Potential bedrock source of groundwater arsenic anomaly in northeastern Chihuahua City, Chihuahua, Mexico

Contaminant sources of arsenic are often very difficult to identify. It is rare that specific rock units can be identified to which groundwater anomalies can be attributed. In this study, primary arsenic minerals, such as a Y-As bearing phase and a Sr-Al-As phase, have been identified in the Cenozoic volcanic tuff from El Mimbre area, at the northeast part of Tabalaopa Basin, the City of Chihuahua. Tabalaopa Basin is one of the sources for groundwater of the City of Chihuahua. The volcanic strata and the unconsolidated Quaternary deposit serve as the groundwater reservoir. The El Mimbre area demonstrates elevated groundwater arsenic concentrations, with 5 wells having values greater than 20 ppb. Small hills of Cenozoic volcanic tuff lie immediately up gradient to the northeast adjacent to the groundwater anomaly. Electron microprobe elemental x-ray maps have been applied to detect arsenic distribution in the samples. The volcanic rocks are reddish welded ash flow tuff and rhyolite with mainly sanidine, quartz, and biotite. The Y-As phase (a possible hydrated chernovite YAsO4) exists as inclusions in the biotite. The Y-As phase could be the primary arsenic mineral formed in the crystallization stage of the magma. After the eruption, the vapor phase from the diagenetic welding of volcanic tuff formed the euhedral Sr-Al- As phase in the lithophysal cavities. The wavelength peak scan and quantitative analysis present that the euhedral arsenic minerals are mainly arsenogoyazite (SrAl3[(OH)5(AsO4)2])H2O. There are euhedral alkali feldspars and ilmenite co-existing with arsenogoyazite, so the arsenogoyazite could be the vapor phase derived. The arsenic mineral petrogenesis sequences can be 1) the formation of chernoviet, 2) vapor stage alteration and the precipitation of arsenogoyazite in the cavities. It is hypothesized that the relative soluble arsenogoyazite is leached by meteoric water and provides the arsenic in the nearby groundwater. Based on current study, the arsenic anomaly in the Tabalaopa Basin aquifer can be mainly from the arsenic minerals in the adjacent volcanic rocks.