C - Hanford Site

WHC-SD-EN-TI-136, Rev. 0
following day. Visual inspection revealed the tank to be in very good
condition with no holes or damage that would compromise the integrity of the
tank. However, the bottom half of the tank was somewhat dirty, indicating
that it was in contact with liquid. The excavated area also looked wet in the
centerline of the pit. Additional investigation showed a loose pipe
connection on the delivery line leading from the tank to the pump island. An
eroded area of soil directly underneath the loose connection was observed. A
clay layer of saturated soil 15 to 30 cm (6 to 12 in.) deep at the bottom of
the efcavation appeared saturated with petroleum product. Approximately
2.7 m (3 yd 3 ) of this soil was removed and packaged into barrels for
disposal. Samples were collected at this time (IFigure 6). Another Unusual
Occurrence Report was completed on July 17, 1991, (RL--WHC-NREACTOR-1991-
1030). Analytical results verified the soil to be contaminated with petroleum
product. Maximum levels of toluene were 100 mg/kg and xylene 1800 mg/kg (See
Table 2 for sample analyses results and Appendix C for Unusual Occurrence
Reports).
Excavation of 100-N-SS-28 tank site resumed on September 14, 1991, to
remove the contaminated soil. Backhoe limitations prevented excavating deeper
than 7.6 m (25 ft) below grade surface (BGS). Using the bag technique, OVM
readings were approximately 312 ppm. There was no indication that the extent
of the contamination plume had been determined. Work was abandoned at this
time pending further evaluation. Excavation was continued on April 29, 1992.
A bench for the backhoe was excavated 1.5 m (5 ft) BGS in an attempt to remove
the remaining contaminated soil. At 10 m (33 ft) BGS, the OVM bag technique
was utilized. Readings of 440 ppm were detected in the head space of the bag.
Backhoe limitations were once again encountered at 10.0 m (36 ft) BGS, at
which point OVM bag readings were 760 ppm. Samples were collected for offsite
laboratory analysis (Figure 7). These sample results confirmed petroleum
product in the soil. Maximum Levels were toluene, 4.7 mg/kg; diesel, 1000
mg/kg; and kerosene, 3085 mg/kg (See Table 2).
On September 9, 1992, borehole sampling was implemented. The borehole
was positioned over the previous excavation area and soil borings were
collected to assess remaining contamination in the soil. The borehole started
at ground level, drilling through the backfill, and was to extend 1.5 m (5 ft)
below the lowest point of contamination detected by field instruments, or a
maximum of 16.8 m (55 ft) BGS. Sample collection began near contact between
clean fill and native soil, which was approximately 11 m (36 ft) BGS. Samples
were collected from the borehole cuttings at approximately 12.2, 14.3, and
16.8 m (-40, 47, and 55 ft) and sent offsite for analysis (Figure 8).
Analytical results from these samples were less than detectable (See Table 2).
As the borehole was abandoned, three vapor probes were installed at
approximately 6.1, 10.1, and 12.5 m (-20, 33, and 41 ft) depths. Field
screening analysis of vapors through these probes have been below detection
levels.
During the most recent excavation of this tank site (April 29, 1992),
petroleum product was present in the soil. The lack of contamination found
during soil boring activity could be a result of volatilization and
biodegradation of the petroleum contaminants. It is also possible that the
borehole did not intersect the remaining contamination.
Groundwater in the vicinity of the excavation is at approximately 18.2 m
(60 ft). Four groundwater wells are located near the old 100-N fuel station
(Tanks 100-N-SS-27/100-N-SS-28). These wells are 199-N-54, 199-N-55, 199-N-
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