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California Department of Transportation June 19, 2001 Stockton Maintenance Station, Stockton, California Project No 400296001 In well MW-6, TPH-G concentration decreased from 300 µg/1 to non-detectable Ievels In well VW-1, concentration of TPH-G decreased from 350 pg/l to 200 pg/I Benzene concen- tration increased from non-detectable levels to 7 4 pg/l Total xylene concentration increased from non-detectable levels to 5 8 pg/1 In well VW-2, concentration of TPH-G decreased from 10,000 pg/1 to 130 pg/l Benzene de- creased from 824 pg/1 to 807 pg/1 Toluene increased from 62 pg/l to 158 pg/l Ethyl-benzene decreased from 339 pg/1 to 31 pg/l Total xylenes increased from 372 pg/l to 497 pg/1 MTBE concentration decreased from 38 pg/1 to non-detectable levels According to laboratory analytical data for VOCs using EPA method 8260B, 1,2,4- trimethylbenzene was detected in samples from on-site well MW-I �. 6 RECOMMENDATIONS Based on the results of the Fourth Quarter 2000 groundwater monitoring analytical results, which indicated elevated concentration of TPH-G, TPH-D, and BTEX Ninyo & Moore recommends that quarterly groundwater monitoring continue at the site 7 LIMITATIONS The environmental services described in this report have been conducted in general accordance with current regulatory guidelines and the standard-of-care exercised by environmental consult- ants performing similar work in the project area No other warranty, expressed or implied, is made regarding the professional opinions presented in this report Please note this study did not include an evaluation of geotechnical conditions or potential geologic hazards Our conclusions, recommendations and opinions are based on an analysis of the observed site conditions and the referenced literature It should be understood that the conditions of a site can change with time as a result of natural processes or the activities of man at the site or nearby sites In addition, changes to the applicable laws, regulations, codes, and standards of practice 0290 D I r SIoLkton FN L IQ2000 dog 5