Conditions and processes of precipitation of iron compounds upon discharge of high-mineralized artesian water from artesian borehole R-30, Staro Oryahovo, Bulgaria

The artesian borehole R-30-Staro Oryahovo with total depth of 1740 m gives waters of chlorine-sodium type with a mineralization of 31 g/L from formations of Paleogene age. These waters contain high concentrations of iron and because of this, intense precipitation of iron compounds takes place at and around the wellhead. To clarify the ongoing processes, samples of precipitates and overflow water were taken. It was shown that the predominant form of iron in the water before reaching the surface is Fe2+-85-90% and FeCl+-4-11%. The calculated saturation index for a number of Fe2+-containing minerals showed that hydroxides and sulfates of Fe2+ are the possible candidates for sources of iron in the water. The reductive dissolution of wide spread Fe3+-containing minerals (hematite, goethite) was assumed as a possible alternative process releasing Fe2+ into water. At the surface, due to the contacts with the atmospheric oxygen, the environment is sharply changed to oxidizing one thus forming a geochemical barrier and causing mass precipitation of Fe3+ oxides. Simultaneous Fe2+ oxidation, hydrolysis of Fe3+ and co-precipitation with silica and other components are assumed as the major processes causing the formation of low-crystalline Si-containing ferrihydrite or ferrihydrite-like phase. After formation, the precipitated gels due to the processes of aging and continued interaction with the outflowing water have suffered the further change including the formation of goethite. It was shown that due to the intensive precipitation processes, the iron migration to the surface is limited to a very small area and does not affect adjacent agricultural territories with Fe.

Automated summary

The deep Artesian borehole R-30-Staro Oryahovo provides chlorine-sodium water with high iron concentrations, originating from Paleogene formations. Iron exists mainly as Fe2+ and FeCl+ in the water. The potential sources of iron in the water include hydroxides and sulfates of Fe2+ or reductive dissolution of Fe3+-containing minerals; at the surface, a geochemical barrier is formed due to the oxidation of Fe2+ to Fe3+, causing precipitation of Fe3+ oxides.