Result 3 · Figure 5
PHREEQC-2 computes the saturation index of calcite at each sample. Where it falls below zero, the water is calcite-aggressive — and the wells with elevated nickel sit at the deepest undersaturation.
Saturation index of calcite against pH for 55 raw-water samples across ten production wells. The SI values are computed with PHREEQC-2 using the WATEQ4F thermodynamic database; samples with SI < 0 are undersaturated with respect to calcite, meaning the water tends to dissolve the carbonate cement of the gravel matrix and release the trace nickel bound to it. The nickel-bearing wells BTB 1, BTB 2 and KTB 4 sit at the deepest undersaturation (SI down to −3.6); the seven low-nickel Kaldenkirchen wells (KTB 1–3, 5–8) straddle equilibrium at neutral pH. SI values digitised at ±0.1. *