TPH CWG Criteria Working Group

The TPH Criteria Working Group was set up in the US to address a large disparity between the clean up requirements for sites contaminated particularly with hydrocarbon fuels. A comprehensive study was undertaken and the data was condensed into 5 volumes. This is probably still the most comprehensive set of TPH documentation available today and can be accessed via the aehs website. The Environment Agency’s UK Framework is a modified version of the TPH CWG and this can be downloaded from the Environment Agency website.

Analysis
To analyse TPH CWG two samples are required; a small 60ml jar for the volatile hydrocarbons (C5-C10) and a 125ml jar for the extractable hydrocarbons (C10-C40).

Volatiles: Headspace GCFID analysis for total C5 to C10 with quantitation of the BTEX aromatics, the aliphatic is then calculated as the difference between the total and the aromatics.

Extractables: The hydrocarbons are extracted from the ‘as received’ sample. The extract is split into the aliphatic and aromatic fractions by silica column chromatography. The aliphatics are eluted using a non polar solvent and the aromatics are eluted separately using DCM (see diagram). Each eluate is analysed by GCFID and the data split into the required carbon bands.

Banding Options
TNRCC UK Human Health Fractions Aliphatic Aromatic Aliphatic Aromatic
>C5-C6 C6-C7 (benzene) >C5-C6 >C5-C7 (benzene)
>C6-C7 >C7-C8 (toluene) >C6-C8 >C7-C8 (toluene)
>C7-C8 >C8-C10 >C8-C10 >C8-C10
(ethyl benzene & xylenes) (ethyl benzene & xylenes)
>C8-C10 >C10-C12 >C10-C12 >C10-C12
>C10-C12 >C12-C16 >C12-C16 >C12-C16
>C12-C16 >C16-C21 >C16-C35 >C16-C21
>C16-C21 >C21-C35 >C35-C44* >C21-C35
>C21-C35 >C35-C44*

* Current limit is to C40

Beware!
If an air dried sample is used for the extractable analysis, losses up to C14 can occur. This will result in erroneous data and all your petroleum hydrocarbons will look like weathered diesel. Single run TPH. If the volatile part is done on the same run as the extractables, the lower fractions cannot be resolved from the solvent peak, and losses may occur during the extraction process. A total TPH will give a higher result than the total of aliphatic and aromatic fractions. This is because very polar compounds (eg phenols) sorb so strongly to the silica that they are not removed by the DCM.