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implies a very high concentration gradient between these samples The CPT-1 result is also not consistent with soil data that were collected from GT-11, which is no more than 15 feet away A soil sample collected at a depth of 71 feet in GT-11 had a diesel concentration of 740 parts pei million, and even if all of this leached into groundwater and remained there, it could not yield a groundwater concentration greater than 740,000 ppb The highest TPH-d concentration detected in GT-I1 was 18 000 ppm at a depth of 56 feet, and even this concentration could not yield such an F. elevated groundwater concentration Therefore, the validity of the laboratory data for CPT-1-70' is highly questionable One possible explanation for the value is that hydrocarbons desorbed from the abundant sand and mud in the unfiltered sample prior to laboratory analysis, raising the concentration Diesel was also detected in the water sample from the lit Riverbank Channel in CPT-2, CPT-3, and CPT-4 (Table 2) Concentrations of 140-570 ppb were detected in these samples These values are similar to those that have been found in monitoring wells GT-5 and GT-6 in the past few years and are considered reliable No hydrocarbons were detected in CPT-5 or CPT-6 , r j L hut- tn-&A r 'I) N t: The analytical data for the I" Riverbank Channel are contoured in Figure 8 All data points are shown, even though we have a low degree of confidence in the data from CPT-I The plume is elongate in a general east-west direction but it-apparently spreads laterally eastward, suggesting that the predominant migration direction is eastward The shape of the plume resembles the facies distribution within the channel deposit (Figure 9), but the contaminant axis appears to be offset to the south of the channel axis, which is defined by the bar-shaped lobe,of very coarse-grained sand Planometric methods were used to estimate the areal extent of the impacted groundwater The plume occupies approximately 5,000 square feet ot7-1, ` pa��i a�. how c�,t4„s�� t 5Ct >ILO x >179 = > I'7,8oa Si Z l��0 3.4.2 Second Riverbank Channel k Seven CPT samples were collected in the depth interval between 75 and 100 feet, which spans the P Llr9depth range of the 2nd Riverbank Channel All of these were analyzed for TPH-d (except CPT-2- 401 80', which was broken during transport), and four�were analyzed for TPH-g and BTEX (Table 2) PT As shown m'the cross sections, samples CPT-3-95' and CPT-5-80' were collected fiom fine- grained intervals that are interpreted to lie above oz below the channel deposit, the rest of the COTE samples were collected from said-dominated intervals that are within the channel In addition, the well samples from GT-i and GT-3 also come from the 75-100 foot depth range Hydrocarbons were detected only in CPT-1-80' TPH-d was reported at 140 ppb, and TPH-g was detected at 110 ppb These concentrations are less than 0 01% of those reported from the l" Riverbank Channel 10 feet above this sample, and appear much more reasonable No isocontour map was prepared for the 2"d Riverbank Channel, because the impact appears to be limited to the immediate vicinity of the UST facility 3.4.3 Third Riverbank Channel Two samples were collected below I00'feet, but only CPT-I-114' was collected from a zone that appears to be sand-dominated (cross section F-G) This interval appears to correlate with a thick sand body in GT-11, which is tentatively referred to here as the 3rd Riverbank Channel (cross i 6