Laserfiche WebLink
Whetstone <br />Associates <br />Technical Memorandum <br />A plan for long-term maintenance and bonding will be prepared for the selected mitigation option(s) <br />as part of the feasibility studies, engineering design, and permitting for the mitigation system(s). <br />22) On page 15-6, Operator proposes installation of a permeable reactive barrier (PRB). Please <br />describe in detail the composition and construction of the PRB, the proposed loading rates, the <br />expected geochemical attenuation processes, and the expected water quality that will ultimately be <br />discharged from the PRB at the down gradient terminus. Bonding for installation will also be <br />required. <br />Detailed engineering studies will be performed for the PRB, if this alternative is selected for long- <br />term sustainable treatment of groundwater. <br />23) On page 15-6, Operator presents a chemical reaction for immobilization of uranium in a PRB <br />composed of ZVI. One of the products of the reaction is Fe 2+ resulting from the oxidation of FeO. <br />What is the fate of the ferrous iron; is it mobilized to the aqueous phase and transported <br />downgradient, or is it left behind on the PRB? Provide a prediction of the concentration of any <br />mobilized iron in solution downgradient of the PRB. <br />The Fe 2+ that results from the oxidation of FeO will enter the aqueous phase and will remain in <br />solution as groundwater is transported through the PRB. The aqueous Fe 2+ may precipitate <br />downgradient of the PRB, as siderite (FeC03) under reducing conditions downgradient of the PRB <br />if sufficient carbonate is present in groundwater, and as amorphous Fe[OH]3 or ferrihydrite <br />(Fe[OH]3) under oxidizing conditions at more distal locations from the PRB. <br />The Adventus Group (2009) reported on field trends for dissolved iron downgradient of PRBs and <br />found that: <br />Rarely are the dissolved iron concentrations found to be problematic. For example, <br />Peale et al. (2008) performed a pilot-scale evaluation at a site in Oregon and <br />provided a detailed analysis of dissolved iron trends within and downgradient of a <br />[PRB] treatment zone. Because the pilot was located about 70 ft upgradient of a <br />river, dissolved iron fate was a part of the technology evaluation. The reported iron <br />data showed that dissolved iron concentration decreased gradually with time and <br />along groundwater flow path. ...the observed initial iron concentration increased <br />within the [treatment] zone to about 1, 000 mg/L. Iron concentration decreased to <br />below background levels of 46 mg/L...within 9 months after installation. The trends <br />in a downgradient monitoring well located 20 ft from the edge of the [treatment] zone <br />showed an initial iron concentration of 300 mg/L and a subsequent decrease to below <br />the background value within 6 months. Notably, the long-term dissolved iron levels in <br />the pilot and directly downgradient were consistently below the background level <br />within the 2 year observation period. An equilibrium geochemical modeling of the <br />pilot geochemical data indicated that precipitation of mineral phases, including <br />carbonates and sulfides, were likely controlling the long-term fate of dissolved iron in <br />the vicinity of the installation. No river impacts due to dissolved iron were detected <br />downgradient of the pilot. <br />(Adventus, 2009) <br />4109C.100731 10