By Greg Glass
We completed our review of the first of two “As-Built” reports describing the Interim Action cleanup activities at the site. That report covers cleanup actions through the Spring of 2008. It provides an effective summary of those actions. Our technical review comments were mostly editorial in nature and were submitted to Ecology and Chevron’s consultant, ARCADIS, on February 12, 2008. The second As-Built report should be available within a few months.
The results of Round 2 ground water sampling, which occurred in December 2008, were submitted to Ecology by Chevron on February 13th. A total of 49 wells were inspected; no LNAPL product was observed in any of the wells. [The correct number of wells being observed is 49, not 50 as previously cited]. Water samples from 40 wells were chemically analyzed for Total Petroleum Hydrocarbons (TPH) and related constituents. More than half of those analyzed samples (21 of 40) had no detected TPH concentrations. Included among the wells with no detected TPH in Round 2 were the two recently installed wells in the southeast Lower Yard that had elevated TPH in Round 1 – MW-500 and MW-501. This reinforces the notion that conditions are better evaluated only after several rounds of sampling are completed. It is noteworthy that the ground water levels in those two wells showed large changes between the October and December sampling rounds, unlike almost all other monitoring wells. How that relates to the variation in TPH, if at all, is still unclear.
TPH is typically measured as one or more of three types based on composition: gasoline, diesel, or heavy oil. Total TPH is the sum of these three analytical measurements. When all three types are reported as not detected (ND), the total TPH is also reported as ND. When some but not all of the three types are ND, the total TPH value includes one-half of the detection limit for ND types. For example, a sample with TPH-G, TPH-D, and TPH-O results of 100, <250, and <500 ug/L, respectively, would have total TPH reported as 100 + 125 + 250 = 475 ug/L, even though the only TPH actually measured and quantified was 100 ug/L TPH-G. In some cases where the ground water cleanup level for the UNOCAL Edmonds site is exceeded this is largely because of the inclusion of one-half the detection limit for ND TPH results. In an attempt to achieve lower detection limits and better characterization of site conditions, samples being collected in February 2009 will be sent to a different analytical laboratory.
Most wells with detected TPH were in the western half of the Lower Yard. A few wells showed increases in total TPH from Round 1 to Round 2; more showed a decrease. However, it is premature to evaluate any true trends in ground water contamination. All but one of the monitoring wells where TPH was detected were mostly TPH-G. The exception is MW-510, located at the former slops pond along the site boundary near Detention Basin 1 (DB-1). TPH at that well is mostly TPH-D. MW-510 had the highest reported total TPH concentration in Round 2 at over 5,000 ug/L, as well as the largest increase in total TPH. Four other wells exceeded 1,000 ug/L: two perimeter (compliance) wells, MW-104 and MW-518, and two interior wells (monitored for natural attenuation of ground water contamination), MW-502 and MW-514.
Benzene is another constituent being analyzed. One well, MW-20R along the western site boundary near the tidal basin, had a large increase in benzene concentration to 22.2 ug/L, more than 7 times the result from Round 1. This concentration is still well below the ground water cleanup level for benzene.
The results for Round 3 ground water sampling should be available by mid-April.
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