By Greg Glass
The results from Round 10 sampling of ground water monitoring wells at the UNOCAL Edmonds site were submitted to Ecology by Chevron on September 13, 2010. Round 11 sampling, the last planned sampling event for the 2-year post-cleanup ground water monitoring program, will occur later in October. We expect a Year 2 ground water monitoring report to be completed early in 2011. It is already clear that some of the wells will not be in compliance with ground water cleanup levels for TPH. The monthly progress report submitted to Ecology by Chevron on October 7, 2010 noted that Ecology and Chevron have already begun discussing further cleanup activities for the site.
The results of Round 10 sampling conform to the general established pattern seen in recent sampling rounds. MW-510, at the location of the former slops pond along the western site boundary, continues to have product (NAPLs) detected. Of all of the wells with continued elevated TPH concentrations, MW-510 is the one for which the source seems clearest: residual slops pond contamination. MW-510 may not be located at the point of maximum residual slops pond contamination.
Most of the interior wells are at low TPH concentrations. The exception is MW-143 in the southwestern Lower Yard, which had total TPH of 1,585 ug/L in Round 10. There are no monitoring wells being sampled upgradient of MW-143; the source of the continuing TPH contamination is unclear.
Four of the perimeter (point of compliance) monitoring wells exceeded 1,000 ug/L in Round 10. [TPH cleanup levels depend on the composition of total TPH and will generally be in the 500 to 700 ug/L range]. LM-2, at the northern corner of Detention Basin 1, had 2,075 ug/L total TPH. The interior wells upgradient of LM-2 do not show elevated TPH concentrations, and DB-1 was cleaned out in an interim remedial action with no significant residual contamination detected. The source for ongoing LM-2 contamination is unclear, but may possibly be related to the slops pond contamination demonstrated at MW-510.
Three additional perimeter wells exceeding 1,000 ug/L in Round 10 are located in the southeastern corner of the Lower Yard. MW-129R had the highest total TPH concentration in Round 10, at 2,225 ug/L. MW-135 and MW-136 were at 1,625 and 1,575 ug/L, respectively. The contamination at MW-129R may be related to operations at the former asphalt warehouse, where asphalt was pumped in a line located in a trench in the warehouse floor. Alternatively, past disposal practices along the edge of DB-1 may have contributed to contamination near MW-129R. No wells upgradient of MW-129R, MW-135, or MW-136 are currently being sampled.
In summary, then, recent sampling rounds show the major remaining areas of interest for perimeter (point of compliance) wells to be near the slops pond (MW-510), along the southeastern portion of the Lower Yard (MW-129R, MW-135, and MW-136), and at the northern side of DB-1 (LM-2). The primary interior location of interest is at MW-143, in the southwestern part of the Lower Yard near the toe of the Upper Yard slope.
The high variability, or “up and down” pattern, in total TPH concentrations at some wells has been noted in several previous postings. Examples of wells where relatively low TPH results have been followed by concentrations more than 5 times higher occur at locations across the site:
MW-147 351 ug/L in August 2009, then 2,720 ug/L in October 2009 (x7.7)
MW-104 464 ug/L in August 2009, then 3,105 ug/L in January 2010 (x6.7)
MW-143 258 ug/L in October 2009, then 1,585 in July 2010 (x6.1)
LM-2 386 ug/L in August 2009, then 2,225 ug/L in October 2009 (x5.8)
MW-502 199 ug/L in August 2009, then 1,230 ug/L in January 2010 (x6.2)
MW-501 442 ug/L in April 2009, then 3,385 ug/L in August 2009 (x7.7)
A brief examination of results at these wells suggests that the variations reflect something other than simple seasonality or a relationship to ground water levels.
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