By Greg Glass
Chevron submitted a report to Ecology on February 12, 2010 summarizing and discussing the ground water monitoring data collected for the first year after Interim Action cleanup activities were completed (“2009 Annual Groundwater and Geochemical Parameter Monitoring Report”). The analytical results from the first seven sampling rounds (October 2008 through October 2009) at 40 monitoring wells (21 Point of Compliance “perimeter” wells and 19 additional “interior” wells) are included in this report.
The Year 1 ground water monitoring report includes discussions of ground water elevations and flow directions, ground water contaminant chemistry, time trends in contaminant concentrations, and natural attenuation (geochemical) indicator parameters. ECAC will be submitting review comments on this report to Ecology in the near future.
The results from Round 8 ground water sampling, conducted in January 2010, were submitted to Ecology by Chevron on March 12th. Three additional sampling rounds are planned for April, July, and October 2010 to complete the scheduled two-year post-Interim Action ground water monitoring program.
The monitoring well at the former slops pond location – MW-510 – continued to show measurable levels of product, and therefore no lab analyses were performed for that well in Round 8. The apparent product thickness at MW-510 increased in Round 8 to approximately 1.5 inches.
Overall, the perimeter well system continues to show higher TPH concentrations than the interior wells. One perimeter well, MW-104 on the western site boundary near the tidal basin on Willow Creek, had a new maximum TPH concentration in Round 8; at 3,105 ug/L total TPH this was also the maximum value reported for any sampled well in Round 8. Monitoring wells showed a mixture of increasing and decreasing TPH concentrations in Round 8 compared to Rounds 6 and 7. As noted in earlier postings, meaningful trend analyses should be based on much longer records than just two successive sampling rounds a few months apart. Numerous wells have shown an up-and-down pattern for TPH rather than a steady increase or decrease over time.
Of the 14 wells along the western site boundary, between LM-2 at Detention Basin 1 (DB-1) and MW-150 in the far southwestern corner, 9 had total TPH concentrations less than 360 ug/L. Besides MW-510 (product) and MW-104 (3,105 ug/L), the 3 additional wells in this area with higher TPH ranged between approximately 900 and 1,600 ug/L.
Wells in the southeastern Lower Yard continued to show elevated TPH levels. The two wells (MW-500 and MW-501) closest to the ground water mound that developed after soil excavations were completed in the southeastern Lower Yard had low TPH concentrations in Round 8, well below Cleanup Levels. Well MW-129R (southeast corner of DB-1), with 2,475 ug/L total TPH, had the second highest result of any Round 8 wells. Wells MW-135 and MW-136, farther to the southeast, had TPH of 845 and 1,545 ug/L, respectively.
The highest TPH concentrations at interior wells occurred at MW-502 (1,230 ug/L) and MW-143 (975 ug/L). Both of these wells are close to the Upper Yard hillside, on the upgradient side of much of the Lower Yard ground water flowpaths. Only two additional interior wells marginally exceeded 500 ug/L TPH. Well MW-507, located in the north-central Lower Yard, continued its pattern of alternating increasing and decreasing TPH concentrations in successive rounds; in Round 8 it was at only 383 ug/L TPH, compared to three previous values over 1,000 ug/L.
The analytical results for Round 9 ground water sampling are expected to be available by mid-June 2010. No decisions on the need for additional cleanup actions are expected from Ecology until after the results from the complete two-year ground water monitoring program are evaluated.
(For well orientation check the well map found in categories on the left.)
Feel free to contact: ( HYPERLINK "mailto:gglassenviro@comcast.net" gglassenviro@comcast.net)
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