ABUNDANCE, RESIDENCE, AND MOBILITY OF ARSENIC IN SANTA FE GROUP SEDIMENTS, ALBUQUERQUE BASIN, NEW MEXICO

Groundwater and core samples have been characterized to better understand the abundance, residence, and mobility of arsenic (As) and related elements (e.g., iron, Fe) in Santa Fe Group sediments of the Albuquerque Basin, New Mexico. Because these sediments host major aquifers, much interest has centered on the origin and geochemistry of arsenic in the subsurface.

Groundwater at 4 levels in a recently-completed monitoring well is distinguished by different total dissolved arsenic [(As(tot), <2 - 42 ug/L] and redox species [arsenite, As(III); arsenate, As(V)] concentrations, total dissolved Fe concentrations [<0.01 - 0.044 ug/L], pH [8.3 - 9.9], and dissolved oxygen (DO). Redox conditions based on DO range from oxidizing (>1 mg/L) to mildly-reducing (0 - <1 mg/L), and both As(III) and As(V) are generally present.

Arsenic is relatively low to moderate in abundance (<2 - 25 ppm) within the sediments. A 5-step sequential partial extraction technique (SPE) was applied to fresh cores to determine As distribution and relative mobility among 5 operationally-defined solid phases. Step 1 mimics a water-soluble extraction and produces no detectable arsenic. Step 2 produces the highest soluble arsenic concentrations (<=2.2 mg/L in the extract solution). This “anion-exchangeable” arsenic is associated with clay and secondary iron oxide surfaces, and is the most likely source of soluble As in the aquifer. More-aggressive leach conditions that result in As solubilization from resistate minerals (e.g., silicates) are unlikely to exist in the aquifer.

Fission-track methods using uranium as a proxy for arsenic support iron oxides as major hosts for arsenic. Grains composed dominantly of iron oxide produce more fission tracks than iron oxide coatings on mineral grains, which in turn, show more tracks than grains from silicates. The results indicate that aqueous arsenic is produced by intrinsic rock-water interaction under geochemical conditions that allow As to be mobilized by desorption from mineral surfaces.