Question: Are there ways to trace detected soil contaminants back to the polluting source?

@amocorro has marked @cherokeeconcernedcitizens as a co-author.

One common test for water is total organic carbon(TOC). Usually, it's a relatively small instrument that takes water and measures the amount of carbon in it. If it's a small amount, the instruments will do it by mixing the water with air and hitting it with UV . The amount of CO2 is then measured. But for higher amounts, there are usually chemical oxidants that are mixed with the water sample. The range that can be covered is usually large. These instruments were moderately expensive-don't know what they run now.

Some also claim to be able to handle soil samples. Sorry. I haven't used those. Although rinsing a soil sample is a possibility.

As far as tracing... to the best of my knowledge, you need to take a number of samples and find a pattern. If you can get access to data that has already been run, so much the better.

To identify individual contaminants, you are talking about GC/MS or other techniques. That will probably not be cheap.

A local electroplating plant discharged directly into the wastwater. Because of the color of electroplating solutions, this was visibly tracible. Hopefully, you will have the same luck.

This is a great question! Yes, to a certain extent you can trace contaminants back to the source. You can do this with a combination of approaches and both are typically needed if this is potentially a legal situation: 1] fate and transport modelling of releases from the source (ie- a plant or factory) to predict where the pollutants released from industrial emissions would end up and 2] taking samples (air, water, soil depending) and analyzing them by GC/MS as @Ag8n suggested. Finding black carbon, which is a by-product of burning diesel fuel, hints at pollution from big rigs and manufacturing whereas finding silica or chloride ions suggest natural sources, for example.