Hydrochemistry and stratification of the Blondis lake: the “invisible fingerprint” of historical iron mining in La Arboleda (Bizkaia)

Abstract

The Blondis mine pit Lake is a small artificial lake of 1.64 hm² surface and maximum depth of between 23-27 m, formed after the exhaustion and abandonment of the orconera V Mine, in the iron mining area of La Arboleda (Bizkaia). Physicochemical monitoring and sampling of lake waters was done in two different campaigns in order to characterize the hydrology, hydrochemistry and influence of mining operations on water quality. This monomictic lake has a marked thermal and chemical stratification in summer and experiences a full turnover of the water mass in autumn-winter. The two deepest metres of the hypolimnion in summer reach complete anoxia, being characterized by a marked increase in total dissolved gas pressure (mostly CO₂), specific conductivity (2200 µS/cm), low redox potential (210-290 mV) and lower pH (6.7 vs. 8.4 in the epilimnion). The waters are calcium-magnesium and bicarbonate-rich and contain small quantities of chlorides and metals, although the summer anoxic layer may reach concentrations of 610 mg/L So₄^2-, 63 mg/L Fe(II) and 4.6 mg/L Mn. Globular-like chemical precipitates of ferrihydrite have been identified by SEM and TEM, which are associated with suspended solids of the anoxic layer. The essential nutrient contents are moderate to low, with values of 2-6 mg/L N, <0.04 mg/L Po₄^3-, 0.7 to 2 mg/L ToC and low phytoplankton biomass, so this oligotrophic lake presents good quality of the bulk water mass. The type of ore exploited -hematite, goethite and siderite, and minor to trace pyriteand the host-rock type (Lower Cretaceous limestones), has favoured a high degree of environmental recovery, in which the mining fingerprint on the lake water quality is only evident in the Fe(II) and sulfate contents detected in the deepest layer during the summer.