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Corrective Action Plan Page 24 ' analysis of samples taken from a sampling port at the mid point of the piping connecting the first unit to the second unit When the adsorption capacity of the first unit is exhausted, contaminants break through and are detected in these samples However no contaminants will be discharged to the environment because water flowing out of the first carbon unit is passed through the, until then, unused carbon in the second unit ' When contaminant breakthrough is observed at the sampling port at the mid-point between the pair of GAC units, arrangements are made to remove the exhausted carbon from the upstream unit and replace it with fresh material Both units have the same adsorptive capacity and are sized such that, when breakthrough occurs at the mid-point, the downstream unit captures all detectable contaminants that pass though the system and has sufficient reserve capacity to provide ample time for a new charge of carbon to be procured and placed in the upstream unit The plumbing for the GAC units includes a system of pipes and valves that permit any one of the pair to be upstream of the other After contaminant breakthrough occurs at the mid- point and the exhausted carbon in the upstream unit has been replaced, the valve settings are changed so that the unit that was previously on the downstream side of the mid-point is on the upstream side and the one previously downstream is upstream This arrangement permits the GAC vessel having the greatest reserve of adsorptive capacity to operate, at all times, as the primary unit and provides maximum redundancy against unexpected failure of any upstream process The GAC units are sized such that, when the groundwater treatment system is operating at its design capacity of 30 gpm, breakthrough past any one freshly charged unit does not occur for approximately 90 days when the treatment system is operating normally If all upstream processes were to entirely fail at a time when contaminant concentrations in the influent to the treatment system were at the anticipated maximum and, as is extremely ' unlikely, the upset condition was not detected and the system automatically shut down, breakthrough of a single, freshly charged unit would not occur for 15 days With two GAC units installed in series, even under such extreme failure conditions, contaminants ' would not breakthrough to the discharge to the storm-water sewer before 30 days had elapsed This provides a very large margin of safety given the 2 working days notice required to arrange for recharging of the units with fresh carbon Confirmation that no impermissible concentrations of contaminants are present in the treatment-system discharge where it enters the line to the storm-water sewer is obtained by analysis of samples taken from a sampling port located immediately downstream of the GAC units ' Exhausted carbon from the treatment units is removed from the vessels at the time they are recharged and held on site in United States Department of Transportation (DOT) ' approved containers until the material has been profiled When profiling is complete, the spent-carbon containers are transported to the carbon vendor's facility under control of the requisite DOT waste transport manifest On arrival at the vendor's facility the adsorbed