By Greg Glass
Ecology approved modifications to the ground water monitoring program on December 7th. The frequency of sampling will be reduced from six times per year to quarterly; the next sampling event will occur in January 2010, with results reported around mid-March. The final round in October 2010 will include all of the analytical constituents that have been measured up to now, including geochemistry parameters for natural attenuation evaluations. That will provide a complete “snapshot” of ground water conditions at the end of the planned two-year monitoring program. The next three of four quarterly rounds will have a reduced set of analytical parameters, including TPH but dropping the natural attenuation parameters and PAHs. With the extensive excavations that occurred during the 2007-2008 Lower Yard Interim Actions, the “source – plume” model assumed for monitored natural attenuation evaluations for ground water is no longer representative of site conditions, a conclusion shared by all parties involved with the site.
The reduced frequency for ground water sampling will provide a total of 11 rounds of data instead of the 13 originally planned. Sampling will still cover a two-year period and provide a means to address seasonality effects, if any, on ground water contaminant concentrations. Statistical evaluations of the complete data set – for example, tests of trends in ground water TPH concentrations at individual wells – will have to take into account the change in sampling frequency.
The results from Round 7 ground water sampling, conducted in October 2009, were submitted to Ecology by Chevron on December 9th. Time trend plots for total TPH concentrations at selected wells for Rounds 1 through 7 are provided below; see the monitoring well map to locate individual wells. These time trend plots will be updated periodically to visually summarize some of the ground water data.
Well MW-510, located at the former slops pond, had measurable product (LNAPL) observed in the October 2009 sampling event. No ground water sample was submitted for laboratory analysis because the presence of product indicated TPH concentrations would be significantly elevated; well MW-510 has had the highest TPH levels of any well sampled through Round 6. The results to date indicate further remedial action will likely be needed for the former slops pond.
Consistent with previous results, the Round 7 data show generally higher TPH levels along perimeter (Point of Compliance) wells than in the interior of the site. Ten of the 21 POC wells (not counting MW-510) exceeded [default] cleanup levels for TPH in Round 7. [NOTE: the default cleanup levels based on assumed TPH composition are 706 ug/L on the west side and 506 ug/L on the east side of the Lower Yard]. Several wells, including POC wells LM-2, MW-147, and MW-522 along the western margin of the Lower Yard, had their maximum TPH levels so far in Round 7. Two wells marginally exceeded the [default] TPH cleanup level for the first time in Round 7, both located at the far southwestern corner of the Lower Yard (MW-149R and MW-150).
Several wells have shown markedly inconsistent or intermittent patterns for TPH contamination through the first 7 rounds of sampling. Examples include MW-147, LM-2, MW-522, and MW-501 (all included in time trend plots below). These patterns point to the need for caution in assuming that TPH concentrations will remain low after one or two successive low rounds, whether the reason(s) for the intermittent patterns are identified or not. A similar intermittent pattern was noted from the long-term ground water monitoring record prior to extensive Lower Yard excavations at other wells, including MW-136.
In the southeast Lower Yard, 4 of 5 wells (MW-129R, MW-500, MW-135, MW-136) continue to show elevated TPH concentrations (approximately 1,500 to 2,600 ug/L). The exception is MW-501, where TPH dropped from over 3,300 ug/L in Round 6 to less than 150 ug/L in Round 7. That well also had an anomalously high ground water elevation, associated with the ground water mound in the southeast Lower Yard after excavations and backfilling. The variability in TPH at southeast Lower Yard wells may be influenced by this ground water mounding.
In the northern part of the Lower Yard MW-510 remains highly contaminated (product observed in Round 7). TPH at well LM-2 has varied over time but had its highest concentration yet (2,225 ug/L) in the most recent sampling round. Well MW-507, located in the site interior just south of DB-1, has had an alternating pattern in TPH between about 500 and 1,100 ug/L. (Note: two versions of the time trend plot for northern wells are provided below. The second one with a change in Y-axis scaling makes the trends at LM-2 and MW-507 easier to see, without the dominating effect of MW-510).
Southwest of the slops pond, along the western site perimeter, 5 POC wells exceed the [default] cleanup level for TPH. Two wells in the far southeast corner had TPH between 700 and 800 ug/L, exceeding the [default] cleanup level for the first time (MW-149R and MW-150, not included in the time trend plot below). The highest TPH occurred at MW-147, at over 2,700 ug/L – a new maximum level for that well and, except for MW-510, the highest to date along the western site boundary. Wells MW-518 and MW-522 have been comparatively more consistent than MW-147 over time.
Most wells in the interior of the site have had TPH concentrations below cleanup levels. At interior wells with higher TPH levels, the concentrations have been more consistent, even slowly declining, through Round 7 (for example, MW-512, MW-513, MW-514). MW-502, located near the WSDOT storm drain line where remaining soil TPH contamination has been documented, has been somewhat more variable.
Chevron is expected to submit a Year 1 ground water monitoring program evaluation report early in 2010.
Feel free to contact: ( HYPERLINK "mailto:gglassenviro@comcast.net" gglassenviro@comcast.net)
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