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With regard to the quality control,samples, the analysis of MW-1 (the primary sample from monitoring well MW-1) and the blind duplicate (labeled MW-8) had comparable analytical results indicating very good repeatability of the laboratory analyses. Analytical results of the field decontamination sample, collected after sampling MW-1 (which was the first well sampled during the sampling event) indicated that there were low values of detectable BTEX constituents present in the field equipment after'completion of field decontamination procedures. However, as shown by the absence of petroleum hydrocarbon compounds in the samples taken from wells sampled after the field decontamination sample was collected, no cross-contamination of the other wells occurred. Therefore, the analytical results for this sampling event are considered to be representative of groundwater conditions at the site. Field decontamination procedures are being revised to improve cross coniamination protection for future monitoring events. 3.0 TIDAL INFLUENCE STUDY On July 8 and 9, 1992, WESTON collected field data to complete a tidal influence investigation 1 at the EBMUD facility. investigation ty This was performed to measure the effects on the 1 groundwater gradient beneath the :;site from tidal fluctuations in the Stockton Deep Water Channel which is located approximately 550 feet north of the site (Figure 1). Field work consisted of collecting;;nine complete rounds of groundwater levels from seven monitoring wells (MW-1 through MW-7) over a twelve hour period (one complete tidal cycle). , Figure 2 depicts the monitoring well locations. The schedule of groundwater level measure- ments at EBMUD was chosen to correspond to high, low, and intermediate tidal stands in the I Stockton Channel as shown on Figure 3. The dates and times chosen for the tidal study were { also selected to correspond to one df the highest high tides of the summer. Each round of groundwater levels was collected within a 10 minute period in order to minimize F groundwater level fluctuations during the measurement process. Water levels were measured with an electronic well sounding device and recorded to the nearest 0.01 feet. Relative groundwater elevations were calculated and the results are presented in Table 3. From the results presented in Table 3, three groundwater surface maps were generated (Figures 4, 5, and 6) as well as a graph (Figure 7) showing fluctuations in groundwater elevations with time in the seven wells. {� 3.1 Tidal Study Results I L 1; During the twelve hour study period, the tide was receding for 5.25 hours and was rising for 6.35 hours. The results of the measurements show minor fluctuations in groundwater elevations in all seven wells. The maximum fluctuation in groundwater elevations ranged from 0.24 inches in MW-5 and MW-7 to 0.48 inches in MW-4. Although these groundwater level variations indicate the diurnal tides effect groundwater elevations at the site, the effect is minimal ' particularly when viewed in comparison to the channel tide variation of 4.0 feet observed during { the evaluation period. Of particular importance to the EBMUD site, the minimal tidal effect observed did not change the groundwater gradient direction as shown on Figures 4, 5 and 6. " "rSAMMUDRMW 4 US/27M 1 L