LOWER YARD 2007-2008 CLEANUP (INTERIM ACTIONS): LONG-TERM GROUND WATER MONITORING PROGRAM UPDATE, June 2009

By Greg Glass (gglassenviro@comcast.net)

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Preliminary Notes. The second of two reports describing the Interim Action cleanup

activities in the Lower Yard during 2007-2008 was submitted by Chevron to Ecology on

May 13, 2009. That Phase II As-Built Report covers the final soil excavation activities,

Willow Creek sediment remediation and restoration, and the installation of new ground

water monitoring wells. ECAC review comments on the Phase II Report will be

submitted later this summer. The Phase I As-Built Report has been revised in response to

review comments from Ecology, ECAC, and the Washington State Department of

Transportation (WSDOT) and is now Final.

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ECAC submitted comments to Ecology on the recent Chevron report titled “2008

Additional Site Investigation and Groundwater Monitoring Report” (March 27, 2009).

Our comments, submitted on April 23, 2009, are provided in a separate posting.

Chevron’s report summarized the results of “data gaps” soils investigations as well as the

results from the first two rounds of the planned two-year ground water monitoring

program. I note that while we identified a potential data gap for soils characterization at

the former slops pond location, no additional soil samples were collected there. One new

ground water monitoring well was installed in that area (MW-510). One portion of this

recent Chevron report addressed the remaining soil contamination that was not removed

during Lower Yard excavations because of concerns over damaging the large WSDOT

storm drain line that traverses the Lower Yard. Soil TPH concentrations near that line

ranged up to almost 17,000 ppm.

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ECAC’s current Public Participation Grant from Ecology ends on June 30, 2009.

Ecology has notified ECAC that it will be receiving an additional grant for the two-year

period through June 30, 2011. That continuation grant will support continued review and

evaluation of the long-term ground water monitoring results as well as any additional

cleanup actions that may be required. We plan to continue periodic postings to this blog

as a primary means of keeping the community informed of the status of site cleanup

activities and remaining issues.

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Ground Water Monitoring Results. Round 4 sampling of 40 ground water monitoring

wells occurred in April 2009; results have now been submitted to Ecology. Since the

long-term ground water monitoring program started in October 2008 15 wells have had

total TPH concentrations above 1,000 ug/L at some point. In Round 4 there were 9 wells

above that level, with 7 of the 9 around the perimeter of the Lower Yard rather than in the

interior. One well – MW-510, at the location of the former slops pond and along the

western site boundary between Willow Creek and Detention Basin 2 (DB-2) – has shown

steeply increasing TPH concentrations over time. In Round 4 it had a TPH value of

22,930 ug/L, more than 10 times the second highest result and by far the highest single

result in any sampling round.

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After a change in analytical laboratory before the third round of sampling (see the April

2009 posting), the reported dominant type of TPH (gasoline versus diesel) changed at

several west-side perimeter wells. Those changes persisted through Round 4. The mix of

TPH types will affect cleanup levels, so this apparent change in type will need to be

carefully reviewed. So far the earlier “default” cleanup levels for the east and west sides

of the Lower Yard (506 and 706 ug/L, respectively) are just being carried forward in all

reporting and data summaries. I expect this issue will be addressed in the Year 1 ground

water evaluation report early in 2010.

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In general, the TPH concentrations in perimeter wells have been more variable (up and

down at a single well over the repeated rounds of sampling) than the interior wells. The

layout of the interior wells was meant to provide information to track changes in ground

water contaminant “plumes” and residual contamination that could move with ground

water to and beyond the property boundary. For each of three “plumes” the set of

monitoring wells included one upgradient well and six additional wells covering both

downgradient and cross-gradient areas. Conceptually this would support tracking

changes in both space and time as any remaining ground water TPH attenuated naturally.

The cumulative results through Round 4 are not very consistent with the assumption of

spatial “plumes”. For two of the three sets of plume wells TPH has been detected at the

upgradient and far-downgradient wells, but not at any wells in between. Single well

(within-well) data evaluations will always be possible, but it remains to be seen whether

multi-well analyses for spatially-extensive “plumes” (as anticipated in an Ecology

software package for natural attenuation evaluations) will be meaningful.

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One well (MW-20R along the western property boundary) remains a high outlier for

benzene concentration. The level in Round 4 declined somewhat to just below the

benzene cleanup level for ground water. The second highest benzene level was at MW-

510 (slops pond), at less than one-third the MW-20R value.

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The ground water conditions in the Lower Yard can be illustrated using some simple data

visualization plots. These plots are intended for data descriptions and not formal data

evaluations (which are initially scheduled to be provided in Chevron’s Year 1 ground

water monitoring report). The X-axis in these plots is time, reflecting ground water

sampling dates; the Y-axis is total TPH concentrations in ug/L. [Any not-detected results

for component TPHs (e.g., TPH-D or TPH-G) are included in total TPH at one-half the

detection limit. Note that those detection limits changed when the analytical lab changed.

The fractional contribution of not-detected results, and instances of all TPH types being

not-detected, are not identified in these simple plots].

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A few of the wells currently being sampled were installed many years earlier for initial

site investigations. The long term data sets for such wells provide a historic context for

the recent monitoring results. Figure 1 shows time trend plots from 1995 on for three

wells: LM-2, at the northwestern corner of DB-1, MW-104 on the western boundary near

Willow Creek, and MW-136 in the far southeastern corner. All three wells had much

higher TPH concentrations when first sampled in the mid-1990s. Recent results appear

somewhat elevated compared to pre-2007/2008 Interim Actions.

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Short-term trends over the first four rounds of the current two-year monitoring program

can be illustrated using various groupings of wells. [Look for changes in the Y-axis

scales for TPH concentrations].

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Figure 2 shows recent trends for 5 wells along the western property boundary. From

southwest to northeast they are: MW-147, MW-522, MW-20R, MW-104, and MW-518.

Figure 3 shows recent trends for 3 wells in the northern Lower Yard near DB-1 and DB-

2: MW-510 (slops pond well, which has shown the most dramatic upward trend in TPH),

LM-2, and MW-507.

Figure 4 shows recent trends for 5 wells along the southeastern boundary of the Lower

Yard. From northwest to southeast they are: MW-129R, MW-501, MW-500, MW-135,

and MW-136. The Round 1 TPH value of more than 8,000 ug/L at MW-501 is the

highest TPH at any well apart from MW-510 so far.

Figure 5 shows recent trends for selected interior (“plume” monitoring) wells: MW-502,

MW-512, MW-513, and MW-514.

The Round 5 sampling results should be available by mid-August. 

All questions and comments on these blog postings and the status of the UNOCAL Edmonds site are

welcome.