I was reading up on how to detect arsenic in soil, since the UMASS soil tests offered at such great low cost don't cover arsenic, yet that's one of the contaminants of concern here in Somerville, Massachusetts. I found this great thesis on using UV-Vis spectrometry (the kind we're trying to develop at PLOTS) with a kind of arsenic-binding dye (if I'm reading this right). Pretty dense, but offers some good leads for eventual lower-cost testing. Also a good quote here:
X-ray fluorescence spectroscopy is very well-suited for the analysis of soil samples since it provides a rapid analysis of multiple elements in a non-destructive manner. Portable X-ray fluorescence units are now available in a price range ($20k to $40k) that makes their inclusion in educational settings feasible.
https://kb.osu.edu/dspace/bitstream/handle/1811/45372/ChristinePellegrini_HonorsThesis.pdf (also attached to this post)