Pesticide runoff? Arsenic is commonly used a pesticide. Best of all, it's considered organic, so farmers are encouraged to use it.

Both naturally from rocks and soil, and latent arsenic from decades of arsenic pesticide use. Arsenic pesticides have been banned for a while, but they’re still in the soil.

Rice, for whatever reason, can take up this element more readily than other crops. There has been some research on this: https://pubmed.ncbi.nlm.nih.gov/29749629/

what confuses me is not the origin of the arsenic in rice, but why there's more after you process the rice... where does it come from??

Depends on your definition of "process", but parboiled rice can use the same arsenic contaminated groundwater and can raise the arsenic content depending on the process used, whereas mechanical hulling reduces it.

Sure, I only buy rice grown in italy due to that they have a much lesser amount of arsenic in the ground.

Italian rice also doesn't have the methane problem.