Saturation and equilibration of the borehole

Immediately after installation of the downhole equipment, the three intervals were injected with APW. Its chemical composition was derived from the location of the BN experiment in the clay stratigraphy and the relationship between sulfate, cations and the chloride content (Pearson 1999; Pearson et al. 2003), though without the organic matter (Table 1). This APW was prepared, stored and injected in the intervals anaerobically (Ar atmosphere with [O₂] <5 × 10⁻⁴ vol%) to avoid oxidation of the clay surrounding the borehole. It was injected into the intervals at a water pressure of 300–500 kPa (absolute). Note that no special precautions were taken to avoid contamination with exogenous microorganisms during preparation of the APW.

To achieve full saturation and hydraulic equilibrium of the interval with the surrounding clay, APW was reinjected when the water pressure in the interval(s) decreased below atmospheric pressure and until the water pressure in the intervals remained stable. After the initial injection of Interval 1 with APW, the water pressure rapidly decreased and stabilized at ~250 kPa (absolute). In Interval 2, three consecutive injections were needed until the water pressure stabilized to a pressure of ~130 kPa (absolute). Note that circulation of the solution during the injection tests resulted in a slightly higher water pressure in both intervals.

After static water diffusion into and equilibration with the surrounding clay at a stable water pressure for ~7 months, the APW in each interval was homogenized by circulation at a flow rate of ~5 mL min⁻¹ for 4 weeks. Next, each interval solution was sampled and its chemical composition was determined.