To avoid contamination, Wells in the Bengal Basin can be drilled into deep layers that were oxidized during the last ice age, in which the water is free of arsenic, Berg says.
The presence of exogenous surface contamination, including dirt, grease, free iron from contact with steel tooling, and so forth, may interfere with the formation of the passive film.
外生的表面污染，包括：尘土、油脂、来自钢铁加工的游离铁等等，可以影响到钝化膜的生成。
3
The standard states that the citric acid-sodium nitrate treatment is the least hazardous for removal of free iron and other metallic contamination and light surface contamination.