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10/8/2020 Federal Register::National Oil and Hazardous Substance Pollution Contingency Plan-Partial Deletion of the California Gulch Superfund... <br /> indicated that the Yak Tunnel discharge had a significant detrimental impact on the aquatic environment at <br /> the site.In addition,the Yak Tunnel discharge presented a potential public health risk based on exposure to <br /> affected surface and groundwater at the California Gulch Site. <br /> The EPA released the Yak Tunnel Feasibility Study(FS)in June 1987 and a proposed plan for the Yak Tunnel <br /> in August 1987. <br /> 0U1 Selected Remedy <br /> The EPA issued the Record of Decision(ROD)for OUi(1988 OU1 ROD)on March 29,1988.The remedy <br /> chosen in the 1988 OU1 ROD was modified in an Amended ROD(AROD)signed on March 23,1989(1989 <br /> OUi AROD)and,further,modified in an ESD signed on October 22,1991(1991 OU1 ESD)and an ESD <br /> signed on July 29,2013(2013 OU1 ESD). <br /> The selected remedy in the 1988 OU1 ROD was narrowly focused on the discharges from the Yak Tunnel as a <br /> major source of contamination to California Gulch and the Arkansas River.Broader issues of water quality <br /> generally in California Gulch and the Arkansas River were addressed as part of remedial actions taken at <br /> other operable units.Thus,the 1988 OU1 ROD identified a single remedial action objective(RAO)of <br /> decreasing the release and threatened release of hazardous substances,pollutants and contaminants from <br /> the Yak Tunnel into California Gulch. <br /> The original remedy selected in the 1988 OU1 ROD consisted of the following remedial components:(1) <br /> Construction of surge ponds to capture drainage from the tunnel and to minimize the impact of surges on <br /> California Gulch and the Arkansas River;(2)Installation of an interim water treatment system to treat water <br /> from the Yak Tunnel before discharge in California Gulch;(3)Sealing of shafts,drill holes and fractured rock <br /> and diversion of surface water from tunnel recharge areas to reduce the amount of water entering the Yak <br /> Tunnel system;(4)Grouting of fractured rock,caved-in areas and drill holes to prevent seepage of <br /> contaminated water to the land surface;(5)Installation of a pumping system to control water levels behind <br /> the portal plug.The pumped water would be routed to the interim treatment system;(6)Construction of a <br /> minimum of three concrete plugs in the Yak Tunnel to seal off the major flow route for groundwater <br /> movement;(7)Establishment of a surface and groundwater monitoring system to detect leakage,seeps or <br /> migration of contaminated groundwater,which may result from installation of the tunnel plugs;(8) <br /> Development and implementation,as necessary,of a contingency plan to address any adverse effects on <br /> surface or groundwater from tunnel plugging;and(9)Operations and maintenance of the remedy. <br /> The 1989 OU1 AROD made the following changes in the remedy:(1)Installation of a permanent water <br /> treatment system to treat contaminated groundwater from the Yak Tunnel before discharge in California <br /> Gulch,as opposed to the interim treatment facility originally proposed;(2)Construction of a surge pond or <br /> ponds to capture drainage from the tunnel and to minimize surges on California Gulch.The original remedy <br /> called for multiple surge ponds and did not consider the use of a single pond;(3)Reduction of seepage and <br /> recharge was made optional.Grouting of fractured rock,cave-ins and drill holes was removed as part of the <br /> remedy;and(4)The portal plug was modified to be a flow-through plug as opposed to a solid plug. <br /> The 1991 OU1 ESD made the following changes in the remedy:(1)Construction of a single surge pond as a <br /> permanent part of the remedy;(2)Construction of a flow-control bulkhead within the tunnel to prevent <br /> surges.Two of three originally planned plugs were removed from the remedy;(3)Identification of <br /> groundwater flow direction and potential gradient reversal as additional element of the monitoring plan.The <br /> monitoring system was proposed to include a minimum of seven groundwater monitoring wells as opposed <br /> to a minimum of 23 wells proposed in the 1989 AROD;(4)Placement of six or more weirs,or other flow <br /> https://www.federairegister.gov/documents/2 01 6/02/09/2 01 6-02 60 1/national-oil-and-hazardous-substance-pollution-contingency-plan-partial-deletion-... 8/16 <br />