2012-11-29_PERMIT FILE - M2012032 (9)

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Although biota appears to be a minor reservoir of zinc relative to soils and sediments, microbial decomposition of biota in water can produce ligands, such as humic acids, that can affect the mobility of zinc in the aquatic environment through zinc precipitation and adsorption (EPA 1979d). In water, zinc occurs in the environment primarily in the +2 oxidation state. It dissolves in acids to form hydrated Zn +2 cations and in strong bases to form zincate anions, which are hydroxo complexes, e.g., (Zn[OH]3) -, (Zn[OH]4)2 -, and (Zn[OH]4[H2O]2)2- (O'Neil et al. 2001). In most waters, zinc exists primarily as the hydrated form of the divalent cation. However, the metal often forms complexes with a variety of organic and inorganic ligands (EPA 1979d, 1984b, 1987c). Zinc can occur in both suspended and dissolved forms in surface water. Dissolved zinc may occur as the free (hydrated) zinc ion or as dissolved complexes and compounds with varying degrees of stability. Suspended (i.e., undissolved) zinc may be dissolved with changes in water conditions (e.g., pH, redox potential, solution speciation) or may sorb on to suspended matter. Gundersen and Steinnes (2003) reported that <10% of zinc was sorbed on particles or colloids in river water from two rivers with average pHs of 3.1 and 5.1 (rivers with mining activity near Roes, Norway), whereas 21% of zinc occurred in sorbed form in six pH neutral rivers. In the aquatic environment, zinc partitions to sediments or suspended solids in surface waters through sorption onto hydrous iron and manganese oxides, clay minerals, and organic material. Reservoirs located downstream from lead -zinc mining and milling areas were found to contain higher concentrations of zinc than reservoirs in other areas, and the zinc was more highly concentrated in reservoir bottom sediments than in the surrounding soils (Pita and Hyne 1975). In addition, the zinc content in sediment closely correlated with the depth, organic content, and clay content of the sediments. Phosphates and iron hydroxides affect the transfer of metals (including zinc) from river water to the sediments, according to a study by Houba et al. (1983). In this study, zinc was bound predominantly to carbonate and amorphous matter (iron, aluminum, and manganese hydroxides). In addition, mobile components of naturally occurring organic matter contributed to the increase in the metal hydroxide -bound fraction. The transport of zinc in the aquatic environment is controlled by anion species. In natural waters, complexing agents, such as humic acid, can bind zinc. The stability of zinc complexes depends on the pH of the water and the nature of the complex. The dissociation of the complex may determine the amount of free zinc ions in solution. Zinc -humic acid complexes may be 50% dissociated at pH 5.5 and the dissociation rate may be higher as the pH decreases (Guy and Chakrabarti 1976). Therefore, as the pH of the water decreases, the concentration of zinc ions in the water phase increases at the same rate as that of the release of zinc from the sediment. The magnesium found in the silicate minerals of igneous rocks is often replaced with the divalent zinc ion; consequently, weathering of this zinc- containing bedrock gives rise to Zn +2 in solution. The hydrated cation is the dominant form when the pH is _S9 (EPA 1979d). The tendency of zinc to be sorbed is affected not only by the nature and concentration of the sorbent but also by pH and salinity (EPA 1979d). Zinc tends to sorb more readily at a high pH (pH >7) than at a low pH (EPA 1979d). Desorption of zinc from sediments occurs as salinity increases (Helz et al. 1975), apparently because of displacement of the adsorbed zinc ions by alkali and alkaline earth cations, which are abundant in brackish and saline waters (EPA 1979d). In column leaching tests with sediment collected from the banks of the Rhone River, the presence of dissolved organic matter and pH was found to be the factors controlling the adsorption and mobility of zinc (Bourg and Darmendrail 1992). Revenue Mine August 2012 T -5