Porewater Fe²⁺, FeTot and sulfide (S²⁻) were measured by spectrophotometry. The 1,10-phenanthroline method was used to determine porewater Fe²⁺ and, after reducing Fe³⁺ by adding hydroxylamine, FeTot (APHA, 2005). Aqueous S⁻² was analysed using the methylene blue method (Cline, 1969). Groundwater total alkalinity was determined by a Gran titration using a titrator (Metrohm 888 Titrando with Tiamo light) with a precision better than 5 µM. Drifts and deviations in the acid concentration were corrected using certified reference materials (CRM batch 175 and CRM batch 178), as described by Dickson (2010).