These background values/reference values can be used for calculation of different indexes, viz. enrichment factor, geo-accumulation index, contamination factor, pollution load index and many more.
You can find them at CBSQG(Consensus-Based Sediment Quality Guidlines of Wisconsin),2003/ Department of natural resources , Wisconsin, December : (35 pp).
Besides the other sources, you can also search for articles that studied the background/ reference values of different heavy metals for a particular country or region of your interest.