Sulphur springs of the Craven Basin, NW England: indicators of natural methane leakage?

Sulphidic springs are a common feature of the Craven Basin and have long histories, indicating that they are natural phenomena, and not a result of recent anthropogenic contamination of groundwater. This study identified several extant sulphur springs and these were sampled and analysed for hydrochemistry and isotopic composition (δS of sulphate and sulphide, δO of sulphate and δC of total dissolved inorganic carbon (TDIC)). The springs are associated with limestone units and are located in anticlinal crests that are sometimes faulted. Sulphide concentrations at these 'sulphur springs' range from <0.5 to 96 mg l and light sulphide isotopic compositions indicate that sulphide originates by microbial sulphate reduction. One site, 'Stinky Bottoms', with high sulphide concentration (46 mg l) also has very low δC-TDIC (-25.3‰ V-PDB) characteristic of a significant component of TDIC generation via methane oxidation. Other sites have δC-TDIC more typical of shallow groundwaters (-14.2 to -16.3‰ V-PDB) and two sites with highest Cl concentration and elevated Sr:Ca have heavier δCTDIC (-11.3 and -6.8‰ V-PDB), indicative of a more evolved, long residence-time brine component. At Stinky Bottoms a strong case can be made that generation of sulphidic groundwater is related to subsurface methane. At other sites, sulphide generation may also be related to hydrocarbon or methane seeps (in some cases associated with a brine component) but the δC-TDIC values are more equivocal.