I found a paper reporting Lemna gibba as good boron accumulator. I do not know if this Lemna species is growing in Chille but may be some other Lemnaceae could be also used? The paper is attached - check the discussion section where might be information also about other boron-accumulating plants. In some other papers Lemna gibba and also other free-floating aquatic plants (especially Eichhornia crassipes) are reported to be a good arsenic-accumulator (see the attachment). Of course, you should consider the native flora of your region because if the species reported in the literature are not native, you should not use them (all of them may be invasive or expansive). Good luck!
Katerina
Article Boron removal by the duckweed Lemna gibba: A potential metho...
Article Accumulation of arsenic in Lemna gibba L. (duckweed) in tail...
Article Phytoremediation of Mercury and Arsenic from Tropical Openca...
you could contact katherine Lizama, from the university of Santiago, she's an expert in Arsenic en Boron (phyto)remediation in CWs (may be you already work with her...)