Sources, occurrence and mobilization of arsenic related to iron oxide concretions in former redbed sandstones, Colorado Plateau 

Tong Guo¹, Grant Ferguson^1,2, Coleman Hiett³, Christabel Obi¹, David Vinson⁴, Peter Reiners³, Jennifer McIntosh^1,2 

¹Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA, ²Department of Civil, Geological and Environmental Engineering, University of Saskatchewan, Saskatoon, SK, Canada, ³Department of Geosciences, University of Arizona, Tucson, AZ, USA. ⁴Department of Earth, Environmental, and Geographical Sciences, The University of North Carolina at Charlotte, Charlotte, NC, USA.

Natural arsenic in drinking water remains a worldwide public health concern with arsenic often associated with iron oxide minerals. In the Colorado Plateau, the Navajo Sandstone aquifer is an important water source for rural populations and commonly contains elevated dissolved As, yet the source and mechanisms for arsenic mobilization are relatively unknown. Arsenic in bulk Navajo Sandstone is low (~10 ppm), but higher in siltier layers and the underlying Kayenta and Chinle formations (up to 500 ppm). Iron oxide cements collected along joints in the bleached Navajo Sandstone near the Escalante Anticline are enriched in As (>7000 ppm). This study combines new rock and water chemistry data with reactive transport modeling (RTM) using PFLOTRAN to constrain paleofluid flow and fluid-rock reactions that caused arsenic accumulation on iron oxides and eventual release to groundwater dependent on source, flow paths, and hydrogeochemical conditions.

Preliminary RTM results show that arsenic can be adsorbed onto iron oxides along joints in the Navajo Sandstone from upward migration of As-bearing reducing fluids in discharge areas. The Navajo Aquifer is relatively oxic with near-neutral pH, indicating arsenate (HAsO₄^2-) is the dominant arsenic species. The absence of correlation between dissolved As and Fe suggests As is not from reductive dissolution of As-bearing iron minerals (e.g., arsenopyrite). Rather, elevated As is associated with higher Na/(Ca)^0.5 ratios and longer residence times, suggesting cation exchange and desorption reactions with iron oxide concretions and/or other clay-rich units may release arsenic to groundwater, similar to other semi-arid oxic aquifer systems.

Current words count: 247 (Word limit: 250)