After a year of sampling (5 quarters), the benzene concentration at MW-4 (Monitoring Well # 4) has not decreased and instead has increased by about 17%. However, there has been decreases at MW-3 (17%), MW-2 (60%), and MW-1 (60%). These percentages were calculated based on initial samples taken in December 2003 with those taken in December 2004. Seasonal variation in watertable depth makes it difficult to assess absolutely improvements.
The increase at MW-4 is likely because there has been no pumping at the West Trench since October. The PRP had been renting the sparge tank and compressor and for cost saving reasons was going to purchase his own equipment. Unfortunately, before he could set up his own equipment there was a personal family tragedy which diverted his attention. I spoke with the PRP today and he intends to get the pump and sparge system back in operation within the next two weeks.
In any case, there has been no visible seeps or sheens into the South Yamhill River according to the PRP’s consultant. This likely is because the collection and sparge system was taken off line in the fall when the water table was at its seasonal low point.
The gasoline constituent (BTEX) concentrations at MW-3 and MW-4 have generally been found in these relative concentrations (from highest to lowest) with each of the quarterly samples: toluene, benzene, xylene, and ethylbenzene. At MW-2 the xylene is present in higher concentrations than benzene. With MW-1, all the constituents have generally been found at concentrations an order of magnitude less than the other three monitoring wells. Benzene is found at the highest concentration. Interestingly toluene, which has consistently been the highest concentrated constituent at the other monitoring wells, drops to become the lowest.
Generally, the concentrations of all constituents from highest to lowest concentrations at any one time are found at MW-4, MW-3, MW-2, and MW-1. This is as to be expected as MW-4 is closest to the release followed by MW-3. MW-2 is between the East Trench and the river and is the closet monitoring well to the seep into the river.
Quarterly Ground Water Sampling Results
MW-1
12-21-04: 1150/28.3/20.4/54.6
10-06-04: -/-/-/-
6-29-04: -/-/-/-
4-2-04:4480/950/1100/1860
12-30-03:2920/500/760/1340
MW-2
12-21-04: 7230/18600/1890/9790
10-06-04: -/-/-/-
6-29-04: -/-/-/-
4-2-04: 9990/27900/4080/18900
12-30-04: 18800/21900/3660/22500
MW-3
12-21-04: 18000/34500/2480/12000
10-06-04: 24200/26500/3440/19100
6-29-04: 22600/22500/1920/11300
4-2-04: 18000/24000/2340/13800
12-30-04: 21800/21800/2380/14400
MW-4
12-21-04: 36600/41100/2730/19600
10-06-04: 10900/19500/1860/13500
6-29-04: 17500/27800/1760/10900
4-2-04: 28100/43100/3130/18200
12-30-03: 33800/48800/3320/22800
Note: All results are provided in order of Benzene/Toluene/Ethylbenzene/Xylene and in units of micrograms per liter (ug/l)
- indicates that the well was dry and not sampled
Continue pumping and sparging ground water.
In 2 weeks, assess that pump and sparge system is back in service.
Sample monitoring wells at the end of March.
Enuring PRP rapidly puts the pump and sparge system back into service.
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