|
downhole sampling assemble with vacuum vials, steel prone rods with an
<br /> ' outside diamet„r of 1.75 inches, ar,1 a driving unit.
<br /> Soil gas probes will be advanced 5 re:. 20 feet belo-e grade at
<br /> approximately 40 )Pcations, szown in Figure 2. In oxdsr to select the
<br /> appy, ;riate sampling depth's), initial probes x4111 be advarced adjacent r-o
<br /> tha locations of the urd.trground storage tanks, Soil gas sampler dill be
<br /> obtained from -pproxim,te depths of. 7, 10, lJ, ar:d 20 feet cc evaluate
<br /> hither there as-e vertical concentration Gradients which should ce
<br /> con:*dared during L.1e sampling prograa, Additional, sampling locations are
<br /> everay distribut,d across the site, concentrating primarii.y on those areas
<br /> where hydrocarbons wage previrusl,� detected, namely M?d-1 and MW-2. Where
<br /> possible, a�hand-held slide harmer will be used to minimize disturbance to
<br /> ( landScaDe plantings, and the p;:oroserl grid pattern will be designed to 1•sve
<br /> minimal imp.-,et on the Holt Bro,,..hrr operations.
<br /> At eixch location, the ,mall dismne« p~obe and drive-tip
<br /> will be driven to a predetermined depth above Cie wat-er table. The '_o're-
<br /> evacuated s7,-Sli_.g assembly will be lowered througn the hollow cr,itzr of the
<br /> i prsbe rod tc collect the vapor sample, {Ln,.s
<br /> p procedure virtually eliminates .
<br /> contamination by a mvspheric gas giving t; rcl;scls and:reprortucible. raading
<br /> aL the concentrations of the hydrocarbcn vapor in chi. soil it the sample
<br /> iµ depth. Mach sample vigil is the same size, r, ntains a constant volume (if
<br /> gas, and is evacuated to -.he same cegres.: 11iireforp, the t:umbPt' of
<br /> variables associated with vapor sampling are raduced. Reducing variaciori in
<br /> the vFpor sampling pra,�edure provides consistency in sampling at di Uerent
<br /> depths.
<br /> After the sampling asseml-ly has filled with soil gas, the 'assembly is
<br /> 3-� retrieved from the probe, a syringe is inserted into t:he Sampling assembly
<br /> t� and a sample of the soil pore gas is withdrawn. The. gas sample is then
<br /> analyzed immediately for hydrocarbons and r0l. et .s using a Photnvac gas
<br /> chromatograph O.Q. After the raLtal probe is wiOidravn, rhe hole will be
<br /> tS backfilled :.with bento,,ite pellets to within a few inches of the a round
<br /> surface and then topped off with asphalt to the surface
<br /> neCection limits of the GC area function of zhs injection •v�olucia,
<br /> 6;t standards used for calibration of the GC, and detector sensitivity zor
<br /> individual compounds. Detection limit varies with the sample size; larger
<br /> i.tjectior. ,size generally results in greater sensitivity, If . ary
<br /> .� cornstitueni being analyzed is :not detacted, the detection limit for the~,
<br /> c:
<br /> compound in analysis is given as a "less than" ?value (<O.l ug/L) , ;This
<br /> number is calculates: from the current response factor, the sample size, and
<br /> the estimated minimum peak size (area) rhai, would have been vis;,bli .under
<br /> !.� the Conditlons of measurement, It shr-%lei be noted that coucentrr.L•ions of
<br /> VOCs detected in soil gas do no', dire,;cly correlate to concentr.-arions of
<br /> ! ' such VOCs in soil or grouna water. In addition, the soil gas probes cannot
<br /> h.6 describe whether the detected Lnmpounds reside in soile or groundwater.
<br /> The analytical methodology is only able to qualirate compounds pre-
<br /> - ;. 3 programmed into the internal library. 1z is not possible to quantitate total hydrocarbons since individti;1 alkane compounds (e.g. . pentane,
<br /> s hexane) will show a characteristic' peak. In ocher words, the soil gas
<br /> LD01.022 !,
<br /> tn4
<br />
|