ARCHIVED REPORTS_XR0006042 - Laserfiche WebLink

Home Browse Search

3.3 Analytical Results Diesel was detected in only one of the five monitoring wells (Table 2) The laboratory reported a TPH-d concentration of 320 parts per billion (ppb) in GT-9, which is up from the 61 ppb reported in March and May Concentrations in GT-5 and GT-6 declined below the detection limit of 50 ppb This is the second consecutive quarter when diesel was below the detection limit in GT-1 Tile concentration in the Gillies domestic well (G-1) also remained below the detection limit 4.0 CONCLUSIONS In the First Quarter 2003 Report we concluded that hydrocarbon concentrations have been stable fol the past nine months, and in the second quarter we extended that conclusion to the past twelve months Concentiations have apparently declined in the past three months to below the detection limit, except peihaps in GT-9 Perhaps this is related to the sharp drop in the water table, which is now near the top of the Riverbank Formation and at least 10 feet below the base of the zone of soil contamination in the Modesto Formation In our work plan dated January 22, 2003 we presented data that argued strongly against the possibility of significant groundwater contamination in the Riverbank Formation, and the results from the September sampling appear to support that argument As we have stated repeatedly in the past, it is our opinion that neither soil nol groundwater in the Riverbank Formation is significantly impacted by petroleum hydrocarbons and further investigation of the vertical extent of groundwater contamination is unnecessary As a further corollary, destruction and replacement of the Gillies domestic well would be frivolous waste of financial resources, because TPH-d was detected in only 1 of the past 11 samples from this well, at a concentiation of only 64 ppb (14 ppb above the detection limit), and groundwater elevation contour maps prepared for the past three quarters have shown no evidence that the domestic well has had any effect on the groundwater flow direction, which has been in the opposite direction Therefore, we recommend postponing all of the work tasks requested by EHD in November 2002, including further drilling, downhole camera surveys, and well destruction and replacement for at least one additional quarter to confinn the sample results obtained this quarter If the samples collected in the next quarter show that concentrations have remained below the detection limit, the wisdom of further investigation at this site should be reexamined I I 3 I