Chapin, C. E., and Dunbar, N. W.,

New Mexico Bureau of Geology and Mineral Resources, Socorro, NM 87801

Arsenic may provide a natural tracer of use in both surface and groundwater hydrology. The dissolved arsenic content of the Rio Grande and its tributaries in northern New Mexico is low (averaging 2 ppb). But, dissolved arsenic in the Rio Grande increases markedly at its confluence with the Jemez River, which drains the young Jemez volcanic field, and then remains elevated as the river flows along the eastern edge of the Datil-Mogollon volcanic field (averages between 4 to 5.3 ppb). Arsenic enrichment is typically associated with silicic volcanic rocks and associated water because such rocks are relatively high in As and because As is preferentially mobilized by magmatic and hot meteoric fluids associated with silicic intrusions. Arsenic is also enriched in rocks during the process of potassium metasomatism, which is widespread in the Basin and Range province of the western United States. The high arsenic values of the Rio Grande River south of Albuquerque, plus the lack of perennial tributaries in this reach, indicate that the Rio Grande is substantially augmented by groundwater. Arsenic concentrations in the Snake River are elevated in a similar fashion where the Snake flows through silicic volcanic terranes in southwestern Idaho. In contrast, arsenic concentrations in the Colorado River remain at low levels where the Colorado flows through an arid region containing silicic volcanic rocks, areas of potassium metasomatism, and basins containing volcanic-rich sediments between Las Vegas, Nevada, and Yuma, Arizona. Isotopic and hydrological studies show that the Colorado is a loosing stream along this reach.

The sorption properties of arsenic onto Fe and Mn oxide phases in oxidizing surface waters provide a mechanism for removing dissolved arsenic from river water. The dissolved arsenic in large rivers seldom exceeds 6 ppb, and although dissolved arsenic increases when a river passes through a silicic volcanic terrane, sorption processes cause As to decrease when there is no longer a direct input of As-rich water. Dissolved arsenic contents of small tributary streams, however, may be very high, such as Jemez River (28-66 ppb), which drains the Jemez volcanic field, New Mexico. A plume of higher temperature and high-arsenic groundwater extends southward from the Jemez Mountains to beyond Albuquerque, and helps to delineate groundwater flow and mixing paths in the northern Albuquerque Basin.

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