Water Nitrate Effects Arsenic Valence and Concentration in an Urban Lake

Harold F. Hemond
Massachusetts Institute of Technology
P42ES04675

Background: Anyone who has ever read a mystery novel knows that arsenic is acutely toxic and that its harmful effects have been known and exploited for hundreds if not thousands of years. However, at doses found in some drinking water supplies and private wells in parts of the United States and other parts of the world such as Chile, Taiwan, and Bangladesh, arsenic can have chronic, long-term effects such as causing skin, bladder, and prostate cancer. In the environment, inorganic arsenic is found in oxidized and reduced states. The oxidized form is known as arsenate and this valence state is much more carcinogenic than the reduced form known as arsenite.

When lakes and streams become anoxic, underlying sediments tend to release arsenic and iron compounds. In most cases, the arsenic originates from industrial pollution. Arsenate can be bound up by iron particles rendering the arsenic unavailable to wildlife and humans who may drink or otherwise come into contact with the water. Therefore, any agent that changes this intricate balance or the valence state of the arsenic compounds may have important public health consequences.

Advance: This publication describes how nitrate contamination alters the balance of arsenic and iron complexes. Nitrates accumulate in water systems through agricultural and lawn fertilizer use and from animal waste runoff. Nitrate is a powerful oxidant and thus has the potential to oxidize arsenite released from lake and stream sediments to the more toxic arsenate. Depending on the concentration of iron in the water, arsenate may accumulate in the water.

Implication: With the new drinking water standard for arsenic of 10 G/l, some municipalities will have to pay high costs to bring their water supplies into compliance to protect the health of the people they serve. Understanding the causes of arsenic contamination and other factors influencing the concentration of arsenic and its oxidation-reduction cycle may influence the method used to reduce the arsenic concentration. Methods to prevent the introduction of nitrates into drinking water supplies may be important in keeping the arsenic in the lake and stream sediments and out of the water supply.

Citation: Senn DB, Hemond HF. Nitrate controls on iron and arsenic in an urban lake. Science 2002, vol. 296:2373-2376.