By Greg Glass
The ninth round of ground water sampling was performed in April 2010, with lab results reported to Ecology by Chevron on June 17th. With only 2 more scheduled rounds of sampling (in July and October), we are nearing the end of the planned two-year, post-Interim Action excavations ground water monitoring program. A Year 2 ground water monitoring report will be prepared after the October 2010 results are compiled.
Given the results so far, with product (LNAPLs) consistently being observed in one well and multiple perimeter (Point-of-Compliance, or POC) wells still above ground water TPH cleanup levels, Chevron has started some preliminary work on how the residual contamination could be addressed.
Round 9 results continue to reflect the same general pattern seen previously. The Interior wells as a group show less TPH contamination than the perimeter wells. Well MW-510 at the location of the former slops pond still has observable LNAPLs. Six additional perimeter (POC) wells are above the (default) ground water TPH cleanup levels: Southeast Lower Yard wells MW-136, MW-135, and MW-129R; well LM-2 at the northwest corner of DB-1; and west-side wells MW-518 and MW-147. Apart from MW-510, the highest TPH concentration in a POC well is 2,015 ug/L at MW-129R. The highest Interior well TPH concentration is at MW-143 at 1,565 ug/L [and 1,875 ug/L for a duplicate sample]; only 3 additional Interior wells exceeded the TPH cleanup level, all at less than 850 ug/L.
A preliminary look at some monitoring results, and a little math:
After removal of most of the source(s) of TPH contamination, the residual ground water TPH levels at a well would be expected to decrease fairly steadily over time; an exponential decay model is often used as a first approximation for this time trend. The UNOCAL Edmonds data over the first 9 sampling rounds show examples of such patterns, as well as highly variable patterns that do not reflect such steady decreases. For example, compare the total TPH results for these three wells:
MW-518 MW-143 LM-2
10/2008 1,142 <400 <389
12/2008 1,162 <386 <389
2/2009 1,403 2,005 1,835
4/2009 1,202 770 1,355
6/2009 675 1,175 915
8/2009 1,066 421 386
10/2009 1,007 258 2,225
1/2010 924 975 1,625
4/2010 908 1,565 [1,875] 1,585
TPH at MW-518 shows a fairly steady decline over time, with one lower reading on 6/2009; MW-143 and LM-2 have much more “up and down” patterns. The interpretation of the highly variable time trends may need to consider whether there are ongoing sources affecting TPH concentrations at the wells, the possible effects of seasonal changes in infiltration and ground water levels, varying ground water flow directions seasonally, or other factors causing deviations from a “steadily decreasing concentrations” model.
Detailed evaluations and interpretations of the monitoring data are some months in the future. It is interesting to note that MW-143 is on the upgradient side of Lower Yard ground water flow directions, and that LM-2 may (at least at times) be downgradient of MW-510 where LNAPLS and elevated TPH levels have been observed. Most of the POC wells with higher residual TPH concentrations are also seen to have relatively higher TPH-heavy oil (TPH-O) levels. The highest TPH-O concentration in the ground water monitoring program so far has been at MW-510; perhaps elevated TPH-O levels are a marker for the past (or continuing?) proximity of LNAPLs or near-saturated soil TPH as a source of ground water contamination.
A quick summary of the progress in ground water cleanup, post-Interim Actions, is provided by comparing the Round 9 TPH results (in ug/L) to the maximum TPH for any round, for selected wells:
MW-136 1,535 / 3,725 41.21%
MW-135 1,035 / 2,625 39.43%
MW-129R 2,015 / 4,425 45.54%
LM-2 1,585 / 2,225 71.24% ***
MW-518 908 / 1,403 64.72% ***
W-147 1,128 / 2,720 41.47%
MW-143 1,565 [1,875] / 2,005 78.05% [93.52%] ***
MW-512 670 / 748 89.57% ***
MW-513 487 / 932 52.25%
MW-514 646 / 1,578 40.94%
MW-502 844 / 1,697 49.73%
MW-507 406 / 1,142 35.55%
The maximum values for Interior wells appear to be lower than those for perimeter (POC) wells; maybe this is a result of the greater removal of ground water from the interior of the site during soil excavations. For both groups of wells, the percentages of maximum TPH still present in Round 9 show two clusters, one around 40% and one around 70% or higher (noted by ***).
Returning to the simple model of exponential decay in TPH concentrations over time, and assuming the reductions illustrated by remaining percentages as shown above reflect 18 months (the duration of monitoring so far), the exponential decay parameter can be estimated and from that the duration until TPH has been reduced to any specified fraction. [NOTE: using the full 18 months in these calculations, even when the maximum TPH occurred some months into the monitoring program, is conservative in that it gives a high-bound number for the months required to reach any specified level of TPH. Using less than 18 months would mean a faster reduction in TPH concentrations].
An exponential decay function reduces a starting concentration (in our case, the maximum TPH over the first 9 rounds of sampling) by a factor related to time, t, and a decay constant, k. After t units of time, e-kt of the starting concentration remains. If we let t equal 18 months, the percentages shown above for Round 9 versus previous maximum TPH concentration let us calculate k. For example, for a percentage of 45%
e-kt = e-k*18 = 0.45
k = 0.044, approximately
For a TPH cleanup level of about 500 ug/L, reductions from a starting value of 1,500 to 3,000 ug/L would mean remaining fractions of 0.33 to 0.167 (or a 3-fold to a 6-fold reduction). For a TPH cleanup level of about 700 ug/L, those same remaining fractions would mean starting concentrations of 2,100 to 4,200 ug/L. These are roughly the starting (maximum) TPH values shown above.
So here is the payoff: our simple exponential decay model, with the parameter value k derived from the reductions so far in ground water TPH values [as a range of k values], lets us estimate the number of months after the start of the ground water monitoring program when the reductions needed to hit the 500 to 700 ug/L range of cleanup levels will be achieved.
Target fraction remaining:
Round 9 fraction k parameter 0.33 0.25 0.167
0.40 0.051 21.7 27.2 35.1
0.45 0.044 25.2 31.5 40.7
0.50 0.039 28.4 35.5 45.9
0.65 0.024 46.2 57.8 74.6
0.70 0.020 55.4 69.3 89.5
0.75 0.016 69.3 86.6 111.9
These simply calculated values should be considered illustrative, and not predictive; as noted above, the simple exponential decay model may not be applicable at all wells, especially those that are still being influenced by ongoing sources of TPH.
What does this table show? For wells that have achieved reductions to 40 to 50% by Round 9, about 22 to 28 months would be needed for a 3-fold reduction from a starting concentration, 27 to 36 months for a 4-fold reduction, and 35 to 46 months for a 6-fold reduction. Appropriate ranges can also be read off for cases where reductions so far are less, to only 65 to 75% of starting values; obviously, the months needed to achieve 3-fold to 6-fold reductions are longer in those cases. These results suggest that the POC wells will not all be in compliance at the end of the 24-month monitoring program. And remember that by using 18 months for our exponential decay formula we end up with upper-bound values for the months required.
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