Mainly Roundup residues on GMO foods, as well as farm runoff in groundwater. EPA estimates we're all exposed to 0.058–0.230 mg/kg/day and they set the maximum acceptable daily dose at 1.75 mg/kg/day. But the available dosage science is weak.
Why mainly through GMO foods? I am wondering if other non-GMO ones would use mostly non Roundups, which would be a surprise to me given the availability of Roundups for vegetation control purpose around houses.