Heavy metals are present in the soil/water in many regions. It's often a natural accumulation. For example the arid production zones in California and Arizona were highly contaminated naturally (and salty). When they started irrigating the land they deep plowed it to breakup the caliche layers. They then continuously flooded it to push the salt and water soluable heavy metals lower in the soil profile. Much of it ended up in the groundwater.

Some heavy metals like lead and arsenic were used as pesticides for almost 90 years. Most productions zones were contaminated during this period. Of course once you apply them, they don't disappear.

At this point, pretty much all production areas are contaminated to some degree. We have no good way to remediate the issue in the soil. So regulators focus on limiting the contamination from controllable sources.