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Report—Site Closure t— New Jerusalem School July 22, 1999 were collected from various depths in each boring. The native soil encountered during the installation of monitor wells MW-1, MW-2, and MW-3 generally consisted of approximately 17 feet of clay overlying sand. Laboratory analyses of the soil samples collected from MW-1, MW-2, and MW-3 indicate that BTEX, TPHG, and MTBE were not detected at or above the laboratory reported detection limits in the samples collected from various depths, with the exception of the soil sample collected from five feet bgs in MW-1. Only total xylenes were detected in this sample at a concentration of 7.3 µg/kg. Laboratory analyses of the groundwater samples collected from MW-1, MW-2, MW-3, and the facility's irrigation well and domestic water supply well indicate that BTEX, TPHG, and MTBE were not detected at or L above the laboratory reported detection limits. A table showing laboratory analytical results are included in Appendix B. The average depth to groundwater was approximately 8.8 feet bgs on December 29, 1998. The calculated groundwater gradient on December 29, 1998,was 0.23 ft/100 ft to the northeast. 3. HYDROGEOLOGIC SETTING Groundwater occurs at approximately 8-12 feet below the surface with slight gradient to the northeast. �.. Topographic relief is small, less than one foot. There is no longer a source of contamination,USTs were removed approximately 10 years ago. Gasoline contamination was detected in the soil and groundwater at the tank excavation only. Trace contamination was indicated in a groundwater grab sample collected in 1996 approximately 15 feet south of the former tank location, but this was not confirmed in a subsequent monitor well sample collected in 1998 at approximately the same location. The geologic material from which the USTs were removed and where the contamination has been detected is clay. Seven feet below the zone of contamination, at a depth of approximately 17 feet, saturated heaving sand occurs. Three monitor wells are located within 20 feet of the original UST excavation. All groundwater samples were non detect for all parameters(TPHG,BTEX and MTBE). 4. GROUNDWATER FLOW VELOCITY AND DIRECTION Groundwater flow velocity (V) is estimated from Darcy's law, and an assumed hydraulic conductivity v.. (K) for clayey sand and porosity(n) as follows: V= 1.03 x 106 (K dh/dl)/n =0.3 ft/yr Where y 1.03 x 106=conversion from cm/s to ft/year K= 5 x 10-5 cm/s dh/dI =0.002 ft/ft `r n=0.35 The estimated velocity of 0.3 ft/yr is consistent with the fact that no contamination from the UST area has reached either of the down-gradient monitor wells in the ten year period since the contamination was discovered. At the estimated velocity calculated above, approximately 67 years would pass before for contamination at the UST would reach the monitor wells 20 feet away. L In general, slight groundwater gradients vary in direction through a wide-swinging arc. It is not uncommon for gradients to vary over more than 90 degrees in a year. Inconsistent gradient direction would further diminish down-gradient travel distance and would disperse contamination in a local pear- shaped area in the immediate vicinity of the source. w L t J CONDOR