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' Pae 4 Report- Pilot Remedial System Test g Former Manna Pro Site Project No 207 3 July 31,2000 wcoeval with six continuous years of greater-than-average rainfall in the northern San Joaquin Valley The highest concentrations of gasoline are known to lie from about 35 to ' 45 bgs or about 10 to 25 below the surface of groundwater Due to the fact that the bulk of the contaminant plume lies 10 to 25 feet below the groundwater surface, we postulated that remedial measures based principally on vapor extraction would not be sufficient to materially reduce the contaminant mass below groundwater in a reasonable period of time Air sparging to introduce oxygen into the saturated subsurface and mobilize contaminants to the groundwater surface was posited as an attractive supplement to enhance the effectiveness of vapor extraction It was anticipated that infected air would encourage the growth of aerobic bacteria and would also mobilize light hydrocarbons and cause them to rise to the groundwater surface As these mobilized hydrocarbons moved ' through, and up, the water column, down-gradient vapor extraction wells could be used to extract them One drawback of this remedial method is that the vapor extraction and the air sparging would both tend to generate a local rise in the level of groundwater causing a ' localized groundwater "mound" It was feared that hydrocarbons would tend to migrate "down-hill" radially away from this localized groundwater mound We suggested that an effective way to capture and destroy such fugitive hydrocarbons would be to contain the ' plume with a passive oxygen barrier This barrier could be constructed by placing magnesium dioxide in soil borings down-gradient the groundwater plume This material lie is available under the trade name, Oxygen Releasing Compound (ORC) 1.2.2 Scope of Work In order to complete the pilot study of the conceptual model outlined above, the following steps were completed ' • A 15 inch diameter boring was drilled to 50 feet below ground surface A 4 inch diameter by 50 foot deep vapor extraction well was later constructed in this boring • A 2 inch by 55 foot air sparge well was constructed approximately 15 feet up-gradient ' from the vapor extraction well. The boring was constructed in such a way that ambient air could be injected into the subsurface at a depth somewhat below the main contaminant mass By doing this, mobilized contaminants would move through the subsurface to the vapor extraction well • In order to adequately monitor the subsurface response to the vapor extraction / air sparge pilot testing, two air monitoring wells were constructed These are 2 inch ' diameter wells constructed in 10 inch diameter borings drilled to 30 feet below the ground surface • Selected soil samples from each of the borings were submitted to test for the presence of petroleum hydrocarbons In addition, several samples (including several from the recently constructed monitonng well MW-11) were submitted for analysis of certain geo-technical soil parameters and bactena counts This was completed in anticipation of future risk assessment and fate and transport modeling 2937 Veneman Ave,#8240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto,CA 95356 Fax(209) 569-0295