Wednesday, April 2, 2008 : 9:00 a.m.

Arsenic in and around Montezuma Well, a Natural Spring in Central Arizona: Evaluating Regional Ground Water Flow and Resulting Arsenic Transport

Raymond H. Johnson, Ed DeWitt and L. Rick Arnold, USGS

Arsenic is found naturally in some ground waters of Arizona at concentrations above the current U.S. EPA drinking water standard (10 µg/L). Arsenic-rich ground water in this area is commonly associated with springs and wells with a deep-seated source component, especially near fracture zones. A detailed study of the ground water emerging from Montezuma Well (a natural spring in Montezuma Castle National Monument) indicates a unique geochemistry for the region with high concentrations of arsenic (up to 110 µg/L) and carbon dioxide. An evaluation of the geology and ground water flow modeling indicate the presence of a basalt dike that influences ground water flow and geochemistry. This basalt dike appears to be a barrier for regional ground water flow and a locus for a component of deep-seated ground water flowing upward along bedrock fractures (hence the formation and unique geochemistry of Montezuma Well). Shallow, upgradient wells have arsenic concentrations near or below the drinking water standard, and no deep wells exist. Downgradient from Montezuma Well, both shallow and deep wells have elevated arsenic concentrations (up to 50 µg/L). This may occur because of ground water mixing due to the basalt dike and/or the screened interval of the wells. Evaluation of geologic scenarios and resulting ground water flow are critical in understanding which aquifers may or may not have high arsenic concentrations. In Arizona, the consideration of existing and future well locations and their pumping effects on ground water flow directions are important in limiting anthropogenic spreading of arsenic. This information will be useful to local ground water users in 1) avoiding the location of new wells in aquifers that could potentially be high in arsenic, and 2) using pumping and well completion scenarios that limit the spread of naturally occurring arsenic.

Raymond H. Johnson, USGS Raymond H. Johnson is currently a research geologist with the USGS in the Central Region Crustal Imaging and Characterization Team. He obtained his Ph.D. in geological engineering from the Colorado School of Mines in 2003 and an M.S. in hydrogeology from the University of Waterloo in 1993. His current research interest includes the use of integrated science approaches using geology, geochemistry, and ground water modeling to understand the transport and fate of metals in the environment.

2008 Ground Water Summit