxidation and recycling in soils produces the "terrestrial" sulphates found in semi-arid regions. Atmospheric sulphur sources include natural and "technogenic" or industrial SO₂, particulate sulphur and aerosols of marine sulphate. The movement of sulphur through these various reservoirs in soils and the hydrosphere constitutes the sulphur cycle.

Sulphur compounds from these various sources participate in the geochemical evolution of groundwater. They also contribute to groundwater salinization. Some of the applications of sulphur isotope geochemistry include the cycling of sulphur in agricultural watersheds, the origin of salinity in coastal aquifers or sedimentary strata, groundwater contamination by landfill leachate plumes, acid mine drainage, and dating sulphate-reducing groundwaters.

Sulphur-34 is highly fractionated between sulphur compounds due to biological cycling. Similarly, the ¹⁸O content of sulphate is an important tool to trace the sulphur cycle. A summary of the ranges for ³⁴S in natural materials is shown in Fig. 6-1. Meteorites and magmatic sulphur are close to the standard Cañon Diablo Troilite (CDT). Values exceeding +20‰ are found in association with evaporites and limestones. Negative d³⁴S values are typical of diagenetic environments where reduced sulphur compounds are formed (Krouse, 1980). The most common reaction product is pyrite, which is present in many shales or other organic-rich sedimentary rocks and is formed by bacteria reducing seawater sulphate in marine sediments.

Fig. 6-1 Ranges in d³⁴S contents of sulphur and sulphur compounds in different materials and environments (modified from Krouse 1980).

 

Marine sulphate

The sulphate concentration in groundwater can readily exceed 1 g/L due to the high solubility of gypsum. Gypsum [CaSO₄·2H₂O] and its unhydrated polymorph, anhydrite [CaSO₄], are principal constituents of marine evaporites in sedimentary strata and can be major contributors to groundwater sulphate. Gypsum also accumulates in the soil of arid regions and in sabkha environments. In addition to dissolution of evaporites, marine sulphate in groundwaters comes from mixing with seawater in coastal aquifers and seawater-derived sulphate that accumulates in soils by evaporation. Sulphur isotopes and geochemistry can be used to distinguish between these sources. Other tools include ionic ratios, mineral saturation indices, carbonate isotope geochemistry, and ³⁷Cl to trace chloride salinity. Table 6-1 gives the ionic composition of seawater, along with values for isotopic composition.
 

The solubility of gypsum in pure water is governed by the equilibrium equation:
 

Table 6-1 The major ion geochemistry and isotopes of seawater (from Drever, 1988; Longinelli, 1989; Tan, 1989; Bassett, 1990; Chan, 1992)

Gypsum and anhydrite are not restricted to major evaporite sequences in the geological record. Minor sulphate nodules and laminations on bedding planes can occur throughout limestone and dolomite sequences, and can be overlooked during geological mapping. The dissolution of sulphate minerals increases permeability, and so can enhance concentrations in groundwater.

In pure water, the Ca²⁺/SO₄^2– molar ratio equal to 1 would distinguish gypsum dissolution from other sources of sulphate salinity such as seawater (0.36) (Table 6-2). Calcite precipitation also affects this ratio. Groundwaters typically move towards calcite saturation within the soil or along the flow path. Dissolution of sulphate then forces calcite precipitation according to the common ion effect, resulting in a disproportionate increase in SO₄^2–:

Ca²⁺ + HCO₃^– + CaSO₄·2H₂O ® Ca²⁺ + SO₄^2– + CaCO₃ + H⁺ + 2H₂O

The dissolution of gypsum or anhydrite occurs without measurable isotope effects, and so the isotope contents of SO₄^2– can be used as a tracer for sulphate origin. The sulphate of modern seawater has a very homogeneous and well-defined isotopic composition (Table 6-2):
 

d³⁴S_SO4 = 21‰ CDT

d¹⁸O_SO4 = 9.5‰ VSMOW

 

 

Fig. 6-2 The ³⁴S and ¹⁸O composition of marine sulphate through geologic time (modified from Claypool et al., 1980, with data from Fritz et al., 1988).

 
This composition was not constant in the past. Pronounced variations exist throughout geologic time (Claypool et al., 1980; Fig. 6-2). These variations are found in all major marine evaporite deposits and were most likely controlled by major inputs or removal of sulphide from the oceanic reservoirs during changes in tectonic activity and weathering rates. Simple removal of sulphate (increase in evaporite formation) would not be accompanied by such dramatic isotope effects. The d¹⁸O content of seawater sulphate has been more stable over geologic time, and is controlled largely by the sulphide weathering reactions that contribute sulphate to seawater.

Comparison of the isotope contents of sulphate in groundwater with the appropriate geological period in Fig. 6-2 can distinguish a geogenic source from other sources of sulphate salinity.