Sample selection
The soil was in a large mound that had been terraced due to soil being removed over several years. Four areas were selected for analysis, with up to four trial pits per area being constructed. Samples were taken at surface and 1m intervals to 4m depth. Composite samples from each pit were also made by mixing portions of sample from each depth together.
12 duplicate samples were taken , with one set being analysed immediately by the on site method and the other set sent to an MCERTS accredited laboratory for high resolution GC-MS analysis for PAH as well as other parameters.
Screening for Benzo-a-Pyrene
The following example shows how on site analysis can be used to screen soils for Benzo-a-Pyrene above 1mg/kg
Site History
A housing development contractor wanted to import soil for a new housing development and was given a BaP limit of less than 1 mg/kg BaP for any imported soil. The soil selected for import was a clay based material with coal particles present, originally from a former brickworks. A concern had been raised that BaP may exceed the 1mg/kg threshold requiring a time consuming and expensive
Summary Findings
Case Study 1
Case Study 2
Sample ID
Total PAH On site
(mg/kg)
Total PAH lab
(mg/kg)
Sum 16 On site
(mg/kg)
Sum 16 lab
(mg/kg)
BaP
On site
(mg/kg)
BaP
Lab
(mg/kg)
A1 TP1 3m
4
33
0.8
2.4
0.05
0.12
A1 TP1 comp
4
0.2
0.7
0.5
0.05
0.03
A1 TP3 0m
22
10
2.6
1.6
0.17
0.04
A1 TP3 comp
7
1.1
1.0
1.7
0.07
0.11
A2 TP1 comp
9
22
1.0
0.3
0.07
<0.01
A2 TP1 2m
3
2
0.4
0.5
0.03
0.03
A2 TP2 comp
16
<0.1
1.7
0.8
0.11
0.02
A3 TP1 comp
23
10
2.7
2.5
0.18
0.10
A3 TP2 1m
46
25
5.3
3.3
0.35
0.18
A3 TP2 4m
8
<0.1
0.9
0.5
0.06
0.03
A3 TP3 comp
18
20
2.4
3.2
0.16
0.12
A3 TP4 2m
12
<0.1
1.7
1.4
0.11
0.49
The correlation for the sum of the 16 EPA PAHs is very good considering the the clay matrix was difficult to completely homogenise on site and in the lab. The presence of coal particles adds to the problem . The coal was thought to be the source of the PAHs
The BaP results from the on site method also shows good correlation with the GC-MS derived laboratory results. The on site method is a screening method and does respond to other PAHs, including Benzo-e-Pyrene, which has the same molecular weight and number of aromatic rings as BaP. For this reason the on site method tends to over estimate the actual BaP content of the soil. The data shows however that the on site screening method can reliably predict if soil samples are likely to pass or fail the 1mg/kg limit for BaP. The slight over estimation is a built in confidence factor to ensure soil exceeding the limit is not imported.
It is interesting to note that the lab data for the total PAH is often lower than the lab result for the sum of the 16 EPA PAHs. The sum of the 16 value is derived from high resolution GC-MS analysis and is likely to be the more accurate of the two analytical techniques used. As a general rule the sum of the 16 EPA PAHs is between 8 and 15% of the total PAH content. This anomaly does however highlight the problems inherent with all analysis, namely that the results are always dependent on the technique used. This is why it is important to know the limitations of any analysis requested and this is true for both laboratory and on site methods.
Case Study 3
Case Study 5