Arsenic in rice originates from groundwater, soil levels, and pesticides previously applied to rice fields. In flooded rice paddies, arsenite, an inorganic more toxic form is prevalent.
Rice absorbs more arsenite in flooded fields because it cannot differentiate arsenite and silicic acid. Rice likes the silicon because it strengthens the stem. It can absorb 10 x's more than other grasses. As it absorbs more silicon, it also absorbs more arsenite. (MaryLou Guerinot, Dartmouth College)
Rice can also be grown in aerobic conditions where fields are not flooded. Arsenate, another toxic form of arsenic is prevalent in this condition. Rice cannot differentiate arsenate from phosphate, a beneficial nutrient available in soil. As rice absorbs phosphate, it also absorbs arsenate.
Rice needs high levels of silicon and phosphate to strengthen, protect, and enhance its nutrition. The most obvious way to lower arsenic absorption is to lower the use of pesticides containing arsenic used in agricultural fields.
Recently, the F.D.A. has solicited pharmaceutical veterinary drug companies to phase out arsenic in more than 100 pet products. The use of arsenic in poultry has already been removed commercially.
Source: Urvashi Rangan, Consumer Reports Food Safety and Sustainability Center