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Page 3 January 30, 1992 Job No 400511-SA ' The monitoring wells which were originally screened below the water table began going dry in July of 1989 as the water table lowered due to drought conditions The bottom of the screen interval in these wells extends to depths of 50 to 70 feet below ground surface By January of 1990, all of the monitoring ' wells had gone dry due to the declining water table This lowering has occurred due to below normal precipitation and continued regional pumping of groundwater ' Free product was found in three on-site wells lust before they went dry Monitoring well MW-5 had an unspecific quantity of free product and MW-15 had 4 8 inches of free product on January 9, 1988 ' Recovery well RW-25 was reported to have 2 inches of free product in July and October 1989 A vapor extraction system was installed at the site to help remediate the hydrocarbons in the vadose ' zone which became larger as a consequence of the declining groundwater table This remediation system has been in operation since August of 1989 The soil remediation system consists of ten vapor ' wells, a vacuum extraction blower, and a thermal incinerator The system was modified on August 14, 1991 to use RW-25 as the sole extraction well with the other wells in the system {MW-7, MW-8, MW-9, MW-10, MW-11, MW-16, MW-18, RW-24, and RW-26} serving as air induction wells In a letter dated January 2, 1991, San Joaquin County Public Health Services requested that Conoco ' submit a proposal for groundwater monitoring DERS submitted a work plan entitled "Groundwater Monitoring Well and Vapor Extraction Well Installation," dated July 12, 1991 to Conoco in response to this request The work plan proposed four groundwater monitoring wells to be constructed as part ' of the groundwater monitoring program, and in addition,nineteen vapor extraction wells were proposed as part of the soil vapor extraction system An addendum to the work plan, dated September 4, 1991 ' submitted at Conoco's request, proposed the installation of three air sparging wells Conoco later decided not to perform the work proposed in the addendum On September 23, 1991, DERS began implementation of the original work plan The results of that investigation are the subject of this ' report ' 3.0 REGIONAL GEOLOGY AND HYDROGEOLOGY ' The site is located near the center of the Great Valley geomorphic province of California, also known as the Central Valley The surface of this lowland area is composed of unconsolidated Quaternary sediments primarily represented by alluvial, flood, and delta plains of the Sacramento and San Joaquin Rivers and their tributaries Where streams have cut channels into these sediments, lake beds are occasionally exposed that include clays, diatomites, and other rocks The site is underlain by Cenozoic nonmanne sedimentary rocks and alluvial deposits 400511, Rev n Du Pont Environmental Remediation Services Printed on Recycled Paper