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' The wells were surveyed after completion under the direction of <br /> GTI . Wellhead elevations used in gradient calculations are those <br /> reported in the GTI report of 21 August 1987 . <br /> Gradient direction from the most recent gauging is shown on the <br />' potentiometric map of Figure 4 . All data from the gauging and <br /> surveying is presented in Table 4 . <br /> 2 . 6 SOIL VAPOR CONTAMINANT ASSESSMENT <br />' EA conducted a soil vapor survey ( SVGA ) at former Exxon RS 7-1050 <br /> on 23 and 24 November and 22 December 1987 . An SVCA is a field <br />' method of collecting samples of soil vapor and analyzing them for <br /> petroleum hydrocarbons with a portable gas chromatograph. Sam- <br /> ples of soil vapors at Exxon RS 7- 1050 were collected by driving <br /> 5/8-inch-diameter steel probes into the soil at 11 locations . A <br /> ac uum pump was used to purge several probe-volumes of vapor from <br /> the probes , thus ensuring that the samples were representative of <br /> the vapors contained in the soil . The vapors were extracted from <br /> the probes with a microsyringe through a septum in the vacuum <br /> pump and infected into a Photovac 10S50 portable gas chromato- <br /> graph for analysis . The Photovac 10S50 is a programmable , inte- <br /> grating gas chromatograph with a photoionization detector ( PID ) . <br /> The instrument was calibrated with standards consisting of 10 ppm <br /> benzene and 10 ppm toluene in air . <br /> On 23 and 24 November 1987 , deep ( approximately 20 feet ) soil gas <br /> samples were collected from six locations on-site ( V1 -V6, Figure <br /> 5 , and monitoring well headspace samples were taken from the five <br /> monitoring wells . A follow-up investigation for the presence of <br /> shallow hydrocarbon levels was conducted on 22 December 1987 . <br /> Shallow samples were collected from five locations ( V7-V11 , <br /> Figure 5 ) <br /> e25/ 71050/8-89/txt 10 <br /> 1 <br />