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Ms. Kim Schwab -2- FOURTH QUARTER AND ANNUAL 2002 MONITORING REPORT REVIEW, FOOTHILL SANITARY LANDFILL Although difficult to discern,please know that there was no attempt on our part to withhold information. The laboratory identifications are shown on Page 2 of the analytical results, and on the chain of custody documents, which were included in the report. To further clarify the designations, we are submitting an Addendum to the 4th Quarter Report presenting the sample designations (attached). We apologize for any misunderstanding this may have caused, and we have contacted Sequoia Analytical to request that the sample designations not be changed by subcontracted firms. Item 4: Removal of Outliers and High Concentration of Total Dissolved Solids (TDS) Thank you for clarifying the need to identify outlier data. As we discussed,the data point (360 mg/L, TDS in MW-1)was included in the 4 1 Quarter Monitoring Report,but removed from the Time Series Plots in the following monitoring report. This data point was removed because it was identified to be a statistical outlier by the Sanitas software, and it was our understanding that outliers identified by Sanitas were to be removed. In the past,the County has generally removed data points which Sanitas determines to be outliers from Time Series Plots because they skew the graph such that it is difficult to see natural behavior of the data which is more representative of the groundwater quality. Per your direction,we will retain statistical outliers in the Time Series Plots in future reports. If you have any additional concerns,please let us know. We have contacted Steve Giacomini with Emcon to determine the apparent discrepancy between the TDS (360 mg/L) and electrical conductivity(250 umhos/cm)reported in that sample, during the fourth quarter 2002 monitoring event. Steve could not identify any potential causes of the high concentration reported, and there was no activity at the site that would explain an increase in TDS. It appears that this concentration is an anomaly not reflecting the true condition of the groundwater. Analytical results from the two subsequent samples reported concentrations similar to what has been reported in the past(approx. 190 mg/L). Per your request the County will resample the upgradient well immediately upon report of concentrations which are above the established limit. Item 5: Resampling Wells while Under Evaluation Monitoring As we discussed, it had been our understanding that resampling during evaluation monitoring is not necessary. As directed, we will resample when VOCs,which have not been previously confirmed to be present in groundwater, are reported in the analytical results and would trigger the resampling requirements as in detection monitoring. Please note that the compound in question(Methyl-tert butyl ether)was not detected in the following monitoring event.