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i <br /> 07 January 1998 <br /> AGE-NC Project No 95-0128 <br /> Page 2 of 7 <br /> addendum <br /> Between 1991 and 1997, four soil borings, one vapor extraction well and seven ground water <br /> monitoring wells were installed at the site The vertical and lateral extent of hydrocarbon-impacted <br /> soil has been delineated, however the lateral extent of hydrocarbon-impacted ground water has not <br /> been delineated to the satisfaction of the PHS-EHD Data collected from the site during the <br /> investigation of the former UST site indicated that hydrocarbon-Impacted soil is present in the <br /> vadose zone from depths of approximately 5 feet to 55 feet bsg Rising ground water elevation has <br /> greatly exceeded the screened intervals of monitoring wells MW-1, MW-2 and MW-3, rendering <br /> data collected form these wells unrepresentative of shallow ground water conditions Hydrocarbon- <br /> impacted ground water has been encountered north and east of the property in the down-gradient <br /> direction from the suspected former source of the unauthorized hydrocarbon release, but is not well <br /> constrained in these directions These factors necessitate installation of the additional ground water <br /> monitoring wells Figures 5 through 16 depict the current contaminant plumes and the initial <br /> dissolved hydrocarbon plume boundaries (See Figure 14 and 15) <br /> A vapor extraction test was conducted in December 1993 Since that time, ground water has risen <br /> . from approximately 55 feet bsg to as high as 30 feet bsg, reducing the vertical extent of the vadose <br /> zone open to the screened interval of the vapor extraction well The rise of ground water elevation <br /> has also eliminated most of the coarse-grained soil from the vadose zone, leaving predominantly <br /> fine-grained soil in contact with the reduced open screened interval of the vapor extraction well <br /> These changes can potentially drastically reduce the zone of influence of the vapor extraction well(s) <br /> and reduce the concentrations of hydrocarbon vapor recovered by the extraction process For this <br /> reason, an updated vapor extraction test should be conducted uhf vV, L V1 wine t5 7 � <br /> � t�. v 2or� �G�►uw�i <br /> 3.0. SCOPE OF WORKy <br /> The scope of the work includes the advancement of eight to ten soil borings for the collection of soil <br /> grab water samples The additional ground water monitoring wells will be installed at the southern <br /> boundary of the property, west of the USTs, north of the property and probably across Pacific <br /> Avenue east of the site Wells will be installed near or beyond the observed edges of the dissolved <br /> hvdrnrarhnn nhimQ hacari An tl,a annhTt1"n1 .1�*o nnlTon+nil r,-- +1.., ---I, . _+__ .1 <br /> r.. ... .... .......�14... uuu uwit a wu lavlll ultb fau wa►c i aaiii��cJ <br /> 3 1 PROBING AND SAMPLING <br /> AGE proposes to initially advance eight soil probe borings surrounding the site Probe borings will <br /> be advanced to a depth of approximately 41 feet bsg (below surface grade) using direct push <br /> technology (DPT) All probe borings will be dvanced using AGES Geoprobe 5400 probing unit <br /> Advanced GeoEnviron mental,Inc <br /> hV& �ON <br /> t <br />