Arsenic and Arsenic Species in the Rio Grande, and the effect on irrigated lands

The Rio Grande was used for centuries and continues to be used for irrigation in New Mexico. In the Middle Rio Grande Valley arsenic concentrations of between 2 and 16 ppb have been reported in river water. Of concern is build up of arsenic in irrigated fields. To determine if arsenic is being adsorbed in the fields, irrigation water and drain water were measured weekly. The chemical behavior and mass balance of arsenic were evaluated to determine changes in the arsenic flux on a spatial and time basis. Waters were sampled and tested for arsenic species arsenite (3+), and arsenate (5+). Arsenic levels in waters used for irrigation had arsenate values as high as 13.4 ppb. For the drain, arsenite values as high as 14.3 ppb were obtained. We estimate that greater than 80 percent of the arsenic in irrigation waters remains in the irrigated fields. Preliminary analysis of irrigated soils indicates they contain 1.6 to 2.7 ppm labile arsenic, which is consistent with a century of arsenic accumulation. A major implication is that irrigated soils in the Middle Rio Grande Valley are an arsenic time bomb that could catastrophically release their arsenic in the future.