By Greg Glass
The Round 3 sampling of ground water monitoring wells took place in February 2009. The results for Round 3 have just been submitted to Ecology by Chevron. Detailed evaluation and interpretation of ground water conditions at the site will not occur until after the first year of monitoring has been completed, when ARCADIS (Chevron’s consultant) will prepare a Year 1 Ground Water Report. Based on my initial review of the most recent data, I provide some brief observations in this note.
From sampling round to sampling round there have been some notable changes in reported TPH concentrations in the monitoring wells. Overall, Round 3 showed an increase in reported levels of contamination. This was more pronounced in the perimeter wells than the interior wells. A total of 12 of the 40 wells where samples were analyzed had total TPH greater than 1,000 ug/L, versus only 5 in the previous round.
Round 3 analyses were performed by a new analytical laboratory, which achieved lower detection limits and therefore more precise measurement of low-level TPH. Since total TPH is calculated using one-half the detection limit for not-detected TPH results (gasoline, diesel, or heavy oil), the lower detection limits should provide better estimates for total TPH. (For an example of total TPH calculations involving not-detected results, see the February 2009 posting).
All TPH represents a mixture of a large number of different but related hydrocarbon compounds. The analytical chemist at the laboratory designates the type of TPH based on a review of the output from the analytical instrument and a comparison to standard profiles; analyst judgment is therefore involved.
It is noteworthy that Round 3 results for a number of wells reflect a change in the dominant type of TPH (from TPH-G to TPH-D) versus earlier rounds and previous site characterization studies. This is important because the cleanup level for ground water depends on the mixture of types of TPH present. The separate numerical cleanup levels being used so far for the east and west sides of the Lower Yard reflect assumed TPH compositions that were developed from earlier ground water analyses. Final compliance will be evaluated based on the actual composition found, so changes in the TPH composition (types) can affect cleanup levels and the status of the site.
The highest concentration of TPH continues to be found at MW-510 at the location of the former slops pond along the western site boundary. The Round 3 result of more than 16,000 ug/L is more than 3 times higher than the Round 2 value. Based on water levels measured in each sampling round, the ground water flow direction at that location is from the perimeter into the Lower Yard. The upgradient side of this flow direction is within Willow Creek, which was substantially remediated during the recent Interim Actions. I found no information indicating that the former slops pond was ever cleaned out. It seems very likely to me that the MW-510 contamination is a localized response to residual contamination at the slops pond.
The southeast Lower Yard showed significant changes from Round 2. TPH was detected at all 5 monitoring wells, from MW-129R at the corner of DB1 to MW-136 in the far southeast corner. Three of these 5 wells had total TPH exceeding 1,000 ug/L, with the highest value of over 3,000 ug/L at MW-136 and the second highest of over 2,600 ug/L at MW-129R. Over the long period of ground water monitoring at the site well MW-136 has shown quite an up-and-down pattern in TPH concentrations, with markedly elevated TPH levels following multiple sampling rounds with not-detected TPH. The Interim Action soil excavations in the southeast Lower Yard covered virtually all of the areas nearby and upgradient from the current set of perimeter monitoring wells. Unlike the central portions of the Lower Yard, there are no interior (Monitored Natural Attenuation: MNA) wells in the southeast Lower Yard.
Several wells showed their first detected results for TPH in Round 3. Monitoring well LM-2 at the northernmost corner of DB1 had over 1,800 ug/L total TPH. Extensive sampling of DB1 soils after it was cleaned out showed almost no remaining TPH contamination. Well MW-143, the most upgradient well in the western MNA system of wells, had over 2,000 ug/L total TPH. With the exception of MW-143, the MNA plume wells had TPH results that were pretty consistent with earlier rounds.
A ground water cleanup level has also been established for benzene. Well MW-20R along the western property boundary near the tidal basin marginally exceeded that cleanup level in Round 3, the first exceedance of the benzene cleanup level in the post-Interim Action monitoring program. Benzene concentrations have increased at MW-20R over the first three rounds to levels much higher than at any other monitoring well.
The results for Round 4 sampling, occurring in April, should be available by mid-June.
The Final Phase I As-Built and Draft Phase II As-Built Reports are due to be submitted to Ecology on May 17th. Chevron submitted another report on March 27th: “2008 Additional Site Investigation and Groundwater Monitoring Report”. The results from Rounds 1 and 2 of the post-Interim Action ground water monitoring program are presented in that report without any attempt to evaluate trends or status. The results of the soil investigations that addressed “data gaps”, between Phases I and II of the Lower Yard Interim Actions, are provided and discussed. Residual contamination near the WSDOT storm drain line is one of the main topics covered. We will be submitting review comments on this report shortly.
More information related to this article can be found under "Additional Info" on the left margin.
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