By Greg Glass
Frances Murphy, Duane Uusitalo and Greg Glass, representing ECAC, met on October 13th with Ecology, Chevron, WSDOT, and Chevron’s consultants ARCADIS to discuss reviewers’ comments on recent site reports and to review recent ground water sampling data. [WSDOT, despite budget pressures, has found resources to support continued involvement on this site].
Review comments on the second of two As-Built Reports on the 2007-2008 Lower Yard Interim Actions, as well as the report on additional site investigations between Phases I and II of the Interim Actions, were discussed. With relatively minor revisions, final versions of those two reports will be prepared by ARCADIS.
A review of site hydrogeology by ARCADIS, including water level data from recently installed piezometers in the southeast Lower Yard where ground water mounding has been observed post-Interim Actions, has resulted in some revisions to the interpretation of ground water flow directions. The results were presented at the October 13th meeting. Under this new interpretation, Detention Basin Number 1 (DB-1) does not act as a sink for Lower Yard ground water, and Willow Creek does not prevent ground water from flowing out of the UNOCAL site to adjacent properties. The interconnection between surface water in Willow Creek and the uppermost ground water at the UNOCAL site is minimal. The mounding of ground water in the southeast Lower Yard appears to be a result of the contrast in soil characteristics between more permeable backfill materials at excavations and the less permeable non-excavated materials adjacent to those backfill materials. Details of this revised model of site hydrogeology will be provided as part of the Year 1 Ground Water Monitoring Report, due to be submitted in early 2010.
We have previously noted that the ground water data from the three “plume” monitoring well systems in the interior of the Lower Yard are not consistent with typical contaminant plume geometry, and that Ecology’s multi-well data evaluation software for plumes would have limited applicability at this site. Only within-well, not between-well, evaluations are likely to be supported. ARCADIS has reached a similar conclusion, which was discussed at the October 13th meeting. ARCADIS provided notice that they will be submitting a request to Ecology to modify the Year 2 ground water monitoring program. Anticipated changes include a reduced frequency of sampling, to quarterly (still providing data to address seasonality); collecting ground water samples for chemical analysis at fewer wells; and potentially dropping some geochemical analyses that were designed to support attenuation modeling. No existing wells will be decommissioned, and all wells will still be observed for LNAPL product and water levels. David South, Ecology’s site manager, suggested that all wells be sampled for the final round to provide a snapshot of total site conditions. We will review the proposal once it is submitted and provide our comments to Ecology. Wells will still be sampled in late October 2009.
The results from Round 6 ground water sampling, conducted in August 2009, were submitted to Ecology by Chevron on October 15th. Year 1 of the planned two-year post-Interim Action ground water monitoring program has thus been completed.
Two wells had reported minor (trace, at <0.01 foot apparent thickness) detections of LNAPL product in Round 6. Those wells were MW-510 at the former slops pond location and MW-129R near the southeast corner of DB-1. Those were also the two wells with the highest reported TPH concentrations in Round 6. Well MW-510 had over 18,000 ug/L total TPH, a reduction of more than 25% from Round 5 and the first reversal from what had been a continuous upward trend over Rounds 1 through 5. All 5 wells in the southeast Lower Yard remained relatively high for total TPH. Well MW-129R at more than 4,400 ug/L was at a new maximum level for post-Interim Action sampling and was the highest of the 5 wells in the southeast Lower Yard; the other 4 ranged from about 2,200 to 3,400 ug/L. The slops pond and the southeast Lower Yard remain the two areas of greatest interest for residual ground water contamination.
A few additional wells had total TPH levels marginally above ground water cleanup levels, with Round 6 results well below previous maximum TPH concentrations for those wells. The highest observed TPH concentrations continue to be found at Point of Compliance wells along the outer property boundary and not in the Lower Yard interior where extensive excavations occurred.
The results from Round 7 sampling in October should be available by mid-December 2009.
Feel free to contact: ( HYPERLINK "mailto:gglassenviro@comcast.net" gglassenviro@comcast.net)
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