WORK PLANS - Laserfiche WebLink

Home Browse Search

� I J 4} 'Ms. Lori Duncan January 11, 2005 Page 3 of 7 Ground Zero installed and sampled four groundwater monitoring wells in January 2001 and conducted four additional rounds of groundwater monitoring andI sampling between June 2001 j and March 2002 to investigate the lateral extent of groundwater contamination at the site. Based upon the analytical results of soil and groundwater samples collected at the site through March 2002 (Table 2), Ground Zero submitted a formal request for site closure as a low risk groundwater case in correspondence dated June 18, 2002. EHD disagreed with Ground Zero's recommendation and requested additional investigation in a letter dated August 1, 2002. In correspondence dated October 3, 2002, Ground Zero responded to EHD's request for additional investigation. The October 2002 document addressed specific concerns raised in EHD's August 2002 letter and included a contaminant partitioning'evaluation demonstrating that the vertical extent of groundwater contamination had been defined and an analysis of the fate and transportation of dissolved MTBE in support of Ground Zero's recommendation for site closure. r The simple fate and transportation analysis predicted MTBE concentrations in the former onsite domestic well and the closest offsite domestic well of 0.037r micrograms per liter (µg/L) and LO µg/L, respectively, assuming. first-order biodegradation rates, and 1.3 µg/L and 4.9 µg/L, respectively, assuming no biodegradation. However, as stated in the October 2002 document, the model was overly conservative, as it did. not account for': the fact that the supply wells are undoubtedly completed much deeper than the shallow aquifer, which persists to less than 25 feet j bgs. The model also-had no provision for the fact that the nearest offsite domestic well is actually J upgradient from the source area. In a letter dated March 21, 2003, EHD rejected Ground Zero's contaminant partitioning analysis and requested additional sampling and monitoring wellsl�to confirm the vertical extent of groundwater contamination and to further define the lateral extent of contamination in the f shallow aquifer. EHD also requested that the nearest supply wells be sampled for gasoline constituents. Pursuant to EHD's directive, the three nearest supply wells;(the offsite well 125 feet upgradient from the UST, the onsite fire suppression well 220 feet downgradient from the UST, and the onsite facility well approximately 500 feet cross gradient from the UST) were sampled for gasoline constituents on September 26, 2003. No gasoline constituents, including benzene or MTBE, were detected in any of the wells, further evidence that the fate and' transport analysis was overly conservative. I Ground Zero also installed and sampled three additional shallow monitoring wells and supervised the drilling of additional borings in January 2004 to:further investigate the lateral extent of groundwater contamination in the shallow aquifer.!'and to investigate the vertical extent of soil and groundwater contamination in the source area.",Confirmation groundwater samples j were collected from site wells in March 2004. An investigation summary report was submitted on April 27, 2004. The report concluded that the vertical extent of groundwater contamination had been defined and the downgradient extent of contamination in the shallow aquifer had been essentially defined, based upon the results of two groundwater samples collected from the down gradient well, MW5. The samples from MW5 in January and March 2004 contained MTBE at s 6:IGROU NDZENTuff$oy\ReporLs\W orkplansl W P I204.doc t f i