Page 51 - Materials Chemistry, Second Edition

34 Practical Design Calculations for Groundwater and Soil Remediation

2.2.7 Determination of the Extent of Contamination:
A Comprehensive Example
This subsection presents a comprehensive example of calculations related to
common site-assessment activities.

Example 2.19: Determination of the Extent of Contamination:
A Comprehensive Example
A gasoline station is located in the greater Los Angeles Basin within
the floor plain of the Santa Ana River. The site is underlain primarily
with coarser-grained river deposit alluvium. Three 5,000-gallon steel
tanks were excavated and removed in May of 2013, with the intention
that they would be replaced with three double-walled tanks within the
same excavation.
During the tank removal, it was observed that the tank backfill soil exhib-
ited a strong gasoline odor. Based on visual observations, the fuel hydrocar-
bon in the soil appeared to have been caused by overspillage during filling
at unsealed fill boxes or minor piping leakage at the eastern end of the tanks.
The excavation resulted in a pit of 20′ × 30′ × 18′ (L×W×H). The excavated
soil was stockpiled on site. Four samples were taken from the piles and ana-
lyzed for TPH using EPA method 8015. The TPH concentrations were ND
(not detectable, <100), 200, 400, and 800 ppm.
The tank pit was then backfilled with clean dirt and compacted (the three
new USTs were installed at a different location that was not impacted). Six
vertical soil borings (two within the excavated area) were drilled to charac-
terize the subsurface geological condition and to delineate the plume. The
borings were drilled using the hollow-stem-auger method. Soil samples
were taken by a 2-in.-diameter split-spoon sampler with brass soil sample
retainers every 5 ft bgs. The water table is at 50 ft bgs, and all the borings
were terminated at 70 ft bgs. All the borings were then converted to 4-in.
groundwater monitoring wells.
Selected soil samples from the borings were analyzed for TPH (EPA method
8015) and B/T/E/X (EPA method 8020). The analytical results indicated that
the samples from the borings outside the excavated area were all ND. The
other results are listed here:

Boring no. Depth (ft) TPH (ppm) Benzene (ppb) Toluene (ppb)
B1 25 800 10,000 12,000
B1 35 2,000 25,000 35,000
B1 45 500 5,000 7,500
B2 25 ND (<10) ND (<100) ND (<100)
B2 35 1,200 10,000 12,000
B2 45 800 2,000 3,000