Iam not sure about it.

But I did this index for water, sediments and although to human through eating fish.

May be you can do it for plant if it consumed by human.

Also you measure the bio availability of metals in wAter and related its concentration to plant.

Please have a look OK my paper.

https://www.researchgate.net/publication/309463664_Environmental_Assessment_of_Heavy_Metal_Pollution_and_Human_Health_Risk?_iepl%5BviewId%5D=Ki80nMQW0zXgkL16uz7ucfAC&_iepl%5BprofilePublicationItemVariant%5D=default&_iepl%5Bcontexts%5D%5B0%5D=prfpi&_iepl%5BinteractionType%5D=publicationTitle

Regards

Article Environmental Assessment of Heavy Metal Pollution and Human ...

http://www.resilience.org/stories/2014-08-11/using-plants-to-clean-contaminated-soil/

Water quality can be indexed using the Bureau of Indian standards (BIS 10500 (2012)). For groundwater quality lot of studies have been done.