SITE INFORMATION AND CORRESPONDENCE

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/fACAWCA Working to Restore Nature the two deepest borings were drilled to approximately 115 feet bsg in June 1990 (boring B- 9) and 110 feet bsg in December 1992 (boring B-26). During drilling at the site (soil borings B-6 to B-28) groundwater was not encountered in sediments above approximately 115 feet bsg (RESNA Report No. 30041-2, 30041-3, 30041- 4, March 20, 1992, November 12, 1993, and March 18, 1993, respectively). Groundwater has not been observed in vapor extraction wells VW-10, VW-13, and VW-17. Based on these data we conclude that sediments underlying the site at depths less than 115 feet bsg are not saturated by groundwater. Because the well screen intervals in the water-supply well are 396 to 416 feet bsg and 496 to 536 feet bsg, and saturated zones were not indicated to be present on the driller's log above these intervals, we therefore infer that saturated unit(s) below approximately 396 feet bsg are providing water to the well and are under confined conditions. The United States Geodetic Survey Midway 7.5-minute Quadrangle shows an artesian well at a similar ground surface elevation, approximately 1,800 feet southeast of the onsite water-supply well; indicating that similar conditions exist in the area. Our previous investigations (RESNA Report No.30041-2,30041-3, 30041-4, March 20, 1992, November 12, 1993, and March 18, 1993, respectively) indicated that trace concentrations of residual gasoline hydrocarbons (total petroleum hydrocarbons as gasoline <2.0 parts per million (ppm) and benzene <0.05 ppm) are present in soil at a depth of 110 feet bsg, but that the bulk of the residual gasoline hydrocarbons beneath the site is in soil above 70 feet bsg, and above 90 feet bsg offsite. Therefore, it appears that soil affected by residual hydrocarbons above 100 feet bsg has not been in contact with groundwater below 115 feet bsg. The driller's log of the water-supply well indicated the annular space from 540 feet to 35 feet bsg is packed with gravel. Groundwater has not been detected in the vapor extraction wells screened collectively over a continuous interval of approximately 10 to 102 feet bsg. Therefore, because groundwater was not detected in the three vapor-extraction wells, it appears that groundwater has not migrated laterally from the annular space in the water- supply well into the adjacent sediments between 10 and 102 feet bsg. It is our opinion that the data gathered to date indicate that groundwater beneath the site is below 115 feet bsg, groundwater is under confined conditions, and the top of the first saturated unit is most likely 396 feet bsg or deeper. In addition, it appears that the confined groundwater has not migrated into sediments affected by residual gasoline hydrocarbons beneath the site. Therefore, groundwater beneath the site probably has not been affected by residual gasoline hydrocarbons. It is our opinion that additional evaluation of the depth to groundwater and assessment of the groundwater beneath the site are not warranted. 0830c r 2 30041