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<br /> Mr. Brian Duggan - 2 - 11 March 2014
<br /> downgradient, from about 40 mg/I to 5 mg/I, 1,2,3-TCP decreased from 2 ug/L to 0.05 ug/L, and
<br /> 1,2-DCP decreased from 35 ug/L to less than the detection limit of 0.5 ug/L.
<br /> Ammonium
<br /> While ammonium is not typically expected to be degraded with insitu anaerobic biodegradation,
<br /> it appears to be decreasing in concentration corresponding to the HRC remediation. For
<br /> example, before the 2002 HRC injection, MW-2 contained ammonium predominately greater
<br /> than 100 mg/I, and two years after injection, ammonium was predominately below 100 mg/I.
<br /> Following the 2007 HRC-X injection, MW-6R, which is also about 5 feet downgradient,
<br /> contained more than 500 mg/I ammonium until 2 years following the injection, then decreased to
<br /> about 200 mg/I. Microorganisms capable of oxidizing ammonium to dinitrogen gas in a process
<br /> called anammox may be responsible for the reduction in ammonium in the HRC injection areas.
<br /> However, reductions in ammonium concentrations are also occurring in MW-3, within the former
<br /> tank farm, which is not near the HRC injections. Unlike the HRC injection areas, ammonium
<br /> trends in MW-3 exhibit strong seasonal variability, but are decreasing overall. Ammonium highs
<br /> have declined from about 1,000 mg/I in 1998 to about 50 in 2013. Nitrate (as nitrogen) has also
<br /> declined from about 900 mg/L to about 30 mg/L. Ammonium and nitrate are either being
<br /> degraded insitu or are migrating downgradient.
<br /> Soil samples obtained between 1 and 3 feet below ground surface (bgs) at the former tank farm
<br /> in 1990, 1995, and 1996 contained between 1,800 to 5,800 mg/kg ammonium. Samples
<br /> obtained from 6 feet bgs contained 2,800 to 4,200 mg/kg ammonium, and a sample obtained at
<br /> 14 feet bgs contained 950 mg/kg ammonium. This area is not paved, and potentially represents
<br /> a substantial source of pollutants to groundwater. CPS needs to determine if nitrate and
<br /> ammonium are still present in the old tank farm, and if nitrogen in groundwater has degraded or
<br /> migrated off-site.
<br /> Next Steps
<br /> While CPS has made great progress in reducing concentrations of nitrate, ammonium, 1,2-DCP
<br /> and 1,2,3-TCP, the concentrations of these compounds exceed the water quality objectives at
<br /> the former evaporation pond (MW-2, MW-61R), and ammonium exceeds the water quality
<br /> objectives at the old tank farm (MW-3). Water quality objectives are shown in Table 1.
<br /> Table 1: Water Quality Objectives, CPS Vernalis
<br /> Constituent Water Quality Objective Basis for Objective
<br /> Nitrate (as nitrogen) 10 mg/L Maximum Contaminant Level
<br /> for Drinking Water
<br /> Ammonium 1.5 mg/L Taste and Odor Threshold
<br /> 1,2-Dichloropropane 0.5 ug/L Public Health Goal
<br /> 1,2,3-Trichloropropane 0.0007 ug/L Public Health Goal
<br /> CPS needs to determine whether ammonium and nitrate are still in soil beneath the former tank
<br /> farm, and if ammonium and nitrate have migrated off-site. When the ammonium distribution has
<br /> been determined, CPS needs to develop a remedial action plan to address nitrate, ammonium,
<br /> 1,2-DCP and 1,2,3-TCP in the former evaporation pond area, and ammonium released from the
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