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_ r£ �.• .,. :. Sy.. �". MMM F .-=` ,�.s., .%. y. �.� = - •a#" ;; -- `„'was' r- ''�c -' : � - _ _ - -.., ...?m�- a. 'a � � � x �PS�-� a�x€ r` �a'c "S.�`,.x� 7 �".m`- -sv -'��r _ ��3�� x ,�^� •_`�' � �- � � r aa'a�� .;�"r"r?`Sirat{'.-� r„�"�-"� � ,'�,, zg �.�rq.��s �x,�'`� ��-, ��•: ��`� , z f�x y A ry For the purpose of groundwater sample quality control and quality assurance, two additional groundwater samples were collected and analyzed for BTEX. Sample MW-8 was a blind duplicate of sample MW-1 and was used to check the red stability of the analytical results, Sample MW-9 was a water sample collected by pourii 11stilled water into the sample collection bailer after it had been cleaned by the stand,}i decontamina- tion techniques. 2.5 Sits GeOloay The lithology beneath the site consists of two distinct sedimentary units which were consistently found at all soil boring and monitoring well locations on the site. The upper unit consists of clay and silty clay which extends from the surface to a depth of approxi- mately ten feet below ground surface(BGS). This clay is firm, cohesive, and had no odor in the borings logged by WESTON, The lower unit consists of fine grained sand and/or silty sand from a depth of approximately 10 to 25 feet BGS. The sand is uniformly saturated from ten feet through the total well depths of 21 to 22 feet BGS. In the boring constructed as part of the installation of MW-7, a sandy clay was also encountered from 25 to 26 feet BGS. Lithologic logs of the monitoring well borings are presented in the Field Report in Appendix A. The subsurface sediment distribution is depicted in cross sections A-A' and B-B', Figures 3,4 and 5. As seen in these cross sections,the distribution and thickness of the clay and sand units appears to be relatively uniform across the EBMUD site. 2.6 Site Hyd[Q92D1= Groundwater was encountered during borehole drilling at a depth of 10 to 12.feet BGS, as seen in cross sections A-A' and 8-13'. Thi depth of the groundwater corresponds generally to the top of the first subsurface sand interval. The thickness of this aquifer zone is not known although a clay was encountered beginning a125 feet BGS in MW-7 which suggests a possible aquitard at that depth. The distribution and/or continuity of that clay is not known at present. The depth to water was recorded in each of the seven monitoring wells prior to groundwater sampling. This depth, together with the relative elevations of the well head measuring points, yields the groundwater elevations in each well. As mentioned previously, the elevations of the well heads were surveyed with respect to an arbitrary datum elevation of 900 feet, rather than to sea level. Consequently, the groundwater elevations are also relative. The groundwater gradient was calculated using several different three-point solutions within the seven well monitoring array. The solutions yielded inconsistent results with gradient magnitudes ranging from 0.0037 ft.1ft.to 0.0065 ft./ft.,and directions ranging from -- - nearly-due-west to due south. The apparent discrepancy in gradient direction indicates p that the groundwater potentiometric surface is not aner. - 6 -