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12 In trench A (we have labeled the five trenches that we observed A through E; these numbers will not correspond to the test pit designation in the Environmen- tal Assessment report) , the glass was observed to extend down to at least 15-ft depth. In trench D, the glass extended below 7-ft penetration. In the "Environmental Assessment" report (Woodward Clyde, 1988) , glass was reported to be present to 12.5-ft depth in monitor well MW-23 and to 8-ft depth in monitor well MW-22. These monitor well locations appear to have been north- west and southeast of the "plant" area. Significant quantities of glass were encountered in borings 1, 2, 3, and 4 to depths of 8, 16, 4, 6 ft, respectively. Materials encountered in some borings appeared to be ash residue from burn pits, where glass by-products were burned. The boring locations, for this study, together with estimated locations of the previous exploration trenches and environmental assessment monitor wells are shown on Plate 36. Information with respect to presence or absence of glass at these locations is shown on this illustration. These data suggest that signifi- cant quantities of glass are present in the southern part of the "plant" area. In this area, the glass apparently is present over a wide area and locally extends down to 16-ft depth. Less evidence of glass was observed at the boring locations in the northern portion of the plant area. Significant glass, however, was observed in monitor well MW-23 and deep surface trenches present in the northwest plant area. Thus, the probable presence of significant glass in these areas cannot be discounted. Only in the northeast corner of the plant area does the probability of encountering glass appear to be lower. Groundwater Groundwater was encountered between 7- and 11-ft depth in the borings drilled during December 1988. Groundwater was observed at about 15-ft depth in the bottom of one of the trenches excavated in late March 1988. Groundwater levels in three monitor wells drilled in July 1987, for the Environmental Assess- ment (Woodward Clyde, 1988) indicated the depth to water was between 7 and 11 ft. The difference in depth to groundwater may be due to seasonal variations. We note that during the two weeks prior to our December 1988 drilling, several significant rainfalls occurred in the Lathrop area. Alternately the water encountered in the glass and surface sand fill may be perched water, while the static water level in the underlying clay and sand layers may be somewhat lower. 1144A/CC-12 ,:i McClelland