Geochemical models of the impact of acidic groundwater and evaporative sulfate salts on Boulder Creek at Iron Mountain, California

During dry season baseflow conditions approximately 20% of the Bow in Bould er Creek is comprised of acidic metals-bearing groundwater, Significant amo unts of efflorescent salts accumulate around intermittent seeps and surface streams as a result of evaporation of acid rock drainage. Those salts incl ude the Fe-sulfates - rhomboclase ((H3O)Fe3+(SO4)(2). 3H(2)O), ferricopiapi te (Fe-5(3+) (SO4), O(OH). 20H(2)O), and bilinite (Fe2+ Fe-2(3+) (SO4)(4). 22H(2)O); Al-sulfates - alunogen (Al-2)(SO4)(3) . 17H(2)O) and kalinite (KA l(SO4)(2). 11H(2)O); and Ca- and Mg-sulfates - gypsum (CaSO4 . 2H(2)O), and hexahydrite (MgSO4. 6H(2)O), The dissolution of evaporative sulfate salt a ccumulations during the first major storm of the wet season at Iron Mountai n produces a characteristic hydrogeochemical response (so-called "rinse-out ") in surface waters that is subdued in later storms. Geochemical modeling shows that the solutes from relatively minor amounts of dissolved sulfate s alts will maintain the pH of surface streams near 3.0 during a rainstorm. O n a weight basis, Fe-sulfate salts are capable of producing more acidity th an Al- or Mg-sulfate salts. The primary mechanism for the production of aci dity from salts involves the hydrolysis of the dissolved dissolved metals, especially Fe3+. In addition to the lowering of pH values and providing dis solved Fe and Al to surface streams, the soluble salts appear to be a signi ficant source of dissolved Cu, Zn, and other metals during the first signif icant storm of the season. (C) 2001 Elsevier Science Ltd. All rights reserv ed.