Natural Arsenic Distribution in the Socorro, NM area of the Middle Rio Grande Rift

Brandvold, Lynn. A. and Frisch, Patricia. L.
New Mexico Bureau of Geology and Mineral Resources, Socorro, NM 87801


Arsenic occurs naturally in groundwaters in the Socorro and San Antonio area of the Middle Rio Grande Rift in central New Mexico. The Socorro geothermal springs (33°C) have arsenic levels of ~40 ppb and have been used by the city as drinking water sources for over 150 years; two other municipal supply wells (20°C -23°C) have levels of 22-25 ppb. Some Bosque del Apache Wildlife Refuge irrigation wells, including one geothermal well (32°C), show elevated arsenic. The high arsenic groundwaters were studied in the field by Eh, pH, temperature, alkalinity, and conductivity. Some samples were also field separated for As(III) and As(V). Arsenic was in the As(V) form in all these samples. Major and trace element analyses were performed in the laboratory. The springs and wells in the study area are located in part of the Middle Rio Grande rift zone, which is characterized by north-south trending high angle extensional faults. Elevated levels of arsenic in the Socorro area are found in waters that spring from silicic volcanic rock and issue from a well-defined rift-fault line. These springs further coincide with an ancient hydrothermal system that formed several small ore bodies; minerals reported in this area include mimetite, (PbCl)Pb4(AsO4)3. In the Bosque del Apache, arsenic levels of 10 to 39 ppb are found in several wells that correlate with basin sediments that include volcanics and a known deep fault. The geothermal well is centrally located in this lineament. Groundwater geochemistry has not been correlative with high-arsenic waters. The relationship between rock type and arsenic concentration in water is not well defined in this study area. What is most evident is the linearity of anomalous arsenic wells. Our data suggest that arsenic is being supplied from deep circulating groundwaters that have long residence times and are enriched in arsenic. These arsenic enriched waters are mixing with shallow recharge and are issuing from springs in the Socorro area. The mid-range arsenic in the two municipal supply wells located down gradient from the springs is possibly a further result of mixing. In the Bosque del Apache, wells that intersect the Rio Grande rift faults show elevated arsenic and it is postulated that these deeper waters are the source of elevated arsenic in this study.