SITE INFORMATION AND CORRESPONDENCE FILE 2

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' I I Rodger Liston Darpetro Inc. 749 E. Charter Way, Stockton, California Page 2 of 3 documents for not decontaminating the sampling bailer and demonstrate how the monitoring events were conducted in accordance with such guidance , EHD will reevaluate the current concern with the data quality obtained to date. The purge procedures were the same for wells with screens across the water table as for wells with water levels above the screened intervals. The pumping unit was lowered approximately 10 to 15 feet below the depth to water and water was purged at a rate of approximately 'Y2 to 1 gallon per minute (three well purges were observed by EHD) . Following the purging of monitoring wells BM-9 and BM-7 , the depths to water (DTW) were measured and found to be within a few hundredths of a foot of the initial DTW . This suggests to EHD that the water in the casings above the screened intervals in the wells was not purged , and samples collected from a bailer would merely collect the old casing water that had been present near the top of the water column in the well before the well was purged . This water is exposed to the atmosphere and foreign materials falling in the well and is not likely to be representative of aquifer conditions. Not only is this water potentially not purged , it may also pick up contaminants from the uncleaned hose and bailer. You can see that it may be quite difficult to interpret the chemical data collected this way. In view of the potential ineffective purging of some wells and the cross contamination issues, EHD approved collecting samples from the purge pump at the end of purging the wells for this event only. This technique has a potential problem of excessively agitating the samples as the purge rate could not be slowed , but the sample may be more confidently assumed to be 'fresh ' and the potential for cross contamination would be minimized . During the start of the 14 April 2004 monitoring event, Mr. John Cussen noted to EHD that monitoring wells BM-10, 11 , 12 and 4 would not be sampled as he had approval from EHD some years ago to drop four wells from each monitoring event. An EHD record for 14 April 2000 shows that Carol Oz of EHD did approve sampling MW-6, 10 , and 11 every other quarter. The concern of EHD at this point with periodically dropping wells from the sampling program is that EHD has not been provided a written proposal and rational for selecting which wells will be sampled at what frequency, and how the monitoring and site characterization objectives for this site will be met while periodically eliminating various wells from the sampling program . To address the concerns noted above, at this time EHD directs you to provide EHD with a ground water monitoring work plan by 21 June 2004 that includes: 1 . A clear statement of the objectives for the monitoring program ; 2 . A monitoring schedule for each well in the current monitoring well network with a rational for the monitoring schedule for each well that is not proposed to be sampled on a quarterly basis; 3. Purge procedures to be utilized for each well or set of wells that will ensure that the samples collected will be representative of aquifer conditions. and 4. A proposed quality assurance and quality control (QA/QC) program to ensure collection of highly reliable data to meet the objectives of the monitoring program.