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L <br /> 4.7 Natural Attenuation <br /> Natural Attenuation is the reduction in mass or concentration of a chemical in groundwater over <br /> time or distance from the source of contamination due to naturally occurring physical,chemical, <br /> and biological processes (Barden,2002). These processes include dispersion,sorption of <br /> contaminants to soil particles,volatilization,biodegradation of contaminants by naturally occurring <br /> organisms,or abiotic degradation/transformation(Wiedemeier,2002). <br /> L Typically,it is not cost effective to perform remedial actions to the point of removing all <br /> contaminants,and natural attenuation is used to demonstrate any remaining concentrations of <br /> contaminants are decreasing. Source removal and or groundwater treatment are often used to <br /> reduce the majority of the contaminant mass,and then the collection of natural attenuation <br /> indicators is used to demonstrate that remaining contaminant concentrations will decrease over <br /> time. <br /> Three lines of evidence (Wiedemeier et al.,1999) that can be used to support natural attenuation <br /> are: <br /> L documented loss of contaminants in monitoring wells over time, <br /> • contaminant and geochemical analytical data,and <br /> • direct microbiological evidence. <br /> Table 4 shows trends expected in groundwater geochemical parameters when natural attenuation is <br /> occurring,and compares data from source area well MW-1 to background conditions at well MW 4 <br /> collected at the site,and indicates that natural attenuation is occurring. <br /> Table 4 <br /> Natural Attenuation Parameter Comparison <br /> Western Oil and Spreadipl Inc.,Acam o Plant Site,Acam o,California <br /> Natural Attenuation Parameter Expected Trend for Source Well Background Source Area Consistent <br /> Related to Background Well MW-4 Well MW-1 with Trend <br /> LDecember 20M <br /> Dissolved Oxygen(mg/L)1 Decreases NM3 NM NA4 <br /> Dissolved Carbon Dioxide(mg/L) Increases 110 210Yes <br /> Oxidation-Reduction Potential(MV)2 Decreases 23 -144 Yes <br /> March 2009 <br /> Dissolved Oxygen(mg/L) Decreases 3.76 2.80 1 Yes <br /> Dissolved Carbon Dioxide(mg/L) Increases 255 560 Yes <br /> Oxidation-Reduction Potential(mV) Decreases -52 -268 Yes <br /> June 2009 <br /> Dissolved Oxygen(mg/L) Decreases 3.34 1.97 Yes <br /> Dissolved Carbon Dioxide(mg/L) Increases 150 255 Yes <br /> L Oxidation-Reduction Potential(mV) Decreases 127 -19 Yes <br /> January 2010 <br /> Dissolved Oxygen(mg/L) Decreases 4.72 3.90 Yes <br /> L Dissolved Carbon Dioxide(mg/L) Increases 55 130 Yes <br /> Oxidation-Reduction Potential(mV) Decreases 85 -105 Yes <br /> 1. mg/L: milligrams per Liter 3. NM: Not Measured <br /> 2. mV: millivolts 4. NA: Not Applicable <br /> \\Eureka\projects\2007\0071.52-WOS\PUBS\rpt\20100324-lstHa1f2olOGWMR&RFC.doc <br /> 10 <br />