Laserfiche WebLink
+f ATC Associates Inc <br /> 1117 Lone Palm Avenge,Suite B <br /> Modesto, California 95351 <br /> 209-579-2221 <br /> A s s o c I a T e s i H c Fax 209-579-2225 <br /> The histograms in Attachment 3 show the contaminant levels for benzene over time Other BTEX <br /> constituents were not analyzed due to being below MCLs Since MWl and MW2 are both below <br /> detection limits for the past three sampling periods, they will not be analyzed for attenuation rates <br /> MW3 was analyzed using the above equation, with C, equal to 2 1(the contaminant level in June <br /> 2004) and C(t) equal to 14 (the contaminant level in March 2005) Using a time difference of <br /> 0 75 years, the equation was solved for k Next, the value determined for k was used to calculate <br /> the amount of time "t" (in years) using a value of 1 for C(t) (the MCL for benzne, in ppb) The <br /> calculated attenuation rate for MM3 is 1 3 years Calculations are shown in Attachment 3 <br /> The histograms in Attachment 4 show the contaminant levels for TPHg over time MWl, MW2, <br /> and MW3 were analyzed using the above equation for degradation rates For MW1, C,was set to <br /> equal 11,000 and C(t) equal to 470 Using the calculated value for k (0 9695) and inserting 100 <br /> for C(t) (the taste and odor threshold for TPHg, in ppb), a time "t" was calculated to be <br /> approximately 4 8 years MW2 and MW3 were calculated in the same way For MW2, the time <br /> was calculated to be 5 8 years, and MW3 was calculated to be 7 2 years Calculations are shown <br /> in Attachment 4 <br /> Mass Balance Calculations <br /> Using current contaminant levels, an isoconcentration map was drawn for benzene(Attachment 5) <br /> and TPHg (Attachment 6) Based on these maps, remaining contaminant mass was calculated <br /> based on plume area and a source zone thickness of 10 feet For benzene, a remaining mass of 16 <br /> kg or 3 5 lbs was calculated (see Attachment 5 for calculations) For TPHg, a remaining mass of <br /> 1,604 kg or 3,535 IN was calculated (see Attachment 6 for calculations) Monitoring well and soil <br /> boring logs are contained in Attachment 7 <br /> CONCLUSIONS <br /> The groundwater flow direction calculated for June 1, 2005, is not consistent with previous <br /> sampling events, as the predominant historical direction of groundwater flow is to the northwest <br /> Water table elevations have slightly decreased beneath the site since March 2005 Benzene has <br /> not been detected in samples collected from MW 1 and MW2 for the last three consecutive <br /> sampling events TPHg concentrations in the groundwater samples collected from MW2 and <br /> MW3 decreased with respect to the previous sampling event, and increased in the sample <br /> collected from MW3 MTBE has never been detected in groundwater samples collected from <br /> MW 1, MW2, and MW3 <br /> RECOMMENDATIONS <br /> Based on recent lack of benzene concentrations at or above the laboratory reported detection <br /> limits in groundwater samples collected from NMI and MW2, Iow benzene concentrations in <br /> groundwater samples collected from MW3, no future use planned for the area around the site, and <br /> the attenuation of groundwater contaminants to levels at or below MCLs as listed above, we <br /> recommend the site be considered for closure <br />