I am providing you the Digestion process for heavy metal analysis in soil samples (Ref., EPA Methods for digestion of soil samples) to assess the heavy metals in soil using AAS and Digestion process for heavy metal analysis in water samples
(Ref., EPA Methods for digestion of water samples) using AAS.
The key item to keep in mind is how much heavy metals and what metals are being leached from soil under various conditions such as temperature, pH and quality of water used for irrigation. One can develop this method and analyze the metals to infer more information.
If you need for analysis heavy metal or metalloid in water samples then you can follow anyone of my following papers.
Article Phytofiltration of arsenic by aquatic moss (Warnstorfia fluitans)
Article Effect of pH, temperature, and oxygenation on arsenic phytof...
So, whatever method you will use, keep one thing in mind it should either be published in peer reviewed journals or recognized as standard from some standard testing organization.