Geochemistry of a large impoundment – Part II: Fe and Mn cycling and metal transport

Abstract

Depletion of oxygen in lakes and reservoirs due to summer stratification has significant effects on many aspects of water chemistry, including lake productivity, elemental cycling and water quality. We have studied the redox conditions of Lake Texoma, a large impoundment lake on the border of Texas and Oklahoma, USA, formed from the confluence of the Red and Washita rivers, in order to understand the impact of summer anoxia on metal distribution. In summer, dissolved oxygen decreases with depth (from c. 7.0 to 0.1 mg/L), whereas dissolved ( 2+ ), Mn and HS − concentrations show complementary increases in the hypolimnion. Summer anoxia is, to a large degree, responsible for vertical variations in Fe and Mn concentrations, and Fe speciation is controlled by Fe-oxyhydroxide reduction and subsequent pyrite precipitation in sulphide-rich bottom waters. Summer anoxia is also responsible for vertical variations in the concentration of other metals including Ba, Pb and Ni in the deepest portion (main lake) of the lake. Lake Texoma and its two river arms show relatively minor variation in δ 34 S CDT (from +11.5 to +13.4‰), mirroring variation in Permian/Cretaceous marine gypsum and anhydrite deposits (δ 34 S CDT +10 to +15‰) in the headwater regions of the catchment. Increasing δ 34 S CDT with depth (>16 m) in the main lake is consistent with fractionation associated with sulphate reduction in anoxic bottom waters. δ 13 C PDB values of dissolved inorganic carbon (DIC) become more negative (from –2.5 to –8.2 ‰) with depth in summer, due to bacterial oxidation of organic matter linked to sulphate reduction. Summer anoxia may induce temporal degradation of water quality in the central three zones of the lake with elevated Fe and Mn concentrations owing to breakdown of oxyhydroxides and release of adsorbed heavy metals, such as Pb and Ni. However, complete turnover of the water column in the autumn lowers dissolved Fe, Mn and heavy metal concentrations by oxidation and formation of oxyhydroxides . The characterization of anoxia in Lake Texoma provides the background for further water quality research and management for the rapidly increasing population of North Texas and improves our understanding of redox cycling and metal mobility in reservoirs.