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Mr Ronald Rowe August 17, 1999 C A M B R I A Figure D in November 1993 (Attachment C) The highest Benzene Concentrations in Groundwater benzene concentration detected was 29,000 ppb in (Well MW-2) 3 000 80 well MW-9 in January 1991 ° - --- - -�� - 60 E 2000 -- — 40 100020 20 LU Although elevated hydrocarbon concentrations 0 0 c� have been detected in several wells at the site, mg Co 4{ cz, CP gP' CP concentrations have decreased by several orders of 40 40 40 40 10 40 40 magnitude as indicated in Figures D, E, F and G ©�rizene GWE Tor,of scre&n" Additional concentration trend figures are included in Attachment D — - -- Figure E I Benzene Concentrations in Groundwater � As can be seen in the adjacent figures, 4 000 (Well MW-4) 80 .. concentrations were decreasing before the water .. 3 000 ---------- ----- 60 E level rose above the top of the well screens There d ' 2000 -- ---- -- ao c 1 000 - - 20 3 is not a good correlation between rising water table v -o 0 and concentration decreases Therefore, it appears �� I:o C;1 ati CP C�' Ih that the concentration decreases are due to 40 01 40 40 40 10, 40 skimming and SVE removing non-aqueous phase --- Benzene OWE Top of Screenl liquid and aqueous-phase hydrocarbons from the Subsurface, resulting in decreasing aqueous-phase --� hydrocarbon concentrations Figure F Benzene Concentrations in Groundwater (Well MW 8) The highest hydrocarbon concentrations detected c 10000 --- -- so ^ o - --- - H in the wells during the May 25, 1999 sampling _ 7500 - - 60 r 5000 40 event were 1,700 ppb TPHg and 56 ppb benzene in 2500 - -- _ zo 3 the sample from well MW-9 The water table 0 o o � during this sampling event was above the top of the 40 40 �bA ed 1g (P��` ati�1P`�a well screens However, the concentrations 404 4t4�'�'�,a le,4a� �a i currently detected are consistent with Benzene —GWE ---Top of Screen concentration trends established prior to the water table rising above the well screen Therefore, it is Figure G difficult to assess whether the concentrations Benzene Concentrations in Groundwater (Well MW-3) currently detected are indicative of ongoing = 30 000 - - -- ---- -_—_—___ ao concentration reductions or dile to the rising water ro 20000 60 E 40 table 10,000 - 20 3 U 0 L0 Since no hydrocarbons have been consistently �4�bA �0�%0��P`1�pit- 1P�aR detected in down-gradient well MW-10, even �a�a� ��a'a� � � before the water table rose above the top of the ©Benzene OWE ----Top of Screen 4