SITE INFORMATION AND CORRESPONDENCE

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f fi 1740 HOUSTON,STOCKTON CLOSURE CHRONOLOGY Page 2 I I� 04/27/99 5 soil borings B1 to B5 were advanced at the site. B4 and B5 were converted to MW4 and MW5,respectively. Analyses included the 5 oxygfenate compounds. MTBE was detected in both soil and groundwater samples. f; 1 11/28/00 3 wells MW-6 to MW-8 were installed. i f 02/05/01 Date of the report of the November 2000 work. 05/09/01 EHD received a report dated April 25,2001 that included a calculation of the remaining dissolved MTBE mass prepared by COS consultant Advanced GeoEnvironmental,Inc. (AGE) EHD did not accept this ckculation because the extent of the plume has not been defined. e i 07/12/01 EHD met with the City of Stockton and discussed that the MTBE groundwater plume continued to migrate down gradient toward the north-"northwest. EHD requested additional investigation to define the vertical extent and lateral extent of 1 the contamination. ; 02/19/02 Three soil borings 138,B9,and B10 were advanced at the site. B9 and B10 were , subsequently converted into deep wells MW9 and MW10,respectively, to investigate the vertical extent of the contamination based on the 24-hour turn 1 around analyses of the grab groundwater samples collected from B8. 10/08/02 . Date of the report of the February 2002 work. 03/12/03 Regional Water Quality Control Board (RWQCB) staff and EHD agreed in a meeting that the MTBE plume has not been defined. I 07/17/03 Two deep Cone Penetrometer Technology(CPT) borings were advanced down gradient to assess the vertical extent of the contamination. i 07/24/03 EHD agreed the plume has been defined. EHD discussed with RWQCB for potential site closure concurrence. RWQCB recommended a`'fate and transport model of the dissolved MTBE plume be submitted. 09/02/03 Date of CIosure Summary Report with soil and groundwater tabulated results,CPT 4 work performed on July 2003,and boring logs. I 10/10/03 Date of closure report that includes a Non-Steady State Spreakdsheet Analytical Model of the MTBE plume prepared by AGE. The model demonstrated a travel time of 4.3 years for a distance of 1,700 feet at a velocity of 0.07 feet per day(sandy silt to silty sand) before.the concentration of MTBE reach 4.96 micrograms per liter. i The report also included an estimate of attenuation of the diss€olved MTBE by dispersion in a down-gradient direction to be less than 1260 days based on historical quarterly monitoring well results of MW 3,MW-4,MW-6,M`Y/-9,and MW-10.