Ideally, you should be using the source rock/background composition if you know it. In most of the cases, people however use a shale or UCC data for convenience. If someone has used PAAS, and if you want to compare with them, you may use PAAS.

If you are dealing with a sediment core chemistry, the sediments which are presumably not contaminated may be there deep down. If you have sediment core of say 200 years old, you may use the elemental data of older sediments as a background value.

Please keep in mind that these are only indicative values. Certain elements may vary geographically because of changes in source rock lithology. The index/contamination values can only be compared after accounting for these changes in source rock composition. The excess values you may see after using PAAS/UCC may not necessarily mean "contamination" always.

Ideally, you should be using the source rock/background composition if you know it. In most of the cases, people however use a shale or UCC data for convenience. If someone has used PAAS, and if you want to compare with them, you may use PAAS.

If you are dealing with a sediment core chemistry, the sediments which are presumably not contaminated may be there deep down. If you have sediment core of say 200 years old, you may use the elemental data of older sediments as a background value.

Please keep in mind that these are only indicative values. Certain elements may vary geographically because of changes in source rock lithology. The index/contamination values can only be compared after accounting for these changes in source rock composition. The excess values you may see after using PAAS/UCC may not necessarily mean "contamination" always.

I also agree that the best choice would be the average composition of your source basin. But this information is often not available. UCC and PAAS are different, but not so much. You should consider your geological and petrological context. UCC is well suited for fresh rocks, being related to the upper continental crust as a whole. On the opposite PAAS is more appropriate when dealing with exposed and weathered rocks. But the differences between the 2 standards are not so relevant. You should obtain similar results when referring to typical elements related to pollution. Probably the only significant differences concern mobile elements, as Na, K etc, being them more concentrated in UCC than in PAAS.

Well,

the attached file is useful

Sincerely