A Mass Balance Approach to Evaluate Release of Mining-Derived Metalsfrom Lakebed Sediments due to Hydroelectric Development Operations

摘要

Avista Corporation operates the Post Falls HydroelectricDevelopment (HED) near the outlet of Lake Coeur d'Alene, Idaho, USA.Historic mining activities in the Coeur d'Alene River drainage basin haveresulted in metal loading and accumulation in lakebed sediments. The mainmining district, often referred to as the "Silver Valley", has been in operationsince 1880 and was one of the major commercial sources of silver, lead andzinc in the United States. Lake Coeur d'Alene is a thermally stratified natural lake that is prone to influxof metals from upstream mining areas and low dissolved oxygen. The PostFalls HED imposes temporal and spatial changes on lake levels, which werethought to potentially affect the lake's water quality dynamics. The FederalEnergy Regulatory Commission (FERC) and Idaho Department ofEnvironmental Quality (IDEQ) required an evaluation of the possible effectsof the Post Falls HED on aqueous metal concentrations in Lake Coeurd'Alene. This paper presents a mass-balance approach to evaluate the effectsof these changes on metals mobility. The study approach focused on identification of the chemical reactionsresponsible for metals transfer between the lake sediments and the overlyingwater column and the geochemical conditions controlling these reactions.Historical data were reviewed to characterize sediment metal concentrationsand the phases in which metals are present. The CE-QUAL-W2 water qualitymodel was used to characterize the distribution of temperature, dissolvedoxygen, and pH with and without HED operations. Geochemical modelingwas then used to simulate the effects of HED-related changes in lake pH,dissolved oxygen, and redox on the stability of metals present in the lakebedsediments in association with iron oxides or as sulfides. Differences in metalflux between lakebed sediments and the water column relative to naturalconditions (i.e., in the absence of HED operations) were quantified.